Gumboro disease in broilers continues to be a problem , and the best advice is to stay
using the hotter strains of vaccine , and to ensure by blood testing day old chicks that
the timing of vaccination is correct. Many sites which have vaccinated for considerable
time, still suffer the disease on removing the vaccine, and we now have to plan for the
continual use of Gumboro vaccine in most broilers.
CHAPTER 3.6.1.
INFECTIOUS BURSAL DISEASE (Gumboro disease)
SUMMARY
Infectious bursal disease (IBD) is caused by a virus which was difficult to
classify, but is now considered to be a member of the genus Avibirnavirus.
Although turkeys, ducks and guinea fowl may be infected, clinical disease
occurs solely in chickens. Only young birds are affected. Severe acute
disease is associated with high mortality, but a less acute, or subclinical,
disease is common. This can cause secondary problems due to the effect of
the virus on the bursa of Fabricius. IBD virus causes lymphoid depletion of
the bursa, and if this occurs in the first 2 weeks of life, significant
depression of the humoral antibody response may result. Two serotypes of
IBD virus are recognised; to date, clinical disease has only been associated
with, and vaccines made against, IBD type I. Recently it has been shown
that serological variants of IBD type I occur. These may require special
vaccines for maximum protection. Very virulent strains of IBD virus have
emerged and caused serious disease in many countries over the past
decade.
Clinical disease due to infection with the IBD virus, also known as
Gumboro disease, can usually be diagnosed by a combination of
characteristic signs and post-mortem lesions. Laboratory confirmation, or
detection of subclinical disease, can be carried out by demonstration of a
humoral immune response or by detecting the presence of viral antigen in
tissues. In the absence of such tests, histological examination of bursae may
be helpful.
Identification of the agent: Isolation of IBD virus is not usually carried out
as a routine diagnostic procedure. When this is required, some difficulty
must be anticipated. Cell cultures, chickens or embryonating eggs from
specific pathogen free or specific antigen negative sources may be used for
attempted virus isolation. The virus can be identified by the virus
neutralisation (VN) test.
The agar gel immunodiffusion (AGID) test can be used to detect viral
antigen in the bursa of Fabricius. A portion of the bursa is removed and
homogenised, and used as antigen in a test against known positive
antiserum. This is particularly useful in the early stages of the infection,
before the development of an antibody response.
Serological tests: Either an AGID, VN or enzyme-linked immunosorbent
assay may be carried out on serum samples. The infection usually spreads
rapidly within a flock of birds. Because of this, only a small percentage of
the flock needs to be tested to detect the presence of antibodies. If positive
reactions are found, then the whole flock must be regarded as infected.
Requirements for vaccines and diagnositc biologicals: Both live
attenuated and inactivated (killed) vaccines are available to control the
disease. It is important that live vaccines be stable, with no tendency to
revert to virulence on passage. To be effective, the inactivated vaccines
need to have a high antigen content.
Live vaccines are used to produce an active immunity in young chickens.
An alternative to this is to provide chickens with passive protection by
vaccinating the parents using a combination of live and killed vaccines.
Effective vaccination of breeding stock is of the greatest importance.
Live vaccines: Attenuated strains of IBD viruses are used. These are
referred to as either mild, intermediate, or 'intermediate plus' ('hot')
vaccines. The mild vaccines cause no bursal damage, while the
intermediate vaccines cause some lymphocytic depletion in the bursa of
Fabricius. None of the vaccine types results in immunosuppression when
used in birds over 14 days old.
Mild vaccines are rarely used in broilers, but are used widely to prime
broiler parents prior to inoculation with inactivated vaccine. Intermediate
and 'hot' vaccines are more capable of overcoming very low levels of
maternally derived antibodies (MDA). They may be administered by
intramuscular injection, spray or in the drinking water. In the absence of
MDA, the vaccines are given at 1-day old. When 1-day-old maternal
antibodies are present, vaccination should be delayed until MDA in most of
the flock has waned. The best schedule can be determined by serological
testing of the birds to detect the time at which MDA has fallen to a low
level.
Killed vaccines: These are used to produce high and uniform levels of
antibody in parent chickens so that the progeny will have high and uniform
levels of MDA. The killed vaccines are manufactured in oil emulsion and
given by injection. They must be used in birds already sensitised by primary
exposure, either to live vaccine or to field virus. This can be checked
serologically. High levels of MDA can be obtained in breeder birds by
giving, for example, live vaccine at approximately 8 weeks of age, followed
by inactivated vaccine at approximately 18 weeks of age.
A. DIAGNOSTIC TECHNIQUES
Two distinct serotypes of infectious bursal disease (IBD) virus are known to exist. Type I
virus causes clinical disease in chickens. Antibodies are com-monly found in other avian
species, but no signs of infection are seen. Type II antibodies are very widespread in turkeys
and are sometimes found in chickens and ducks. There are no reports of clinical disease
caused by infection with Type II virus.
Laboratory diagnosis of IBD depends on detection of specific antibodies to the virus, or on
detection of the virus in tissues, using immunological methods. Isolation and identification of
the agent are not usually attempted for routine diagnostic purposes (5).
1.
Identification of the agent
Clinical IBD has very characteristic signs and post-mortem lesions;
confirmation or detection of subclinical disease is best done by using
serological methods. The virus of IBD is difficult to isolate, and this is
not usually attempted in routine diagnosis. When there are special
reasons for attempting virus isolation, the methods described below
should be followed. Differentiation between serotypes I and II should
be undertaken by a specialist laboratory (e.g. the OIE Reference
Laboratories for Infectious Bursal Disease – see Table pages 707-721).
a)
Sample preparation
Remove the bursae of Fabricius aseptically from
approximately five affected chickens in the early stages of
the disease. Chop the bursae using two scalpels, add a small
amount of peptone broth containing penicillin and
streptomycin (1,000 µg/ml each), and homoge-nise in a
tissue blender. Centrifuge the homog-enate at 3,000 g for
10 minutes. The super-natant fluid is harvested and used for
the following investigations.
b)
Isolation of virus in cell culture
Inoculate 0.5 ml of sample onto each of four confluent
chicken embryo fibroblast cultures (from a specific
pathogen free [SPF] source) in 25 cm2 flasks. Adsorb at
37°C for 30 minutes, wash twice with Earle's balanced salt
solution and add maintenance medium to each flask.
Observe daily for evidence of cytopathic effect (CPE). This
is characterised by small round refractive cells. If no CPE is
observed after 6 days, freeze and thaw the cultures and
inocu-late the resulting lysate onto fresh cultures. This
procedure may need to be repeated at least three times. If
CPE is observed, the virus should be tested against IBD
antiserum in a tissue culture virus neutralisation (VN) test
(see below).
c)
Isolation of virus in embryos
Inoculate 0.2 ml of sample into the yolk sac of five 6-8day-old, specific antibody negative (SAN) chicken embryos
and onto the chorio-allantoic membrane of five 9-11-dayold SAN chicken embryos. Candle daily and discard deaths
up to 48 hours post-inoculation. SAN embryos are derived
from flocks shown to be serologically negative to IBD
virus. Embryos that die after this time are examined for
lesions. IBD produces dwarfing of the embryo,
subcutaneous oedema, congestion and haemorrhages. The
liver is usually swollen, with patchy congestion producing a
mottled effect. In later deaths, the liver may be swollen and
greenish, with areas of necrosis. The spleen is enlarged and
the kidneys are swollen and congested, with a mottled
effect.
IBD virus usually causes death in at least some of the
embryos on primary isolation.
d)
Isolation of virus in chickens
Inoculate, by eyedrop, five susceptible chic-kens and five
that are IBD immune (3-7 weeks of age) with 0.05 ml of
sample. Kill the chic-kens 72-80 hours after inoculation,
and exam-ine their bursae of Fabricius. The bursae of IBD
virus-infected chickens appear yellowish (sometimes
haemorrhagic) and turgid, with prominent striations.
Peribursal oedema is sometimes present, and plugs of
caseous material are occasionally found. The plicae are
petechiated.
The presence of lesions in the bursae of susceptible
chickens along with the absence of lesions in immune
chickens is diagnostic for IBD.
2.
Serological tests
a)
Agar gel immunodiffusion test
The agar gel immunodiffusion test (AGID) is the most
useful of the serological tests for the detection of specific
antibodies in serum, or for detecting viral antigen in bursal
tissue.
Blood samples should be taken early in the course of the
disease, and repeat samples should be taken 3 weeks later.
Because the virus spreads rapidly, only a small proportion
of the flock needs to be sampled. Usually 20 blood samples
are enough. For detection of antigen in the bursa of
Fabricius, the bursae should be removed aseptically from
around ten chickens at the acute stage of infection. The
bursae are chopped using two scalpels in scissor movement,
then small pieces are placed in the wells of the AGID plate
against known positive serum.
•
Preparation of positive control antigen
Inoculate 3-5-week-old susceptible chickens, by eyedrop,
with a clarified 10% (w/v) bursal homogenate known to
contain viable IBD virus1. Kill the birds 3 days postinoculation, and harvest the bursae aseptically. Discard
haemorrhagic bursae and pool the remainder, weigh and
add an equivalent volume of cold distilled water and an
equivalent volume of undiluted methylene chloride.
Thoroughly homogenise the mixture in a tissue blender and
centrifuge at 2,000 g for 30 minutes. Harvest the
supernatant fluid and dispense into aliquots for storage at –
40°C.
•
Preparation of positive control antiserum
Inoculate 4-5-week-old susceptible chickens, by eyedrop,
with 0.05 ml of a clarified 10% (w/v) bursal homogenate
known to contain viable IBD virus. Exsanguinate 28 days
post-inoculation. Pool and store serum in aliquots at –20°C.
•
Preparation of agar
Dissolve sodium chloride (80 g) and phenol (5 g) in
distilled water (1 litre). Add agar (12.5 g) and steam until
the agar has dissolved. While the mixture is still very hot,
filter it through a pad of cellulose wadding covered with a
few layers of muslin. Dispense the medium into 20-ml
volumes in glass bottles and store at 4°C until required for
use.
•
Test procedure
i)
Prepare plates from 24 hours to 7 days before
use. Dissolve the agar by placing in a steamer or
boiling water bath. Take care to prevent water
entering the bottles.
ii)
Pour the contents of one bottle into each of the
required number of 9 cm plastic Petri dishes laid
on a level surface. (Some laboratories prefer to
place the gel on 25 x 75 mm glass slides, with
wells 3 mm in diameter and up to 6 mm apart.)
iii)
Cover the plates and allow the agar to set, and
then store the plates at 4°C. Poured plates may
be stored for up to 7 days at 4°C. (If the plates
are to be used the same day that they are poured,
dry them by placing them opened but inverted at
37°C for from 30 minutes to 1 hour).
iv)
Cut three vertical rows of wells using a template
and tubular cutter.
v)
Remove the agar from the wells using a pen and
nib, taking care not to damage the walls of the
wells.
vi)
Using a pipette, dispense the test sera into the
wells as shown in Figure 1 so as to just fill the
wells.
OR
Dispense small pieces of finely chopped test
bursae by means of curved fine pointed forceps
into the wells as shown in Figure 2 to just fill the
wells.
vii)
Dispense the positive and negative control
reagents into the relevant wells.
viii)
Incubate the plates at between 22°C and 37°C for
48 hours in a humid chamber to avoid drying the
agar.
ix)
Examine the plates against a dark background
with an oblique light source.
•
Quantitative agar gel immunodiffusion tests
The AGID test can also be used to measure antibody level,
by using dilutions of serum in the test wells, and taking the
titre as the highest dilution to produce a precipitin line (2).
This can be very useful for measuring maternal or vaccinal
antibodies and deciding on the best time for vaccination.
b)
Virus
neutralisation
tests
VN tests are
carried out in cell
culture. The test is
more laborious and
expensive than the
AGID test, but is
more sensitive in
detecting antibody.
This sensitivity is
not required for
routine diagnostic
purposes, but may
be useful for
evaluating vaccine
responses.
First, 0.05 ml of
virus dilution
containing
100 TCID50 (50%
tissue culture
infective doses) is
placed in each well
of a tissue-culture
grade microtitre
plate. The test sera
are heat inactivated
at 56°C for 30
minutes. Serial
doubling dilutions
of the sera are
made in the diluted
virus. After 30
minutes at room
temperature, 0.2 ml
of SPF chicken
embryo fibroblast
cell suspension is
dispensed into each
well. Plates are
sealed and
incubated at 37°C
for 4-5 days, after
which the
monolayers are
observed
microscopically for
typical CPE. Endpoints are
determined using
the SpearmanKärber (1) or the
Reed & Muench
(7) method to be
the reciprocal
(log2) of the final
dilution which did
not show CPE.
c)
Enzyme-linked
immunosorbent
assay
ELISA tests are in
use for the
detection of
antibodies to IBD.
Coating the plates
requires a purified,
or at least
semipurified,
preparation of
virus, necessitating
specialist skills and
techniques.
Methods for
preparation of
reagents and
application of the
assay were
described by
Marquardt et al. in
1980 (6).
Commercial kits
are available.
d)
Interpretation of
results
The AGID test is
surprisingly
sensitive, though
not as sensitive as
the VN test which
will often give a
titre when the
AGID test is
negative. Positive
reactions indicate
infection in
unvaccinated birds
without maternal
anti-bodies. As a
guide, a positive
AGID reaction in a
vaccinated bird or
young bird with
maternal antibody
indicates a
protective level of
antibody. ELISA
gives more rapid
results than VN or
AGID and is less
costly in terms of
man-hours,
although the
reagents are more
expen-sive. VN
and AGID titres
correlate well, but
as VN is more
sensitive, AGID
titres are proportionally lower.
Correlation
between ELISA
and VN and
between ELISA
and AGID is more
variable depending
on the source of the
ELISA reagents. A
formula has been
devised that allows
ELISA titres to be
used to calculate
the optimal age for
vaccination (4).
B. REQUIREMENTS FOR VACCINES AND
DIAGNOSTIC BIOLOGICALS
Two types of vaccine are available for the control of IBD. These are live, attenuated vaccines
or inactivated oil emulsion adjuvanted vaccines (10). To date, IBD vaccines have been made
from type I IBD virus only, although a type II virus has been detected in poultry. The type II
virus has not been seen to be associated with disease, although its presence will stimulate
antibodies. Type II antibodies do not confer protection against type I infection, neither do they
interfere with the response to type I vaccine. Recently there have been descriptions of
serological variants of type I virus. Cross protection studies have shown that inac-tivated
vaccines prepared from 'classical' type I virus require a high antigenic content to provide good
pro-tection against some of these variants. Con-sideration is therefore being given to making
IBD vaccines that contain both classical and variant IBD type I viruses.
•
Live vaccines: methods of use
Live IBD vaccines are produced from fully or partially attenuated strains of virus, known as
'mild', 'intermediate' or 'intermediate plus' ('hot'), respec-tively.
Mild vaccines are used in parent chickens to produce a primary response prior to vaccination
near to point of lay using inactivated vaccine. They are susceptible to the effect of maternally
derived antibody (MDA) so should be administered only after all MDA has decayed.
Application is by means of intramuscular injection, spray or in the drinking water, usually at 8
weeks of age (8).
Intermediate vaccines are used to protect broiler chickens and commercial layer replacements.
They are also used in young parent chickens if there is a high risk of natural infection with
virulent IBD. Intermediate vaccines are susceptible to the presence of MDA, but are often
administered at 1-day old, as a course spray, to protect any chickens in the flock that may
have no, or only minimal, levels of MDA. This also establishes a reservoir of vaccine virus
within the flock which allows lateral transmission to other chickens when their MDA decays.
Second and third applications are usually administered especially when there is a high risk of
exposure to virulent forms of the disease. The timing of these will depend on the antibody
titres of the parent birds at the time the eggs were laid. As a guide, the second dose is usually
given at 10-14 days of age when about 10% of the flock is susceptible to IBD, and the third
dose 7-10 days later. The route of administration is by means of spray or in the drinking
water. Intramuscular injection is used rarely. If the vaccine is given in the drinking water,
clean water must be used, free of smell or taste of chlorine or metals. Skimmed milk powder
may be added at a rate of 2 g per litre. Care must be taken to ensure that all birds receive their
dose of vaccine. To this end, all water should be removed (cut off) for 2-3 hours before the
medicated water is made available. It is preferable to divide the medicated water into two
parts, giving the second part 30 minutes after the first.
Live IBD vaccines are generally regarded as compatible with other avian vaccines. However,
IBD vaccines that cause bursal damage could interfere with the response to other vaccines.
Only healthy birds should be vaccinated. Vaccine should be kept at temperatures between 2°C
and 8°C up to the time of use.
•
Inactivated vaccines: method of use
Inactivated IBD vaccines are used to produce high, long-lasting and uniform levels of
antibodies in breeding hens that have previously been primed by live vaccine or by natural
exposure to field virus during rearing (3). The usual programme is to administer the live
vaccine at about 8 weeks of age. This is followed by the inactivated vaccine at 16-20 weeks of
age. The inactivated vaccine is manufactured as a water-in-oil emulsion, and has to be
injected into each bird. The preferred route is intramuscular, into the leg muscle, avoiding
proximity to joints, tendons or major blood vessels. A multidose syringe may be used. All
equipment should be cleaned and sterilised between flocks, and vaccination teams should
exercise strict hygiene when going from one flock to another. Vaccine should be stored at
4°C-8°C. It should not be frozen or exposed to bright light or high temperature.
Only healthy birds, known to be sensitised by previous exposure to IBD virus, should be
vaccinated. Used in this way the vaccine should produce such a good antibody response that
chickens hatched from those parents will have passive protection against IBD for up to about
30 days of age (11). This covers the period of greatest susceptibility to the disease and
prevents bursal damage at the time when this could cause immunosuppression. It has been
shown that bursal damage occurring after about 15 days of age has little effect on
immunocompetence, as by that time the immunocompetent cells have been shed out into the
peripheral lymphoid tissues. However, if there is a threat of exposure to infection with very
virulent IBD virus, live vaccines should be applied as described above. The precise level and
duration of immunity conferred by inactivated IBD vaccines will depend mainly on the
quantity of antigen present per dose. The manufacturing objective should be to obtain a high
antigen concentration and hence a highly potent vaccine.
1.
Seed management
a)
Characteristics of the seed
•
Live vaccine
The seed virus must be shown to be free of extraneous viruses, bacteria,
mycoplasma and fungi, particularly avian pathogens. This includes
freedom from contamination with other strains of IBD virus. The seed
virus must be shown to be stable, with no tendency to revert to virulence.
This can be done by carrying out at least six consecutive chicken-tochicken passages at 3-4-day intervals, using bursal suspension as inoculum.
It must be shown that the virus was transmitted. A histological comparison
is then made to show that there is no difference between bursae from birds
inoculated with initial and final passage material. Bursal scoring and
imaging tech-niques have been developed.
Test for immunosuppression: An important characteristic is that the virus
should not produce such damage to the bursa of Fabricius that it causes
immunosuppression in suscep-tible birds. The vaccine is administered by
injection or eyedrop, one field dose per bird, to each of 20 SPF chickens, at
1-day old. A further group of birds of the same age and source are housed
separately as controls. At 2 weeks of age, each bird in both groups is given
one field dose of live Newcastle disease vaccine by eyedrop. The
haemagglutination inhibition (HI) response of each bird to Newcastle
disease vaccine is measured 2 weeks later, and the protection is measured
against challenge with 106.5 ELD50 (50% embryo lethal doses) Herts 33/56
strain (or similar) of Newcastle disease virus. The IBD vaccine fails the test
if the HI response and protection afforded by Newcastle disease vaccine is
significantly less (p <0.01) in the group given IBD vaccine than in the
control group. In countries where Newcastle disease virus is exotic, an
alternative is to use sheep erythrocytes or Brucella abortus killed antigen
as the test material, measuring the response with haemagglutination or
serum agglutination test, respectively.
•
Killed vaccine
For killed vaccines the most important characteristics are high yield and
good antigenicity. Both virulent and attenuated strains have been used. The
seed virus must be shown to be free of extraneous viruses, bacteria,
mycoplasma and fungi, particularly avian pathogens (9).
b)
Method of culture
Seed virus may be propagated in various culture systems, such as SPF
chicken embryo fibroblasts, or chicken embryos. In some cases,
propagation in the bursa may be used. The bulk is distributed in aliquots
and freeze-dried in sealed containers.
c)
Validation as a vaccine
Data on efficacy should be obtained before bulk manufacture of vaccine
begins. The vaccine should be administered to birds in the way in which it
will be used in the field. Live vaccine can be given to young birds, and the
response measured serologically and by resistance to experimental
challenge. In the case of killed vaccines, a test must be carried out in older
birds which go on to lay, using the recommended vaccination schedule, so
that their progeny can be challenged, to determine resistance due to MDA
at the beginning and end of lay.
•
Live vaccine
Efficacy test: Administer one field dose of the minimum recommended titre
to each of 20 SPF chickens of the minimum age of vaccination. Inoculate
separate groups for each of the recommended routes of appli-cation. Leave
20 chickens from the same hatch as uninoculated controls. After 14 days,
challenge each of the chickens by eyedrop with a virulent strain of IBD
virus such as CVL 52/70 (see footnote 1). Observe the chickens for 10
days. The vaccine fails the test unless at least 90% of the vaccinated
chickens survive without showing either clinical signs or severe lesions in
the bursae of Fabricius and if more than half the controls do not show
severe lesions of the bursa of Fabricius. Lesions are considered to be severe
if at least 90% of follicles show greater than 75% depletion of
lymphocytes. Providing results are satisfactory, this test need be carried out
on only one batch of all those prepared from the same seed lot.
•
Killed vaccine
Efficacy test: At least 20 unprimed SPF birds are given one dose of vaccine
at the recommended age (near to point of lay) by one of the recommended
routes, and the antibody response is measured by serum neutralisation with
reference to a standard antiserum2 between 4 and 6 weeks after
vaccination. The vaccine must induce mean antibody levels of at least
10,000 Ph. Eur. units per ml.
Eggs are collected for hatching 5-7 weeks after vaccination, and 25
progeny chickens are then challenged at 3 weeks of age by eyedrop with
approximately 102 CID50 (50% chicken infec-tive doses) of a recognised
virulent strain of IBD virus, such as strain CVL 52/70 (see footnote 1). Ten
control chickens of the same breed but from unvaccinated parents are also
challenged. Protection is assessed 3-4 days after challenge by removing the
bursa of Fabricius from each bird; each bursa is then subjected to
histological examination or tested for the presence of IBD antigen by the
agar gel precipitin test. No more than three of the chickens from vaccinated
parents should show evidence of IBD infection, whereas all those from
unvaccinated parents should be affected.
These procedures should be repeated towards the end of the period of lay,
but challenging the progeny when they are 15 days old. The test needs to
be performed once only using a typical batch of vaccine.
2.
Method of manufacture
The vaccine must be manufactured in suitable clean and secure accommodation,
well separated from diagnostic facilities or commercial poultry.
Production of the vaccine should be on a seed-lot system, using a suitable strain
of virus of known origin and passage history. SPF eggs must be used for all
materials employed in propagation and testing of the vaccine. Live vaccines are
made by growth in eggs or cell cultures. Inactivated IBD vaccines may be made
using virulent virus grown in the bursae of young birds, or using attentuated,
laboratory-adapted strains of IBD virus grown in cell culture or embryonated
eggs. A high virus concentration is required. These vaccines are made as waterin-oil emulsions. A typical formulation is to use 80% mineral oil to 20%
suspension of bursal material in water, with suitable emulsifying agents.
3.
In-process control
Antigen content: Having grown the virus to high concentration, its titre should
be assayed by use of cell cultures, embryos or chickens as appropriate to the
strain of virus being used. The antigen content required to produce satisfactory
batches of vaccine should be based on determinations made on test vaccine
which has been shown to be effective in laboratory and field trials.
Inactivation of killed vaccines: This is frequently done with either betapropiolactone or formalin. The inactivating agent and the inactivation procedure
must be shown under the conditions of vaccine manufacture to inactivate the
vaccine virus and any potential contaminants, e.g. bacteria, that may arise from
the starting materials.
Prior to inactivation, care should be taken to ensure an homogeneous suspension
free from particles that may not be penetrated by the inactivating agent. A test
for inactivation of the vaccine virus should be carried out on each batch of both
the bulk harvest after inactivation and the final product. The test selected should
be appropriate to the vaccine virus being used and should consist of at least two
passages in susceptible cell cultures, embryos or chickens, with ten replicates
per passage. No evidence for the presence of any live virus or microorganism
should be observed.
Sterility of killed vaccines: Oil used in the vaccine must be sterilised by heating
at 160°C for 1 hour, or by filtration, and the procedure must be shown to be
effective. Tests appropriate to oil emulsion vaccines are carried out on each
batch of final vaccine as described, for example, in the European Pharmacopoeia.
4.
Batch control
a)
Sterility
Tests for sterility and freedom from contam-ination of biological materials
may be found in Chapter I.4.
b)
Safety
•
Live vaccine safety test
Ten field doses of vaccine are administered by eyedrop to each of 15 SPF
chickens of the minimum age recommended for vaccination. The chickens
are observed for 21 days. If more than two chickens die, the test must be
repeated. The vaccine fails the test if any chic-kens die or show signs of
disease attributable to the vaccine or if, 21 days after inoculation, more
than moderate bursal lesions are present in any of the chickens. The test is
performed on each batch of final vaccine.
•
Killed vaccine safety test
Ten SPF birds, 14-28 days of age, are inoculated by the recommended
routes with twice the field dose. The birds are observed for 3 weeks. No
abnormal local or systemic reac-tion should develop. The test is performed
on each batch of final vaccine.
c)
Potency
•
Live vaccine potency test
The method described in Section B.1.c. may be used. The test need be
carried out on only one batch of all those prepared from the same seed lot.
•
Killed vaccine potency test
Twenty SPF chickens, approximately 4 weeks of age, are each vaccinated
with one dose of vaccine given by the recommended route. An additional
ten control birds of the same source and age are housed together with the
vaccinates. The antibody response of each bird is determined with
reference to a standard antiserum 4-6 weeks after vaccination. The mean
antibody level of the vaccinated birds should not be significantly less than
the level recorded in the test of protection. No antibody should be detected
in the control birds. This test must be carried out on each batch of final
vaccine.
d)
Duration of immunity (killed vaccine)
Evidence should be provided to show that progeny hatched from eggs
taken at the end of the laying cycle are as adequately protected as those
taken soon after vaccination. Information should be provided on the
duration of antibody levels in the breeders throughout the laying cycle. The
test may be performed on primed birds vaccinated by the recommended
schedule, but the final dose of vaccine is given at the earliest recommended
age and the final observations of progeny protection and anti-body levels
are made when the vaccinated birds are at least 60 weeks of age.
e)
Stability
Evidence should be provided on three batches of vaccine to show that the
vaccine passes the batch potency test at 3 months beyond the requested
shelf life.
f)
Preservatives
A preservative is normally required for vaccine in multidose containers.
The concentration of the preservative in the final vaccine and its
persistence throughout shelf life should be checked. A suitable preservative
already established for such purposes should be used.
g)
Precautions (hazards)
Oil emulsion vaccines cause serious injury to the vaccinator if accidentally
injected into the hand or other tissues. In the event of such an accident the
person should go at once to a hospital, taking the vaccine package with
him. Each vaccine bottle and package should be clearly marked with a
warning of the serious consequences of accidental self-injury. Such
wounds should be treated by the casualty doctor as a 'grease gun injury'.
5.
Tests on the final product
a)
Safety
See Section B.4.b.
b)
Potency
See Section B.4.c.
REFERENCES
1.
American Association of Avian Pathology (1989). Chapter 43. In Laboratory
Manual for the Isolation and Identification of Avian Pathogens. 3rd edition.
Kendall/Hunt Publishing, Dubuque, Iowa, USA.
2.
Cullen G.A. & Wyeth P.J. (1975). Quantitation of antibodies to infectious
bursal disease. Vet. Rec., 97, 315.
3.
Cullen G.A. & Wyeth P.J. (1976). Response of growing chickens to an
inactivated IBD antigen in oil emulsion. Vet. Rec., 99, 418.
4.
Kouvenhoven B. & Van der Bos J. (1993). Control of very virulent infectious
bursal disease (Gumboro disease) in the Netherlands with so called 'hot'
vaccines. Proceedings of the 42nd Western Poultry Disease Conference,
Sacramento, California, USA, 37-39.
5.
Lukert P.D. & Saif Y.M. (1991). Infectious bursal disease. In Diseases of
Poultry, 9th edition. Calnek B.W., ed. Iowa State University Press, Ames,
Iowa, USA, 648-663.
6
Marquardt W.W., Johnson R.B., Odenwald W.F. & Schlotthoken B.A. (1980).
An indirect enzyme-linked immunosorbent assay (ELISA) for measuring
antibodies in chickens infected with infectious bursal disease virus. Avian Dis.,
24, 375-385.
7.
Reed L.J. & Muench H. (1938). A simple method of estimating fifty per cent
end points. Am. J. Hyg., 27, 493-497.
8.
Skeeles J.K., Lukert P.D., Fletcher O.J. & Leonard J.D. (1979). Immunisation
studies with a cell-culture adapted infectious bursal disease virus, Avian Dis.,
23, 456-465.
9.
Thornton D.H. & Muskett J.C. (1982). Quality control methods for inactivated
infectious bursal disease vaccines. Dev. Biol. Stand., 51, 235-241.
10.
Thornton D.H. & Pattison M. (1975). Comparison of vaccines against
infectious bursal disease. J. Comp. Pathol., 85 (4), 597-610.
11.
Wyeth P.J. & Cullen G.A. (1979). The use of an inactivated infectious bursal
disease oil emulsion vaccine in commercial broiler parent chickens. Vet. Rec.,
104, 188-193.
1
2
A suitable strain of IBD virus (type I) is the strain 52/70, obtainable from CVL
Weybridge, New Haw, Addlestone, Surrey KT15 3NB, United Kingdom (UK).
For quantitative agar gel immunodiffusion tests, the British Standard serum is
available from CVL Weybridge (see footnote 1).
Infectious Bursal Disease
DESCRIPTION OF THE DISEASE
Infectious bursal disease (IBD) was discovered in 1957 in Gumboro, Delaware, USA.
As a result, the disease is often referred to as Gumboro. Not long after IBD was first
reported, it was being recognized in poultry populations throughout the world.
IBD is caused by a virus classified as a birnavirus. There are two basic serotypes, I
and II. Most isolates of serotype I are of chicken origin and most isolates of serotype II
are of turkey origin. Within each serotype, a variation in antigenic structure exists.
Variants within serotype I have been studied extensively since their discovery in 1985.
When IBD was first recognized, it was characterized by whitish or watery diarrhea,
anorexia, depression, trembling, weakness, and death. This clinical IBD was generally
seen in birds between three and eight weeks of age. The course of the disease runs
approximately 10 days. Mortality usually ranges from 0-30 percent. Field reports
suggest leghorns to be more susceptible to IBD.
Subclinical IBD was later recognized and is a greater problem in commercial poultry
than the clinical disease. It is generally seen in birds less than three weeks of age. No
clinical signs are generally seen. This early infection results in a lymphoid depletion of
the bursa of fabricius. The bird is immunologically crippled and unable to fully
respond to vaccine or field virus. In addition, the bird may be susceptible to agents that
are not normally pathogenic (adenovirus, clostridial infections).
In susceptible chickens, damage from IBD can be seen within two days of exposure to
virulent virus. Upon ingestion, the virus reaches the bursa via the blood stream or
through the opening that exists between the gut and bursa. Upon entering the bursa,
extensive replication occurs. Initially, the bursa swells (3 days post-exposure) and then
begins to atrophy (7-10 days). The bursal wall becomes thin and the internal folds may
be seen through the wall. Occasionally, hemorrhages may occur within the bursa and
the birds may pass blood with their feces. Variations in this progression of events have
been noted. Variant strains of serotype I, isolated in 1985, in the Delmarva area
(USA), have been shown to cause bursal atrophy in as little as three days postinfection.
The bursa is responsible for taking embryonic stem cells, received from the yolk sac,
and turning them into competent B-lymphocytes. From day 8 to 14 of incubation,
these stem cells enter the bursa. Once inside the bursa, these stem cells begin the
maturing process to B-lymphocytes. At day 17 of incubation, those B-lymphocytes
that have matured begin to migrate, through the blood stream to secondary lymphoid
organs. Migration of B-cells is also seen in the thymus. Secondary lymphoid organs
include the spleen, hardarian gland, cecal tonsil, and gut-associated lymphoid tissue.
This migration of B-lymphocytes peaks at three weeks and is complete by sexual
maturity.
IBD virus is cytopathic to only certain B-lymphocytes. The highest concentration of
these specific B-lymphocytes is found in the bursa. Destruction from IBD field virus
results in an incomplete seeding in the secondary lymphoid tissue. As a result, the bird
is immunocompromised and not capable of responding to other pathogenic agents or
vaccines
IBD virus can be found throughout the world. The occurrence of clinical IBD is
relatively low compared to the prevalence of subclinical Gumboro. The IBD virus is
very resistant to common disinfectants and has been found in lesser meal worms,
mites, and mosquitoes. These facts correlate with field experience of reoccurring IBD
problems on a farm, despite clean up efforts.
Infection with IBD results in a strong antibody response. Even in birds that have been
compromised by an earlier IBD exposure go on to produce high levels of antibodies
against IBD. But, response to other viruses will be negatively affected.
FACTORS TO CONSIDER BEFORE DESIGNING AN IBD
VACCINATION PROGRAM
The more information acquired prior to designing a vaccination program, the better the
chance of a successful program. Some of these factors include:
1. Virus quantity in the environment
2. Characteristics of the virus in the environment
3. Level of maternal immunity
4. Genetic resistance
5. Mixing numerous breeder flock progeny
Virus Quantity
IBD is a stable virus and is resistant to most common disinfectants. Phenolic
disinfectants have had some efficacy as well as formaldehyde fumigation.
Formaldehyde fumigation has the advantage of being able to permeate into areas that
are not accessible to liquid disinfectants. Once a house is seeded with IBD, it should
be considered; thereafter, to be a house problem.
Increasing the 'down time" between growouts has also been reported to reduce the
IBD challenge somewhat. By allowing the house to remain empty for 2-3 weeks,
removing old litter, and washing and disinfecting IBD challenge has been less evident.
Brooding on paper has met with varied success. In some operations, it has helped
reduce early exposure to Marek's Disease and IBD. This is especially true where built
up litter has been used. A disadvantage of putting down paper is it tends to trap
moisture resulting in high levels of ammonia and eventually caked litter.
Virus Characteristics
IBD viruses are not all the same. The field IBD viruses may vary in their virulence,
immunogenicity, and antigenic make up. Virulence refers to a viruses' ability to enter
the bird and destroy target cells and tissue (B-cells and bursa in the case of IBD). This
variation has been demonstrated most frequently by determining a viruses' ability to
infect a bird possessing varying amounts of maternal antibody (MA). The more
virulent the viruses are, the more capable they are of establishing themselves in the
face of high MA. Once they are established, seroconversion is noted.
Maternal Immunity
Vaccination of breeder hens with vaccine containing IBD has become widespread
practice throughout the world. This is done because of the ability of the hen to transfer
antibodies against IBD from her bloodstream to the chick's yolk sac. The transfer of
antibodies, from hen to chicks, is an efficient process.
Maternal antibodies (MA) are efficient neutralizers of IBD. This passive protection,
provided by MAs, prevents bursal atrophy and immunosuppression. These two criteria
(bursal atrophy and immunosuppression) need to be distinguished, because bursal
atrophy may be seen without immunosuppression. However, the levels of MA's
necessary to neutralize IBD vary with the invasiveness and pathogenicity of the virus
strain. In practical terms, if a "hot" (invasive, pathogenic) IBD challenge is present,
higher MA levels and/or an effective progeny vaccination program will provide the
desired protection.
Vaccination
The goal of vaccinating for IBD is prevention of subclinical and clinical Gumboro and
the economic aspects of each. In reality, vaccinating for IBD is not 'all or nothing'.
Instead of preventing infection of IBD, we are attempting to minimize the effects of its
infection. Due to our management systems and the biology of the virus itself,
prevention is often impractical.
Vaccination for IBD is approached by using two basic concepts.
1. High levels of maternal antibodies are protective
2. Effective vaccination in the field induces protection starting before 5 days
post-vaccination.
As has been discussed, maternal antibodies make vaccinating in the field difficult.
Because of this difficulty, there are a variety of vaccines available. Knowledge of
these vaccines is essential to effectively design a vaccination program for IBD.
MODIFIED LIVE VACCINES
There are two things to consider when examining a modified live vaccine. These
include:
1. Invasiveness - addresses the ability of the virus to replicate in the face of
maternal antibody.
2. Spectrum of antigenic content - addresses original seed strain and vaccine
preparation technique.
A vaccine virus' ability to replicate in the face of maternal antibodies allows live
vaccine to be categorized into three groups: mild, intermediate, and strong. These were
developed at different times in the history of IBD research and for specific reasons.
The initial vaccines for IBD were of the strong variety. These were often used in
breeder programs to induce high levels of circulating antibodies. However, when
given to a young bird with moderate (100-200 on serum neutralization (SN)) or low
levels (<100 on SN) of maternal antibodies, these vaccines could cause extensive
bursal atrophy resulting in immunosuppression. Mild vaccines were developed to be
used in young birds. These vaccines are not immunosuppressive even when used in
birds having no maternal antibodies. However, they are easily neutralized by moderate
and high levels (<100 on SN) of MA. As breeder programs developed (including the
use of adjuvanted killed vaccines), higher levels of maternal antibodies where
generated in progeny. This reduces the effectiveness of these mild vaccines.
Intermediate strength vaccines were developed to overcome the inadequacies of the
mild vaccines. They are capable of establishing immunity in birds with moderate
levels of maternal antibodies (100-200 on SN). These vaccines will cause some bursal
atrophy in MA negative birds. However, research and field experience has shown
them not to be immunosuppressive. The following table summarizes these
characteristics.
Type of Vaccine
Moderate MA
Mild
Intermediate
Strong
Ability to Overcome In Birds With
Low To Moderate MA
+
+
SPECTRUM OF ANTIGENIC CONTENT
Immunosuppressive
+
The spectrum of antigenic content of a live IBD vaccine is a newer characteristic to
consider. It has become evident that a variation in antigenic content exists in IBD field
isolates in some areas. There is also a variation in antigenic content within vaccines.
This variation is dependent upon the original seed strain-selected for the vaccine as
well as the technique used in the production process.
Techniques used in manufacturing IBD vaccines may involve some manipulation of
the original seed strain. Some manipulations may limit the variation in antigenic
content which naturally exists within the vaccine seed strain. An example of this is
cloning.
KILLED VACCINES
Inactivated IBD vaccines are used in broiler breeders throughout the world. They
differ in some of the same ways as live vaccines. Their efficacy depends upon
spectrum of antigens they contain. This is related to the original seed strain and the
manipulation of that seed strain. The wider range of antigenic spectrum, the increased
chance that the antibodies passed to progeny will neutralize the existing field
challenge viruses.
There are three basic ways antigen for killed vaccines are grown. These include tissue
culture origin (TCO), chick embryo origin (CEO) and bursal tissue derived (BTO).
BTO produces the highest quality antigen and the best immune response. This is
followed by CEO and then TC, being the least effective.
APPLICATION TECHNIQUES OF IBD VACCINE
Commercially available IBD vaccines vary on recommended application method.
Possible routes for application of live vaccines include subcutaneous, eye drop/nasal
drop, spray, and water. Injectable oil-emulsion products may be given subcutaneously
or intramuscularly.
Live vaccines must be given in a way in which the virus will reach the bursa where it
will multiply and initiate an immune response. When given subcutaneously, the
vaccine virus enters the blood stream and is transported to the bursa to replication.
This same scenario is also seen by eye drop/nasal drop, spray and water methods. Eye
drop/nasal drop and spray first are inhaled before entering the blood stream. IBD
vaccine given via the drinking water (as well as any virus swallowed in spray and
eyedrop/nasal drop applications) reaches - the bursa two ways. As it is swallowed,
some virus is absorbed through the gut lining into the blood stream. Virus that stays
within the gut can enter the bursa through the communication which exists between
the bursa and gut.
All of these methods of application are capable of working. However, it is best to
follow the manufacturers recommendations for each product. The application routes
on the label have been proven safe and efficacious.
Inactivated IBD vaccines are generally licensed with both subcutaneous and
intramuscular routes approved. Administration should be done carefully, as with all
injections.
VACCINATION
Effective vaccination for IBD can be divided into four categories:
1. Protecting the developing bursa (broilers, breeders, layers)
2. Preventing clinical disease (broilers, breeders, layers)
3. Priming (breeders)
4. Boosting (breeders)
PROTECTING THE DEVELOPING BURSA
The bursa needs to be protected from the immunosuppressive effects of IBD. This is
accomplished by preventing significant bursal atrophy. Immunosuppression resulting
from IBDV is age, dose, and strain related. The younger the bird, the more extensive
the immunosuppression. Protection from bursal atrophy for the first 14 days of life
prevents any permanent immunosuppression from occurring.
The higher the dose , the more permanent the immunosuppression. Birds exposed to a
high dose of very pathogenic IBD virus may be permanently immunosuppressed.
Birds exposed to a lower dose of the same IBD may not respond properly to the initial
Newcastle disease (NDV) vaccination but when vaccinated later are capable of
responding well. In other words, the immunosuppression was temporary.
APPROACH
To minimize the immunosuppressive effects of IBDV, each of these points (age, dose,
and strains) should be addressed.
Age
Protection to the very young can be achieved through maternal antibodies passed from
the breeder hen to her progeny. This requires an aggressive and well implemented
breeder vaccination program.
Vaccination of the very young chick itself may not be successful. Onset of protection
after vaccination is between three and five days. When a bird lacking MA protection is
introduced to a pathogenic field strain of IBD, the damage will be done in 24-48
hours. Field experience with vaccination in the very young (within the first week of
life) has yielded variable results due to MA interference and the points mentioned
above.
Dose
The dose of pathogenic IBDV, the young chick receives can be reduced through
management. The management practices that have helped reduce the quantity of IBD
field challenge include:
1. Cleaning and disinfecting between growouts (including removal of old litter).
2. Allowing the house to remain empty at least two weeks between growouts.
3. Brooding paper placed prior to housing new chicks
Strain
Attempt to replace the field strain of IBDV with a vaccine strain that is not
immunosuppressive. Once IBD invades the bursa, the virus is shed into the
environment in large numbers. This multiplier effect will be seen with either the field
strain or the vaccine strain. In field situations where vaccines have been used for
several consecutive growouts, a reduction in strain pathogenicity has been
experienced. Although not scientifically proven, the vaccine strain seems to replace
the field strain. With this in mind, vaccination programs should be evaluated over
three growouts.
PREVENTING CLINICAL DISEASE
Effective vaccination, avoiding MA interference, will help prevent clinical IBD.
Clinical IBD is typically seen between three and six weeks of age. This also coincides
with the time period where MA are rarely present. The immune response of the chick
must be stimulated as the passive protection (MA) is metabolized. By using
(estimating - where titers are not available) the breeder or chick titer and a MA halflife of four days, the timing of initial vaccination may be estimated.
MA tend to vary within population. This is due to the breeder hen variation as well as
progeny from several breeder flocks are often grown together. For this reason, it is
recommended that the initial 'vaccination be followed with a second vaccination 4-10
days later.
Clinical IBD is rarely seen after 8 weeks of age. One effective vaccination is sufficient
to protect birds for this time period.
PRIMING
Priming is a term that refers to preparing the immune system for a killed vaccine. This
involves introduction of live vaccine or field challenge a number of times so the bird
responds and makes memory cells to IBDV. By doing this, the optimal response is
seen from the administration of the inactivated antigen.
The early vaccinations serve as primers, although they may not be enough to create an
optimal amount of memory cells. The bird responds to the early vaccination by
creating memory cells and plasma cells and eventually, antibodies. The more effective
and complete these early vaccinations are, the more complete the priming. In most
situations, this is not considered adequate.
Field challenge may be suggested as the 'best primer". In actuality, it may well be;
however, it is unreliable. If relying on field challenge, a third of the flock may be well
primed, a third of the flock may be moderately primed, and a third of the flock may be
poorly primed. This results in an uneven boosting with the inactivated vaccine.
BOOSTING
Boosting is the term commonly associated with the administration of a final
vaccination prior to the onset of lay. This is done to increase the circulating antibody
in the hen. This, in turn, raises the MA passed to her progeny. Both inactivated and
live products have been used for this purpose, with inactivated being the more popular.
Live boosting was popular prior to the development of inactivated vaccines. The
strongest product available was often used at 3X dose. The use of a live vaccine in an
older bird will result in a boost; however, large variations are often seen. These
variations resulted in progeny becoming susceptible to field challenge from as early as
a few days of age out to 14 days of age.
The use of inactivated IBD vaccines gave a higher titer as well as decreased the
amount of variation seen between birds' responses. It is difficult to quantitate how
much higher, due to the many variables involved, but progeny were protected for as
many as 10 days longer. Progeny from breeders that are properly primed and boosted
with an inactivated IBDV vaccine are generally protected from 7 to 21 days within a
given operation. This variation may be 14-21 days for younger breeder hens (<40
weeks) and 7-14 days for older breeder hens (>40 weeks).
As breeders age, the titer to IBDV deteriorates slowly. Again, variations exist, but an
estimate of 1 log base 10 every 8-10 weeks may be a useful tool. Due to this aging and
the initial variations which existed, mixing progeny from old and new breeder flocks
complicates IBD field vaccination.
Modifications in boosting programs have been used in high challenge areas. In some
places, two inactivated boosters are administered 6 weeks apart with the last being 4
weeks prior to egg production. This is done to reduce the variation within the flock
even further. A second modification is to administer another inactivated boost while
the hens are in mid-production. This reduces the variation seen in MA titers from
progeny of old and young flocks. There are places in the world where both of these
modifications are practiced.
The most common mistakes seen in breeder IBD programs stem from inadequate
priming and poor injection technique with the booster. Priming needs to set the
foundation for the booster. If it is not solidly in place, disappointing results will be
seen thereafter. Administration of inactivated vaccines, in general, must be done
carefully and properly placed within the muscle of the breast or subcutaneously along
the neck. (Follow label directions.)
There are many inactivated products which contain multiple antigens, including IBD.
These products, if they have been USDA licensed, have had to pass efficacy tests
concerning each antigen they contain. Therefore, they are safe and effective. This does
not mean mixing two products within a syringe produces the same efficacy.
HOW TO MEASURE RESULTS
Performance
An IBD vaccination program is best evaluated by examining overall performance.
This must include livability, feed conversion, weights, and condemnations. An
effective program, when instituted on a problem farm, is capable of making
improvements in all the above categories.
Serology
Serology for IBDV has been done with several tests. These include mainly the enzyme
linked immunosorbent assay (ELISA) and virus neutralization (VN) tests. The two
have been found to correlate to some degree. It has been seen that VN tend to correlate
better with protection than ELISA. Quantitative agar gel precipitin (QAGP) test has
also been used. However, the QAGP is not as sensitive as the other two.
Serology for IBD must be examined critically. Monitoring titers to IBD is useful in
estimating when MA's reach a level that vaccination can be done effectively. This has
been mentioned previously. IBD serological tests are also useful in determining the
virulence of field challenge. Monitoring MAs throughout their decline, seroconversion
will eventually be seen from field strain. If this happens while MAs are still relatively
high ( > 200 on SN), the field challenge should be considered strong. The majority of
field strains seem to be intermediate strength and above.
IBD serology may be used in evaluating the priming vaccinations in breeders. Most
companies consider titers of 1:150 to 1:200 (on a VN test) appropriate priming. In
some areas, the cost of doing serology is prohibitive. In these situations, an "assurance
primer" should always be given at 10-12 weeks.
Breeder hen titers are often taken periodically to assure the desired MA's in progeny.
Most vaccination programs using an inactivated vaccine put out progeny with MA
lasting from 7 to 21 days depending on age, breed, environment, and other factors.
Examination of Bursal Atrophy
Examination of bursae as a bird ages is a useful, but often confusing, parameter to
examine. Bursal atrophy may be done by gross examination, bursa to body weight
ratio or histologic bursal score. Gross examination of the bursa is very subjective.
Experienced service people and flock owners commonly "have an idea" that the bursal
size is abnormal. Examination of the bursa should be done during post mortems but
future decisions should only be made after large numbers of birds have been examined
(as well as using a more exacting measurement tool, if possible). Examination of only
dead or cull birds must be taken in context. A bird may have atrophied bursa from a
number of other conditions including an excessive amount of stress, Marek's disease,
and aflatoxin.
Histologic Bursal Scores
A third method of quantitating bursa damage is by histologic examination. This may
then be quantitated on a 0 to 4 scale with 4 being the worst. A general scoring criteria
is listed below.
Bursal Lesion Scoring System
Level of Severity
Description of Lesions
0
No lesions
1
Mild, scattered cell depletion in a few follicles
1.5
25% of follicles are depleted of lymphocytes
2
Moderate, 1/2 of the follicles have atrophy or depletion of cells
2.5
75 of follicles are depleted of lymphocytes
3
Diffuse, atrophy of all follicles or depletion of cells in all follicles
4
Acute inflammation and acute necrosis typical Of IBD
The scores assigned to particular bursa may vary according to pathologist doing the
examination.
Interpreting Bursal Size
Correlating bursal size to performance at processing has not proven to be useful. The
important point seems to be when the bursal damage appeared and what caused this
damage. If the damage to the bursa was done prior to two weeks of age, the birds may
be immunosuppressed. If the bursal damage was done at five weeks on top of a
vaccination, there may be no effect from challenge at all. The point that becomes
evident here is in strong challenges, bursal damage may not be prevented by even the
best vaccination program. However, the birds process better and clinical IBD is not
seen. Correlating the time of bursal damage by field challenge with performance
would be a more useful tool.
Vaccines cause some bursal damage. In order to make intermediate vaccines effective
in the face of some MA, they had to be invasive. They also must not be
immunosuppressive when given to birds with no MA. The intermediate vaccines are
proven non-immunosuppressive by vaccination with Newcastle vaccine following
IBD vaccination and comparing the resulting titer with the titer of birds not given the
IBD vaccine. However, bursal damage will be seen.
There have been three parameters mentioned in evaluating an IBD vaccination
program: Performance, Serology, and Bursal Size. By far, the most meaningful is
performance. This remains the bottom line. As indicated, performance of a vaccine
should be done over at least three growouts.
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Waterville, Maine 04903-0255 USA
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or 1-800-639-1582
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Tech Services: Tech Services Vet@mainebiolab.com
PREVENTION AND CONTROL OF IBD
An effective IBD prevention and control program must involve an
effective breeder vaccination program, an effective biosecurity program,
and an effective broiler vaccination program. Immunization of breeders is
an important part of the IBD control program. Antibodies produced by
the hen are passed through the egg to the broiler chick. These maternal
antibodies, if present in adequate levels, protect the chicks against
subclinical IBD. An example of a comprehensive breeder vaccination
program where subclinical IBD is a problem might have a vaccine
schedule such as this: at 12 to 15 days of age -- IBD live; at 30 to 33 days of
age -- IBD live; at 85 days of age -- IBD live or inactivated; and at 120
days of age --IBD inactivated.
Revaccinate at 38 to 42 weeks of age with an inactivated IBD vaccine if
breeder titers are low or of poor uniformity. Routinely monitor breeder
IBD antibody titers to ensure vaccines are administered properly and that
the chickens respond appropriately.
Effective control of IBD in commercial broilers requires that field virus
exposure be reduced by proper clean-up and disinfection between flocks,
and that traffic (people, equipment and vehicles) onto the farm be
controlled. The development and enforcement of a comprehensive
biosecurity program is the most important factor in limiting losses due to
IBD.
Phenolic and formaldehyde compounds have been shown to be effective
for disinfection of contaminated premises. Efforts at biosecurity (cleaning,
disinfecting, traffic control) must be continually practices, as
improvement is gradual and often only seen after 3 or 4 flocks.
A third factor to consider in the IBD prevention and control program is
vaccination of the broilers to prevent clinical IBD. Three categories of
vaccines, based on their pathogenicity, have been described: 1) mild, 2)
intermediate, and 3) virulent. The intermediate type IBD vaccines are
most commonly used. These vaccines can stimulate the broiler to produce
antibodies earlier than the mild-type vaccines, without significant damage
to the BF as may occur with the virulent type vaccines.
The timing of broiler vaccination depends on the level of maternal
antibody present in the chicks. High levels of maternal antibody at the
time of vaccination will neutralize the vaccine virus. Thus, only a limited
active immune response results, and chickens will be susceptible to disease
as maternal titers decrease. If low levels of maternal IBD titers are present
in the chicks, vaccination may not be effective on farms contaminated with
virulent field virus.
Approximately 10 to 12 days are required after vaccination for chickens
to develop minimal protective titers. During this "lag time," chickens are
susceptible to IBD. In addition, virulent IBD viruses are able to break
through higher maternal titers than milder vaccine viruses. Thus, if IBD
field virus contamination on a broiler farm is high, nor broiler vaccination
can stimulate protection in the flock before damage occurs.
If the maternal antibody titer is not uniform in the broiler flock, multiple
costly vaccinations will be required. For example, some producers may
vaccinate broilers at one day of age and again at fourteen days of age. This
multiple IBD vaccination would be recommended when maternal titers
are poorly uniform, which results from poor vaccine administration in
breeders or when mixing broilers from different breeder flocks. In a
recent study, even a group of breeders that had fairly uniform IBD titers
had chicks with titers that were variable, with many chicks have little or
no maternal antibody protection.
Although the 1 day of age vaccination would be of little direct benefit to
broilers with high maternal titer levels, multiple vaccinations would
provide some protection to chicks with lower levels of maternal antibody
and would help reduce replication of IBD field virus and subsequent shed
in the poultry house environment.
The important factors to consider in the control of IBD are the prevention
of broiler losses through an effective IBD breeder vaccination program
(maternal titers) and decreasing exposure through a comprehensive
biosecurity program. Relying on broiler vaccination has met with only
limited success when not coordinated with effective breeder vaccination
and biosecurity programs.