1. MAIN FISH DISEASES
Vibriosis is one of the most prevalent fish diseases caused by bacteria belonging to the genus
Vibrio. Vibriosis caused by Vibrio anguillarum has been particularly devastating in the marine
culture of Salmonid fish.
Vibriosis occurs in cultured and wild marine fish in salt or brackish water, particularly in
shallow waters during late summer. It was originally believed that scavenger fish feeding
around the farms were the natural reservoir of V. anguillarum, and contact between fish seems
to be an important factor for the spread of this pathogen. However, there is evidence that V.
anguillarum is normally present in the intestinal microflora and food of cultured and wild
healthy fish. The temperature and quality of the water, the virulence of the V. anguillarum strain
and stress on the fish are important elements influencing the onset of disease outbreaks.
Etiology
The bacterium Vibrio anguillarum is a polarly flagellated, gram-negative, curved rod. The
causative agent, of this Vibriosis disease: V. anguillarum, was first described in 1909 as the
aetiological agent of the 'red pest of eels' in the Baltic Sea. An earlier report from the early
1800's, describing epizootics in migrating eels (Anguilla vulgaris) implicated a bacterium
named Bacillus anguillarum. The pathology of the disease and the characteristics of the
bacterium in these two reports suggested that the etiological agents were the same.
Vibriosis was not reported in North America until 1953, when V. anguillarum was isolated
from chum salmon (Oncorhynchus keta).
Vibrio anguillarum belongs to one of the halophilic groups of Vibrios and survives at different
salinities. Studies have shown that it is able to survive in sea water for more than 50 months.
More than twenty different serovars of V. anguillarum (designated O1 to O23) have been
described (Pedersen et al., 1999). Serovars O1 and O2 occur world-wide and are those most
often found in connection with diseases in fish (Toranzo, 1997), particularly in salmonids and
species of cod fish.
Transmission
Epidemiology
Outbreaks affecting close to 50 species of fresh and salt-water fishes have been reported in
several countries in the Pacific, as well as the Atlantic coasts. The losses produced by this
disease are so disastrous that Vibriosis caused by V. anguillarum has been recognized as a
major obstacle for Salmonid marine culture.
External & Internal Signs
The characteristic clinical signs of Vibriosis include red spots on the ventral and lateral areas
of the fish and swollen and dark skin lesions that ulcerate, releasing a blood exudate. There are
also corneal lesions, characterized by an initial opacity, followed by ulceration and evulsion of
the orbital contents. However, in acute and severe epizootics, the course of the infection is
rapid, and most of the infected fish die without showing any clinical signs.
Prevention and Control
General
Control of furunculosis and vibriosis is best achieved by maintenance of water quality, good
husbandry and low stocking densities. This is not, however, always possible, and where
outbreaks occur, treatment with antibiotics is the only option (Hjeltnes & Roberts 1993). In
areas where a disease is not endemic it is possible to exclude the causative agents by a
legislative policy such as


1) restrictions on importation/movement of live fish/eggs and
2) slaughter and disinfection in infected fish farms (Ellis, 1988).
Antibiotics
Treatment of established infections with antimicrobial compounds has been used for the control
of many infectious diseases (Ellis, 1988). However, its value is limited since clinically affected
fish do not eat and therefore cannot be treated. A successful treatment is dependent on a rapid
diagnosis and immediate treatment.
Vaccination
Vaccination has proven to be an efficacious method in preventing furunculosis and vibriosis.
Vaccines are preparations of inactivated antigens derived from pathogenic organisms, which
will stimulate the immune system to increase the resistance to disease from subsequent
infection by a pathogen.
The method of choice for prevention of the disease is therefore vaccination with inactivated
whole cells or subunits and an appropriate adjuvant for enhancing the immune response.
Pasteurellosis ( Photobacteriosis)
Introduction
Since 1963 when Pasteurellosis was first detected in striped bass and white perch in the United
States, this disease has become a worldwide concern in aquaculture. Photobacterium damsela
subsp. piscicida was known until recently as Pasteurella piscicida. It is a gram-negative rod
which causes a disease in fish known either as Pseudotuberculosis or Fish Pasteurellosis.
This serious problem in Japanese yellowtail culture can result in losses on individual farms of
up to 50%. The bacterium's taxonomic position as a bonafide Pasteurella has been questioned
for a number of years and based on small subunit ribosomal ribonucleic acid (rRNA) sequences;
whole deoxyribonucleic acid (DNA) relatedness and biochemical characterization, Gauthier et
al. (1995) have reassigned the bacterium to a subspecies of P. damsela. Some fish farmers and
scientists may still refer to the disease as Pasteurellosis.
Etiology
Photobacterium damsela subsp. piscicida is a Gram-negative, pleomorphic, non-motile
bacterium that grows slowly on NaCl-supplemented media.
Serologically, the strains found all over the world are highly homogeneous and there seems to
be only one serotype. However, ribotyping revealed genetic variation of different geographical
isolates.
Epidemiology
Photobacterium Damsela infections have been confirmed in the USA, Japan, Taiwan, Spain,
Greece, France, Portugal and Norway. Though previously confused with the other common fish
pathogens, Vibrio anguillarum and Aeromonas salmonicida subsp. Salmonicida, this bacteria
differs from the others as well as Pasteurella spp. Researchers have found that isolates from
several European countries, Japan and the USA are biochemically and antigenically similar,
with homogeneous lipopolysaccharide (LPS) electrophoretic patterns and membrane protein
profiles.
The disease is usually diagnosed at water temperatures above 20-22°C but the pathogen can be
isolated from clinical cases with low chronic mortalities in nurseries even at a water temperature
of 15°C. Therefore, Pasteurellosis is a "summer window" disease in the cage farms while it can
be diagnosed throughout the year in hatcheries especially if broodstock fish are carriers or when
water input is not UV or ozone treated.
External & Internal Signs
Clinical
Diagnosis
Photobacterium damsela causes a disease sometimes referred to as Pseudotuberculosis or Fish
Pasteurellosis. It can cause massive losses in fish populations, both cultured and in the wild.
The disease is characterized by numerous, white bacterial colonies throughout the internal
viscera, especially the kidneys and spleen.
The following external and internal signs and symptoms of Pasteurellosis can occur with the
two forms of the disease.
Acute septicaemia:



Dark coloration
Focal gill necrosis
Dark enlarged spleen and congestion
Chronic form:


Granulomata on kidney and spleen with greyish-white bacterial colonies of 0.51.0mm2
Purulent material in abdominal cavity
The pigmentation of the skin is usually altered (becomes darker or lighter). Slight bleeding can
be observed in the fin and head areas, while the gills become paler.
Laboratory
Tests
Isolation on NaCl-supplemented media shows the growth of small, shiny, translucent colonies
after 48 to 72 hours when incubated at 26° C. It does not grow on TCBS. When using the API20 E identification kit, a specific code (2005004) is obtained. It is sensitive to vibriostatic
compound O/129.
Gram staining reveals bipolar staining.
Commercial test kits, e.g., those based on agglutination with specific antibodies, are available
and easy to use (not requiring a specialised lab), using either infected fish tissue or purified
colonies. Specific antibodies can also be used in immunohistochemistry, ELISA,
immunofluores-cence, etc., but these techniques require specialised lab equipment.
PostMortalDiagnosis
Internal organs such as the spleen and kidney are often enlarged and paler. Haemorrhages are
sometimes observed on the liver. In the chronic stage of the disease, white nodules typically
appear on an enlarged spleen.
Histopathological examination shows an inflammatory, necrotic reaction caused by bacterial
septicaemia, eventually leading to granuloma formation. The bacteria are easily observed in
macrophages.
Viral Encephalopathy and Retinopathy
What is it?
Viral encephalopathy and retinopathy, also known as viral nervous necrosis, is a disease
affecting a wide range of fish, caused by a Betanodavirus, which is in the family Nodaviridae.
The disease has been seen to have the biggest impact in populations of sea bass.
Where and When Might it Occur?
The disease is thought to mainly affect fish in the larval or fry stage, but has been seen in older
fish.
The disease has been reported in all inhabited continents apart from Africa.
Transmission is believed to occur horizontally, through the water column and vertically (parent
to
offspring).
The rate of transmission may be influenced by stressors, including handling, repeated
spawning, high stocking densities, high ambient temperature and virulence of the particular
Betanodavirus
strain.
Sand worms of the family Nereidae, genus Nereis, collected in proximity to an infected farm
have
had
positive
detection
of
Betanodavirus.
Virus can survive for one year in the right environmental conditions (pH 2–9 and 15°C) and
can persist subclinically in infected live fish. Therefore, fish products and byproducts may
facilitate the spread of virus to unaffected areas.
Diagnosis
The disease often results in 50–100 per cent cumulative mortality over a period of 48 hours to
several
weeks.
Fish infected with the disease may show anorexia, abnormal swimming behaviours, including
erratic, uncoordinated darting, spiral and/or looping swim pattern; corkscrew swimming.
Hyperactivity, sporadic protrusion of the head from the water and fish resting belly-up (loss of
equilibrium)can
also
be
seen.
In some fish, a change in colouration is an important indicator of disease. Species differ in how
they are affected (e.g. barramundi show lighter colouration when affected).
Gross pathological signs are:





colour change—larval barramundi become lighter, but groupers become darker
blindness
abrasions
emaciation
overinflated swim bladder (the only significant internal gross pathological sign).
Microscopic pathological signs are:


vacuolation of central nervous tissues, including retina
intracytoplasmic inclusions in brain tissues as crystalline arrays or aggregates.
Lymphocystis Disease of Fishes
Description
Lymphocystis is an infectious viral disease of freshwater and saltwater fishes that causes cell enlargement
(many more times than normal cell size), also called hypertrophy, usually on the skin and fins. The enlarged
cell nodules may each reach 0.3 mm to greater than 2.0 mm in diameter, each becoming a virus factory.
Lymphocystis is the most common viral infection of aquarium fish, and has been reported in over 125 species
of freshwater and marine fishes.
The disease usually runs its course in 4 or more weeks (depending on species involved, water temperature,
and other variables) and then the enlarged cells rupture or slough off and release the viral particles into the
water. While infected, the fish may become slowed or weakened, or more visible, and thus be more prone to
predation or attack. If there are mouth lesions, the fish may have difficulty in feeding or may not be able to
feed. The low mortality rate some attribute to lymphocystis is mostly due to secondary bacterial or fungal
infections. I have worked with many thousands of fishes of various freshwater and salt water species and do
not recall a single death being directly due to a lymphocystis infection.
After lymphocystis lesions are lost, the host tissue heals up. Adhesions and scarring can occur during healing.
If the gills are affected, the fish can have difficulty breathing, especially if gill surface areas are destroyed
(no longer present), or adhesions or scarring occur and gill surfaces are thus reduced in surface area or
functional quality for oxygen uptake.
The viral particles in the water can go on to infect another fish of the same or closely related species. I suspect
the virus can also become dormant and remain viable in sediments for years. The virus may be stored for
years (for future research) by either freezing or freeze-drying the separated viral particles, infected tissues,
or infected whole fish.
The disease poses no known health hazard to humans. To decrease spreading this disease to other fishes,
infected fish should be buried or burned, and not thrown back into the water.
The viral agent of lymphocystis disease is an iridovirus (of the family Iridoviridae) called Lymphocystivirus
(genus name). Iridoviruses range from 120-300 nm (nanometer = one billionth of a meter) in diameter.
Iridoviruses have an icosohedral or 20-sided shape, and a DNA core.
I have worked on the disease in silver perch (Bairdiella chrysura), white-tailed damselfish (Dascyllus
aruanus), black-tailed humbug (Dascyllus melanurus), copper banded angelfish (Chelmon rostratus), Koran
angelfish (Pomacanthus semicirculatus), Moorish idol (Zanclus canescens), foureye butterflyfish
(Chaetodon capistratus), orbiculate bat fish (Platax orbicularis), queen angelfish (Holacanthus ciliaris), and
warmouth or goggle-eye (Lepomis gulosus).
Infection
Lymphocystis does show some host-specificity, i.e., each strain (or species) of lymphocystis can infect only
its primary host fish, or some additional closely related, fish.
DNA studies have showed that there are different species of the virus. This has been suspected for some time
because the viral particles from different fishes vary in size plus the virus from a fish usually will infect only
that species of fish or a few other species closely related to the primary host.
The virus enters through broken skin or injured tissue (usually skin or fin). If the virus gets into the blood
(usually via gill infections) then various internal organs can be infected. In l974 I showed that the spleen,
tissues behind the eye, eye, and many other internal organs can be infected via systemic infection. One can
easily infect fish by putting them into a bucket of water, introducing the virus, then injuring fish by vigorously
swirling a stiff bottle brush in the bucket. One can also run a sharp probe on the skin or tail (see warmouth
picture below) and expose the fish to the virus to infect them.
Incubation times (until lesions are visible to eye) range from about 10 days at 25 C to longer, depending on
species involved, temperature, and other variables.
Important or valuable affected fish should be isolated and monitored for secondary bacterial or fungal
infections that should be treated with appropriate drugs.
In 1979 I discovered that goggle-eye (warmouth), when subjected to heavy rains and sediment loads, came
down with lymphocystis. It is unknown, in this case, whether stress or physical injury led to the lymphocystis
infections. Stress to the fish, in this case, could be from exposure to sediments in the water leading to
breathing problems, from getting tired trying to maintain their position in the swift water, from being exposed
to toxins swept in by the water, etc. There is also the possibility that sediment or debris particles hitting the
fish in the swiftly moving water caused injury that led to the infections (similar to injuring fish in a bucket
by swirling a brush).
Control
There is no way to cure a viral disease in any organism yet, no matter what some of the fish medications
claim. Some virus diseases in various organisms can be prevented by vaccination, or slowed down by
medications. There is no vaccine for lymphocystis yet.
Some medications claim to cure lymphocystis in several days to a week. However, depending on the stage
of the disease, many enlarged cells will burst and disappear on their own (without using the "cure"), making
some think their chemical has cured the disease. One can also excise the enlarged cells, affecting an apparent
"cure," but, in reality, just removing most of the infected cells (which then can no longer be seen).
Lymphocystis cannot be "cured."
2. Vaccinations (general )
Vaccines are marketed in Greece to protect against serious bacterial systemic diseases of farmed
sea bass (Dicentrarchus labrax) and sea bream (Sparus auratus). They comprise formalin killed
bacterins or oil adjuvanted products. Products that may be administered orally by mixing with
the fish feed are also gradually appearing on the market. The available vaccines may offer
protection against several pathogenic serotypes of Vibrio anguillarum causing vibriosis to sea
bass, and/or Photobacterium damsela subsp. piscicida (Pasteurella piscicida) causing
pasteurellosis (or pseudotuberculosis) to both sea bass and bream.
The different products that are marketed at present are either monovalent, that is, offering
protection against one bacterial strain or multivalent, that is, protecting against more than one
bacterial strains. The application methods and the vaccination schemes that are suggested by
the vaccine distributors differ, but all employ either the immersion of the fish in groups in a
suitable vaccinal dilution and/or the intraperitoneal injection of individual fish with a specific
dose of undiluted vaccine. Oral vaccination is by far less common since oral vaccines have yet
to realise consistent good results.
The practical needs in terms of implements, workforce and time, differ according to fish size,
the type of holding unit (net pen, tank, raceway) and the application method employed. Injection
vaccination on the cages at sea is considerably more time consuming and labour intensive, since
the fish have to be completely anaesthetised and treated individually. On the other hand,
immersion vaccination of the fish proceeds fast as groups of fish are added simultaneously to a
vaccine dilution. The vaccination schemes depend on the epizootiology of the site and the
duration of the production cycle compared to the expected period of immunity cover as well as
on the availability of time and labour.
General principles for vaccination
Vaccination is recommended only to healthy fish, which are not under any form of stress.
Vaccination is not indicated when fish are suffering a disease outbreak or have been through
recent severe handling or other environmental stress.
The fish should be deprived of food prior to vaccination in order to have empty gastrointestinal
tract at vaccination. The smaller the fish size is and the higher the water temperature the smaller
the required fasting interval. Fasted fish suffer less handling stress and respond better to
anaesthetics. Vaccination must be performed in a disease free environment and precede
exposure to disease or transfer to a disease prone site by 12 days when water temperatures range
around 17oC ( 200 degree-days). Smaller intervals are required for the establishment of
immunity at higher water temperatures.
At dip vaccination, the temperature difference between vaccinal dilution and holding water
should not exceed 2oC.
Immersion (dip) vaccination
Large groups of fish are cut off from the rest in a cage and enclosed in a tarpaulin where a slight
dose of diluted anaesthetic is added to sedate them. Air or oxygen is continuously pumped in
to avoid anoxia.The proper vaccine quantity is calculated according to the estimated biomass
of the fish to be vaccinated (usually for every 100kg of fish one litre of vaccine is required).
The vaccine is diluted with sea-water in a suitable receptacle (the common dilution rate is 1:10,
that is, 9lt of water added for each 1lt of vaccine). Oxygen may advantageously be trickled
through the vaccine dilution to reduce stress.The sedated fish are netted out of the tarpaulin in
lots of approximately 0.5kg, avoiding overcrowding or crushing. Holding water is drained from
the fish, which are then placed in the vaccine dilution where they are allowed to swim for a
certain minimum time, usually  30 seconds.
Common practice is to place the netted fish from the tarpaulin in a perforated plastic bowl
within the dilution container. When immersion time is up, the fish are withdrawn from the
vaccine dilution and released in their holding facility.
Groups of caged fish are cut-off in a tarpaulin where oxygenation is provided and a light
dilution of anaesthetic is added for sedation. The fish are netted out in small groups, drained
for a few seconds from sea water and immersed in the dilution of vaccine. Usually a perforated
plastic container is used to hold the fish in the vaccinal dilution.
The sedated fish remain in the perforated bowls for at least 30sec. Then the bowl is up-lifted
from the vaccine dilution and is left to drain for a few seconds prior to transferring the
vaccinated fish to their cage or raceway.
After drainage, the vaccinated fish are carefully released into the receptor raceway or cage,
situated next to where the immersion takes place.
Intraperitoneal injection
When the sea bass are sufficiently large to be individually handled (>10g average weight) the
vaccine may be safely administered undiluted by injecting them intraperitoneally using
injection guns. Groups of fish are cut off from the rest in a cage and enclosed in a tarpaulin as
for immersion, but because the fish are larger, a smaller number of them is enclosed. Extra care
is also required when supplying the anaesthetic since the larger the fish the grater the risk of
self-injury due to stress reactions.
Subsequent to sedation in the tarpaulin, small groups of fish are netted out and placed in a
container with a higher dose of anaesthetic dilution until completely immobile.
The anaesthetised fish are then taken to a "vaccination table" where they are handled
individually. The vaccination table consists of one or more troughs filled with water where the
immobile fish are presented to the operators floating belly up.
A certain dose of vaccine (usually 0.1ml to 0.2ml) is injected in the abdominal area of each fish
held with the ventral side up and the head away from the operator΄s body. The needle is inserted
into the peritoneal cavity at a 45o angle. Needle lengths range from 3mm to 6mm depending to
fish size at injection. Shorter needles for small fish require short bevel lengths and suitable
gauge calibre. Thus, proper needle selection has to be seriously considered. Automatic injection
guns accepting luer lock hypodermic needles are used for this purpose.
Subsequent to injection the fish are released into their holding unit where they recover from
anaesthesia in a few minutes.
Usually, the vaccination table is constructed in a way that allows simultaneous easy grading of
the injected fish into size groups. The fish are released according to size class into distinct
channels. Water is pumped from the sea into the channels and flushes the fish along tubes
leading them to different cages.
The table, pumps, tubing and all other implements are often installed on a floating platform.
Costing immersion (dip) vaccination
Appreciating the cost of immersion vaccination is relatively easy, since all necessary
implements are cheap and either used for other tasks and hence, readily available, or can be
quickly made in-house. For example, ordinary household plastic bowls can be purchased from
super markets and perforated with an electric drill on farm.
The major cost items comprise consumables, such as vaccines, anaesthetics, fuel, oxygen, etc.
Labour costs may not be included in the calculation when the vaccinators are drawn from the
regular farm workforce. Fish losses due to handling stress or mishaps are often negligible when
immersion vaccination is performed methodically and not in a haphazard manner. Anyway, the
fish are still relatively small and of low value.
Costing injection vaccination
Apart from the consumable items, which coincide with those of the immersion administration
(vaccines, anaesthetics, fuel, oxygen), injection vaccination requires considerable investment
in infrastructure. A spacious, steady working platform (raft) is paramount. The vaccination
table has to be ergonomically designed and made according to its expected use, that is, not only
for injecting fish but also for grading and counting. A powerful enough water pump is important
in order to supply plenty running water to the grading channels. Injection guns do not comprise
a major cost element but need to be meticulously maintained after use in order to last. 5 to 6
“guns” plus at least one spare have to be available for each vaccination crew of 8 people.
On large-scale fish farming operations it has been tried to adjust the automatic injection
machines that are used for salmon vaccination in Northern Europe. Success with sea bass has
been limited however, because it is a scaly fish with less skin mucus and far more vulnerable
than salmon to handling stress and injury. Hence, injection vaccination of sea bass remains a
labour intensive operation and often, casual labour is employed. Labour constitutes a serious
cost element when budgeting for vaccination costs.
In addition, fish losses due to handling stress and trauma should be accounted for. Not only can
they reach 1% of injected fish or more, but also the fish are larger and hence of greater value.
Vaccination strategies for sea bass and sea bream
Vaccination strategies for sea bass against vibriosis
Sea bass frequently suffers vibriosis outbreaks at any stage during their grow-out period. On
the other hand, pasteurellosis may become a problem for bass mainly during the first summer,
or until the fish reach about 70g of body weight.
Immunity lasts longer the older/heavier the fish are. Immersion vaccination of small bass, 1.5g
to 2g average weight, against vibriosis should provide effective cover for about six months,
whereas when the fish are larger at vaccination, between 10g to 20g average weight immunity
against vibriosis should last for a whole year.
Therefore, when the production cycles are short, that is, bass is grown to market size in 16 to
18 months, then two vaccinations by immersion would suffice to protect the stocks against
vibriosis. That is, fry are immersion vaccinated at 1.5g to 2g and a repeat vaccination, also by
immersion, is performed when the fish reach about 15g to 20g of average weight.
When the production cycle of bass until market size is considerably longer, or in the cases
where the fish are to be marketed at much larger sizes than the usual 350g, injection vaccination
is necessary.
It may be combined with counting and grading of the fish by size into distinct groups. In such
cases, the vaccination plan consists of an immersion application when the fish are about 2g to
8g and a second application by injection when the fish obtain an average weight of between
60g and 100g. Since injection vaccination is inherently a precisely dosing technique applicable
to larger fish with mature anosopoietic system, immunity is expected to last for more than a
year post injection. Thus, this plan ensures immunity cover over longer production cycles, even
up to 24 months. However, vaccination schemes involving larger fish have been compromised
after the emergence of large, open-sea circular cages in modern fish farming holding hundreds
of thousands of fish.
After the fish are transferred into these cages (often 120m circumference) vaccine
administration becomes a practically difficult process. It is excessively hard for the operators
and stressful to the fish to administer vaccines when the fish are held in such large
confinements, unless “helper cages” are used to break down the population increasing the time
and labour needed. Therefore, the fish have to be injected early and as soon as they reach a size
suitable to be handled and injected individually.
Currently, many farms growing millions of fish in large cages inject their sea bass fry when
only around 10-15g. Thus, vaccine administration is performed once during the production
cycle. Due to labour scarcity, fish growers start injecting their fish as soon as they reach
“handling size” and continue injecting for months utilising vaccination crews as the young fish
come out of the smaller holding units and prior to their transfer into the large unmanageable
cages. Timing is crucial for such long lasting work.
Vaccination strategies for sea bream against pasteurellosis
Sea bream is a far more resistant fish to environmental stress and suffers from bacterial diseases
mainly when young. As it grows its resistance to bacterial infections strengthens thus, sea bream
is perceived as a "safe" fish to grow.
Sea bream naturally resists vibriosis, but suffers from acute pasteurellosis with very high
mortality frequently when very young (0.1g to 1.5g) and still in the hatchery. Pasteurellosis
outbreaks in hatcheries usually decimate stocks. Therefore, measures to protect it from the
disease have to be taken early. Later, when the bream exceed 4g of weight, they become capable
of resisting acute infections but remain considerably vulnerable until about 8-12g of weight.
Later during production, bream may also suffer from chronic pasteurellosis with moderate
losses. Antibiotic treatments are effective against such relatively mild outbreaks.
In order to effectively protect bream, it would necessitate vaccination of fry against
pasteurellosis in the hatcheries when very small (0.1g to 0.5g). Unfortunately, efforts to
immuno-stimulate and vaccinate bream at such an early stage of its life have been unsuccessful.
Trials have shown that such young bream are incapable of effectively utilising the vaccine
antigens, thus, casting doubts on whether vaccination of bream would be beneficial in hatchery
practice.
The merits of bream vaccination against pasteurellosis for the on-growers are debatable, unless
bream could be effectively vaccinated by immersion in a monovalent vaccine dilution either
prior to delivery or shortly after delivery at the on-growing sites when the fish are still between
1g and 2g. A six-month immunity cover (protection from mid-May through to mid-October)
would suffice.
.Vaccination evaluation
The result of vaccination, be it a laboratory or field trial or a commercial application, is
evaluated according to the "Relative Percent Survival" formula or RPS.
RPS = 1- (vaccinated fish mortality % / non-vaccinated mortality %) x 100 %
RPS expresses the percentage of fish, which would have died from the disease if not protected
against it. That is, the proportion of fish saved due to vaccination. An economically acceptable,
successful vaccination should exceed a RPS value of 70%, meaning that in case of a disease
outbreak the unvaccinated fish against this particular disease would suffer at least a three-fold
mortality loss than the vaccinates.