Bluetongue (BT)
Author: Prof. N. J. Maclachlan
Adapted from:
1. Verwoerd, D.W., Erasmus, B.J. 2004. Bluetongue, in Infectious diseases of livestock, edited by J.A.W. Coetzer & R.C.
Tustin. Oxford University Press, Cape Town, 2: 1201-1220.
2. Maclachlan, N.J.: Bluetongue: history, global epidemiology, and pathogenesis. Prev Vet Med., 102:107-111, 2011.
3. Maclachlan, N.J., Drew, C.P., Darpel, K., Wourwa, G.: The pathology and pathogenesis of bluetongue. J. Comp. Pathol.,
141: 1-16, 2009.
Licensed under a Creative Commons Attribution license.
Introduction
typically undergo subclinical or asymptomatic infections
Bluetongue is an insect-transmitted viral disease of
with the virus. However, sporadic but authentic
domestic and wild ruminants. Twenty six different
instances of bluetongue do occur in cattle, and disease
serotypes of the causative agent, bluetongue virus
of cattle was commonly reported during the recent
(BTV), are now recognized globally. BTV is usually
epidemic of BTV serotype 8 infection of livestock in
transmitted to livestock by the bites of hematophagous
Europe.
Culicioides midges, and the global range of BTV
coincides with that of competent midge vectors. Of the
>1000 species of Culicoides midges that occur
worldwide, only approximately 30 have been strongly
implicated as vectors of BTV. These midges occur on
all continents except Antarctica.
Salient features of BT
Bluetongue disease typically presents as acute febrile
illness with facial swelling and oedema that is
accompanied by reddening
(hyperaemia)
of
the
coronary bands of the feet and mucosal lining of the
Acute BT. Severe swelling of tongue,
and submandibular oedema
mouth and respiratory passages. Ulcers may develop
in the oral cavity of affected animals and, in rare
instances, the tongue becomes severely cyanotic –
BTV infection also has been described in wild and
thus the name, “blue tongue”. Severely affected
domestic carnivores such as lions, lynx and dogs,
animals become increasingly reluctant to stand or
either as a consequence of vector-mediated or oral
move, and will refuse to eat or drink. Certain breeds of
transmission of the virus. Young ruminant livestock
sheep, in particular European fine-wool breeds such as
also are susceptible to oral infection with BTV and the
the merino, and some species of non-African wild
virus can be spread in colostrum.
ungulates such as white-tailed deer are most likely to
express clinical signs of disease following BTV
Transplacental BTV infection is characteristic of only
infection, whereas cattle and non-African ungulates
some strains of the virus, particularly certain live-
attenuated vaccine strains. Transplacental infection of
replication of the virus within its biologic insect vector.
ruminants in early gestation (up to the end of the first
In contrast, infection and disease in livestock are very
trimester) leads to fetal loss and return to service,
infrequent or entirely absent in temperate regions of
whereas infections from the beginning of the first
the world during the remainder of the year – the so-
trimester through mid-gestation result in developmental
called “over-wintering” period. It is also clear that
abnormalities – specifically, cavitating brain lesions that
different strains of BTV have different virulence for
manifest as “dummy” newborn calves or lambs.
livestock, so particularly virulent viruses such as the
strain of BTV serotype 8 that recently emerged in
Where does BT occur?
Europe
BTV infection occurs throughout tropical and temperate
production. In contrast, BTV infection of livestock in
regions of the world. The global range of BTV has
endemic regions is often subclinical or entirely
traditionally been
asymptomatic, especially in cattle and wild African
considered
to
reside
between
latitudes of approximately 40 – 50° North and 35° S,
have
a
profound
impact
on
livestock
ungulates.
although within this broad zone different serotypes of
BTV are transmitted by different species of Culicoides
Prevention and control
insects in distinct global ecosystems. Thus, for
Prevention of bluetongue in livestock is usually
example, a different midge vector species transmit
accomplished by vaccination. Vaccines must be
different serotypes of BTV in much of North America as
administered to susceptible livestock prior to the
compared to Central America, despite the lack of
seasonal occurrence of BTV infection, and the vaccine
significant
two
must include all serotypes to which the susceptible
regions. The global distribution/range of BTV infection,
animals will be exposed. A variety of vaccine
however, has recently altered dramatically; since 1998,
formulations are commercially available, depending on
at least 8 different BTV serotypes have invaded
region and virus serotype. Inactivated vaccines have
extensive portions of Europe, including continental
been most widely used in Europe because of inherent
northern Europe, England and Scandinavia. Prior to
potential problems associated with live-attenuated
1998 only transient incursions of single serotypes of
vaccines, especially the ability of the latter to be spread
BTV into limited areas of Mediterranean Europe had
naturally and to reassort gene segments with field
been described. Similarly, 10 additional BTV serotypes
strains of the virus.
geographic
barriers
between
the
have been identified in the southeastern United States
since 1998, presumably as a result of incursion from
Rapid and accurate diagnosis also is central to control
the adjacent Caribbean ecosystem. Climate change
of outbreaks of bluetongue in livestock, and highly
has been implicated as the cause of this dramatic
sensitive
alteration of the virus’ global range.
immunosorbent (cELISA) and quantitative reverse-
and
specific
competitive
enzyme-linked
transcriptase polymerase chain reaction (qRT-PCR)
What triggers an outbreak of BT?
assays are available, respectively, for detection of
As an arthropod-borne virus, spread of BTV is
BTV-specific antibodies and viral nucleic acid.
associated with appropriate climatic conditions that
favour expansion of populations of the midge vector.
Control
of
the
Culicoides
vector
is
considered
Thus bluetongue very characteristically is a disease of
impractical in most situations, because of their
the late summer and autumn, a time when vectors are
ubiquitous presence and high population density in
most abundant and higher temperatures promote
many regions.
Animal movement and trade restrictions are widely
used by BTV-free countries/regions to prevent the
introduction of BTV.
Find out more
The CPD module on BT provides a detailed account of
the disease and its epidemiology, particularly in
southern Africa. Other aspects are the pathogenesis of
BT, the diagnosis and differential diagnosis, including
clinical
signs
and
pathology,
and
methods
for
controlling the disease. The module concludes with a
section on the effect of BT on trade in animals and
animal movement, i.e. the area in which this disease
likely has its greatest impact.