Yet another virus affecting livestock, and the food you eat.
Just what is in out meat?
Where do you think these infected animals really end up? In your
burger, Sunday roast etc.
ANIMAL HEALTHDISEASE CARDS
Bluetongue
Names
Pathogen(s)
Preferred Name : Bluetongue virus
Disease/Parasitosis
Preferred Name : Bluetongue
Common Names :
Spanish: Lengua azul
French: Fievre catarrhale du mouton
Overview
Bluetongue virus is an arbovirus (arthropod borne) that naturally
infects domestic and wild ruminants, camelids and some other
herbivores such as elephants. Bluetongue virus is transmitted by
several species of Culicoides (biting midges). Bluetongue is almost
exclusively a disease of sheep, although white-tailed deer, pronghorn
and desert bighorn sheep may suffer disease in North America. In
cattle and goats clinical disease is rare, and, when present, is much
milder than in sheep (Verwoerd and Erasmus, 1994).
Bluetongue can cause spectacular disease outbreaks and is placed in
the Office International des Epizooties (OIE) List A disease category.
Affected sheep may die after acute or chronic disease, or may recover
with weight loss and/or wool breaks.
Bluetongue was first described in South Africa after Merino sheep from
Europe were introduced in the late eighteenth century (Verwoerd and
Erasmus, 1994). The disease was considered confined to South Africa
and for many years research efforts on the virus and the disease were
exclusively undertaken in that country, mostly at the Onderstepoort
Veterinary Institute. The viral nature of the disease was established,
as was its insect spread and multiple virus s*****ypes (Howell, 1960;
1970). There are now 24 s*****ypes of bluetongue virus recognized
worldwide.
Bluetongue virus is the type species of the genus orbivirus in the
family Reoviridae. Initially the virus was classified as an arbovirus
but it appeared to share some properties with reoviruses and was
provisionally classified as a reovirus. However, bluetongue virus
differs in some respects from reoviruses and, along with a number of
other related viruses, was classified in a separate genus by Borden et
al. (1971).
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Animals Affected Table
See Table
Animals Affected
Bluetongue is an arbovirus, infecting vertebrates and invertebrates
cyclically.
Bluetongue virus naturally infects domestic and wild ruminants,
camelids and some other herbivores such as elephants. Historically,
the primary cycle may have involved species of African antelope, but
this role has now been taken over by cattle (Erasmus, 1990).
Midges of the genus Culicoides act as biological vectors of bluetongue
virus. Of the approximately 1400 species of Culicoides world-wide,
less than 20 are considered actual or possible vectors (OIE, 1998;
Mellor, 1990). The most well-studied vector species are C. variipennis
and C. insignis in the USA, C. fulvus, C. wadai, C. actoni and C.
brevitarsis in Australia, and C. imicola in Africa and the Middle East
(Erasmus, 1990). It is possible that additional vector species will be
identified in countries such as China and Bulgaria where bluetongue
has been recognized only recently.
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Epidemiology
Bluetongue virus is an arbovirus that has evolved a life cycle where
alternate cycles of virus replication in vertebrate and invertebrate
hosts are essential for virus persistence. There is no evidence of
vertical transmission of the virus in the invertebrate host.
Observations on the placental transmission of virus in the vertebrate
host are contradictory (Roberts, 1990) and therefore any vertical
transmission in vertebrates is considered to be of no consequence to
virus ecology. There is little evidence of direct or indirect contact
transmission in either host, other than rare instances of seminal
transmission in vertebrates (OIE, 1998). The virus cannot be spread by
meat, milk or dairy products. Cattle are the primary vertebrate hosts
(Erasmus, 1990) and a small number of species of Culicoides midges are
the only insect hosts (Mellor, 1990). The rare recovery of bluetongue
virus from other insects is of no ecological significance.
The insect vectors of bluetongue virus breed in moist conditions in a
variety of habitats, particularly damp, muddy areas and in faecal and
plant matter. They have nocturnal feeding habits, preferring still,
warm conditions, pastures and open pens. At least some
speciesØpreferentially feed on cattle. Females take a blood meal prior
to egg laying, feed at roughly 4-day intervals and live for aboutØ2
toØ3 weeks. The eggs hatch inØ2 toØ3 days and depending on the
temperature, the larval stage lasts 12 to 16 days. Adults emergeØ2
toØ3 days after pupation and take a blood mealØ1 day later and they
also mate during this time (Roberts, 1990). The activities of the
midge are influenced by temperature and the optimum lies between 13o
and 35oC (Sellers, 1981).
As summarized by Gibbs and Greiner (1994), bluetongue is a common,
generally subclinical infection of ruminants throughout the tropics
and subtropics, within a number of separate ecosystems. Seasonal
incursions of the virus into more temperate latitudes, sometimes
accompanied by disease, may occur under favourable climatic conditions
at certain key locations. There is evidence that infected midges are
carried on the wind for long distances (Sellers, 1981). It has been
postulated that the major epidemics of bluetongue, in regions where
disease occurs only s****adically, can often be traced to windborne
carriage of infected Culicoides from distant areas (Gibbs and Greiner,
1988).
Critical in the understanding of the epidemiology of bluetongue is
knowledge of the virus competence of the Culicoides species in
different ecosystems, but vector competency research is a very
specialised discipline. Not only may different populations of a
species of midge have varying susceptibilities to a strain of virus,
but a single strain of the vector may have differing susceptibility to
different virus s*****ypes (Mellor, 1990).
Competent midges may be infected when biting viraemic vertebrates. The
chance of infection depends in part on the genotype of the midge, the
strain of virus, the level of viraemia, and environmental factors
(Mellor et al., 2000). The extrinsic incubation period (the period
between feeding on infected blood and the appearance of virus in the
saliva of the midge) is 1-2 weeks.
The colonised USA vector, C. variipennis, is able to ingest
approximately 10-4ml of blood (Mellor, 1990), whereas the most widely
distributed Australian vector, C. brevitarsis, has a blood meal volume
of around 10-4.5ml (Muller et al., 1982). Therefore viraemia must be
of the order of 104 infectious units of virus per ml or greater for
feeding midges to have much chance of infection. OIE (1998) summarized
re****ted peak levels of viraemia, in virus infectious units per ml of
blood, as 104.4 to 106.3 for cattle, 106.4 to 108.0 for sheep and
106.0 for goats, though levels reached are mostly much lower. Viraemia
peaks in the first two weeks after infection, before the appearance of
serum antibody. Virus titres then drop rapidly and are very low if
infections persist for a month or more.
The duration of viraemia in the infected vertebrate is an im****tant
factor in the transmission of bluetongue virus to biting, competent
midges. Bluetongue is no longer considered a persistent infection of
ruminants, especially cattle (MacLachlan, 1994). Singer et al. (2001)
analyzed a large volume of existing data on the length of bluetongue
viraemia of cattle and concluded that this was equal to or less than 9
weeks in >99% of adults. OIE (1998) re****t the viraemia of most cattle
as less than 4 weeks with fewer than 1% exceeding 8 weeks. The maximum
viraemia re****ted for sheep is 54 days (Koumbati et al., 1999), but
this is exceptional.
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Distribution Table
See Table
Distribution
In 1943 bluetongue disease was re****ted in Cyprus, and outbreaks were
subsequently re****ted in Israel, the USA, ****tugal, Spain, Pakistan
and India (Verwoerd and Erasmus, 1994). Over the past 30 years
evidence of regular virus activity, but not necessarily disease, has
been found in most countries in the tropics and subtropics with
substantial populations of ruminants. The virus may be found in a
geographic band between latitudes 40oN and 35oS. The presence of
bluetongue virus within this band, whether year round or seasonal,
depends on the climatic zone type. Genetic studies (topotyping)
indicate that the virus exists in discrete, stable ecosystems,
probably the result of co-evolution of different strains of the virus
and vectors (OIE, 1998). Numerous countries in the tropics and
subtropics have bluetongue virus unknowingly circulating subclinically
in cattle and other ruminants. A properly designed serological survey
would reveal the presence of the virus. The virus is endemic in areas
of some countries, being more or less continuously active. Depending
on climatic factors affecting the vector, in most years the virus will
seasonally extend to adjacent areas (Gibbs and Greiner, 1988). In
exceptionally favourable years the virus will spread even further,
such as to ****tugal and Spain in 1956, to British Columbia in 1988, to
Bulgaria, continental Greece and Tunisia in 1999 and to Algeria,
Sardinia, Corsica, Majorca, Minorca, Sicily and continental Italy in
2000.
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Economic Im****tance and Impact
Bluetongue can be a costly infection for several reasons. The clinical
disease in sheep can be severe, resulting in deaths, weight loss and
wool break. In some countries where disease is endemic (South Africa
and some States of the USA), vaccination is a recurring cost. However
the greater cost of bluetongue is to infected countries which ex****t
live animals, germplasm and some animal products such as foetal calf
serum. Here the presence of bluetongue virus, even if wholly
subclinical, causes loss of trade due to restrictions on the source of
animals, and the costs of health testing. It has been estimated that
in the late 1970s, the ban on US cattle semen ex****ts resulted in an
annual loss of $24 million (Gibbs and Greiner, 1988).
Bluetongue is included in the OIE List A diseases, largely because of
dramatic outbreaks of disease in Cyprus in 1943 and ****tugal and Spain
in 1956. The Cyprus outbreak was due to a particularly virulent strain
of the virus causing between 60 and 70% losses in some flocks
(Gambles, 1949). The Iberian outbreaks were also spectacular. Within
the firstØ4 months 46,000 sheep had died in ****tugal and 133,000 in
Spain (Roberts, 1990). This listing of bluetongue in the most serious
of animal diseases exacerbates the trade sensitivity and associated
costs to countries with the infection, and has been challenged by some
(Gibbs and Greiner, 1994).
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Disease Systems
See Table
Disease Course
Natural bluetongue infection is usually subclinical. Bluetongue
disease is the result of a complex interaction between the animal, the
virus and the environment. Bluetongue is almost exclusively a disease
of sheep, with European breeds most susceptible. Most breeds of sheep,
especially in regions where the virus is endemic, are resistant to
disease though there is increasing information that native breeds in
India and China can be clinically affected. Outbreaks of disease
typically occur either when susceptible sheep are introduced to
endemic areas, or when infected midges carry the virus from endemic
regions to adjacent areas containing populations of
immunologically-na¨ve, susceptible sheep.
Many strains of bluetongue virus appear incapable of causing
significant disease following natural or experimental infection of
breeds of sheep known to be susceptible to disease. Experimental
reproduction of disease can be inconsistent, except with the most
virulent strains of virus. This could be because exposure of sunlight
can have a marked influence on the severity of disease (Erasmus,
1990). Passage of virulent field virus in cell cultures rapidly
reduces virus virulence (Gard, 1987).
After introduction by the bite of an infected midge, bluetongue virus
first replicates in the local lymph nodes and subsequently induces a
primary viraemia which seeds other lymph nodes, spleen, lung and
vascular endothelium (Gibbs and Greiner, 1988). Circulating virus
associates with blood cells, mostly with erythrocytes and platelets,
though virus associated with mononuclear cells is critical for
dissemination of virus throughout the animal. Later in viraemia, the
virus is exclusively associated with erythrocytes (MacLachlan, 1994).
Virus particles appear to be sequestered in invaginations of the
erythrocyte membrane, allowing prolonged viraemia in the presence of
neutralizing antibodies (OIE, 1998).
Fever is usual but not invariable. Other common clinical signs include
oedema (of lips, nose, face, submandibulum, eyelids and sometimes
ears), congestion (of mouth, nose, nasal cavity, conjunctiva, skin and
coronary bands), lameness and depression. The oedema of lips and nose
can give the sheep a 'monkey-face' appearance. There is frequently a
serous nasal discharge, later becoming mucopurulent. The congestion of
the nose and nasal cavity produces a 'sore muzzle' effect, the term
used to describe the disease seen in sheep in the USA before its
bluetongue virus aetiology was realized. The mouth is sore and the
sheep may champ to produce a frothy oral discharge. Sheep are not
strictly anorexic, but eat less because of oral soreness and will hold
food in their mouths to soften it before chewing. Affected sheep
occasionally have swollen, congested, cyanotic tongues. Lameness, due
to coronary band congestion, may occur early in the disease and
lameness or torticollis, as a result of skeletal muscle damage, may
occur later (OIE, 1998).
If fever occurs, sheep are first pyrexic 4-10 days after infection.
The other clinical signs soon follow with acute deaths occurring
during the second week following infection. Many of these deaths are
the result of pulmonary oedema and/or cardiac insufficiency. Further
sheep may die from chronic disease 3 to 5 weeks after infection with
bacterial complications, especially pasteurellosis. Under-nutrition
arising from lameness and depression may be contributing factors. The
production loss due to bluetongue may be the result of deaths,
unthriftiness during prolonged convalescence, wool breaks and possibly
reproductive wastage (OIE, 1998).
Although the frequency of infection of cattle with bluetongue virus is
generally higher than in sheep, disease in cattle is rare. Clinical
infection is actually a hypersensitivity reaction, including fever,
stiffness or lameness and increased respiratory rate. There may be
lacrimation and increased salivation. The skin of the muzzle is often
inflamed, and may crack and peel. The lips and tongue may be swollen,
with ulcers on the oral mucosa. Similarly, the skin of the neck,
flanks, perineum, and teats may be affected (Erasmus, 1990).
Hydranencephaly and congenital deformities may develop in bovine and
sheep foetuses of bluetongue virus-infected dams, the severity of
lesions depending on the stage of gestation. Foetuses seem to be most
susceptible during the period of active brain development (Erasmus,
1990). It is clear that cell culture-adapted virus more readily
crosses the placenta than unadapted virus, suggesting that the
occasional instances of natural virus-induced teratogenesis may be due
to strains of virus derived from live virus vaccines (MacLachlan,
1994).
Bluetongue in dogs associated with use of a contaminated vaccine was
re****ted by Akita et al. (1994). Only pregnant *****es were affected.
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Pathology
All of the pathology of bluetongue can be assigned to vascular
endothelial damage resulting in changes to capillary permeability and
fragility, with subsequent disseminated intravascular coagulation and
necrosis of tissues supplied by damaged capillaries. These changes
result in oedema, congestion, haemorrhage, inflammation and necrosis.
In animals dying acutely, the oral mucosa is hyperaemic and petechiae
or ecchymoses may be present. Excoriations may be in areas subject to
mechanical abrasion; the edges of lips, dental pad, tongue and cheeks
opposite the molar teeth. There may be hyperaemia in the
fore-stomachs. The lungs may be hyperaemic with severe alveolar and
interstitial oedema, froth in the bronchi, and excess fluid in the
thoracic cavity. The pericardial sac may have petechiae and excess
fluid. A variable sized haemorrhage in the tunica media near the base
of the pulmonary artery is almost pathognomonic. Subepicardial and
subendothelial haemorrhages, particularly involving the left
ventricle, are common. Generalized damage to the cardiovascular system
is evidenced by widespread hyperaemia, oedema and haemorrhage
(Erasmus, 1990).
Animals that die later than 14 days after infection often show
dramatic degeneration and necrosis of the skeletal musculature.
Muscles lose pigmentation and the inter-muscular fasciae are
infiltrated with a clear gelatinous fluid (Erasmus, 1990).
Microscopic examination of mucosal lesions shows mononuclear cell
infiltration, degeneration and necrosis of epithelial cells in which
large acidophilic intra-cytoplasmic m***** ac***ulate. Affected
muscles have oedema, haemorrhage, hyaline degeneration and necrosis.
Infiltration by neutrophils, macrophages and lymphocytes is present in
acute cases (Verwoerd and Erasmus, 1994).
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Symptoms Table
See Table
Diagnosis
There is aØsummary of recommended procedures for bluetongue serology
and virus isolation (Afshar and Gard, 1995), while Afshar (1994)
provides details of the diagnosis methods. The isolation and
identification of bluetongue virus is also described in detail by
Clavijo et al. (2000).
The recommended tests for the detection of bluetongue
serogroup-specific antibodies are agar-gel-immunodiffusion and
competitive ELISA, with the latter becoming more popular because of
its greater accuracy and adaptation to conventional laboratory rapid
testing and reading technology. The recommended test for the detection
of s*****ype-specific antibodies is the virus neutralization test.
Bluetongue virus is usually isolated from tissues, or preferably red
blood cells washed free of any antibody, in embryonated chicken eggs.
Bluetongue virus may be detected in the inoculated eggs by antigen or
nucleic acid detection procedures or by passage to susceptible cell
cultures. Cell culture isolates are identified as bluetongue viruses
by tests based on group-specific antibodies and using fluorescent or
enzyme conjugates. Isolates placed in the bluetongue serogroup are
typed by virus neutralization tests using s*****ype-specific antisera.
Differential diagnosis should include contagious ecthyma, foot and
mouth disease, photosensitization, pneumonia, polyarthritis, footrot,
foot abscesses, plant poisonings, peste des petits ruminants,
coneurosis and epizootic haemorrhagic disease of deer.
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Disease Treatment Table
No data
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Disease Treatment
There is no treatment for bluetongue disease. The recovery of affected
animals will be aided by the provision of shade, water, feed and
shelter.
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Disease Prevention and Control
Bluetongue is a disease of sheep, but cattle are the principal
vertebrate reservoirs of the virus. Once established, it is impossible
to actively eradicate bluetongue virus. The virus will circulate,
generally subclinically, in cattle and other ruminants, and in midges.
In countries marginally suitable for virus persistence, the virus may
be maintained for several years before dying out. Bluetongue entered
****tugal and Spain in 1956 and appears to have persisted in ****tugal
until 1959 and in Spain until 1960 (Roberts, 1990). In seasonally
infected areas, the onset of cold weather will reduce midge
populations to ineffective levels and cause the virus to retreat to
regions of year-round activity.
The bluetongue virus cycle could be interrupted by the immunization of
vertebrate hosts, especially cattle, removal of vectors or prevention
of vector attack. Understandably, the immunization of animals that
will not suffer from the disease is not acceptable to farmers. The
control of midges by the application of insecticides and larvicides to
insect resting and breeding sites, or systemically to cattle, has not
been fully investigated but is likely to have local success only.
Protecting sheep from exposure to midges is a more practical approach
and can be achieved by moving sheep from insect resting and breeding
sites, stabling animals overnight or the use of insect repellents.
Mixing cattle with sheep will draw vectors with a host preference for
cattle from sheep, but may raise the virus infection level of the
midge population.
Prophylactic immunization of sheep is the most practical and effective
control measure, especially when the threat is from an epidemic due to
a single s*****ype, such as the type 10 outbreak in ****tugal and Spain
in the 1950s. However, multiple s*****ypes of virus are usual in
endemic situations (Hawkes, 1996), requiring multivalent vaccines
because bluetongue vaccines are s*****ype specific. However,
multivalent vaccines have attendant problems resulting from
interference between virus strains, differences in immunogenicity and
growth rates between various strains, as well as differences in the
response of individual animals to the components of such vaccines
(Verwoerd and Erasmus, 1994). Additionally, there is growing concern
by some scientists about the use of live attenuated bluetongue
vaccines. Murray and Eaton (1996) summarized these concerns into four
areas. These areas are: the known teratogenicity of attenuated virus
for the developing foetus; the propensity for vaccine virus to be
excreted in the semen of bulls and rams; the possibility that vaccine
virus will infect vectors and establish in the environment; and the
generation of recombinant progeny virus with novel genetic and
biological properties after the reassortment of genes from wild and
vaccine virus in the vaccinated animal or the vector.
Alternatives to live attenuated vaccines are described by Murray and
Eaton (1996). Vaccines based on inactivated whole virus, recombinant
virus-like particles or recombinant core-like particles all show
promise, but require more research. If a commercial product of any of
these is achieved, it will likely cost considerably more than a live
attenuated vaccine.
Live attenuated bluetongue vaccines have wide use in South Africa, and
more limited use in USA and a few other countries. The vaccines are
compromises between attenuation and immunogenicity and may have
residual pathogenicity for some vaccinated sheep. The application of
the vaccines has to be well managed. Colostral immunity in young sheep
can interfere with the development of active immunity to the vaccine
and breeding ewes and rams should be vaccinated before mating.
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Vaccines
See Table
Zoonoses and Food Safety
Bluetongue is not a zoonosis.
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Taxonomic Tree
Kingdom "Viruses" [01"VIR]
Family Reoviridae [41REOV]
Genus Orbivirus [51ORBI]
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Hosts Table
See Table
Vectors and Intermediate Hosts
See Table
Pathogen Characteristics
Bluetongue virus is an icosahedral-shaped particle consisting of a
segmented double-stranded RNA genome, encapsidated in a double-layered
protein coat. Removal of the outer protein layer activates a
viral-associated RNA polymerase which transcribes the ten genome
segments into 10 mRNAs which are in turn translated into at least
seven structural (VP1-VP7) and three non-structural (NS1-NS3) proteins
(Huismans and Dijk, 1990). The virions have a diameter of 68-70 nm,
comprising an outer capsid around a 54 nm core (Verwoerd and Erasmus,
1994).
The genome segments vary in size from 0.5 kDa to 2.7x10 3kDa, and the
viral proteins range in size from 25,000 to 144,000 daltons. VP2 and
VP5 form the outer capsid and the other five structural proteins are
in the core. VP2 is primarily responsible for the induction of
type-specific neutralizing antibodies and its variable sequence
results in the 24 recognized s*****ypes of bluetongue virus (Verwoerd
and Erasmus, 1994). The 24 s*****ypes are designated BLU 1- BLU 24.
Variations in other proteins of the virus are responsible for the
innumerable strains of the virus and for their varied biological
properties. This genetic diversity of bluetongue virus is a
consequence of both drift and reassortment of individual gene segments
(OIE, 1998).
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