Bluetongue is a non-contagious, arthropod-borne viral disease
of both domestic and wild ruminants. Bluetongue virus (BTV)
is endemic in some areas with cattle and wild ruminants serving
as reservoirs for the virus. Epizootics of Bluetongue virus
killing approximately 179,000 sheep within 4 months have threatened
the livestock industry in recent years. For this reason,
regulatory veterinarians have heightened their interest in
this devastating disease.
The threat of decreased trade associated with Bluetongue
outbreaks has become an even bigger threat to the livestock
industry than the actual disease itself. According to Kahrs,
bluetongue is a major obstacle to exportation of U.S.
ruminants and ruminant products and probably affects the United
States more than most countries.
This is because of the prevalence of BTV in conjunction with
competent vectors within the U.S.,
vague surveillance and reporting policies, and extensive BTV
research emanating from U.S.
Bluetongue is an orbivirus which cross-reacts with many
antigenically related viruses including Palyam virus and the
viruses that cause epizootic hemorrhagic disease of deer and
African Horse sickness. Bluetongue virus replicates in both
arthropod and mammalian host cells. The virulence of BTV
varies quite markedly; even strains with matching serotypes
have variable virulence. A total of 25 serotypes have been
identified worldwide with only 5 recognized within the United
Testing diagnostically for BTV can be difficult. Two types
of viral antigen are used for BTV testing. All Bluetongue
serotypes share a common antigenic determinant called antigen
protein P7, while the antigen protein Ps is variable and is
used to determine the specific serotype of a virus (1-25).
The serum is often tested by complement fixation, AGID, or
one of several Elisa techniques. The AGID test can detect
antibodies that have persisted for years in BTV-exposed animals
and can cross-react with related orbiviruses, thereby producing
high numbers of false negatives (low sensitivity and specificity).
While difficult to perform, complement fixation is still used
to determine BTV exposure status for export since it can detect
shorter-lived antibodies. However, because of BTVs
wide pathogenic variability and the fact that cross-reaction
may occur between other orbiviruses (especially EHD), a positive
result on Bluetongue group test does not mean clinical signs
seen were caused by BTV itself. The competitive ELISA (C-ELISA)
has proven to be the best serologic test for BTV antibody
detection. Monoclonal antibody detection is used with ELISA
to decrease the chance of cross-reaction. In any case, the
detection of BTV antibodies is poorly correlated with BTV
Virus isolation from blood of a viremic animal is the most
definitive means of BTV diagnosis. Virus isolation can be
labor intensive, time-consuming, and expensive; however, spleen
and brain tissues (often from aborted fetuses) are used to
isolate the BTV. According to James Mechan, Isolation
of BTV has traditionally relied on inoculation of cell cultures,
embryonated chicken eggs, or sheep with blood from infected
animals or with homogenates of insects collected in endemic
areas. Currently, PCR is taking over previous methods
of virus isolation. The PCR tests have proven to be very
sensitive and specific for BTV RNA. A positive PCR is not
always indicative of infection, however, since viral RNA can
be detected in some tissues after viremia has passed.
BTV can replicate in a variety of mammalian cells. According
to Smith, Clinically, the BTV seems to present as underlying
endothelial cell damage resulting in a vasculitis causing
edema and eventually necrosis of epithelial and mucosal surfaces.
Teratogenesis occurs in the developing fetus due to virally
induced disruptions of organogenesis.
It has recently been hypothesized that development of clinical
disease in cattle may be mediated through Type 1 hypersensitivity
(aranylaris). Likewise, clinical disease in sheep appears
to be most severe when previous exposure has occurred.
BTV infection occurs in both wild and domestic ruminants/camelids
from the bite of the vector midge of the genus Culicoides.
The Culicoides vector infects most species during mid-summer
to early fall when it is most active. The virus can also
be transmitted sexually in infected semen and transplacentally
from dam to offspring. Transmission via embryo transfer may
also be a concern if the embryo is not washed at least ten
times. Culicoides transmission is by far the most important
method of transmission in endemic areas. BTV is mostly seen
in the southern United States
where Culicoides are widespread. In the absence of
competent vector populations, animal to animal transmission
is not capable of maintaining an endemic state. The overall
seroprevalence of cattle in the United
States is >18%.
Bluetongue is clinically manifested as two syndromes: 1)
vascular insult of several organ systems and 2) a reproductive
syndrome. Sheep are commonly seen with clinical disease,
but other domestic ruminants such as cattle and goats only
rarely show clinical signs. Differential diagnoses of Bluetongue
in sheep include Orf (contagious ecthyma), foot and mouth
disease, any vesicular disease, and sheep pox.
After a prepatent period of 3-8 days, sheep may begin to
show clinical signs such as transient fever (up to 106F),
edema of the face, lips, muzzle and ears, excessive salivation,
and hyperemic oral mucosa. The disease name stems from the
fact that affected sheep begin to develop a mucopurulent nasal
discharge after the first few days and the tongue may become
cyanotic. This is actually an infrequently reported sign;
however, the oral lesions may progress to petechial hemorrhages,
erosions, and ulcers. A marked pulmonary edema is often seen.
Late in the disease (7-12 days), lameness characterized by
petechial hemorrhages at the coronary band may occur and the
hooves may eventually slough. Fragile wool and diarrhea are
commonly seen. Many affected animals become depressed and
die while others make a full recovery.
The reproductive portion of the disease varies greatly.
Signs include abortions, stillbirths, and weak dummy
lamb live births. BTV can be both abortigenic and teratogenic
in cattle experimentally, but neither is commonly seen in
field conditions. Early embryonic loss and decreased reproductive
efficiency is a more frequently seen manifestation of the
disease in cattle and can be devastating to their calf/milk
production. Clinical signs in cattle also include hyperemia
and necrosis of the muzzle (burnt muzzle) and
patchy dermatitis. Differentials for BTV in cattle include
Bovine Viral Diarrhea virus, Malignant Catarrhal Fever, vesicular
diseases, Rinderpest, photosensitization, Bovine Papular Stomatitis
and Infectious Bovine Rhinotracheitis. Regulatory officials
should be notified if an outbreak in cattle occurs or is suspected.
Unfortunately, no single gross or histologic lesion points
with certainty towards BTV. Some animals appear normal at
necropsy, while most show hemorrhage in some organ, most frequently
the heart. Petechial and ecchymotic hemorrhages are also
seen under the tongue, on the hard palate, esophagus, forestomach,
lymph nodes, bladder, and spleen. Erosions and ulcers can
be seen anywhere in the oral cavity. Gelatinous subcutaneous
edema of the head, neck, forelimbs, and trunk is commonly
Supportive treatment is used since no antibiotic for BTV
exists. Since animals with severe oral lesions are reluctant
to eat, they should be fed via stomach tube or encouraged
to eat soft feedstuffs. Muscle and coronary band pain limits
mobility and therefore shade and water should be made readily
available. Sulfas may be administered to treat secondary
bacterial pneumonia and NSAIDs are commonly used to control
Environmental elimination is usually not possible so sheep
should be kept indoors during peak midge activity, e.g. dusk.
Environmental control using Ivermectin can be attempted, but
transmission of BTV can occur before the insects demise.
Some modified live vaccines are available and should be based
upon the local strains and serotypes.
-by Lisa McDill, Class of 2002
-edited by Dr. Theresa Boulineau, ADDL Graduate Student
Kahrs, Robert: 1998. The Impact of Bluetongue on International
Trade. Proceedings of the Annual Meeting of the USAHA.
MacLachlan, Pierce and deMattos: 1997. Evolution of Bluetongue
Virus in the Western United States.
Proceedings of the Annual Meeting of the USAHA.
Mecham and Wilson: 1994. Strategies for Improved Bluetongue
Diagnostics. Proceedings of the Annual Meeting of the USAHA.
Monke, Donald R: 1996. Bluetongue Virus: Review of Regulations
and Diagnostic Tests Pertinent to the International Exchange
of Bovine Semen. Proceedings of the Annual Meeting of the
Parsonson, Ian M.: 1993. Bluetongue Virus Infection of Cattle.
Proceedings of the Annual Meeting of the USAHA.
Smith: 2001. Large Animal Internal Medicine.