Since there is no curative treatment for BTV-infected animals, prophylactic immunization of susceptible species remains the most effective and practical control measure against bluetongue in endemic regions. In the U.S., the only vaccine approved for national use is a monovalent attenuated modified-live vaccine against serotype 10. Unfortunately, if the BTV serotype causing infection is different than the one in the vaccine being used, use of the vaccine is expected to provide little or no protection.
Control of vectors by using insecticides or protection from vectors may lower the number of Culicoides spp. bites and subsequently the risk of exposure to BTV infection. However, these measures alone are unlikely to effectively halt a bluetongue epidemic and should be regarded as mitigation measures to be used alongside a comprehensive and vigorous vaccination program.
Clinical signs of bluetongue reflect the damage that this virus causes to vascular endothelium and the resultant changes to capillary permeability and subsequent intravascular coagulation. This results in edema, congestion, hemorrhage, inflammation, and necrosis of affected tissues. The clinical signs in sheep are typical of these pathologic changes. After an incubation period of 4–6 days, a fever of 40.5°–42°C (105°–107.5°F) develops. The animals are listless and reluctant to move. Clinical signs in young lambs are more apparent, and the mortality rate can be high (up to 30%). Approximately 2 days after onset of fever, additional clinical signs may be seen, such as edema of lips, nose, face, submandibular area, eyelids, and sometimes ears; congestion of mouth, nose, nasal cavities, conjunctiva, and coronary bands; and lameness and depression. Some sheep may exhibit severe swelling of the tongue, which may become cyanotic (hence, the name “blue tongue”) and even protrude from the mouth. In most affected animals, abnormal wool growth resulting from dermatitis may be seen.
Clinical signs in cattle are rare but may be similar to those seen in sheep. They are usually limited to fever, increased respiratory rate, lacrimation, salivation, stiffness, oral vesicles and ulcers, hyperesthesia, and a vesicular and ulcerative dermatitis. Susceptible cattle and sheep infected during pregnancy may abort or deliver malformed calves or lambs. The malformations include hydranencephaly or porencephaly, which results in ataxia and blindness at birth. White-tailed deer and pronghorn antelope develop severe hemorrhagic disease leading to sudden death.
Animals that become infected with BTV typically develop a serologic response within 7–14 days after exposure; these circulating antibodies generally remain lifelong. Consequently, as a means to document exposure to BTV, agar gel immunodiffusion and competitive ELISA tests are used to detect BTV antibody. Laboratory confirmation of BTV is based on virus isolation in embryonated chicken eggs or mammalian and insect cell cultures, or on identification of viral RNA by PCR.
To date, out of more than 1,400 Culicoides spp. worldwide, fewer than 30 have been identified as actual or potential vectors of BTV. In the U.S., the principal vectors are C. sonorensis and C. insignis. Because the range of these two vectors is characteristically limited to southern and western regions of the U.S., bluetongue can be an important disease in susceptible species in these geographic regions.
Currently, there is no coordinated surveillance for bluetongue virus in the U.S. to detect potential introductions of new virus serotypes or document their spread. Nor, are there Federal regulations specific to curbing spread of BTV in the U.S. cattle population. However, bluetongue is of interest to APHIS in as much as when it occurs in cattle, sheep, goats, and farmed cervids, it must be reported to the World Organisation for Animal Health (OIE). APHIS’ interest in this disease has also spawned exploration of the best available countermeasures to rapidly and effectively control and, where feasible, eradicate outbreaks of bluetongue should one occur in the United States.
Bluetongue is an arthropod-borne non-contagious infectious disease of domestic and wild non-African ruminants. Under field conditions, the causative agent, bluetongue virus (BTV), is typically transmitted by Culicoides spp. insects (biting midges) following a bite to consume a blood meal from susceptible animals. Mechanical transmission of the virus by other bloodsucking insects is of minor significance. Of the domestic ruminants that become infected with BTV, sheep are most likely to exhibit clinical disease; most infections in cattle are subclinical. In North America, white-tailed deer (Odocoileus virginianus), pronghorn antelope (Antilocapra americana), and desert bighorn sheep (Ovis canadensis) are also susceptible to BTV infection.
At least 26 serotypes of BTV (genus Orbivirus in the family Reoviridae) have been identified worldwide. In the U.S., 13 serotypes (1, 2, 3, 5, 6, 10, 11, 13, 14, 17, 19, 22, and 24) have been reported; Types 2, 10, 11, 13 and 17 are considered endemic. Distribution of BTV throughout the world parallels the spatial and temporal distribution of Culicoides spp. biting midges, which are the biologic vector of the virus. Continued cycling of the virus among competent Culicoides spp. vectors and susceptible ruminants, particularly sheep, are critical to maintaining a virus presence. To wit, bluetongue typically occurs when susceptible sheep are introduced into areas where virulent strains of BTV circulate, or when virulent strains of BTV extend their range into previously unexposed populations of ruminants.