Bluetongue Disease

Bluetongue disease is caused by an orbivirus in the family Reoviridae which infects ruminants, notably sheep and cattle. This disease occurs in many parts of the world, including temperate and southern regions of North America; parts of Central America and South America bordering the Caribbean; and Europe, the Middle East, Asia, Australia, and southern Africa. It was first recognized in sheep and cattle in South Africa in the early 1930s. In the United States bluetongue (BLU) virus was first isolated in 1952 in Texas from sheep exhibiting a condition known as soremuzzle. It is now known to occur throughout most of the southern and western states, where prevalence of antibody to BLU virus in cattle is commonly 20—50%. No cases have been reported in Alaska or Hawaii. Canada remains largely bluetongue -free; detection of infected animals has been reported there only in the Okanagan Valley of British Columbia during localized outbreaks in 1975 and 1987. Both of these instances apparentiy originated from cattle imported from the United States.

Occurrence of clinical bluetongue disease in cattle tends to be sporadic, often involving only one or a few animals in a given herd. Occasionally, however, epizootics do occur. Following outbreaks in Cyprus in 1943, Turkey in 1944, and Israel in 1951, major epizootics occurred in Europe in the late 1950s, where an estimated 179,000 sheep died in Spain and Portugal. The mortality rate among infected animals was 75%. BLU virus has since been introduced to the Caribbean islands, bordering

development can result in aborted or stillborn calves. Infections which occur later in gestation are more likely to result in congenital deformities and underweight calves at birth. The virus is also found in semen of infected bulls. Consequently, restrictions are placed on the exportation of semen and live animals from endemic bluetongue areas for artificial insemination and other breeding purposes. The resulting economic impact on the livestock industry in the United States for mandatory testing of animals and losses in the foreign market is substantial, totaling millions of dollars annually.

BLU virus represents an antigenic complex with at least 25 currently recognized serotypes, which vary significantly in their pathogenicity. Five serotypes are known to occur in the United States, with serotypes 10, 11, 13, and 17 being the most widely distributed. The most recently introduced serotype is BLU-2, which was first isolated from cattle in Florida in 1982. It is reported to have caused clinical disease and death in cattle only in Alabama. Based on surveys of sentinel cattle and sheep in the Caribbean Basin, eight serotypes of BLU virus are known to occur in that region (serotypes 1, 3, 4, 6, 8, 12, 14, and 17). However, despite widespread infections of animals, notably cattle, clinical disease is largely absent in the Caribbean area.

The common occurrence of multiple serotypes of BLU virus in the same geographic area has hampered the use of immunization for protecting livestock from infection. Polyvalent vaccines have been developed but generally have not been effective. Natural immunity is acquired by sheep following their recovery from this disease but is limited to the particular serotype with which the animal was infected. Cattle apparently do not develop significant immunity following infection.

The primary mode of transmission of BLU virus is by the bite of infected Culicoides midges. Based on experimental studies with members of the C. variipennis complex, the virus acquired while the midge feeds on a viremic animal invades the salivary glands of the midge and there multiplies. The intrinsic incubation period is 10-20 days, after which the midge can transmit the virus at subsequent feedings. Following infection, the Culicoides host remains infective throughout its life. There is no evidence that either transovarial or transstadial transmission occurs. Infection rates are highly variable, depending on the Culicoides species and the geographic populations involved. Selective breeding in laboratory colonies has produced both susceptible and highly resistant lines, indicating the complexity of factors influencing the vector competence of Culicoides species involved in the epizootiology of this disease.

Transmission of BLU virus also occurs venereally via semen of infected rams and bulls. When introduced into the female genital tract, the virus potentially can infect the adult animal and, if she is pregnant, the developing embryo or fetus. No methods have been developed to destroy the virus in semen of infected animals. The virus can survive indefinitely in frozen semen samples.

Biting midges of the genus Culicoides are the only known vectors of BLU virus. Only a few species appear to be responsible for the major epizootics of blue-tongue which have occurred worldwide. The two most important vectors are C. imicola in Africa and the Middle East and C. sonorensis in the southern and western United States. C. imicola also has become established in Europe, where it now occurs in Portugal, Spain, and some of the Mediterranean islands. Although other Culicoides species have been found to be naturally infected with BLU virus, their importance as vectors remains uncertain. Some of them are likely to serve as secondary vectors which can augment transmission of the virus during local outbreaks.

Known or suspected vectors in South Africa are C. imicola, C. bolitinos, C.gulbenkiani, C. magnus, C. pycnostic-tus, C. zuluensis, and members of the C. shultzei group. In Australia C. actoni, C. brevitarsis, C.fulvus, and C. wadai are considered to be likely vectors, with C. brevipalpus, C. oxystoma, and C. peregrinus possibly playing minor roles in transmission. In addition to C. imicola, the following species are regarded as potential vectors in Europe and the Middle East: C. nubeculosus, C. obsoletus, and C. pulicaris. In North America, suspected species other than the principal vector C. sonorensis include C. insig-nis, C. debilipalpis, C. obsoletus, and C. stellifer. The most likely vectors in the Caribbean Basin appear to be C. fi-larifer, C. insignis, and C. pusillus.

EHD is very similar to bluetongue in many respects, the major difference being that it occurs primarily in wild ruminants, notably deer. It is caused by an orbivirus very closely related to BLU virus. Two strains or serotypes of the virus are recognized in North America, designated EHD-1 and EHD-2. EHD-1, known as the New Jersey strain, was first isolated from white-tailed deer in New Jersey during an outbreak in 1955. EHD-2, commonly referred to as the Alberta strain, was first isolated during an epizootic in that Canadian province in 1962. At least eight other EHD serotypes have been isolated in South Africa, Nigeria, Australia, and Japan.

The clinical signs in EHD cases are virtually indistinguishable from bluetongue. Isolation and identification of the etiologic agent is usually required to determine with certainty which virus is involved. Because of the similarities of these two diseases in wild ruminants, cases are often referred to simply as hemorrhagic disease. It also is

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