Epizootiology of bluetongue virus: Transmission cycle, vectors, and serotypic distribution in the Americas

Abstract

Bluetongue is an infectious, insect-transmitted viral disease of domestic and wild ruminants which was first described by Spreull in South Africa in 1905. The virus was first isolated and identified in the USA from sheep in California in 1952, and from cattle in Oregon in 1959, although “mycotic stomatitis” a clinical syndrome identical to bluetongue had been described as early as 1904. Since then bluetongue virus activity has been confirmed serologically or virologically in almost every part of the USA. Bluetongue virus appears to be enzootic in ruminants in the Western USA and in Florida, with occasional epizootic incursions into the northwestern, north central, and eastern states. The northeastern USA and Canada generally appear to be bluetongue free. Serologic or virologic documentation of bluetongue activity in ruminants is available from some of the Caribbean islands, Brazil, Colombia, Costa Rica and Mexico. The primary transmission cycle in the USA involves the biting gnat, Culicoides variipennis . Experimentally, the virus has been isolated from the semen of rams and bulls, and a transplacental transmission cycle has been demonstrated from infected pregnant cows to the fetus. Calves which are infected in utero may become persistently or latently infected carriers and viremia may be activated by the feeding of noninfected C. variipennis . Field evidence documenting the natural occurrence of these phenomena are available. The major proven biologic vector of BTV in the USA, C. variipennis , is found throughout the USA except for southern Florida, and occurs north into Canada and south into Mexico. This species is not found in the Caribbean islands, or in Central or South America. The vectors outside of the USA in the western hemisphere are not known. Experimentally, the oral susceptibility of C. variipennis females to infection with bluetongue virus has been shown to be genetically and environmentally influenced. Naturally occurring populations have been shown to vary in their oral susceptibility to BTV according to their geographic distribution. In the Western states where BTV is enzootic, the indigenous populations of C. variipennis are moderately to highly susceptible to oral infection with BTV. In the northeastern USA where BTV is not active, the natural C. variipennis populations are resistant to oral infection with BTV. Laboratory studies suggest that environmental conditions influence this resistance. Currently in the USA, four BTV serotypes, 10, 11, 13, and 17, are known to exist. All four serotypes have been isolated from ruminants or insects in California, Colorado, Montana and Nevada. Serotype 4 has been isolated from cattle which were being imported to the USA from Brazil. Viral isolates have not been reported from animals elsewhere in the Western hemisphere. Multiple serotypes have been shown to occur in the same ruminant at the same time.