The history of bluetongue and a current global overview.
Bluetongue (BT) was first reported more than 125 years ago when European breeds of sheep were introduced into southern Africa. BT viruses (BTV) have been identified in many tropical and temperate areas of the world. BT, the disease, is a phenomenon of ruminants in the temperate zones. There is little clinical disease in the tropical and subtropical areas of the world. At least 24 serotypes of BTV have been described. While the viruses are classified antigenically and taxonomically as BTV, each serotype is unique and may not cause BT, the disease. The BTVs are transmitted among ruminants by competent vector species of the genus Culicoides, i.e. biting gnats or midges. BTV serotypes exist with vector species of Culicoides in predictable, but finite, geographic and ecological cycles or ecosystems around the world. Despite the almost certain movement of livestock and Culicoides species between these ecosystems, there is little evidence that introduced BTV serotypes have been established in these ecosystems. Rather, periodic cyclic extensions and remissions of these virus-vector ecosystems permit the viruses and the disease to move into and recede from adjacent non-endemic areas in a pattern characteristic of many other known arthropod-borne viruses (arboviruses). Earlier publications suggested that a carrier state occurred in cattle infected as foetuses with BTV. No subsequent natural experiences or research support the hypothesis which has not been validated. The conclusions of the research are not accepted by the scientific community. It is logical, therefore, to propose that regulatory restrictions against the movement of cattle from BTV-affected countries be relaxed or eliminated.
Walton
T E
TE
Centers for Epidemiology and Animal Health, Veterinary Services, Animal and Plant Health Inspection Service, United States Department of Agriculture, Fort Collins, Colorado 80526-8117, USA.