Source: msdvetmanual.com
Overview
- Bluetongue is an infectious arthropod-borne viral disease primarily of domestic and wild ruminants. Bluetongue virus is the type-species of the genus Orbivirus in the family Reoviridae. There are at least 24 serotypes worldwide, although not all serotypes exist in any one geographic area; eg, 13 serotypes (1, 2, 3, 5, 6, 10, 11, 13, 14, 17, 19, 22, and 24) have been reported in the USA and 8 serotypes (1, 2, 4, 6, 8, 9, 11, and 16) in Europe. Vector-borne transmission through Culicoides spp is the primary way BTV spreads.
Clinical signs
- The course of the disease in sheep can vary from peracute to chronic, with a mortality rate of 2%–90%. Peracute cases die within 7–9 days of infection, mostly as a result of severe pulmonary edema leading to dyspnea, frothing from the nostrils, and death by asphyxiation. In chronic cases, sheep may die 3–5 wk after infection, mainly as a result of bacterial complications, especially pasteurellosis, and exhaustion. Mild cases usually recover rapidly and completely. The major production losses include deaths, unthriftiness during prolonged convalescence, wool breaks, and reproductive losses.
- In sheep, BTV causes vascular endothelial damage, resulting in changes to capillary permeability and subsequent intravascular coagulation. This results in edema, congestion, hemorrhage, inflammation, and necrosis. The clinical signs in sheep are typical. After an incubation period of 4–6 days, a fever of 105°–107.5°F (40.5°–42°C) develops. The animals are listless and reluctant to move.
- On close examination, small hemorrhages can be seen on the mucous membranes of the nose and mouth. Ulceration develops where the teeth come in contact with lips and tongue, especially in areas of most friction. Some affected sheep have severe swelling of the tongue, which may become cyanotic (‘blue tongue”) and even protrude from the mouth. Animals walk with difficulty as a result of inflammation of the hoof coronets.
Diagnosis and differentials
- The typical clinical signs of bluetongue enable a presumptive diagnosis, especially in areas where the disease is endemic. Suspicion is confirmed by the presence of petechiae, ecchymoses, or hemorrhages in the wall of the base of the pulmonary artery and focal necrosis of the papillary muscle of the left ventricle.
Overview
Bluetongue is an infectious arthropod-borne viral disease primarily of domestic and wild ruminants. Infection with bluetongue virus (BTV) is common in a broad band across the world, which until recently stretched from ~35°S to 40°–50°N. Since the 1990s, BTV has extended considerably north of the 40th and even the 50th parallel in some parts of the world (eg, Europe). The geographic restriction is in part related to the climatic and environmental conditions necessary to support the Culicoides vectors. Most infections with BTV in wild ruminants and cattle are subclinical. Bluetongue (the disease caused by BTV) is usually considered to be a disease of improved breeds of sheep, particularly the fine-wool and mutton breeds, although it has also been recorded in cattle and some wild ruminant species, including white-tailed deer (Odocoileus virginianus), pronghorn antelope (Antilocapra americana), and desert bighorn sheep (Ovis canadensis) in North America, and European bison (Bison bonasus) and captive yak (Bos grunniens grunniens) in Europe. Bluetongue virus is the type-species of the genus Orbivirus in the family Reoviridae. Vectors become infected with BTV by imbibing blood from infected vertebrates; transovarial transmission has not been reported. High affinity of the virus to blood cells, especially the sequestering of viral particles in invaginations of RBC membranes, contributes to prolonged viremia in the presence of neutralizing antibody. Vector-borne transmission through Culicoides spp is the primary way BTV spreads.
Clinical signs
The course of the disease in sheep can vary from peracute to chronic, with a mortality rate of 2%–90%. Peracute cases die within 7–9 days of infection, mostly as a result of severe pulmonary edema leading to dyspnea, frothing from the nostrils, and death by asphyxiation. In chronic cases, sheep may die 3–5 wk after infection, mainly as a result of bacterial complications, especially pasteurellosis, and exhaustion. Mild cases usually recover rapidly and completely. The major production losses include deaths, unthriftiness during prolonged convalescence, wool breaks, and reproductive losses.