Bluetongue in Sheep
By: Tom Drudik
While researching this topic I read in a late 1950's sheep production textbook that bluetongue has not been a major disease problem in the United States prior to that time. The first cases were definitively established in the United States in 1953, but the disease is thought to have been present in the southwestern states much earlier. In the 1956 USDA Yearbook of Agriculture Animal Diseases, a very detailed description of the disease was given. Now the disease is thought to have spread throughout the United States. The disease seems to be more severe in some years than others, suggesting that the disease favors hot dry weather. For example, here in Nebraska in the late 70's it was estimated that 50,000 deer died of the disease in one year and again this year they are asking hunters to report any dead deer. Judging from the posts on the online sheep forums and other reports, this must be a year for more outbreaks in sheep flocks.
Bluetongue is a viral disease that infects sheep. Cattle, goats and wild ruminants can be infected with the virus, but generally their health remains unaffected except in rare instances. Bluetongue virus is classified as an orbovirus, meaning that this virus is not passed directly from sheep to sheep. Bluetongue virus is commonly transmitted to sheep by the biting midge, which is a night-flying insect found throughout the United States during warm weather. The various species of biting midges overwinter as either eggs or larvae, although only the adult biting midges that can spread the virus. Thus, the disease occurs seasonally during the summer months when the adult insect is most active.
During serious outbreaks of the bluetongue, large numbers of sheep may be affected with clinical signs, but the death losses may be low. The greatest economic loss to the producer is in the loss of weight and condition of the affected sheep. Affected rams may become sterile temporarily, for 60 days or more, due to the high fever, resulting in a reduced or delayed lambing rate.
Typical symptoms are fever, depression, and lack of appetite. Inflammation and ulceration of the mucous membranes of the nasal cavity, mouth, and the tongue may occur. The tongue may sometimes be swollen and blue in color (thus the name "Bluetongue"). The virus causes an increase in permeability of blood vessel walls, permitting leakage of blood constituents into the surrounding tissue. This results in swollen ears, muzzles, and coronary bands, as well a pulmonary edema.
The first sign is a very marked increase in temperature, usually 106 degrees to 108 degrees F. but in most cases the first visible signs observed is a swollen head, ears swollen and droopy, or lips swollen and reddened. The animal may show excessive drooling and nasal discharge. Some effected animal may show degrees of lameness. Affected sheep have a loss of appetite and may refuse to eat or drink; therefore, weight loss is marked. After a period of two or three weeks, bluetongue-affected sheep begin to recover and usually return to normal condition at the end of six weeks.
One should provide a calm environment for the affected animals and not stress them during the recovery period. The research indicates that under these circumstances the death loss should be under 0.5 percent; however, if the affected sheep are driven, worked, drenched, sheared or handled roughly, the death loss may approach 30 percent. Intensity of Bluetongue signs is increased by solar radiation; one should keep affected animal in a dark location.
There are no known treatments for Bluetongue, but giving antibiotics to control secondary pneumonia is recommended. Although a vaccine is available, it is only effective against certain strains of the virus (there are 5 strains known to the U.S. and 24 strains worldwide) and can only be used prior to contracting Bluetongue. Also, unfortunately, in some cases the vaccine produces reactions as bad or worse than what is seen with the actual virus.
