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Title: Investigation of a bluetongue disease epizootic caused by bluetongue virus serotype 17 in sheep in Wyoming

item Miller, Myrna
item JOHNSON, GREGORY - Montana State University
item Mecham, James
item Reeves, Will
item Wilson, William

Submitted to: Journal of the American Veterinary Medical Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/21/2009
Publication Date: 10/15/2010
Citation: Miller, M.M., Brown, J., Cornish, T., Johnson, G., Mecham, J.O., Reeves, W.K., Wilson, W.C. 2010. Investigation of a bluetongue disease epizootic caused by bluetongue virus serotype 17 in sheep in Wyoming. Journal of the American Veterinary Medical Association. Vol 237:8, 955-959.

Interpretive Summary: Sheep from Wyoming and Montana were tested after an outbreak of bluetongue virus serotype 17 in the summer and fall of 2007. Samples were collected 3-6 months after the outbreak to determine how many sheep had been infected and if they carried the virus for long periods of time. Follow-up samples were taken the next year to determine if the virus persisted in the outbreak area over the winter. Bluetongue virus is a vector-borne disease transmitted by a biting midge and outbreaks require climate conditions that allow for large numbers of the vector. The summer of 2007 was exceptionally warm in the Big Horn Basin and parts of Montana. Infected sheep had unusually severe symptoms and high fatality rate, apparently due to a highly susceptible population that had not previously been exposed to the virus. The mountains surrounding the Big Horn Basin had likely protected the valley from previous spread of the disease. Many sheep in the affected flocks were found not to have antibodies that would prevent future infection and ranchers would need to consider other preventive measures if another mild winter and summer were favorable for the virus and the vector. However, testing done in 2008 found no evidence that the virus was circulating in the valley the next year. None of the sheep tested were found to carry the virus. This supports similar reports from studies done using small numbers of experimentally infected sheep. Disease incidence varied greatly between nearby locations indicating that the mobility of this vector is somewhat limited. This suggests that farm and animal level vector control programs could be useful in controlling local, highly infected vector populations.

Technical Abstract: Objective: To better characterize a 2007 bluetongue virus serotype 17 epizootic in the Big Horn Basin of Wyoming. Design: A study using samples collected 3-6 months post outbreak to determine infection rate, susceptibility to infection in the next summer, and long term presence of virus. Samples from 2008 spring-born lambs were also collected in the fall to test for evidence of the virus successfully overwintering. Sample population: 1356 sheep from Wyoming and Montana. Procedures: Serology, virus isolation, and reverse transcriptase-polymerase chain reaction (RT-PCR). Results: The percent antibody positive closely matched morbidity indicating there were few subclinical infections. Sheep kept in low elevation summer pastures were infected first and flocks from mountain pastures returned in the fall to an epizootic in full progress. Flocks separated by as little as 1 mile had significant variation in infection rate. Rams were infected at a higher rate than ewes. There was no evidence of the virus successfully overwintering and circulating in 2008. Conclusions: This outbreak appears to be a new intrusion into a naïve population previously protected by the mountains surrounding the Big Horn Basin. Most sheep in the affected flocks are not immune to future infection. Animal and ranch level vector control programs may help to control spread of infection. The higher infection rate in rams may be due to increased vector biting opportunity. Clinical Relevance: Information gathered will help ranchers make cost effective management decisions to minimize disease spread and to prepare for possible disease recurrence.