Abstract

Hemmoraghic disease (HD), which is caused by several related Orbivirus, including epizootic hemorrhagic disease virus (EHDV) and bluetongue virus (BTV), is a common disease of deer in the central and southern regions of the United States. Both viruses can infect cattle and sheep and are spread by Culicoides spp. gnats. Both EHDV and BTV have been associated with severe disease in white-tailed deer (Odocoileus virginianus). In domestic livestock, only BTV has been associated with severe disease in sheep.

A large-scale outbreak of EHDV-2 was confirmed in central Idaho along the Clearwater and Salmon Rivers in August and September 2003. Diagnosis was confirmed through necropsy, gross lesions, and virus isolation. Although mortality was widespread, it varied considerably in local areas with known or estimated mortality rates ranging from 20–90+%. About 10% of the total WTD population in central Idaho was affected with approximately 3000–5000 white-tailed deer estimated to have died. At least one mule deer was also confirmed to have died from EHDV-2.

Livestock, including sheep and cattle are present within most of the outbreak area and a flock of sheep in this area developed clinical signs consistent with BTV infection during the same time period. Serologic titers were found to both EHDV and BTV, however, virus isolation indicated BTV-17 as the cause of the clinical signs and limited mortality. EHDV-2 was isolated from one of 17 sheep in the flock. Domestic cattle in the outbreak area were not clinically affected. However, serology and virus isolation from two cattle herds in outbreak area indicated that the cattle were exposed to both BTV and EHDV, but only BTV-17 was confirmed by isolation.

An interesting aspect of this outbreak relates to the differences in virus isolation results between white-tailed deer and livestock species that reside within the same area. All of the virus isolations from white-tailed deer were confirmed as EHDV-2 while all but one isolation from sheep and cattle were identified as BTV-17. Further investigation is continuing and further monitoring of the deer population as well as gnat studies will be done in the near future.