Hemorrhagic Disease: Background

Hemorrhagic disease (HD) is an infectious viral disease transmitted by tiny biting flies (often referred to as midges, gnats or no-see-ums) in the genus Culicoides. HD is caused by two closely related viruses in the genus Orbivirus: epizootic hemorrhagic disease (EHD) and bluetongue virus (BTV). There are two subtypes of EHD virus and five subtypes of bluetongue in North America. Because disease features produced by these viruses are indistinguishable, the general term “hemorrhagic disease” is used when the specific virus is unknown.

EHD and BTV are infectious to a wide range of wild ruminants and susceptibility varies among species. Clinical disease has been reported in white-tailed deer, mule deer, bighorn sheep, elk and pronghorn. Antibodies have been detected in bison and mountain goats; however, these infections were not associated with disease. Suspected die-offs have been reported, but have not been confirmed via testing to be HD.

Infection typically occurs from mid-August through October, coinciding with the abundance of biting midges. The onset of freezing weather stops the midges and typically ends HD outbreaks. It is unclear how the viruses persist during winter when the midges are not active.