Last Update: March 2000
Author: F. A. Leighton
Reviewer: H. Artsob
SSH virus is classified in the Genus Bunyavirus of the family Bunyaviridae. It is an enveloped, single-stranded RNA virus. SSH virus is one of a large group of related arboviruses called "California Serogroup" viruses. This group includes Jamestown Canyon virus, the other California Serogroup member associated with human disease in Canada. The California Serogroup derives its name from the first virus of the group that was recognized - California Encephalitis Virus - which was isolated from mosquitoes in California in 1943. SSH virus was first identified in a Snowshoe Hare (Lepus americanus) in Montana in 1958. It was first recognized as a cause of human disease in 1978.
SSH virus has been reported from all 10 provinces, the Yukon, the Northwest Territories and Nunavut. Its world-wide distribution includes Canada, some adjacent areas of the northern tier States, Alaska and some of eastern Asia.
SSH virus persists in cycles of transmission among wild mammals and mosquitoes. A wide range of wild mammal species can be infected with SSH virus, and, for most geographic areas in which the virus occurs, it is not clear which species are the principal hosts. The Snowshoe Hare (Lepus americanus) is thought to be important is some areas of Canada, but other species may be equally or more important as maintenance and amplifying hosts. Evidence of infection has been found in at least 16 wild species, including Snowshoe Hare, 7 rodents, 4 carnivores, 3 ungulates and Ruffed Grouse, and in 4 domestic species - chickens, dogs, horses and cattle.
Many different species of mosquito can become infected with and transmit SSH virus. These include many species of the genus Aedes, as well as species of Culiseta and of Culex. Species of Aedes have had the highest rates of infection in various surveys and appear to be the most important mosquito hosts for SSH virus. The virus is able to persist in adult mosquitoes during prolonged periods of cold weather and also appears to be passed from an infected adult female to its eggs in some species of Aedes. It may also persist in hibernating small mammals during winter.
Overall, it appears the SSH virus persists in a variety of different ecological associations of wild mammals and mosquitoes and is not limited to a small number of either vertebrate or invertebrate hosts species or to a narrow range of habitats.
Wildlife: Although virus or antibodies to the virus have been found in a wide range of mammals and in a few bird species, there is no evidence to date that wild animals become diseased from infection with SSH virus.
Domestic Animals: Non-fatal clinical encephalitis was reported in two yearling horses in Canada, one in Ontario and one in Saskatchewan. Such clinical disease in domestic animals appears to be very rare, however. As with wild species, domestic animals are often infected, but this seldom results in disease.
Infection of people with SSH virus has been documented in British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, Quebec, New Brunswick, Nova Scotia and Newfoundland, and actual human disease (non-fatal in all cases) has occurred in Ontario, Quebec, New Brunswick and Nova Scotia. Disease in people from infection with SSH was first recognized in Canada in 1978. Surveys in Canada have found from 0.5% to 32% of people to possess antibodies to one of two California Group viruses; in most of these people the virus would have been SSH. Disease in people, when it occurs, takes the form of infection and inflammation of the brain (meningitis and encephalitis). Some cases of clinical illness due to SSH virus probably go unrecognized. However, it appears that infection is much more common than is actual disease.
A variety of surveys for SSH virus have been conducted in various regions of Canada, but SSH has not been the object of general surveillance or public health programs. Its wide distribution in Canada and the low frequency of human disease caused by SSH indicate that specific surveillance programs or management strategies are not warranted. SSH sometimes is detected in surveillance programs for other arboviruses.
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