Powassan Virus

Last Update: March 2000

Author: F. A. Leighton

Reviewer: H. Artsob

The Powassan Virus

POW virus is classified as a member of the family Flaviviridae, and is an enveloped, single-stranded RNA virus. Although in the same family as St. Louis encephalitis virus and West Nile Virus, POW virus is ecologically very different.

POW virus was named for the town of Powassan, Ontario: the home town of a child who died from infection with the virus in 1958 and was the first recognized case of disease caused by the virus. It is now known that a virus isolated from ticks of the species Dermacentor andersoni collected in Colorado in 1952 also was this same virus.

Geographic Range

In Canada, POW has caused clinical disease in people in Ontario, Quebec and New Brunswick, and has been isolated from wild mammals and ticks in Ontario. Antibodies against POW have been reported in wild animals from British Columbia, Alberta, Ontario and Nova Scotia. POW virus has been recognized at several different locations in the United States from coast to coast, and may occur in Mexico. POW virus also has been recognized at multiple locations in southeastern Siberia, northeast of Vladivostok.


POW virus is maintained in cycles of transmission among several species of small wild mammals and several species of tick. The species of mammal and tick most important to the virus appears to vary from region to region. In eastern and central Canada, the Woodchuck (Marmota monax) and the tick Ixodes cookei seem to be particularly important, but infection rates can be high in Red Squirrels (Tamiasciurus hudsonicus), Grey Squirrels (Sciurus carolinensis), Eastern Chipmunks (Tamias striatus), Porcupines (Erethizon dorsatum), Deer Mice (Peromyscus maniculatus), voles (Microtus sp.), Snowshoe Hares (Lepus americanus), Striped Skunks (Mephitis mephitis) and Raccoons (Procyon lotor). POW virus has been isolated from four species of tick in North America - Ixodes cookei, I. marxi, I. spinipalpus and Dermacentor andersoni. The virus can pass from larva to nymph to adult as the tick develops. It is not known whether or not it can be passed from infected adult female to its eggs. Clearly, however, the virus can pass through winter in dormant ticks at various stages in their life cycle, and be transmitted to mammalian hosts in the spring when the tick becomes active again. Thus, the virus appears to persist in a variety of ecological associations among a range of ticks and a range of mammalian hosts that include rodents, lagomorphs (rabbits and hares) and carnivores.

Studies in the former USSR detected POW virus in mosquitoes and antibodies to POW virus in a variety of bird species. Whether or not mosquitoes and birds are important in maintenance of the virus or for transmission of virus to people is not known.

Disease in Animals

Wildlife: Isolation of POW virus from dead or diseased wildlife has been reported twice: from one Grey Fox (Urocyon cinereoargentatus) and one Red Fox (Vulpes vulpes), both in the United States. Whether or not POW virus was responsible for the illness and death of these animals, or was isolated from them incidentally, is not certain. Experimental infections of both species have resulted in infection without disease. No other cases of disease resulting from infection of free-ranging wild animals with POW virus have been reported. Infection of wild mammals within the geographic range of the virus is very common, as noted above (Ecology of POW), but disease appears to be unusual.

Domestic Animals: Disease associated with infection with POW virus has not been recognized in domestic animals. Experimental infections have been done in dogs, cats, pigs, horses, and goats. Experimentally-infected horses developed clinical disease associated with infection and inflammation of the brain (encephalitis). POW virus was found in the milk of one lactating goat during the second week after infection. No clinical disease was evident in cats, pigs or goats. Dogs developed a mild, transient fever.

Human Disease

Human disease from POW is rare. In Canada, 12 cases are known to have occurred in the 41 years from 1958 to 1999. Seven were in children less that 10 years of age. At least four cases were fatal either during the acute disease or subsequently due to chronic debilitation. Four of the 8 surviving patients experienced some form of persistent debility. Disease, when it occurs, takes the form of infection and inflammation of the brain (encephalitis and meningitis). Most infections do not result in disease. For example, among residents of northern Ontario from 1959-1961, studies of the pattern of occurrence of antibodies indicated that 3.3%, 0.5% and 1.9% of the population was infected by the virus in each of those three years. Tick bite is considered the major, possibly the only, significant route of exposure for people, and risk of tick bite is the major risk factor for people within the geographic range of the virus.

General References

Artsob, H. 1988. Powassan encephalitis. In: The Arboviruses: Epidemiology and Ecology, Vol. IV. (Monath, T.P., ed). CRC Press Inc., Boca Raton. p. 30-49.

Artsob, H. 1990. Arbovirus activity in Canada. Archives of Virology . Supplement 1: 249-258.

Calisher, C.H. 1994. Medically important arboviruses of the United States and Canada. Clinical Microbiology Reviews 7: 89-116.

Gholam, B.A., S. Puksa, and J.P. Provias. 1999. Powassan encephalitis: a case report with neuropathology and literature review. Canadian Medical Association Journal 161(11): 1419-1422.

Hardy, J.L. 1994. Arboviral zoonoses of North America. In: Beran, G.W.(Editor-in-chief). 1994. Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton. p.185-200.

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