Powassan

Powassan (POW) virus is a North American tickborne flavivirus. POW virus was named for the town of Powassan, Ontario, where the first recognized case of disease caused by the virus occurred in 1958. From 1958 -- 2001, 31 human POW encephalitis cases were reported from Canada and the northeastern United States. POW encephalitis is associated with significant long-term morbidity and has a case-fatality rate of 10%-15%.

In North America, POW virus has been isolated from four tick species, including Ixodes cookei, Ix. marxi, Ix. spinipalpus, and Dermacentor andersoni. A variant POW virus also has been isolated from Ix. Scapularis. Evidence of infection has been found in 38 mammal species. Unlike Ix. scapularis, the primary vector for Lyme disease, Ix. cookei rarely search for hosts on vegetation and are often found in or near the nests or burrows of medium-sized mammals, such as woodchucks. Infections have occurred from May to December, with a peak during June--September when ticks are most active. As with many infectious agents transmitted by Ixodid ticks, few infected persons recalled tick bites because these ticks are small.

Because of the lack of awareness and the need for specialized laboratory tests to confirm diagnosis, the frequency of POW encephalitis may be greater than previously suspected. POW encephalitis should be included in the differential diagnosis of all encephalitis cases occurring in the northern United States, especially the northeast. Laboratory tests for POW virus infection are not commercially available but can be requested through state public health laboratories for testing at CDC. Awareness should be promoted among clinicians and public health staff, and tick-bite prevention strategies emphasized for the general public.

Because there is no vaccine or specific therapy for POW encephalitis, the best means of prevention is protection from tick bite. This includes using insect repellents, avoiding walking through tall grass and vegetation, wearing light-colored clothing with long sleeves and pants tucked into socks or boots, clearing brushy areas, and removing ticks before they attach or as soon after attachment as possible. Checking family pets also can prevent ticks from entering the home. Because Ix. cookei are often found on woodchucks and skunks and may be the primary vector of POW virus, environmental controls reducing human contact with small and medium-sized mammals should reduce risk for exposure to POW virus-infected ticks. Persons should keep areas adjacent to their home clear of brush, weeds, trash, and other elements that could support small and medium-sized mammals. When removing rodent nests, avoid direct contact with nesting materials and use sealed plastic bags for disposal and to prevent direct contact with ticks.