Powassan virus encephalitis
P.K.Ghatak, MD
Powassan is a town in Ontario, Canada. This virus causes febrile illness and in some cases encephalitis, meningitis, and Polio like illness. The POW virus was first isolated in Powassan in 1932 from the brain tissue of a 8year old boy. The virus is named POW virus (POWassan). POW virus is an arbovirus and belongs to the family Flaviviridae and genera Flavivirus. It is transmitted by a deer tick (Ixodes ) from the small ground animals to human. In the USA the two species of deer ticks are responsible for transmission of POW virus - Ixodes scapulars and Ixodes cookie. The deer tick in addition to POW virus, transmit Lyme disease (a spirochete) and Anaplasma phagocytotrophism previously known as Ehrlichiosis (a bacterium) and also Babesiosis ( an intracellular parasite).
People living in the northern hemisphere that includes Canada, Russia and the USA are at risk of infection while working outdoors, camping, fishing or other outdoor activities. In the USA land around the Great Lakes have a large deer population and so the POW virus, and people in New England Sates have seen in the increase incidence of POW infection.
Incidence in the USA:
On the average 20 to 40 people are infected with POW virus each year.
The incidence of encephalitis among the infected group is not known. Fatality rate of those who develop neurological complication is 10%. And 50% of those recovered show neurological impairment. Incubation period is 1 to 5 weeks. The bite of tick is usually short and in 15 minutes viral entry to body is complete.
Clinical features:
The initial symptoms are similar to Flu. The majority of the patients recover within 10 days. A handful of recovered patients develop the second phase of illness marked by severe headaches, high fever, meningeal irritation, various degree of confusion, coma, polio like illness. Diagnosis and management are no different than other viral encephalitis mentioned before. Humans are the end host and human blood does not carry an enough load of virus to infect a tick.
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