Powassan Encephalitis

                           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 an 8-year-old boy. The virus is named POW virus (Powassan). POW virus is an arbovirus and belongs to the family Flaviviridae and genus Flavivirus. It is transmitted by a deer tick (Ixodes) from the small ground animals to humans. In the USA, the two species of deer ticks are responsible for the transmission of POW virus - Ixodes scapularis and Ixodes cookiei. The deer tick, in addition to POW virus, transmits Lyme disease (a spirochete), Anaplasma phagocytotrophum, previously known as Ehrlichiosis (a bacterium) and also Babesiosis ( an intracellular parasite). 

 People living in the Northern Hemisphere, which 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 has a large deer population and so the POW virus, and people in the New England States have seen an increase in the 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 complications is 10%. And 50% of those recovered show neurological impairment. Incubation period is 1 to 5 weeks. The bite of the tick is usually short and in 15 minutes, viral entry to the body is complete.

Clinical features:

The initial symptoms are similar to those of the 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 degrees of confusion, coma, polio like illness. Diagnosis and management are no different from other viral encephalitis mentioned before. Humans are the end host, and human blood does not carry enough load of virus to infect a tick.

*********************