Is a flavivirus and is a tick-borne transmitted arbovirus occurring in the United States and Canada.

The woodchuck and the snowshoe hare are common hosts. Coyotes, foxes, racoons, skunks, domesticated cats and dogs are less common hosts. The Flaviviridae are 40 to 50 nm and does not survive outside the host.

Recently a Powassan-like virus was isolated from the deer tick, Ixodes scapularis. The range of Powassan virus in the United States is primarily in the upper tier States.�

Powassan virus is related to Russian Spring-Summer encephalitis, Japanese encephalitis and other TBE viruses. Like other Arbovirus members, it is carried by Ixodid ticks.

Four cases of encephalitis attributed to Powassan virus arose in Maine and Vermont between September 1999 and July 2001.� (The first cases in the United States since 1994).�

Arbovirus encephalitis is caused by a virus from the Arbovirus group. The term arbovirus stands for Arthropod-borne virus.�

Approximately 80 types of arboviruses are responsible for human disease. When the viruses are present in a tick or mosquito, the viruses are passed to a bird or small mammal when the arthropod feeds on the blood of that creature.

Infection of Powassan encephalitis and other Tick Borne Encephalitis (TBE) is by the bite of infected ticks.

Larval ticks ingest the virus by feeding on rodents, other mammals and certain birds. Ticks infected at any stage remain infectious for life. Consumption of raw (unpasteurized) milk from infected cows, goats, and sheep can transmit the disease.

Viral transfer from blood to the CNS through the olfactory tract has been reported but not proven. Powassan encephalitis and other TBE's are not directly transmitted from person to person.

The major causes of arbovirus encephalitis include the members of the viral families alphavirus (causing Eastern equine encephalitis, Western equine encephalitis, and Venezuelan equine encephalitis), flavivirus (Responsible for, West Nile, St. Louis encephalitis, Japanese encephalitis, Tick-borne encephalitis, Murray Valley encephalitis, Russian spring-summer encephalitis, and Powassan), and bunyavirus (causing California encephalitis).

Laboratory diagnosis of human arboviral encephalitis has changed greatly over the last few years.

In the past, identification of antibody relied on four tests: hemagglutination-inhibition, complement fixation, plaque reduction neutralization test, and the indirect fluorescent antibody (IFA) test.

Positive identification using these immunoglobulin M (IgM) - and IgG-based assays requires a four-fold increase in titer between acute and convalescent serum samples. With the advent of solid-phase antibody-binding assays, such as enzyme-linked immunosorbent assay (ELISA), the diagnostic algorithm for identification of viral activity has changed.

Rapid serologic assays such as IgM-capture ELISA (MAC-ELISA) and IgG ELISA may now be employed soon after infection. Early in infection, IgM antibody is more specific, while later in infection, IgG antibody is more reactive. Inclusion of monoclonal antibodies in these solid phase assays has allowed for a level of standardization that was not previously possible.

Clinically, Powassan virus resembles mosquito-borne encephalitis.

The disease has a variable incubation period of 7 to 34 days. The majority of human infections are asymptomatic or may result in a nonspecific flu-like syndrome. The onset may be insidious or sudden with a sore throat, drowsiness and headache. This may progress to altered mental status.

As the disease escalates, there can be increased lethargy, vomiting, respiratory distress, fever and convulsions. Patients may become semicomatose and paralysis has also been seen. The virus affects the grey matter throughout the brain, spinal cord and meninges and this may explain why survivors can have persistent neurological problems such as severe headaches, memory impairment and muscle atrophy.

Powassan virus has the highest case fatality rate among Arboviruses. Fortunately, only a few infected persons progress to frank encephalitis. In survivors, infection leads to immunity of the disease.

The Post-Encephalitic Syndrome (PES) after the acute stage of the disease can present with cognitive dysfunction's, permanent paresis, balance and co-ordination disturbances, chronic cephalgia (severe headaches), tinitis and hearing loss.

Currently, there is no vaccine for Powassan and treatment is to respond to symptoms. As a relatively unknown virus with long-term effects, Powassan virus is a agent of real concern.

Treatment is supportive during the course of the illness. The main concerns of treatment involve lowering fever, pain relief, avoiding dehydration, metabolic maintenance, and decreasing swelling in the brain with steroids.

Travelers are advised to avoid tick-infested areas and to protect themselves from tick bites by dressing appropriately and using repellents.