Powassan virus (POW) is a flavivirus transmitted through tick bites, first discovered in the town of Powassan, Ontario in 1958 when it was identified in a young boy. This virus is transmitted more rapidly from tick to human than Lyme disease (it can take only minutes) and more likely to be fatal. As of now, it remains relatively rare.
Over the last 10 years, approximately 75 cases of POW virus have been diagnosed in the US. Most cases were discovered in the Northeast and Great Lakes region. Although rare, a rise in the number of cases is noted in recent years.
POW is the only tick- born flavivirus that is transmitted mainly through the bite of the Ixodes cookei tick with the groundhog being the reservoir. Deer tick virus (DTV) is a genotype of POW that is frequently isolated from Ixodes scapularis (deer ticks). DVT is deemed to be the greater threat since it is associated with the tick vectors responsible for the epidemics of Lyme disease, babesioisis, and human granulocyte anaplasmosis.
At least 38 species of mammals have demonstrated evidence of infection, including small and medium-sized wild animals and domestic animals. Its prototype has been isolated from three trick species: Ixodes scapularis, I. marxi, and I. spinipalpus. POW can result in encephalitis and neuroinvasive disease. Most cases of POW are considered to be asymptomatic. Evidence supporting this is the fact that there is a 3% seroprevelance rate in Ontario and a 0.7% rate in the state of NY. It, however, is unclear what is the relationship between seroprevalence and infection.
The incubation period of POW can range from 1 week to 1 month from the time of the tick bite to the onset of disease. Many of those infected develop no symptoms. Symptoms can include fever, headache, vomiting, weakness, confusion, coordination abnormalities, speech abnormalities and seizures. POW can affect the central nervous system leading to encephalitis and/or meningitis. Of those who develop encephalitis, approximately 10% of cases will be fatal. Another 50 % of those will develop permanent neurologic symptoms, which can include memory difficulties, muscle wasting or chronic headaches.
Diagnosis can be difficult to make. CSF studies and brain imaging will reveal signs of a viral encephalitis. The preferred diagnostic tests include reverse transcription PCR of CSF, serologic CSF testing, and serologic testing of the serum. Because these tests are not so readily available, these should be sent to state and federal labs to ensure accuracy of the results. The pathogenesis of this disease occurs due to lymphocytic infiltration of the perivascular neuronal tissue, especially in the gray matter of the brain.
There is no treatment of POW currently available. Supportive care may be needed depending in the severity of symptoms. Patients may need to be hospitalized for IV fluids, respiratory support, and/or medications to reduce cerebral edema. There are no vaccines currently available.
Scientists are particularly worried about POW this summer because they are predicting it to be one of the worst for increase in the tick population. The area where ticks transmit disease has been increasing over recent years. Reforestation and increased deer population contribute to the expanding geographic range. Of note, the number of cases of Lyme disease has tripled since the late 1990’s. Additionally, the number of counties in the Northeast and upper Midwest considered high risk for Lyme disease has increased by 300%. This is significant because the same ticks that spread Lyme disease also transmit POW.
Experts predict that the incidence of POW will rise. It is noted that the numbers of cases of Lyme disease in deer ticks is increasing and now is estimated at 38%.
Recently, POW made headlines when a 5 month in Connecticut became infected. Minnesota has seen the highest number of cases, 20. Following Minnesota, the state with the next highest number is Wisconsin and New York with 16 cases each, Massachusetts with 8, and a few cases noted in New Jersey, New Hampshire, Pennsylvania, and Virginia.
While POW remains a rare infection, its consequences can be deadly. As there is no treatment or vaccine available, the best therapy remains prevention and taking steps to avoid tick bites.
About the Author
Linda Girgis MD, FAAFP is a family physician practicing in South River, New Jersey. She was voted one of the top 5 healthcare bloggers in 2016. Follow her on twitter @DrLindaMD.