Powassan Virus: What to Know about this Rare Tick-Borne Disease

A life-threatening illness that can cause severe neurological symptoms, Powassan cases are on the rise in parts of the U.S., says an expert at Cummings School of Veterinary Medicine

On paper, Powassan virus sounds like your basic nightmare. A tick-borne infection with no vaccine and no cure, it kills 1 in 10 people who get it and causes long-term neurological problems in half of reported cases. For decades, Powassan virus disease had affected only about one person a year in the U.S.—most likely because it was typically transmitted to humans by a tick that rarely bites people. But cases are slowly rising, especially in New England and the Great Lakes region, and human Powassan virus infections have now been diagnosed in the U.S., Canada, and Russia. 

In 1997, Sam Telford III, a professor in the Department of Infectious Disease and Global Health at Cummings School of Veterinary Medicine at Tufts University, found a genetically distinct strain of Powassan virus in deer ticks—the bloodsuckers notorious for spreading Lyme disease. He initially worried about its implications for Powassan virus in humans, as he found the virus in 1 out of every 100 deer ticks he sampled. But at the time, there were no reports of people with swelling in the brain—or encephalitis, a key symptom of Powassan—in places where deer ticks were common. He and his fellow researchers surmised that people just didn’t come down with Powassan virus disease from the strain carried by deer ticks.

“It turned out we were wrong,” he said.

In 2008, an elderly immunocompromised woman died from the deer-tick-carried subtype of Powassan virus. Since then, the number of cases has been slowly increasing each year. In 2022, 44 cases of Powassan virus were reported to the Centers for Disease Control and Prevention, and seven of those people died. So far this year, 29 cases have been reported. Massachusetts has been a particular hot spot. The state Department of Public Health has reported 16 cases in the last decade.

Telford said he suspects that there has been some sort of natural selection for a variant of Powassan virus that’s more easily transmitted by deer ticks to people and causes severe disease. Such is likely the case regarding the recent emergence of babesiosis—a rare and sometimes fatal tick-borne disease that attacks human red blood cells—in the northeastern U.S. “Babesiosis used to be a coastal disease,” explained Telford, who is also part of the Tufts Lyme Disease Initiative. “Now it’s all over the place, from New Jersey to Maine.”

Since 2017, Telford has conducted research under a grant from the National Institutes of Health to try to explain the epidemiological paradox of why they’ve found so much of the virus present in ticks but relatively few cases in humans. The objective is to determine whether some strains are less likely to cause disease. “This seems to be the case, but more work is needed,” he said.

Symptoms and Transmission

Whereas a deer tick needs to attach itself to a human host for 36–48 hours to spread Lyme disease, it may only need to be attached for 15 minutes or so to transmit Powassan virus.

“Powassan is different from other diseases spread by ticks because it lives in the salivary glands of the tick,” said Telford. “That means it doesn’t take much time for the virus to exit the tick’s mouth and transmit to a person. Lyme and other tick-borne diseases are caused by viruses that live in ticks’ guts.”

Powassan virus cannot be transmitted between people via coughing or close contact like the flu. Clinical symptoms begin with a high fever and vomiting and progress to neurologic signs such as headache, weakness, confusion, seizures, memory loss, and encephalitis. Half of those who survive have persisting neurological problems such as memory issues and even paralysis.

“It’s scary what this virus can do to people,” said Telford. “If they survive, some patients will have severe neurological damage for the rest of their life.”

If you’re experiencing any of the initial, flu-like symptoms and you may have recently been bitten by a tick, it’s a good idea to see a doctor. A blood test or spinal fluid test can diagnose Powassan virus. There are no medications that can prevent or treat Powassan, and since it’s a virus, antibiotics (which treat bacterial illnesses) won’t help.

All that said, it is extraordinarily unlikely for a person to encounter a tick carrying Powassan virus. There are 3,000 to 5,000 cases of Lyme disease reported to the Massachusetts Department of Public Health each year, but the greatest number of Powassan cases ever reported to the agency was seven in 2015.

Telford issued a word of caution for parents and dog owners, in particular. In 2021, he published this case report about two infants in Connecticut who contracted Powassan virus after being bitten by a deer tick that entered their home on the clothing of a parent who had been outside. In one case, the parent was hunting in the woods, and in the other case, the parent had been walking the family’s dog. Both patients survived, but the cases demonstrate the importance of preventative measures.

One way to avoid bringing home ticks on your clothes is to spray your clothing, shoes, and gear with insecticides that contain 0.5% permethrin,” he said. “It’s not tick-ageddon. People should continue to enjoy the outdoors. Just protect yourself against tick bites.”

Editor's note: This story was originally published in July 2017 and was updated with new information in October 2023.

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