Powassan encephalitis, a disease caused by Powassan virus, could become the next tick-borne epidemic in the U.S. If it does, it could surpass Lyme disease in its impact upon public health. Transmitted by the same tick that causes Lyme disease, Powassan encephalitis is fatal in about 10% of cases, and half of its survivors suffer permanent brain damage. Worse, there is no treatment or vaccine for it.
Since the discovery of the first and fatal case in Powassan, Ontario in 1959, only sporadic reports of Powassan encephalitis have occurred in southern Canada and some of the bordering U.S. states. But within the past 10 years, the number of reported cases has skyrocketed to more than 75 among 12 states, mostly in the densely populated U.S. Northeast. Massachusetts alone has reported 13 cases with three fatalities within the past three years.
Recent field studies in the Northeast have shown that as many as 5% of deer ticks are infected with Powassan virus. Millions of people are bitten by deer ticks each year, accounting for the more than 30,000 cases of Lyme disease and other tick-borne diseases annually reported by the Centers for Disease Control (CDC). But we do not know what happens to all of the people who must be getting bitten by ticks infected with Powassan virus. The cases being reported are likely to be just the tip of an iceberg.
Public health agencies could be doing much more to respond to yet another public health threat from ticks. In the short term, more clinical research is needed to determine the outcome of infection with Powassan virus in people bitten by infected ticks. As with all new diseases, only the most serious cases receive the attention of public health agencies. The full spectrum of illness caused by Powassan virus is poorly understood, especially when combined with the many other tick-borne pathogens that coexist with Powassan virus in ticks.
Additionally, more field studies are needed to determine where ticks infected with Powassan virus occur within the U.S. so that physicians know where to look for human cases and people will know where to take greater precautions against tick bites. In order to do this, health agencies will need more staffing of medical entomologists who know how to collect and identify ticks.
A vaccine to prevent Powassan encephalitis could be made available by testing and approving the vaccine currently used in Europe to prevent a similar disease known as tick-borne encephalitis. These are closely related viruses and the existing vaccine should have efficacy against Powassan. Vaccines against tick-borne diseases have not been successful in the U.S. mostly because of medical liability costs. But the untreatable and much more serious nature of Powassan encephalitis should change the equation.
In the long term, public health agencies need to get serious about addressing the core of the tick-borne disease problem in the U.S., which is how to control ticks in the environment. One would think that after 30 years of epidemic Lyme disease and other new tick-borne diseases, we would have better knowledge of how to manage the increasing population of infected ticks. But basic field studies on ticks has never been a priority at the National Institutes of Health and the CDC has too little funding to support the major research effort that is needed for our academic institutions to solve the tick problem.
Until this happens, we will continue to lose the battle against new tick-borne diseases.
Durland Fish is professor emeritus of epidemiology at the Yale School of Public Health.
