A site in Pennsylvania recently recorded the highest-ever concentration of ticks carrying a variant of potentially fatal Powassan virus called deer-tick virus (DTV). This rare virus has the potential to cause deadly infections with lasting neurological effects, and officials fear it and other serious tick-borne illnesses may become more common in the future.
Powassan virus is transmitted to humans bitten by infected female black-legged ticks (Ixodes scapularis). Between 2008 and 2017, most of the cases were diagnosed in and around the Great Lakes region.
The virus, which has two lineages — one of which is DTV — was first identified in 1952. While many cases of Powassan virus are asymptomatic, those that are symptomatic can be deadly, according to the Centers for Disease Control and Prevention (CDC). Initial symptoms include headache, fever and vomiting, with the most severe cases involving neurological complications such as encephalitis (inflammation of the brain) and meningitis. Roughly 1 in 10 neuroinvasive cases of Powassan virus are fatal, and about half of the survivors of these cases experience long-term neurological health impacts.
"The infection rate of ticks sampled from the Lawrence Township Recreational Park is extremely high; deer tick virus transfers very quickly through the bite from an infected tick, and the health outcomes from the deer tick virus are more severe than [those of] other tick-borne illnesses typically seen in Pennsylvania," DEP Secretary Patrick McDonnell said in a statement.
The alarming finding was reported in January, when the Pennsylvania Department of Environmental Protection (DEP) reported that its vector management program detected "an unusually high infection rate" with DTV.
Of 25 ticks sampled from the park, 92% tested positive for DTV. In comparison, the highest infection rate of DTV among ticks previously measured at a single U.S. site was 25%. While this is a rather small sample size, the nearly ubiquitous infection of these ticks has Pennsylvania officials concerned.
The virus can be transmitted within 15 minutes of a tick bite, which is much faster than the time from bite to infection for many other tick-borne diseases, such as Lyme disease, in which a tick needs to be latched onto a person for more than 24 hours to transmit the disease. Because Powassan virus historically has been rare, there are no specific treatments, although a vaccine is in development, according to a 2020 study in the journal PLOS Neglected Tropical Diseases.
This rare tick-borne disease has become more common in the U.S. in recent years. Between 2016 and 2020, 134 cases of Powassan virus were reported in the U.S. compared with 44 cases between 2011 and 2015.
In fact, annual cases of all tick-borne diseases reported to the CDC more than doubled between 2004 and 2019, rising from 22,527 in 2004 to more than 50,000 in 2019. According to a 2021 paper in the CDC journal Emerging Infectious Diseases, the CDC numbers are likely vastly underreported, at least for Lyme disease.
This increase in infection rates is likely due to the expansion of the black-legged tick, the vector for both Powassan virus and Lyme disease. In a 2019 study in the Canadian Journal of Infectious Diseases and Medical Microbiology, researchers predicted that "the number of [Lyme disease] cases in the United States will increase by over 20 percent in the coming decades." Increased temperatures and humidity are likely to contribute to increased reproduction, survival and expansion of ticks. A 2021 review in the journal Insects predicts that black-legged ticks will continue to expand farther northward across Minnesota, the Dakotas and Alaska by 2050.
However, climate change is just one part of the problem; changes in land use across North America also may play a role. Matthias Leu, an ecologist and associate professor at the College of William & Mary, told The Washington Post in 2019 that "urbanization has led to a growing population of deer and mice, which are used by the ticks for blood meals, and that in turn increases the tick population."
Originally published on Live Science.
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Cameron Duke is a contributing writer for Live Science who mainly covers life sciences. He also writes for New Scientist as well as MinuteEarth and Discovery's Curiosity Daily Podcast. He holds a master's degree in animal behavior from Western Carolina University and is an adjunct instructor at the University of Northern Colorado, teaching biology.