Powassan: Explaining the Rise of This Tick-Borne Virus

Deer tick, Ixodes scapularis, on a fingertip
Lauree Feldman / Getty Images

Although Powassan virus is rare, each year, there are more people being diagnosed with it.

This virus was first discovered in Powassan, Ontario, in 1958, when it was isolated from the brain of a young boy who died of encephalitis, which is the hallmark of infection with Powassan virus. Between 1958 and 1998, only 27 people were diagnosed with Powassan virus infection. Between 2003 and 2017 this number has more than tripled, with 85 people being diagnosed.

According to the authors of a 2017 review titled “Powassan Virus: An Emerging Arbovirus of Public Health Concern in North America,” this rise “may be due to increased surveillance and testing for arthropod-borne viruses, an actual emergence of the disease, or a combination of both factors.”

Background

Like Zika virus, dengue virus, and West Nile virus, Powassan virus is a type of flavirus. It is similar to another tick-borne virus that causes encephalitis: tick-borne encephalitis virus (TBEV). Infection with TBEV is much more common than Powassan virus, with several thousand people in Europe and Asia affected with encephalitis and meningitis each year.

Much of what we know about Powassan virus’s ability to replicate and potential to cause illness is actually based on TBEV, which is considered an international health concern and much better studied than Powassan virus. Notably, vaccines have been created for TBEV.

Powassan virus is a single-stranded RNA virus. Technically, Powassan virus comprises two lineages: the Powassan virus prototype lineage and deer tick virus (DTV). In this article, we’ll refer to both of these very similar (yet ecologically and genetically distinct lineages) as Powassan virus.

Powassan virus is carried by the Ixodes genus of ticks.

The distribution of Powassan virus runs from Virginia all the way north to Nova Scotia. It is also found in the interior of the United States and Canada, including New York, Pennsylvania, Quebec, Michigan, Wisconsin, Minnesota, and Ontario. Powassan virus has even been found as far west as Colorado, California, and British Columbia.

Typically, Powassan virus is distributed in the Northeast and Great Lakes regions, with the Hudson Valley in New York and New England reporting the most cases.

Interestingly, Powassan virus is also found in Russia; however, this specific distribution is likely due to a single introduction more than 100 years ago—probably from imported mink destined for the late-1800s fur trade.

The Ixodes tick that carries Powassan virus is harbored by many animals, including red squirrels, chipmunks, groundhogs, skunks, voles, white-tailed deer, and white-footed mice. However, humans rarely come into contact with animals that tend to burrow, such as groundhogs and skunks. Instead, humans are likely bitten by Ixodes ticks after passing through leaf litter that’s visited by white-footed mice and white-tailed deer.

Clinical Symptoms

Typically, people can’t remember when they’ve been bitten by an Ixodes tick.

Most people who are bitten by the tick don’t develop symptoms. In those who do develop symptoms, it can take between one and five weeks for infection to take hold. Notably, it takes only 15 minutes for a tick to attach and transmit the Powassan virus.

Initially, people who become symptomatic develop a flu-like illness with fever, which can be accompanied by headache, gastrointestinal symptoms, drowsiness, disorientation, and rash. Within several days of these initial symptoms, encephalitis transpires. Encephalitis refers to inflammation of the brain. Additionally, the spinal cord can also become inflamed leading to meningitis and myelitis.

Once the brain and spinal cord are inflamed, the following symptoms manifest:

  • headache
  • lethargy
  • seizures
  • speech problems
  • paralysis
  • vision problems
  • slurred speech
  • vomiting
  • trouble breathing
  • impaired coordination

About 10 percent of people who develop encephalitis die. Moreover, 50 percent of people go on to develop permanent neurological problems. Permanent neurological problems include recurrent headaches, muscle wasting, and memory difficulties.

Diagnosis

Powassan virus infection is diagnosed based on clinical exam and laboratory testing.

Clinical diagnosis must fulfill three criteria:

  1. fever greater than or equal to 38 degrees Celsius
  2. any signs of nervous system impairment
  3. no other likely clinical diagnosis

Serological testing is the primary method of detecting Powassan virus in the lab. Serological testing identifies antibodies to the virus in a sample of spinal fluid, blood, or tissue.

According to the CDC, here is some other diagnostic information concerning the Powassan virus:

Electroencephalography (EEG) in patients with POW virus encephalitis reveals generalized slow wave activity and results can resemble those seen in herpes simplex virus encephalitis. MRI of the brain in patients with POW virus encephalitis shows changes consistent with microvascular ischemia or demyelinating disease in the parietal or temporal lobes; results of brain CT scans have not been particularly useful.

Treatment

There is no cure for Powassan virus. Instead, symptoms are treated supportively with intravenous fluids, artificial ventilation, and medications that reduce brain swelling. Furthermore, there isn’t enough evidence to develop formal treatment recommendations.

Steroids and intravenous immunoglobulin (IVIG) have both been observed to be effective in the treatment of infection. It is unclear whether antiviral treatment (i.e., ribavirin) is effective. Specifically, one patient who was treated with pegylated interferon and ribavirin still died of the disease. Of note, steroids, IVIG, interferon, and ribavirin all attenuate immune system response and are immunomodulators.

Although not diagnostic, brain imaging with MRI could be prognostic and indicate which patients will fare worse in the long-term.

Prevention

Currently, there is no vaccine for Powassan virus. There are vaccines for TBEV, which is similar to Powassan virus, and these vaccines are available in Russia and Europe. However, these TBEV vaccines haven’t proven effective in preventing infection with Powassan virus.

Moreover, Powassan virus is a type of flavivirus, and in addition to TBEV vaccine, there are vaccines for other types of flavivirus, including yellow fever and Japanese encephalitis. Nevertheless, Powassan virus is least similar to other types of flavivirus in molecular terms so these other vaccines are also of limited use.

For now, the best way to avoid infection with Powassan virus is by following preventive guidance. There are various personal and property measures that you can take to reduce your risk for tick exposure including the following:

  • Avoid direct contact with ticks by avoiding woody areas with high leaf litter.
  • Walk towards the center of trails.
  • Use repellant that contains 20 percent or more DEET, picaridin, or IR3535. Apply repellant to exposed areas of skin.
  • Wear clothing that’s long sleeved and light colored. Also, tuck your pants into your socks.
  • When applying repellant to children, avoid application to the eyes, hands, and mouth.
  • Use clothing and gear that contain permethrin.
  • Shower as soon as you return home and then perform a tick check using a mirror. Remove all ticks immediately.
  • Examine all clothing, gear, and pets for ticks.
  • To kill ticks on clothing, wash the clothing in hot water and dry on high heat.
  • Check your pets for ticks after they spend time outdoors.
  • Remove tick habitat areas, such as tall grass, brush, and leaf piles, from your property.
  • Mow your lawn frequently to reduce tick habitat areas.
  • Construct fences to keep wild animals off your property.
  • Remove discarded furniture and other trash in which ticks inhabit from your yard.
  • Stack wood in neat piles to prevent infestation with rodents that carry ticks.
  • Place patio and playground equipment away from trees and brush areas.
  • Consider using tick pesticides (i.e., acaricides) or repellents on your dog, including dusts, impregnated collars, sprays, or topical treatments. Speak with your veterinarian first.
  • Consider having an exterminator spray your home.

Cats are very sensitive to pesticides; thus, don’t use an insecticide on your cat without speaking with your veterinarian first.

Future Directions

In recent years, there has been an increase in research efforts targeting Powassan virus.

Looking forward, in the battle against Powassan virus, it’s going to be important for scientists to analyze adult and immature Ixodes ticks procured from the field to better understand the life cycle and transmission cycles of these ticks as well as evolutionary factors that shape the virus. Of note, some experts suggest that recent shifts in the life cycle of these ticks could explain the increased virulence of this pathogen.

Furthermore, scientists need to better study the antiviral response to the Powassan virus in mammals that serve as hosts. Specifically, researchers need to better understand the replication cycle of the virus not only in ticks but also in mammals. These insights will help scientists identify therapeutic targets and figure out how the Powassan virus persists in nature.

To better understand the long-term neurological repercussions of infection, experiments need to be designed that follow groups of patients over time (i.e., cohort studies).

In Wisconsin, certain ticks can transmit both Powassan virus and Lyme disease, making co-infection possible. This co-infection could explain the lingering symptoms of Lyme disease and needs to be further studied.

A Word From Verywell

Although rare, the number of people who are becoming infected with Powassan virus is rising and severe illness requires hospitalization. With more testing and increasing prevalence, it’s possible that Powassan virus will become an emerging cause of illness in the future.

On a related note, because of limited public awareness and limited testing for Powassan virus, it’s possible that this virus remains undiagnosed among patients who present with encephalitis. In other words, some people who end up getting encephalitis are never diagnosed with Powassan virus infection.

The best way to manage infection with Powassan virus is to prevent tick exposure in the first place. If you live in areas where Powassan virus is found—especially the Northeast and Great Lakes areas—use repellants when you go outside and perform tick checks when you return.

Keep in mind that there are other tick-borne pathogens that more commonly cause illness, such as Borrelia burgdorferi (i.e., Lyme disease), so repellants and tick checks are a good idea for the prevention of other types of illness, too.

It’s important to understand that even if a tick has been on your body for only a very short time, by the time you notice it, you could be infected with Powassan virus. Other tick-borne pathogens, such as Borrelia burgdorferi, have a longer grace period of around one day.

If you suspect that you or a loved one has been infected with Powassan virus, immediately seek medical attention. Additionally, let your physician know why you suspect infection with this virus and detail any history of potential tick exposures. Furthermore, let your provider know about your activities and where you traveled—especially outdoor environments in which the tick harboring Powassan virus is found. Your physician should be alerted both because there’s a special test for Powassan virus and the condition is typically managed by infectious disease specialists.

Sources:

Bennett, N. Powassan Virus Disease in an Infant—Connecticut, 2016, MMWR. 2017; 66: 408-409.

Doughty, CT, Yawetz, S, Lyons, J. Emerging Causes of Arbovirus Encephalitis in North America: Powassan, Chikungunya, and Zika Viruses. Current Neurology and Neuroscience Reports. 2017: 17;12.

Hermance, ME, Thangamani, S. Powassan Virus: An Emerging Arbovirus of Public Health Concern in North America. Vector-Borne and Zoonotic Diseases. May 12, 2017.

CDC. Powassan Virus. www.cdc.gov.

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