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What is Powassan Encephalitis?

Powassan encephalitis is a serious, but rare, viral disease found in North America and South Eastern Siberia.  Powassan (POW) virus is one of many viruses and bacteria spread by ticks.  There are other types of encephalitis viruses spread by mosquitoes in Canada that are more common, including West Nile Virus and Eastern Equine Encephalitis virus.   

POW virus was named for the town of Powassan, Ontario where the virus was first isolated in 1958 from a patient who died from encephalitis.  Between 1958 and 1999, only 12 human cases of POW encephalitis have been known to occur in Canada.  However, with climate change and ongoing surveillance for tick-borne disease in Canada, the number of new cases of POW encephalitis may increase over time. 

What are ticks?

Ticks are tiny insects about the size of a sesame seed and they feed on blood. They cannot fly. They move about the ground slowly or settle on tall grass and bushes until they attach themselves to a person or animal passing by.  While most tick bites do not cause disease in humans, some do.  The groundhog or woodchuck tick (Ixodes cookei) is commonly found throughout Ontario near nests or burrows and prefers to feed on groundhogs, squirrels, chipmunks, mice, porcupines, raccoons and skunks.  This species of tick rarely contacts and bites humans.  Other ticks that could potentially transmit POW virus include the squirrel tick (Ixodes marxi), Ixodes spinipalpus and the Rocky Mountain wood tick (Dermacentor andersoni).

What do ticks look like?

How do ticks cause disease?

Not all ticks carry POW virus. Even with a bite from an infected tick, there is only a small chance of getting POW encephalitis. Ticks feed on blood by inserting their mouth, not their whole bodies, into the skin of a person or animal. They are slow feeders so it takes time before the virus can be transmitted to your blood. Ticks are most likely to transmit infection after being attached for at least two days of feeding. A complete blood meal can take several days. The tick's body slowly enlarges as it feeds, making it seen more easily.  Ticks tend to attach in warm, moist places – behind the ears, in the armpits and groin, but can be found anywhere on the body.  In Ontario, tick exposure occurs most commonly from June to October.

What are the symptoms?

Human disease from POW virus is rare and most infections do not cause disease. The POW virus causes inflammation and swelling in the lining of the brain and spinal cord (encephalitis and meningoencephalitis).  Symptoms of POW encephalitis begin suddenly, 7 to 14 days following infection and may include headache, fever, stiff neck, drowsiness, nausea and vomiting.  More severe symptoms can develop including difficulty breathing, tremors, disorientation/confusion, seizures, paralysis and sometimes death.  Survivors of POW encephalitis can suffer life-long debilitation.

What is the treatment?

There is no specific treatment or medication for Powassan encephalitis.   Individuals bitten by a tick should remove the tick and have the tick tested through their family doctor or Public Health. Medical attention should be sought if symptoms develop over the next two weeks.  

If POW encephalitis develops, supportive care is indicated to relieve the symptoms and may include hospitalization on intravenous fluids, ventilator support and prevention of secondary infections.  Medications may also be used to reduce swelling in the brain and spinal cord.

How do I avoid ticks?

• Wear lightly coloured clothing. It makes ticks easier to spot.
• Wear long pants and a long-sleeved shirt.
• Tuck pant legs into your socks.
• Use a tick repellent containing DEET and follow the manufacturer's directions. Apply it to your skin and outer clothing more frequently than you would for mosquitoes or biting flies. Avoid your eyes and mouth as well as cuts and scrapes.
• Use veterinary-recommended tick control products on your pets and check them for ticks periodically.
• If you are in an area where you could get a tick bite, search your body well for ticks. It is important to do this each day. Pay special attention to areas such as the groin, scalp and armpits. Use a mirror to check the back of your body or have someone else check it for you.

What do I do if I spot a tick on my skin?

Prompt removal of ticks from your skin will help prevent infection since transmission of viruses or bacteria is unlikely to occur within a day or so of tick attachment. Remove a tick from your skin as soon as you notice it.  Your family doctor or Public Health can submit the tick for identification and testing.

How do I remove a tick attached to my skin?

CDC

• Use fine-tipped tweezers to firmly grasp the tick very close to your skin. Don't squeeze it as squeezing can speed up infection.
• With a steady motion pull the tick’s body away from your skin. Then clean your skin with soap and warm water.
• Avoid crushing the tick’s body. Do not be alarmed if the tick’s mouthparts remain in the skin. Once the mouthparts are removed from the rest of the tick, it can no longer transmit disease.
• If you accidentally crush the tick, clean your skin with soap and warm water or alcohol.
• Don’t use petroleum jelly, a hot match, nail polish, or other products to remove a tick.

For more information contact Public Health at (519) 376-9420 or 1-800-263-3456.

Additional References

Calisher, C.H. 1994. Medically important arboviruses of the United States and Canada. Clinical Microbiology Reviews 7: 89-116.

Hardy, J.L. 1994. Arboviral zoonoses of North America. In: Beran, G.W.(Editor-in-chief). 1994. Handbook of Zoonoses. 2nd Edition. Section B Viral. CRC Press Inc. Boca Raton. p.185-200.

CDC information on Arboviruses http://www.cdc.gov/ncidod/dvbid/arbor/arbdet.htm