Powassan virus disease: For health professionals

On this page

• Key information
• Transmission
• Clinical manifestations
• Diagnosis
• Treatment
• Laboratory testing
• Surveillance
• Subscribe to the Zoonoses Bulletin

Key information

Powassan virus disease is a tick-borne zoonotic disease caused by the Powassan virus, which is an arbovirus within the genus Flavivirus. It is mainly transmitted by infected blacklegged ticks (Ixodes scapularis), groundhog ticks (Ixodes cookei) and squirrel ticks (Ixodes marxi). Consult with your provincial or territorial public health authorities to find out where ticks are most likely to be found.

Powassan virus disease, like most tick-borne disease infections, occurs during the warmer months, but infections can occur throughout the year. Ticks can be active whenever the temperature is consistently above freezing, and the ground isn't covered by snow.

It's critical to remove attached ticks promptly as the risk of transmission of Powassan virus increases the longer the tick is attached. Infected blacklegged ticks, groundhog ticks and squirrel ticks need as little as 15 minutes of attachment time to transmit the virus that causes Powassan virus.

Individuals may not be aware of or remember being bitten by a tick. Therefore, it's important that health professionals conduct a detailed patient history, including history of exposure to ticks, when assessing individuals with signs or symptoms suggestive of Powassan virus disease.

There's no specific treatment for Powassan virus disease. Patient management is focused on supportive care for symptoms.

There's currently no vaccine to prevent Powassan virus. The best way to prevent tick-borne diseases is to prevent tick bites.

For individuals presenting with signs or symptoms of Powassan virus disease, we encourage health professionals to consider the possibility of other tick-borne diseases, such as:

• anaplasmosis
• babesiosis
• Lyme disease
• tick-borne relapsing fever

Learn more about:

• Ticks in Canada
• How to remove a tick
• How to prevent tick bites
• Provincial and territorial public health authorities

Transmission

Powassan virus is primarily acquired through the bite of an infected tick. The ticks known to transmit Powassan virus are:

Transmission of Powassan virus can occur in as little as 15 minutes of attachment of an infected tick.

Learn more about:

• Ticks in Canada

Clinical manifestations

The incubation period for Powassan virus disease is 1 week to 5 weeks.

Symptoms may be absent or mild, flu-like symptoms. However, Powassan virus can cause severe, potentially fatal disease in some individuals.

Individuals who develop symptoms days or weeks after a tick bite may not remember being bitten or associate symptoms with the bite.

If symptomatic, individuals typically experience an initial febrile or prodromal phase (lasting 1 to 3 days) which may include:

• chills
• fever
• nausea
• malaise
• vomiting
• headache
• sore throat
• somnolence
• disorientation

Powassan virus disease can progress to neuroinvasive disease, including encephalitis, meningoencephalitis or aseptic meningitis. Neuroinvasive disease can manifest as:

• fever
• paresis
• tremors
• vomiting
• speech difficulties
• loss of coordination
• hemiplegia
• facial palsies
• pyramidal tract signs
• respiratory distress
• seizures (particularly in children)
• altered level of consciousness

Case fatality of Powassan neuroinvasive disease has been reported as high as 10%.

Long-term sequelae

Long-term sequelae can occur in 50% of patients who develop neurological complications.

Symptoms may include:

• headaches
• hemiplegia
• ophthalmoplegia
• memory impairment
• muscle wasting and weakness
• direction changing nystagmus

Rashes

While a cutaneous rash can occur at the site of infection, it doesn't have the appearance of an erythema migrans rash as seen in Lyme disease. Instead it's typically a morbilliform (measles-like) rash. The presence of an erythema migrans rash in a patient would be more suggestive of an infection or co-infection with Borrelia burgdorferi (the bacterium that causes Lyme disease).

Hypersensitivity reaction to a tick bite

Individuals may develop a hypersensitivity reaction within 24 hours of a tick bite. A hypersensitivity reaction will produce an erythematous skin lesion less than 5 cm in diameter which doesn't expand and usually recedes within 48 hours.

Hypersensitivity skin reactions shouldn't be confused with erythema migrans as seen in Lyme disease. Individuals who develop erythematous skin lesions which haven't resolved within 48 hours should be reassessed to determine whether an erythema migrans rash has developed.

Diagnosis

Powassan virus disease should be considered in individuals presenting with:

• a history of tick exposure
• clinical signs and symptoms that are consistent with Powassan virus disease

Laboratory testing can be conducted when appropriate.

Electroencephalography and brain magnetic resonance imaging tests can exhibit abnormalities in cases of encephalitis.

History of tick exposure

A history of tick exposure includes:

• a recent tick bite or
• living in or having recently visited a potential blacklegged, groundhog or squirrel tick habitat

While a known history of tick exposure, particularly to blacklegged ticks, groundhog ticks or squirrel ticks, helps with the diagnosis, absence of a history of exposure doesn't rule out Powassan virus disease. Individuals may not recall a tick bite because ticks are tiny, and their bites are usually painless. Furthermore, blacklegged ticks, groundhog ticks and squirrel ticks can be found outside currently identified risk areas.

A tick may carry multiple pathogens and transmit them to humans via a single bite. Therefore, while investigating Powassan virus disease, health professionals should consider infection or co-infection with other tick-borne diseases, such as:

• anaplasmosis
• babesiosis
• Lyme disease
• tick-borne relapsing fever

Consider consultation with an infectious disease specialist when suspecting co-infection.

Treatment

There's no specific treatment for Powassan virus disease.

Patients with severe Powassan virus disease, including neuroinvasive disease, will often need to be hospitalized to receive supportive care such as:

• respiratory support
• treatment to reduce swelling in the brain
• intravenous fluids for rehydration
• antiemetic therapy
• pain control for headaches

Laboratory testing

Laboratories that employ conventional diagnostic assays and interpretive criteria should be the only laboratories conducting diagnostic testing. Health professionals should send samples to their provincial and territorial laboratories. They will coordinate with the National Microbiology Laboratory, when necessary.

Samples to collect for patients being tested for Powassan virus include:

• acute serum sample: collected as early as possible after symptom onset
• convalescent serum sample: collected 2 to 6 weeks after the acute sample

For patients with acute neurological involvement, a cerebral spinal fluid sample should be collected along with the serum sample.

Laboratory tests for Powassan virus disease include:

• screening tests for antibodies
• human immunoglobulin M (IgM) enzyme-linked immunosorbent assay (IgM ELISA)
• hemagglutination inhibition (HI)
• confirmatory testing
  • plaque reduction neutralization test (PRNT) is used as a confirmatory test for samples that screen positive on the ELISA or HI
  • polymerase chain reaction (PCR) can also be performed on cerebral spinal fluid and brain tissue samples to detect viral ribonucleic acid (RNA)

Laboratory testing should be done through a licensed and accredited public health laboratory.

Surveillance

Powassan virus disease is a nationally notifiable disease. Nationally notifiable diseases are infectious diseases that have been identified collectively by the federal, provincial and territorial governments as priorities for surveillance and control efforts.

The national notification system receives cases reported through provincial and territorial public health authorities. Both confirmed and probable cases of Powassan virus disease are reportable. The national case definition for Powassan virus disease is used to classify cases reported to the Public Health Agency of Canada.

Provinces and territories have their own legislation for reporting of priority infectious diseases. Consult provincial or territorial public health authorities for reporting requirements in your jurisdiction.

Learn more about:

• Notifiable diseases online
• National case definition: Powassan virus

Subscribe to the Zoonoses Bulletin

The Zoonoses Bulletin is an email subscription list will provide you with regular updates from the Public Health Agency of Canada regarding our work on zoonoses. Zoonoses are infectious diseases that can be spread between animals and people, including those that can be spread through the bite of a tick or mosquito.

Subscribe

Related links

• National Collaborating Centre for Environmental Health: A review of ticks in Canada and health risk from exposure