Powassan Virus

Powassan virus (POWV) was initially recognized as a human pathogen in 1958 after a boy died of severe encephalitis in Powassan, Ontario, and POWV was isolated from the brain autopsy (Hermance et. al. 2017). Affected by climate change, Powassan virus disease is an emerging tick-borne infection that can cause a fatal neuroinvasive disease in humans. Infection in humans occurs following a bite from one of the Ixodes tick species, particularly Ixodes scapularis (deer or blacklegged tick). Other Ixodes species, such as Ixodes cookie (groundhog tick) and Ixodes marxi (squirrel tick) may carry POWV but rarely bite humans. Blacklegged ticks are the same species that can carry the pathogens causing Lyme disease, babesiosis, and human granulocytic anaplasmosis; coinfection with these diseases is therefore possible. Ixodes tick species carrying POWV have been detected in the province of Manitoba, Ontario and Nova Scotia, extending to eastern and central Canada (Bogaty et. al. 2018).

Reservoir: The mammalian host of POWV includes wild rodents (mice, rats, groundhogs) skunks, red squirrels, and chipmunks. The tick becomes a vector from feeding on these infected animals. 

Modes of transmission: Humans are infected following a bite from an infected tick. The virus may be transmitted from the tick in as little as 15 minutes of attachment as per a mouse model (Ebel et. al. 2004). People are considered dead-end hosts as they cannot develop high enough levels of the virus in their blood to infect biting ticks. There is no person-to-person spread. Rarely, blood transfusion transmission can occur from an infected individual; blood and bone marrow donation should be deferred for 120 days following infection.  

Blacklegged ticks in various stages of feeding:

Three nymphs of the blacklegged tick are shown in different stages of feeding.

The following image shows five female blacklegged ticks in different stages of feeding.

For additional photos showing blacklegged ticks in various phases of engorgement from feeding visit the Tick Encounter Resource Center website.

Cases of Powassan disease have been observed in Ottawa, though at an incidence of less than 1/100,000 population per year, meaning the risk of the disease is low. Climate change has increased the distribution and density of ticks responsible for the spread of POWV. Ticks infected with the virus have been isolated in Ottawa and the surrounding regions used for outdoor recreational activities. A tick-surveillance study (Canadian Lyme Sentinel surveillance report, 2019) has identified regions where POWV has been identified in captured ticks. Human cases occur primarily in the late spring, early summer, and mid-fall, peaking between June and September (NCCID Disease Debrief). 

Many people develop mild flu-like symptoms or remain asymptomatic. Symptoms appear from 1 to 5 weeks after the bite from an infected tick and can include low-grade fever, headache, vomiting, weakness. Severe infections can progress to neurological manifestations such as severe confusion, tremors, seizures, paralysis, and coma, which follows high fever. POWV can cause encephalitis (inflammation of the brain) and meningitis (inflammation of the membranes that surround the brain and spinal cord). A rash is infrequent unless there is a co-infection with Lyme disease; however, some cases are accompanied by a fine macular erythematous rash. Approximately 50% of survivors have permanent neurological symptoms, such as recurrent headaches, muscle wasting, paralysis and memory problems. Approximately 10% of POWV virus encephalitis cases are fatal.

Preliminary diagnosis is based on clinical signs and symptoms coupled with the risk of possible exposure from recent travel history to tick endemic areas. The assessment and diagnosis can be challenging since the initial symptoms are non-specific, but there should be high suspicion if the patient is presenting with a fever of unknown origin around the peak season (June – September) after recent outdoor activities in tick endemic areas. When POWV is suspected, consultation with infectious diseases is highly recommended and virus-specific antibody or PCR testing should be initiated as soon as possible. 

Diagnosis of POWV disease can be achieved by testing serum or cerebrospinal fluid (CSF). The request for testing must include relevant travel history, symptoms, and symptom onset date. The general requisition form can be found here: Downloadable Requisition Form.

Instructions for using serum separator tubes (SST) are found in the following document: Sample Collection Instructions

Antibody Testing

Public Health Ontario (PHO) Labs can detect Powassan virus-specific IgM and neutralizing antibodies. Serology is performed using a hemagglutination inhibition assay for the detection of antibodies to Powassan virus. Acute and convalescent serology is recommended, with the convalescent specimen collected 2 to 3 weeks after the initial acute specimen. Specimens should be stored at 2 to 8ºC following collection and shipped to the PHO laboratory on ice packs. Turnaround time is up to 8 days from receipt by the PHO laboratory. 

Other Testing

Molecular tests to detect viral RNA (e.g., reverse transcription-polymerase chain reaction [RT-PCR]) can be performed on plasma, serum, or CSF specimens that are collected early in the course of illness and, if positive, can confirm an infection. Molecular testing is performed at the National Microbiology Laboratory and must be preapproved by the PHO laboratory microbiologist. Specimens for molecular testing should be frozen and shipped on dry ice. Turnaround time is up to 28 days from receipt by the PHO laboratory. 

For further information on POWV diagnostic testing, refer to Public Health Ontario website at Powassan Virus Diagnostic Testing. 

Suspected or confirmed cases of Powassan virus are reportable to local public health under the Health Protection and Promotion Act. 

Monday to Friday from 8:30 am to 4:30 pm: Call 613-580-2424, extension 24224 and leave a detailed, confidential message including your contact information; or fax 613-580-9640. 

After hours, on weekends, or holidays: Call 3-1-1 and ask to speak to Public Health on call. 

For more details on how to report, please visit Reporting a communicable disease.  

No vaccine is available to prevent POWV infections. Minimizing exposure to ticks is the best preventative measure for POWV infection. Patients are advised to adopt the following practices: 

Suspected and confirmed cases of Powassan virus are reportable to local public health under the Health Protection and Promotion Act. 

For Provincial surveillance case definitions and disease-specific direction for the public health management of Powassan Virus, please see the Ontario Ministry of Health’s Appendix 1 - Case Definitions and Disease Specific Information.

For details on how to report, please see the Reporting Requirements section. 

• Powassan Virus,  U.S. Centers for Disease Control and Prevention (CDC)  
• Powassan Virus Disease Fact Sheet. New York State Department of Health 
• Video: How to reduce ticks around your home. Public Health Agency of Canada