Powassan Virus Disease
Reviewed by: Stephen C. Eppes, MD and Clifton Castleman, WEMT & lead acarologist at TickSafety.com
While it’s getting a lot of attention, POWV is still very rare. Only 75 cases have been reported in the U.S. in the last decade (compared to more than 300,000 cases of Lyme disease reported each year), and most cases occurred in the upper Midwestern states around the Great Lakes or in the northeastern states. On average, less than 10 cases are reported each year, and less than 5% of blacklegged (deer) ticks are carrying the virus. Compare this with Lyme disease, where roughly 20% of nymphs and 50% of adult females are infected with the bacteria.
Powassan virus (called POWV or 2-POW) is a tick-borne encephalitic virus (flaviviridae) and is similar to tick-borne encephalitis virus (TBEV), a neurologic infection spread by ticks in Europe and Asia and other encephalitic viruses spread by mosquitoes (e.g. West Nile virus). It is the only tick-borne encephalitic virus in North America.
The first confirmed human case occurred in 1958 when a young boy from Powassan, Ontario died of severe encephalitis and researchers isolated and identified the virus on autopsy. Since then, the majority of POWV cases have been reported from the upper Midwestern and northeastern states. There has not been a single confirmed case in any other state in 2014 or 2015 according to the CDC; and only 8 confirmed cases in 2016 (see map below; click to enlarge).
The virus is maintained in nature through three cycles of mammal host and tick vector: the Groundhog tick (Ixodes cookei) and groundhogs and skunks, the squirrel tick (Ixodes marxi) and squirrels and chipmunks, and the most implicated in human disease, the Blacklegged “Deer” tick (Ixodes scapularis) and white-footed mice.
Field studies have demonstrated that American Dog ticks (Dermacentor variabilis) do NOT carry POWV, mainly because they are not part of these transmission cycles. More study is needed to know the role the Rocky Mountain Wood tick (Dermacentor andersoni) plays in POWV maintenance cycles, however it is currently NOT suspected to transmit the virus to humans.
Powassan virus infection in humans is caused by two closely related, but distinct virus types: Lineage I (Powassan virus or POWV) and Lineage II (deer tick virus or DTV). Humans disease is more often caused by the Lineage II strain because the blacklegged or “deer” ticks carry it are more likely to bite humans than groundhog or squirrel ticks that carry Lineage I. Both lineages have been shown to cause human disease, and no real differences in disease presentation have been noted between the two types.
What makes this virus more concerning than other tick-borne diseases is that it can be transmitted quickly. Unlike most tick-borne bacteria that take hours to days of feeding to transmit an infectious dose, animal models have suggested that it could take as little as 15 minutes to transmit the virus from the bite of an infected tick.
POWV can also be transmitted from an infected female tick to her eggs (i.e. transovarial transmission). However, the chances of infection increase as the tick progressed through each life stage. A bite from a more visible adult blacklegged tick is more risky than a bite from a tiny larval blacklegged tick.
Signs & Symptoms
Fortunately, most people who are exposed to the virus never develop disease. Signs and symptoms of POWV include fever, headache, vomiting, and confusion and disorientation from meningitis and encephalitis (brain swelling). The incubation period ranges from one week to one month from the time of the tick bite.
Roughly 10-15% of people who have developed symptoms have died, and 50% of those surviving a symptomatic illness have long term neurological problems like frequent headaches and memory loss. Since it’s a virus, antibiotics cannot be used to treat Powassan. Treatment for severe symptoms involves supportive care at in a definitive care (hospital) setting. Currently, there is no vaccine to prevent POWV.
There is no specific treatment, but people with severe POW virus illnesses often need to be hospitalized to receive respiratory support, intravenous fluids, or medications to reduce swelling in the brain.
About 10% of patients reported with 2-POW virus infection die from their infection, and long-term problems may persist among those who survive. However, it is possible that some people infected with POW virus experience milder illness or do not have any symptoms. Approximately half of survivors have permanent neurological symptoms, such as recurrent headaches, muscle wasting and memory problems.