The introduction of the West Nile virus into North America in 1999 has stimulated more awareness and tracking of the various arthropod-borne viruses that are capable of causing encephalitis. One in particular is getting more attention recently. It’s called the Powassan virus and it is a tick-borne pathogen in the genus flavivirus.
It was previously found only in animals. The discovery in humans came in 1958 in Powassan, Ontario, after a young boy became infected and died. This was one of only 27 cases reported in humans within the four decades that followed. However, with the continual spread of deer ticks throughout the United States due to ecological changes, there are an increasing number of human cases being reported. Between 2001 and 2014, there were 64 cases reported – and the consequences of infection are quite concerning.
There are two known types of the Powassan virus. Lineage 1 is transmitted by Ixodes cookei or Ixodes marxi, also known as the “squirrel tick” while lineage 2 comes from Ixodes scapularis, known as the common “deer tick.” The incubation period (time between getting infected and showing symptoms) can be anywhere from 8 to 34 days. Transmission from a tick to a human can occur with ticks in any stage of the life cycle and it appears to happen much more quickly than it does with Lyme borreliosis – anywhere from a few minutes to three hours, as opposed to 24 -48 hours (although some studies show that borrelia can be transmitted as early as six hours after a tick bite).
Symptoms & Survival Rate
One of the reasons this virus is causing concern is due to the high fatality rate. It is estimated that 10% of individuals infected will die and 50% of those who survive will have permanent or chronic neurological damage. Some individuals may be asymptomatic; but those who do experience symptoms may have fever, vomiting, muscle weakness, severe headaches, seizures, and speech problems. It is known to attack the central nervous system, causing inflammation in the brain (encephalitis) and in the lining of the brain (meningitis). Studies in animals have demonstrated a 2-phase model of disease progression. The febrile symptoms occur for the first 5-6 days, followed by the central nervous system involvement – and sometimes death. Long-term neurological complications in survivors may include muscle wasting, memory problems, chronic headaches, and paralysis on one side of the body.
Testing & Treatment
As is the case for many other viral infections, there is no current treatment available to directly eradicate the Powassan virus. Medical treatment is largely supportive care and targets include maintaining hydration with IV fluids, reducing brain swelling with medications, and providing respiratory support if needed. Current testing involves measuring serum and spinal fluid for IgM and neutralizing antibodies.
The CDC reports that most cases are being found in the Northeastern and Great Lakes regions, during times when ticks are most active (spring, early summer, and mid-fall). The affected geographical regions are expected to increase, similar to the way Lyme borreliosis has. The good news is that the majority of cases are still mild and the virus does not appear to be transmitted from human to human like other viruses. The bad news is that when we consider the combination of the increasing number of humans becoming infected, the lack of an effective treatment, and the high fatality rate, the Powassan virus may prove to be a bigger public health threat than Lyme disease in the future. The key to factors surrounding transmission and infection can be found in the tick’s saliva. This is a promising target for research on prevention and treatment going forward. The best way to prevent becoming infected is to take steps to avoid a tick bite. When spending time outdoors, particularly wooded or high-grass areas, wear protective clothing, apply insect repellant, and check your body thoroughly, in order to remove ticks immediately in the instance that attachment does occur.
1. Centers for Disease Control and Prevention. The Powassan Virus. Available at: http://www.cdc.gov/powassan/index.html Accessed on April 6, 2016.
2. Hermance, M and Thangamani, S. Tick Saliva Enhances Powassan Virus Transmission to the Host, Influencing Its Dissemination and the Course of the Disease. J Virol. 2015 Aug 1; 89 (15): 7852-60.
3. Katz, D. Lymedisease.org. Hard Science on Lyme: Ticks Can Transmit Infection the First Day. Available at: https://www.lymedisease.org/hard-science-on-lyme-ticks-can-transmit-infection-the-first-day/ Accessed on April 7, 2016.
4. Yale School of Public Health. The Rise of the Powassan Virus. Available at: http://publichealth.yale.edu/news/archive/article.aspx?id=9147 Accessed on April 6, 2016.