The primary vector responsible for spreading the Powassan virus, the black-legged ticks (Ixodes scapularis), can infect hosts within fifteen minutes of attaching themselves [1, 10]. Around 10% of Powassan encephalitis cases turn out to be fatal, and half of those who recover continue to experience neurological symptoms for a longer period [1,3,4].
Cases of Powassan fever peak during the warmer months of the year, between May and November . Reports show that in most cases, the disease occurs during late spring or early summer. The weather during these seasons is ideal for ticks to come out of their hibernation and become active . However, in recent years, due to climate change, tick populations are expanding to non-endemic regions and are active for longer periods.
People who live or work in brushy, grassy, or wooded places are at a higher risk of being exposed to ticks. Men are generally more susceptible to the infection than women because of a higher level of engagement in outdoor activities . The Powassan virus is endemic to the Western Hemisphere. This includes countries such as the United States and Canada. The states of Massachusetts, Minnesota, New York, and Wisconsin are the most affected in the US. New Brunswick, Quebec, and Ontario are Canadian states with higher incidence rates. Another hotspot for Powassan fever is the Russian province of Maritime Territory .
In the late 1950s, around 40 to 45 cases were reported in the United States and Canada. The majority of these cases were in the US, with 60% children under 15 years of age . The incidence dramatically increased during the next decade (from 2007 to 2016). The number of cases documented by the CDC during this period was 98, around 10 cases per year on average . However, this time around, the elderly were the ones most affected .
As of 2020, around 244 cases of human Powassan viral infections were reported. Of these, 205 cases were reported in the US, 21 cases in Canada, and 18 in Russia .
Powassan virus affects the central nervous system in humans and can cause Powassan encephalitis. This disease is often confused with deer tick encephalitis, another tick-borne encephalitis. It can be quite challenging to distinguish the two, especially since the Powassan virus and deer tick virus are genetically similar .
How is it Spread?
Powassan fever is not transmitted directly from one person to another. The Powassan virus is spread by:
- Tick bites: Ticks (usually black-legged ticks) get infected when they feed on the blood of an infected host (groundhogs, squirrels, mice, rodents, etc.). When these infected ticks bite someone, they can transmit the infection to the new individual .
- Blood transfusion: In the recent past, one person in the US was believed to be infected by the Powassan virus during blood transfusion .
- Raw milk: Though there is no conclusive research to back this up, some experts believe that Powassan fever can be spread through the milk of an infected goat .
Since humans do not develop a high concentration of the Powassan virus in their blood, they do not infect ticks that feed on them.
Biology of the Disease
Like other flaviviruses, the Powassan virus is an enveloped, positive, single-stranded RNA virus and a close cousin of tick-borne encephalitis viruses. When an infected tick bites a human, the virus enters the bloodstream and targets macrophages and fibroblasts. Macrophages are vital to maintaining immunity, regulating homeostasis, repairing tissues, and generating new ones. Fibroblasts play a key role in collagen production, maintain the extracellular matrix, and help repair wounds.
The Powassan virus is dangerous because even during the initial phase of the disease (viremic stage), the virus can cross the blood-brain barrier and invade neurons in the central nervous system. During the neurological stage of infection, the virus also targets glial cells in the central nervous system, which, among other things, protect the nervous system .