Powassan Fever: More Deadly Than Lyme Disease With Cases Rising

History

The first human case of Powassan fever was in 1958 when a five-year-old boy died of encephalitis in a town called Powassan, located in Ontario, Canada [3]. The virus, isolated from the deceased child through a brain biopsy, was eventually named ‘Powassan virus’ after the town [2,4]. In the United States, the first case of Powassan virus (POWV) was reported in New Jersey in 1970. After that, in 1978, Russia became the third country to detect the infection in humans [2]. Since then, several human cases have been documented in the US, Canada, and Russia [3].

Epidemiology

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 experience neurological symptoms for longer [1,3,4].

Cases of Powassan fever peak during the warmer months of the year, between May and November [2]. 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 [4]. However, in recent years, due to climate change, tick populations have been 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 [4]. 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 [2].

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 [4]. 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 [2]. However, this time around, the elderly were the ones most affected [4]. 

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 [4]. 

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 [3]. 

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 [6].
• Blood transfusion: In the recent past, one person in the US was believed to be infected by the Powassan virus during blood transfusion [4].
• 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 [5]. 

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 [2]. 

Diagnosis

Diagnosing severe (neuroinvasive) cases of Powassan virus is done by considering clinical symptoms and lab tests. The infected individual’s travel and vaccination history of the patient can also provide valuable information. If the patient has traveled to or stayed in an area endemic to Powassan fever or other flaviviruses, this data can help clinicians during differential diagnosis [3]. Patients should meet one or more of the following diagnostic criteria to confirm Powassan viral infection:

• Isolation of Powassan virus from blood and cerebrospinal fluid samples (effective during the initial viremic phase of disease before antibodies develop in the blood serum).
• Detection of Powassan virus RNA in blood and cerebrospinal fluid samples (effective during the initial viremic phase of disease before developing antibodies in the blood serum).
• Presence of viral antigens in blood, cerebrospinal fluid, tissue, or other body fluids.
• Presence of Powassan virus-specific IgM antibodies in cerebrospinal fluid with a negative result for other IgM antibodies in cerebrospinal fluid for arboviruses endemic to the region where exposure occurred (effective during the encephalitic phase of the disease).
• Presence of Powassan virus-specific IgM and neutralizing antibodies in the serum (effective during the encephalitic phase of the disease) [3].

Symptoms

Most people infected with the Powassan virus have mild symptoms similar to the flu. Following exposure to an infection from the Powassan virus, it usually takes 8 to 34 days for symptoms to appear [4]. Here are a few common symptoms that mark the onset of the disease:

• Fever
• Headache
• Vomiting
• Generalized weakness [1].

As the disease progresses, patients might suffer from some of these clinical conditions:

• Encephalitis (brain inflammation)
• Meningoencephalitis (inflammation of the brain and spinal cord membranes)
• Aseptic meningitis [1,4].

Patients with encephalitis might also have some of these issues:

• Altered mental status
• Seizures
• Cranial nerve palsies
• Aphasia/dysarthria
• Paresis or paralysis
• Movement disorders [1].

Other less common clinical manifestations include:

• Ophthalmoplegia
• Milder febrile illness
• Hemorrhagic encephalitis
• Polio-like illness in cervical spinal cord
• Crohn’s disease
• Diabetes
• Hypertension [4].

Treatment

No specific treatment is available to treat Powassan viral infections though hospitalization is required for severe cases [1]. For those hospitalized, supportive medical care can help address some of the clinical symptoms of the disease. This includes administering intravenous fluids, ventilatory support, and medicine to help reduce swelling in the brain [1,4]. This type of therapy may provide some relief from persistent pain [4]. Corticosteroids can be used in severe cases, such as neuroinvasive Powassan virus disease [3,4].

Prevention

There is no vaccine available to prevent Powassan virus disease [1,4]. Here are a few preventive measures to protect yourself from tick-borne infections:

• Keep your yard clean by regularly removing leaf litter, trimming tall grass, mowing the lawn, and removing trash (old furniture and other unused appliances) [7]
• Avoid visiting brushy or wooded areas filled with thick grass because the density of ticks is usually high in these places.
• Cover your entire body if you have to spend time outdoors or visit places where ticks are prevalent.
• You may spray tick repellent on your clothes or apply DEET, picaridin, IR3535, Oil of Lemon Eucalyptus (OLE), and para-menthane-diol (PMD), or 2-undecanone on the exposed parts of your body for extra protection.
• Sometimes, ticks might cling to your clothes and come in contact with your body when used again. So, it is better to tumble-dry your clothes at high heat once you return from outdoor activity.
• Make it a practice to examine your body after spending time outdoors and remove any ticks attached to the body. Checking and removing ticks early can prevent you from being infected. 
• Have a shower once you are back from outdoor activity. Studies have shown that taking a shower within two hours of coming indoors reduces the risk of Lyme Disease [8].

These guidelines can help you lower your risk of getting other tick-borne illnesses like Lyme Disease, and Rocky Mountain spotted fever, Colorado Tick fever, Ehrlichiosis, Babesiosis, Tularemia, anaplasmosis, and more. Since Powassan is relatively rare compared to other tick fever viruses, it often does not get the attention it may need. Diseases spread by ticks need better research, funding, and awareness-building to reduce their incidence and prevent full-blown public health crises.