Powassan encephalitis in winter challenges the assumption that tick-borne infections only occur during warm months. A male patient was admitted to a New York hospital in December with altered mental status, dysarthria, and left facial droop after reporting multiple recent tick bites. His case demonstrates that Powassan virus transmission can occur year-round, not just during peak summer and fall tick season, and emphasizes why clinicians must maintain high suspicion for tick-borne encephalitis even in winter months when ticks are presumed dormant. With 10% mortality and 50% of survivors experiencing permanent neurologic damage, delayed diagnosis from seasonal assumptions can be fatal.

December Presentation: Altered Mental Status and Facial Droop

Powassan encephalitis can occur even in winter months. In their article “Powassan Encephalitis: A Case Report from New York, USA,” Bazer and colleagues describe a male patient who was admitted to the hospital in December due to altered mental status, dysarthria, and a left facial droop.

The man also had a history of multiple medical problems including a right putamen infarct, hepatitis C, hypertension, and substance abuse.

Clinicians considered a tick-borne disease since the patient had reported having several recent tick bites.

The December timing is critical. Most clinicians don’t consider tick-borne infections in winter, assuming ticks are inactive. But blacklegged ticks can quest for hosts whenever temperatures rise above freezing — and even during mild winter days, active ticks can attach and transmit infection.

Clinical Presentation of Powassan Encephalitis

“The majority of patients who are infected with Powassan are asymptomatic. When patients are symptomatic from Powassan virus, they will present with encephalitis and altered sensorium.”

The Powassan virus can cause serious, long-lasting complications. “Roughly, 50% of patients will have long term neurological sequelae of Powassan virus, such as recurrent headaches, cognitive disruption, and focal neurological deficits.”

“It is estimated that the 10% of patients with Powassan will expire from the disease,” wrote the authors.

These statistics are sobering: 10% mortality and 50% chance of permanent neurologic damage means the majority of symptomatic Powassan patients will either die or live with disability. Unlike Lyme disease which is treatable with antibiotics, there is no specific treatment for Powassan virus.

Initial Treatment for Presumed Meningitis

A spinal tap revealed pleocytosis and an elevated protein. “He was empirically treated for possible meningitis with ceftriaxone and acyclovir,” wrote the authors.

He was intubated for airway protection. Unfortunately, his illness was complicated by a recurrent stroke and the need for a feeding tube.

The CSF findings — pleocytosis (elevated white blood cells) and elevated protein — indicate meningeal inflammation but don’t distinguish viral from bacterial causes. Empiric treatment with antibiotics (ceftriaxone) and antivirals (acyclovir) covered common bacterial and herpes simplex virus causes, but offered no benefit for Powassan virus.

The intubation for airway protection signals severe encephalitis affecting brainstem function. The recurrent stroke complicates the clinical picture — was it related to Powassan-induced vasculitis, his pre-existing cerebrovascular disease, or independent event?

Axonal Loss and Demyelinating Polyradiculopathy

The patient was also treated for axonal loss. “An Electromyography (EMG) and nerve conduction studies (NCS) showed a generalized axon loss with demyelinating polyradiculopathy. He received 2 courses of intravenous immune globulin because of mild demyelinating features on EMG/NCS,” wrote the authors.

The EMG/NCS findings reveal peripheral nerve involvement beyond the central nervous system encephalitis. Generalized axonal loss with demyelination suggests Powassan virus affected both central and peripheral nervous systems — similar to severe neurologic Lyme disease which can cause both meningitis and radiculoneuropathy.

The IVIG treatment for demyelinating features is interesting. While IVIG won’t kill Powassan virus, it may modulate immune responses and reduce inflammation-mediated nerve damage. Whether it helped this patient is unclear given his complicated course.

Why Powassan Encephalitis Occurs in Winter

“Although the peak incidence of transmission of the virus is in the summer and fall when the Ixodes species is most active, this does not exclude transmission in other seasons.”

This case of Powassan encephalitis in winter demonstrates that clinicians should consider tick-borne infections year-round, not just during peak tick season.

Several factors explain winter transmission:

• Mild winter days: Blacklegged ticks quest for hosts whenever temperatures exceed 35-40°F (2-4°C)
• Climate change: Warmer winters extend tick activity seasons
• Indoor-outdoor transitions: Pets, wildlife, and humans bring ticks indoors during winter
• Geographic expansion: Ticks spreading to new regions with milder winters
• Delayed presentation: Infection acquired in fall, symptoms appearing in winter

The assumption that “ticks aren’t active in winter” creates diagnostic delays. When patients present with encephalitis in December, tick-borne infection drops off the differential — leading to missed or delayed diagnosis.

Diagnostic Confirmation

Testing “confirmed the presence of active Powassan virus in CSF, a surrogate to diagnose Powassan encephalitis,” according to the authors.

At the time of discharge, notable neurological symptoms included global aphasia. It was not clear from the case report whether the aphasia was related to the Powassan virus disease or the stroke.

The CSF detection of active Powassan virus confirms the diagnosis definitively. The global aphasia — complete loss of language function — represents severe brain damage affecting language centers. Whether from Powassan-induced encephalitis, recurrent stroke, or both, this patient will require lifelong supportive care.

Clinical Perspective

This case highlights several critical teaching points about Powassan encephalitis in winter. First, the December timing challenges standard assumptions about tick-borne infection seasonality. Most clinicians think “tick season” means May through October. But blacklegged ticks can be active year-round during mild weather, and patients may present with symptoms weeks after infection acquired during warmer periods.

Second, the patient’s thorough tick bite history was essential for diagnosis. He reported “several recent tick bites” in December. Without this history, Powassan would never have been considered. This emphasizes the importance of asking about tick exposure regardless of season — a question many clinicians omit during winter months.

Third, the severity of his presentation — altered mental status requiring intubation, recurrent stroke, feeding tube dependence — demonstrates the devastating nature of Powassan encephalitis. This isn’t a mild illness. When Powassan affects the central nervous system, outcomes are often catastrophic.

Fourth, the peripheral nerve involvement (axonal loss, demyelinating polyradiculopathy) suggests Powassan can affect both CNS and PNS, similar to severe neuroborreliosis. This dual involvement may explain why some Powassan survivors have mixed central and peripheral neurologic deficits.

Fifth, the complicated medical history (prior stroke, hepatitis C, substance abuse) may have increased vulnerability to severe Powassan disease. Immunocompromised or medically complex patients may have worse outcomes with tick-borne infections.

Finally, the global aphasia outcome is tragic. Even if he survived, complete loss of language function means profound disability requiring lifelong care. This underscores the 50% statistic — half of symptomatic Powassan survivors have permanent neurologic damage. For this patient, that meant global aphasia and feeding tube dependence.

Authors’ Takeaways

1. “Our patient’s diagnosis demonstrates the importance of obtaining a thorough tick exposure history.”
2. “This case highlights the importance of obtaining Powassan serology in a patient with an unexplained altered mental status.”
3. “It also demonstrates the importance of testing for the virus in the appropriate clinical scenario in Lyme-endemic areas, even outside of the normal tick season.”

Frequently Asked Questions

Can you get Powassan virus in winter?

Yes. While Powassan virus transmission peaks in summer and fall, cases can occur in winter months. Blacklegged ticks remain active during mild winter days when temperatures exceed freezing. Clinicians should consider tick-borne infections year-round.

What are the symptoms of Powassan encephalitis?

Symptoms include altered mental status, difficulty speaking (dysarthria), facial droop, headaches, and cognitive problems. About 50% of patients have long-term neurological complications including recurrent headaches, cognitive disruption, and focal neurologic deficits.

How is Powassan encephalitis diagnosed?

Diagnosis is confirmed through testing cerebrospinal fluid (CSF) for Powassan virus. Clinicians should obtain a thorough tick exposure history even in winter months when ticks are presumed inactive.

What is the mortality rate for Powassan encephalitis?

Approximately 10% of symptomatic Powassan patients die from the disease. An additional 50% of survivors have permanent neurologic damage, meaning most patients with severe Powassan encephalitis either die or live with disability.

Is there treatment for Powassan virus?

No. There is no specific antiviral treatment for Powassan virus. Treatment is supportive — IV fluids, respiratory support (intubation if needed), seizure management. Some patients receive IVIG for immune modulation but evidence for benefit is limited.

Can Powassan virus cause stroke?

Powassan encephalitis can cause stroke-like symptoms and may trigger actual strokes through vasculitis (blood vessel inflammation). This patient had recurrent stroke complicating his Powassan infection, though whether directly caused by virus or coincidental is unclear.

Why is tick exposure history important in winter?

Most clinicians don’t ask about tick exposure in winter, assuming ticks are dormant. But ticks can be active during mild winter weather, and patients may present with symptoms weeks after fall infections. Without asking, tick-borne infections get missed.