Powassan Encephalitis in Winter: Case Report from New York

Powassan encephalitis can occur even in winter, challenging the assumption that tick-borne infections only happen during warmer months. A male patient was admitted to a New York hospital in December with altered mental status, dysarthria, and a left facial droop after reporting several recent tick bites.

This case demonstrates that Powassan virus transmission may occur year-round, not just during peak summer and fall tick season. Clinicians should remain alert for tick-borne encephalitis even during winter months when ticks are often presumed dormant. The stakes are high: Powassan encephalitis carries an estimated 10% mortality rate, and roughly half of survivors experience permanent neurologic damage.

December Presentation: Altered Mental Status and Facial Droop

In their article “Powassan Encephalitis: A Case Report from New York, USA,” Bazer and colleagues describe a patient admitted in December with altered mental status, dysarthria, and left facial droop.

The patient had several underlying medical conditions including a prior right putamen infarct, hepatitis C, hypertension, and substance abuse. Because he reported several recent tick bites, clinicians considered a tick-borne infection as part of the differential diagnosis.

The timing of the illness was notable. Many clinicians assume tick-borne infections occur only during warmer months. However, blacklegged ticks can remain active whenever temperatures rise above freezing, allowing infection to occur even during mild winter days.

Symptoms of Powassan Encephalitis

“The majority of patients infected with Powassan virus are asymptomatic. When symptomatic, patients typically present with encephalitis and altered sensorium,” the authors wrote.

Powassan virus infection can lead to severe neurologic complications. Approximately 50% of symptomatic patients experience long-term neurologic problems including recurrent headaches, cognitive impairment, and focal neurologic deficits. Mortality is estimated at approximately 10%.

Unlike Lyme disease, which can often be treated effectively with antibiotics, there is currently no specific antiviral treatment for Powassan virus infection.

Initial Treatment for Suspected Meningitis

A spinal tap revealed pleocytosis and elevated protein levels. Because bacterial or viral meningitis was initially suspected, the patient received empiric treatment with ceftriaxone and acyclovir.

His illness progressed and he required intubation for airway protection. His hospital course was complicated by a recurrent stroke and the need for placement of a feeding tube.

While empiric antibiotics and antivirals are appropriate when meningitis is suspected, these therapies do not treat Powassan virus infection.

Peripheral Nerve Involvement

Electromyography (EMG) and nerve conduction studies demonstrated generalized axonal loss with demyelinating polyradiculopathy. The patient received two courses of intravenous immune globulin (IVIG) because of the demyelinating features observed on testing.

These findings suggest that Powassan virus infection may affect both the central and peripheral nervous systems. Similar patterns can be seen in severe neurologic Lyme disease, where infection can involve both meningitis and radiculoneuropathy.

Why Powassan Encephalitis Can Occur in Winter

“Although the peak incidence of transmission occurs in summer and fall when Ixodes ticks are most active, this does not exclude transmission during other seasons,” the authors noted.

Several factors help explain winter transmission of Powassan virus:

• Mild winter weather: Blacklegged ticks can become active when temperatures exceed approximately 35–40°F (2–4°C).
• Climate change: Warmer winters extend tick activity seasons.
• Indoor exposure: Pets, wildlife, or clothing can bring ticks indoors during winter.
• Delayed symptoms: Infection acquired in late fall may present clinically weeks later.

Because clinicians often assume ticks are inactive in winter, tick-borne infections may not be considered in the differential diagnosis when patients present with encephalitis during colder months.

Diagnosis

Testing ultimately confirmed the presence of Powassan virus in the patient’s cerebrospinal fluid, establishing the diagnosis of Powassan encephalitis.

At discharge, the patient had significant neurologic deficits including global aphasia. It was unclear whether the aphasia resulted from Powassan encephalitis itself, the recurrent stroke, or a combination of both.

Clinical Perspective

This case highlights the importance of considering tick-borne infections year-round. Powassan encephalitis may present outside the traditional tick season, particularly during mild winters.

The case also demonstrates the severe neurologic consequences of Powassan virus infection. With high rates of mortality and long-term neurologic disability, early recognition and supportive care are essential.

Clinicians practicing in Lyme-endemic regions should ask about tick exposure even during winter months and maintain a high index of suspicion when evaluating unexplained encephalitis.