Powassan encephalitis

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Powassan virus is an emerging tibovirus that can cause aseptic meningitis and meningoencephalitis. Due to a 10 - 15% mortality rate and high incidence of permanent neurological deficits, Powassan virus disease is a nationally notifiable condition .

Epidemiology 

Powassan virus was first isolated in 1958 from CNS tissue of a deceased young male in Powassan, Ontario . Ticks of the Ixodes genus, namely Ixodes cookei and Ixodes scapularis serve as both reservoirs and vectors of this neuroinvasive flavivirus in North America and the Russian Far East . The incidence of Powassan virus disease has increased by approximately 671% since 1999, with 39 cases of neuroinvasive disease reported to the CDC in 2019 . Cases peak in the spring and autumn months .

Clinical presentation

Seroprevalence rates suggest the majority of infections are undetected . The incubation period may be up to 34 days . Symptoms of viral infection, such as fever, rash, malaise and gastrointestinal disturbance, as well as a history of outdoor activity and/or a tick bite, precede the development of neurological symptoms .

Lethargy, reduced consciousness, coma, generalized weakness, hemi-/paraplegia, paralysis, facial palsy, ataxia, meningism and seizures are commonly reported . Once infected, the likelihood of then developing neuroinvasive disease, namely aseptic meningitis and meningoencephalitis, is unknown.

Radiographic features 

MRI is the modality of choice for viral encephalitis. Imaging findings are non-specific and similar to those of the other endemic encephalitides; the presence of Powassan-specific IgM in the CSF or serum is required for confident diagnosis.

MRI
  • T2/FLAIR/DWI: Signal hyperintensities within the basal ganglia and thalami are present in nearly all cases . Lesions may also be present within the brainstem - predominantly affecting the midbrain; cortical lesions are uncommon . Reversible restricted diffusion throughout the cortex has been reported in one patient . 

Treatment and prognosis 

Supportive treatment is the mainstay of care, with avoidance of ticks the primary mechanism of prevention. The mortality is reported as 10-15%; permanent neurological deficits are seen in 50% of patients .

Differential diagnosis 

A history of a tick bite should raise the suspicion of:

However, a tick may well go unnoticed. The history, examination and MRI findings may also cause one to consider related pathogens:

or perhaps:

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