Powassan encephalitis is a rare infection caused by Powassan virus in specific regions of North America, Canada, and Russia. The virus is transmitted by a tick bite, and constitutional symptoms followed by a range of neurological deficits, including altered consciousness, are typical features. The diagnosis rests on data obtained from patient history and specific microbiological studies.
Powassan virus (POWV) belongs to the group of flaviviruses, and is a rare but possibly fatal cause of tick-borne encephalitis (TBE) in the Northeastern United States (New York, Minnesota, Wisconsin, Michigan, Maine, Vermont, Connecticut, Massachusetts, and Pennsylvania), Canada (Ontario, Quebec, and New Brunswick provinces), and certain parts of Russia    . The virus possesses two distinct lineages, both being transmitted from a stable animal reservoir consisting of rodents (eg. white-footed mice) and medium-sized mammals (eg. woodchucks, red squirrels groundhogs, skunks) onto the Ixodes ticks, one of the most important tick species related to human diseases   . Reports have shown that only 15 minutes are sufficient for the tick to transmit the virus to the host once it attaches to the skin  . Wide-scale epidemiologic studies have established that many patients (due to undisclosed reasons) have an asymptomatic form of POWV infection, but in some patients, infection of the central nervous system (CNS), known as Powassan encephalitis, can develop  . The period between the tick bite and the appearance of symptoms (known as the incubation period) typically lasts between 1-5 weeks and a prodromal period of 1-2 days, comprised of nonspecific respiratory and constitutional complaints (fever, malaise, headaches), is common   . Soon after, severe deterioration characterized by ataxia, weakness, hemiplegia, ophthalmoplegia, confusion, somnolence and a severely altered level of consciousness, as well as seizures and other focal neurological deficits, are observed in the case of Powassan encephalitis   . In addition, gastrointestinal irritation and skin rash may be seen . The mortality rates of Powassan encephalitis range from 10-15% and residual neurological impairment (hemiplegia, paresis of the lower extremities, and deficits in memory and other cognitive skills) is noted in up to 50% of patients who survive the infection   .
Powassan encephalitis is a very rare cause of this type of infection and the clinical presentation is indistinguishable from other causes of viral encephalitis. However, its distinct geographical and epidemiological features are quite useful in including POWV in the differential diagnosis of encephalitis. For this reason, a detailed patient history is perhaps the essential part of the diagnostic workup. Firstly, Powassan encephalitis has a seasonal appearance, meaning that most cases are diagnosed from May through September  . Secondly, a positive travel history to the endemic areas of the United States, Canada, or Russia (or if patients are residents of those areas) could significantly aid in suspecting Powassan virus as the causative agent. And thirdly, reports of a recent tick bite must direct the attention toward tick-borne encephalitis viruses (TEBV). Thus, a thorough physical examination is also a vital component of the workup, as it might reveal the site of the tick bite. Lumbar puncture examination is a must in the evaluation of the central nervous system (CNS) infections. Elevated protein levels are infrequently observed, but either lymphocytic or polymorphonuclear pleocytosis (between 200-700 WBCs/µL) is always seen in Powassan encephalitis  . A definite diagnosis, however, can be made through the enzyme-linked immunoassay (ELISA) testing that detects specific immunoglobulin-M (IgM) antibodies to the Powassan virus in the cerebrospinal fluid (CSF), but these tests are only available in specialized laboratories and tertiary care centers    . Imaging studies might also be of use in Powassan encephalitis, as magnetic resonance imaging (MRI) reveals can ischemic or demyelinating lesions in the basal ganglia, thalamus and the brainstem on T2/FLAIR images  .