A 77–year–old man from Putnam County, New York, was hospitalized on December 5, 2010 for one week history of fever, dysuria and confusion. He was treated for a urinary tract infection without improvement. He had type II DM and COPD. He had WBC count of 11000/mm3 and normal chemistry & liver panel. He became obtunded and developed neck stiffness and jerky movements. A lumbar puncture was done, CSF revealed 256 WBC with 95% lymphocytes, protein of 133 mg/dl and glucose of 140 mg/dl. CSF bacterial and fungal cultures, AFB stains, TB PCR, Cryptococcus antigen and serum and CSF Lyme serologies were negative. He was treated with acyclovir, ampicillin, vancomycin, and ceftriaxone without improvement. MRI of the brain revealed enhancement of the cerebellar hemispheres, pons, and basal ganglia. CSF PCR for HSV, VZV, CMV, adenovirus, enterovirus and other flaviviruses were negative. Serum toxoplasmosis, RMSF, syphilis, legionella, and mycoplasma serologies were negative. Patient’s neurological status progressively and rapidly deteriorated and required intubation for airway protection. Search for unusual forms of encephalitis prompted testing for Powassan Virus which was positive by serology and PCR of CSF. The patient remained comatose and ventilator dependent at the time of discharge to a skilled nursing facility.
Powassan virus (POWV) is a rare cause of human encephalitis. POWV belongs to the tick–borne encephalitis group of flaviviruses and is divided into two types, lineage 1 and lineage 2 or deer tick virus (DTV). POWV is maintained in a transmission cycle between Ixodes cookei (for lineage 1) and Ixodes scapularis (for lineage 2) and small & medium–sized mammals. The disease that is caused by POWV is rare but devastating. It has been reported from eastern Canada and the north central and northeastern United States. PCR for DTV performed on ixodes scapularis from different areas of the lower hudson valley showed an infectivity rate of 5%. Recent human serologic surveys are not available, thus the rate of asymptomatic infections is unknown. Infection mostly occurs from June to September. The case–fatality rate is 5 to 10 percent, with a high incidence of residual neurological dysfunction among survivors. There is no vaccine or specific therapy available. Prevention is suggested by reduced exposure to natural ticks habitat, identification and early tick removal.
Encephalitis is not an uncommon cause of hospitalization. Prognosis depends in most cases on early diagnosis and targeted therapy. Hospitalists need to be familiar with appropriate diagnostic work up and empiric therapy when faced with similar situations. Further studies are needed to elucidate the seroprevalence and adequate treatment for DTV encephalitis.