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BACKGROUND: Powassan virus, a member of the tick-borne encephalitis group of flaviviruses, encompasses two lineages with separate enzootic cycles. The prototype lineage of Powassan virus (POWV) is principally maintained between Ixodes cookei and the groundhog (Marmota momax) or striped skunk (Mephitis mephitis), while the deer tick virus (DTV) lineage is believed to be maintained between Ixodes scapularis and the white footed mouse (Peromyscus leucopus). We present a detailed description of the clinical presentation, laboratory diagnosis and outcome of the 14 cases of POWV/DTV encephalitis diagnosed between 2004 and 2012 in New York State, and review the literature to provide epidemiological evidence suggesting that many of these cases were specifically due to DTV rather than POWV.
METHODS: This was a retrospective review of the medical records of all POWV/DTV cases that were diagnosed by the New York State Department of Health (NYSDOH) during the period 2004 through 2012; in addition, published reports of six of these cases were reviewed for additional details. POWV/DTV virus neuroinvasive infection was defined using the 2011 United States surveillance case definition.
RESULTS: Fourteen cases of POWV/DTV encephalitis were identified in New York State from 2004 through 2012 with an all-cause mortality of 36% . Ten patients (72%) were from counties located in the Lower Hudson Valley (LHV), a highly endemic region for Lyme disease in which I. scapularis ticks account for most human tick bites. This finding suggests that many of these cases were caused by DTV rather than POWV. In two patients, DTV infection was confirmed by genetic sequencing.
CONCLUSIONS: Distinguishing between POWV and DTV infection provides epidemiologically relevant information from a public health perspective. Given the high morbidity and mortality associated with these infections and its evolving epidemiology, molecular analysis is essential in the evaluation of POWV/DTV infections.