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BACKGROUND: Powassan virus (POWV) is a tickborne Flavivirus transmitted by various Ixodes tick species. One of the two lineages of POWV, also known as deer tick virus (DTV), is transmitted by Ixodes scapularis, a tick widely distributed throughout Wisconsin. Symptomatic POWV infection can range from mild febrile illness to severe encephalitis; the nationally reported case fatality rate is 10-15%. Since the first diagnosis of POWV disease in a Wisconsin resident during 2003, enhanced POWV surveillance in Wisconsin has resulted in greater awareness and detection of POWV disease. To enhance understanding of epidemiologic features of POWV infection, we reviewed all reported cases of POWV disease among Wisconsin residents during 2003-2014 and conducted environmental investigations in north-central Wisconsin during 2013.
METHODS: Enhanced arboviral surveillance for POWV infection and disease included analyzing reported serologic arboviral testing results from clinical laboratories, and obtaining specimens for confirmatory testing from patients with illnesses or laboratory markers compatible with POWV disease. Specimens were submitted through the Wisconsin State Laboratory of Hygiene to the Centers for Disease Control and Prevention (CDC) for POWV confirmatory testing. Following review of available clinical records and case report data inclusive of demographic and tick exposure data, Wisconsin residents with illness onsets during 2003-2014 were classified as having confirmed or probable neuroinvasive or non-neuroinvasive POWV disease using the CSTE/CDC arboviral diseases case definition. Surveillance for I. scapularis inclusive of tick collection was conducted during 2013 in a Marathon County, Wisconsin park and included RT-PCR testing for POWV/DTV RNA, plaque assays and virus sequencing at CDC.
RESULTS: During 2003-2014, 19 (18 confirmed and 1 probable) cases of POWV disease were detected among Wisconsin residents; 16 (84%) reported known tick exposure. The 19 patients were residents of 14 Wisconsin counties; 15 (79%) patients were male and median patient age was 57 (range 2-76) years. Among patients, 15 (79%) had neuroinvasive disease, 15 (79%) were hospitalized and all survived. Detection of 17(89%) cases was attributed to enhanced arboviral surveillance. Three (2.6%) of 115 adult I. scapularis tested were positive for DTV.
CONCLUSIONS: POWV is the cause of an infrequently occurring but emerging, and often serious, tickborne encephalitic disease in Wisconsin. Results of enhanced surveillance that included confirmatory testing suggest POWV disease is substantially under-recognized. Clinicians and the public should be aware that I. scapularis is the predominant vector that transmits POWV and other tickborne pathogens that cause serious disease among residents of and visitors to Wisconsin.