Short Report: Stable Prevalence of Powassan Virus in Ixodes scapularis in a Northern Wisconsin Focus

Abstract. Deer tick virus (DTV), a variant of Powassan virus (POWV), appears to be maintained in nature in an enzootic cycle between Ixodes scapularis ticks and small mammals. Although POWV infection of human beings is rare, a recent report suggests increasing incidence and the possibility that POWV may be an emerging tick-borne zoonosis. Therefore, we assessed the long-term stability of the POWV transmission cycle in northwestern Wisconsin. Adult I. scapularis and Dermacentor variabilis were collected from Hayward and Spooner, Wisconsin, screened for infection by reverse transcriptase polymerase chain reaction (RT-PCR), and virus was isolated. Seventeen of 1,335 (1.3%) of I. scapularis and 0 of 222 (0%) of D. variabilis ticks were infected. All isolated virus belonged to the DTV genotype of POWV. These findings suggest stable transmission of POWV in this focus over ten years and highlight the potential for this agent to emerge as a public health concern. Viruses of the tick-borne encephalitis (TBE) complex (Fla-viviridae: flavivirus) are distributed across Eurasia and North America and are a causative agent of severe central nervous infection in humans.1 Powassan virus (POWV), originally iso-lated from a fatal case of encephalitis in Ontario, Canada in 1958, is the sole member of the TBE complex circulating in North America.2 The natural transmission cycle of POWV includes the enzootic transmission between Ixodes cookei and Ixodes marxi and medium-sized woodland rodents, mainly sciurid rodents and carnivores.3,4 Ixodes cookei and I. marxi are generally highly host–specific and rarely attack human beings, although in some cases I. cookei in particular may frequently feed on human beings.5,6 This highly host–specific feeding behavior seems to explain the low incidence of human POWV cases in North America; 27 cases were reported be-tween 1958 and 1998 (0.7 cases/year).7 Recently, however, an apparent increase in incidence has been reported: nine POWV cases occurred between 1999 and 2005 (1.3 cases/ year).8,9 The POWV cases occurred in north-central Wiscon