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.

Figure 1.

Phylogenetic analysis of deer tick virus (DTV) strains based on a 257 nucleotide fragment of the E coding region. Newly isolated strains are denoted by “DTV WI07.” The neighbor-joining analysis with branch lengths proportional to Kimura 2-parameter distances. Numbers at each node are bootstrap confidence estimates based on 1,000 replicate analyses. GenBank accession numbers are as follows: DTV WI07*177 (pending submission), DTV WI07*254 (pending submission), DTV WI07*38 (pending submission), DTV WI07*5 (pending submission), DTV SPO B10 (af310921), DTV SPO (af135461), DTV Chippewa Falls (ay004079), POW WV77 (af310920), DTV CT (u93288), DTV Ipswich (u93289), POW ON62 (ay004074), POW LB (l06436), POW ON81 (ay004078).