An Overview of the History, Ecology, and Phylogeny of Powassan Virus in Maine

Importance: Powassan virus is a tick-transmitted flavivirus that is increasingly reported and can cause fatal human encephalitis. Two lineages circulate in nature: lineage I (Powassan virus-1) is transmitted primarily by the woodchuck tick Ixodes cookei and the squirrel tick I. marxi, and lineage II (deer tick virus) is transmitted by the deer tick I. scapularis. Both lineages can cause human disease and are serologically indistinguishable. In Maine, human cases are increasing—35 cases were reported from 2000 to 2024, of which 14 were reported between 2022 and 2024. Objective: This review summarizes our current understanding of the history, ecology, and phylogeny of Powassan virus in Maine, as well as human demographics of Powassan virus disease. Review: Because Powassan virus encephalitis is a rare and emerging disease in the United States, there is limited published literature available. Keywords used in the literature search included “Powassan virus,” “tick-borne encephalitis virus,” “deer tick virus,” “Ixodes scapularis,” and “Maine.” Findings: Human cases of Powassan virus encephalitis are rising, and Powassan virus is now the most common cause of arboviral encephalitis in Maine. Seasonality of cases coincide with all stages of ticks, but fall and spring cases overlap with peak activity of the adult deer tick. Phylogenetic studies have shown that deer tick virus likely emerged in the northeastern United States between 1940 and 1975, and then expanded northward with the reestablishment of the white-tailed deer and I. scapularis ticks. Conclusions: Additional research is warranted to better characterize the transmission dynamics of Powassan virus in nature and further our understanding of human disease.