Source: Masters Abstracts International, Volume: 57-05.

Adviser: Durland Fish.

INTRODUCTION: Powassan virus (POWV) is a tickborne Flavivirus that was discovered in Powassan, Ontario in 1958 and has been found to cause long-term neurological sequelae or death in 50% and 10% of cases, respectively. POWV includes the prototype POWV lineage transmitted by Ixodes cookei and the POWV subtype deer tick virus (DTV) transmitted by Ixodes scapularis . Little is known about POWV prevalence in ticks and in humans.

OBJECTIVES: The primary objective was to review the epidemiology of POWV infection in ticks and humans, including the frequency of infection in ticks and asymptomatic infection in humans. Secondary objectives include exploring the age distribution of infection and current surveillance activities for POWV in the US.

METHODS: This review was conducted by searching SCOPUS for all articles containing original POWV prevalence research, case studies, or literature reviews published in English. For secondary objectives, case studies were supplemented by MMWR data from the Centers for Disease Control and Prevention for POWV and surveillance information from state health department websites. Statistical analysis was conducted using SPSS.

RESULTS: The DTV infection rate in I. cookie appears low but has not been well established while that of I. scapularis ranges between 0.6 and 5%. Human seroprevalence varies by region from 0.3% to 9.4%. Of the five human seroprevalence surveys reviewed, four provided evidence of POWV infection without clinical symptoms. The age distribution of POWV cases appears to have shifted in recent decades, with significantly more individuals over 40 years old diagnosed after 1998 (X2 (6, N=103) = 27.50, p<0.001). Most states are not conducting active POWV surveillance in ticks or humans, which helps explain an underestimation of the true number of human infections.

CONCLUSION: POWV infection in ticks and humans appears to be increasing, although this may in part be a result of increased awareness of these infections. Serological evidence of previous POWV infection in individuals with no disease history suggests that asymptomatic or mild cases occur and that the disease spectrum is more varied than that characterized by reported cases. It is unclear why the age distribution of POWV disease has changed. Possible explanations include the emergence of DTV, which may have different transmission patterns compared with POWV, a more accurate estimate of the true age distribution as the number of reported cases has increased, or an increase in the aging population.