A Recent Epizootic of Skunk Rabies and Associated Spillover in Northern Colorado, USA

Abstract

Rabies is a fatal viral zoonosis that is transmitted by bite contact with saliva of an infected animal. In the U.S., human deaths are rare, but each year over 5,000 rabid animals are reported and over 90% of cases are from wildlife. Multiple species of carnivores and bats are the primary sylvatic reservoirs in the U.S., and several variants circulate independently among these two taxonomic orders. Rabies prevention efforts led by USDA APHIS Wildlife Services using live recombinant vaccine baiting began in 1995 and currently target raccoons and canids, but the territory of the south-central skunk rabies virus variant has been expanding in recent years, leading to incursions into areas previously free of carnivore rabies. Colorado has been one such area, where multiple species of bats were the main rabies reservoir prior to 2007. By 2012, the number of rabid striped skunks exceeded rabid bats in Colorado. Larimer County and Weld County, in Northern Colorado, both witnessed skunk rabies epizootics starting in 2012 that have continued through 2013. Despite spillover events into raccoons, red foxes, companion animals, and livestock during 2012 and 2013, no other host species appears to support independent circulation of the south-central skunk variant in Northern Colorado. Virus isolation from salivary glands of rabid animals will help to quantify the likelihood for secondary transmission from spillover hosts and guide intervention strategies targeting wildlife. We describe the spatiotemporal pattern of the epizootic and predict likely areas of spread along the Front Range of Northern Colorado.