Seasonal Activity of Ixodes dammini in Relation to Lyme Disease-Infected White-Footed Mice, Peromyscus leucopus

Abstract

This study evaluates the seasonal infestation rates of white-footed mice, Peromyscus leucopus, harboring immature stages of the deer tick, Ixodes dammini, at three sites on Prudence Island, Rhode Island, USA, a Lyme disease endemic area. Larval infestation rates varied from 46.0 larvae per mouse in August to none at all sites during November, while nymphal infestations ranged from 11.0 per mouse in June to none at all three sites in September through November. At all three study sites, the white-footed mouse appeared to be the most common host species in the wooded areas where larvae and nymphs of I. dammini are also abundant. The seasonal infectivity of white-footed mice with the Lyme disease spirochete, Borrelia burgdorferi, was also monitored using the larval stage of the deer tick. Mice were trapped at four sites: three on Prudence Island, a Lyme disease end~mic area with a large population of white-tailed deer; and one on Conanicut Island (Jamestown) with no white-tailed deer. All mice taken from Prudence Island were infective to laboratory-reared larvae with the highest infectivity from May to August. Larvae that fed on juvenile mice collected in September, October, and November were not infective. Mice originating from Conanicut Island were also not infective. This suggests, at least at these study sites, that the presence of host-seeking I. dammini is a prerequisite to perpetuating spirochetes in white-footed mice. Furthermore, this study demonstrates that mice are infective to larval ticks during the peak larval season which extends from July to late October. It is presumed that these infected larvae would become infected nymphal ticks which emerge the following season. Risk of Lyme disease transmission can be expressed as an Entomological Risk Index (ERI), a term that describes the relative abundance of infected deer tick nymphs per unit time. Variations in ERI were observed among the three sites and it was also found to change seasonally. The peak in ERI coincided with peak density of nymphal I. dammini and corresponded with the reported occurrence of Lyme disease in humans.