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Environmental management of mosquito-borne viruses in Rhode Island

West Nile Virus (WNV) and Eastern Equine Encephalitis Virus (EEEV) are both primarily bird viruses, which can be transmitted by several mosquito species. Differences in larval habitats, flight, and biting patterns of the primary vector species result in substantial differences in epidemiology, with WNV more common, primarily occurring in urban areas, and EEEV relatively rare, typically occurring near swamp habitats. The complex transmission ecology of these viruses complicates prediction of disease outbreaks. The Rhode Island Department of Environmental Management (DEM) and Department of Health (DoH) provide prevention assistance to towns and maintain a mosquito surveillance program to identify potential disease risk. Responses to potential outbreaks follow a protocol based on surveillance results, assessment of human risk, and technical consultation.

Rhode Island

Potential effects of climate change on tick-borne diseases in Rhode Island

Human cases of tick-borne diseases have been increasing in the United States. In particular, the incidence of Lyme disease, the major vector-borne disease in Rhode Island, has risen, along with cases of babesiosis and anaplasmosis, all vectored by the blacklegged tick. These increases might relate, in part, to climate change, although other environmental changes in the northeast (land use as it relates to habitat; vertebrate host populations for tick reproduction and enzootic cycling) also contribute. Lone star ticks, formerly southern in distribution, have been spreading northward, including expanded distributions in Rhode Island. Illnesses associated with this species include ehrlichiosis and alpha-gal syndrome, which are expected to increase. Ranges of other tick species have also been expanding in southern New England, including the Gulf Coast tick and the introduced Asian longhorned tick. These ticks can carry human pathogens, but the implications for human disease in Rhode Island are unclear.

Rhode Island

Relationships between maternal engorgement weight and the number, size, and fat content of larval Ixodes scapularis (Acari: Ixodidae)

The relationship between engorgement weight of female Ixodes scapularis Say and characteristics of offspring was studied using field-collected females fed on rabbits in the laboratory. The number of eggs laid was positively related to maternal engorgement weight in one trial, and larval size (estimated by scutal area) was positively related to maternal engorgement weight in the other. These results suggest a trade-off in number of eggs produced versus average size of offspring, possibly determined during late engorgement. The adults for the two trials were collected from different sites in southern Rhode Island and in different seasons (the fall adults were newly emerged, while the spring adults had presumably lived through the winter), so it is not clear whether these results reflect genetic differences or subtle environmental differences between trials. Percent egg hatch and average fat content of larvae were not related to female engorgement weight. We present a modified method to measure lipid content of pooled larval ticks.

Journal of Medical Entomology

Distribution of Ixodes dammini (Acari: Ixodidae) in residential lawns on Prudence Island, Rhode Island

The distribution of nymphal Ixodes dammini Spielman, Clifford, Piesman & Corwin in residential lawns was assessed by flagging on Prudence Island, RI. The number of ticks per sample was five times greater in lawns adjacent to woods than in lawns adjacent to other lawns. Relative tick abundance was negatively correlated with distance from the woods, but the decline was gradual. Spirochete prevalence in ticks did not differ among lawn types or at different distances from the woods. Therefore, barriers that keep people away from the wood edge probably lower the risk of acquiring Lyme disease, but there is still a risk. Even with physical barriers at lawn—wood edges, personal precautions to prevent tick bites should be followed.

Journal of Medical Entomology

Reservoir competence of Microtus pennsylvanicus (Rodentia: Cricetidae) for the Lyme disease spirochete, Borrelia burgdorferi

The reservoir competence of the meadow vole, Microtus pennsylvanicus Ord, for the Lyme disease spirochete Borrelia burgdorferi Johnson, Schmid, Hyde, Steigerwalt & Brenner was established on Patience Island, RI. Meadow voles were collected from 5 locations throughout Rhode Island. At 4 of the field sites, M. pennsylvanicus represented only 4.0% ( n = 141) of the animals captured. However, on Patience Island, M. pennsylvanicus was the sole small mammal collected ( n = 48). Of the larval Ixodes scapularis Say obtained from the meadow voles on Patience Island, 62% ( n = 78) was infected with B. burgdorferi . Meadow voles from all 5 locations were successfully infected with B. burgdorferi in the laboratory and were capable of passing the infection to xenodiagnostic I. scapularis larvae for 9 wk. We concluded that M. pennsylvanicus was physiologically capable of maintaining B. burgdorferi infection. However, in locations where Peromyscus leucopus (Rafinesque) is abundant, the role of M. pennsylvanicus as a primary reservoir for B. burgdorferi was reduced.

Journal of Medical Entomology

Relative humidity and activity patterns of Ixodes scapularis (Acari: Ixodidae)

Laboratory studies have shown clear relationships between relative humidity (RH) and the activity and survival of Ixodes scapularis Say (blacklegged tick). However, field studies have produced conflicting results. We examined this relationship using weekly tick count totals and hourly RH observations at three field sites, stratified by latitude, within the state of Rhode Island. Records of nymphal tick abundance were compared with several RH-related variables (e.g., RH at time of sampling and mean weekly daytime RH). In total, 825 nymphs were sampled in 2009, a year of greater precipitation, with a weighted average leaf litter RH recorded at time of sampling of 85.22%. Alternatively, 649 nymphs were collected in 2010, a year of relatively low precipitation, and a weighted average RH recorded at time of sampling was 75.51%. Negative binomial regression analysis of tick count totals identified cumulative hours <82% RH threshold as a significant factor observed in both years (2009: P = 0.0037; 2010: P < 0.0001). Mean weekly daytime RH did not significantly predict tick activity in either year. However, mean weekly daytime RH recorded with 1-wk lag before sample date was a significant variable ( P = 0.0016) in 2010. These results suggest a lag effect between moisture availability and patterns of tick activity and abundance. Differences in the relative importance of each RH variable between years may have been due to abnormally wet summer conditions in 2009.

Rhode Island