Climate, Deer, Rodents, and Acorns as Determinants of Variation in Lyme-Disease Risk

Abstract

Risk of human exposure to vector-borne zoonotic pathogens is a function of the abundance and infection prevalence of vectors. We assessed the determinants of Lyme-disease risk (density and Borrelia burgdorferi-infection prevalence of nymphal Ixodes scapularis ticks) over 13 y on several field plots within eastern deciduous forests in the epicenter of US Lyme disease (Dutchess County, New York). We used a model comparison approach to simultaneously test the importance of ambient growing-season temperature, precipitation, two indices of deer (Odocoileus virginianus) abundance, and densities of white-footed mice (Peromyscus leucopus), eastern chipmunks (Tamias striatus), and acorns ( Quercus spp.), in both simple and multiple regression models, in predicting entomological risk. Indices of deer abundance had no predictive power, and precipitation in the current year and temperature in the prior year had only weak effects on entomological risk. The strongest predictors of a current year's risk were the prior year's abundance of mice and chipmunks and abundance of acorns 2 y previously. In no case did inclusion of deer or climate variables improve the predictive power of models based on rodents, acorns, or both. We conclude that interannual variation in entomological risk of exposure to Lyme disease is correlated positively with prior abundance of key hosts for the immature stages of the tick vector and with critical food resources for those hosts.

The wild life of tick-borne pathogens

“…Only in areas with only Roe deer did we find a correlation between the relative abundance of Roe deer and larval I. ricinus density. These results agree with a study on I. scapularis in which large variation in deer density did not result in changes in questing nymphal density, while there was a weak correlation with larval density 204 . Such a threshold relationship was already suggested by Wilson et al 202 and can explain equivocal effects of deer culling on densities of ticks in the I. ricinus complex 193 .…”

“…Which factors determine parasite load is of key interest in disease ecology and animal health, as many parasites negatively affect the condition of the host or vector pathogens. An increase in vector burden will potentially increase the number of hosts infected with a vector-borne pathogen and subsequently the number of infected vectors in the environment 33 , which is directly related to disease risk 204 . For parasites that are encountered in the environment, parasite burdens on hosts are determined by three factors: 1) the number of parasites in the environment, and 2) the day range of the host, which together determine the encounter rate of parasites and hosts 12 , and 3) the resistance or tolerance of hosts towards the parasite 13 .…”

Human health impacts of ecosystem alteration

“…Disease ecology explores comprehensively how changes in a whole suite of factors such as population dynamics, movement, physiological state, species richness, and relative abundance of species within an ecological community can alter risks of exposure to infectious diseases (49,50). Recently, disease ecologists have shown that the risk of West Nile virus exposure in the United States rises as avian biodiversity falls (51,52), and, similarly, Lyme disease exposure increases with falling mammalian diversity (53)(54)(55). In field experiments, exposure to hantavirus increases when mammalian diversity falls (56).…”

Reservoir Competence of the Redwood Chipmunk (Tamias Ochrogenys) forAnaplasma Phagocytophilum

“…These findings are consistent with results from the eastern United States, where a PCR-positive eastern chipmunk (T. striatus) was reported from Minnesota (Walls et al 1997). In addition to maintaining active infection, chipmunks in the eastern United States support tick numbers, with a strong correlation between chipmunk numbers and the density of nymphal deer ticks one year later (Ostfeld et al 2006).…”