Ecology of Leptospira interrogans in Norway Rats (Rattus norvegicus) in an Inner-City Neighborhood of Vancouver, Canada

Abstract

Leptospira interrogans is a bacterial zoonosis with a worldwide distribution for which rats (Rattus spp.) are the primary reservoir in urban settings. In order to assess, monitor, and mitigate the risk to humans, it is important to understand the ecology of this pathogen in rats. The objective of this study was to characterize the ecology of L. interrogans in Norway rats (Rattus norvegicus) in an impoverished inner-city neighborhood of Vancouver, Canada. Trapping was performed in 43 city blocks, and one location within the adjacent port, over a 12 month period. Kidney samples were tested for the presence of L. interrogans using PCR and sequencing. A multivariable model was built to predict L. interrogans infection status in individual rats using season and morphometric data (e.g., weight, sex, maturity, condition, etc.) as independent variables. Spatial analysis was undertaken to identify clusters of high and low L. interrogans prevalence. The prevalence of L. interrogans varied remarkably among blocks (0–66.7%), and spatial clusters of both high and low L. interrogans prevalence were identified. In the final cluster-controlled model, characteristics associated with L. interrogans-infection in rats included weight (OR = 1.14, 95% CI = 1.07–1.20), increased internal fat (OR = 2.12, 95% CI = 1.06–4.25), and number of bite wounds (OR = 1.20, 95% CI = 0.96–1.49). Because L. interrogans prevalence varied with weight, body fat, and bite wounds, this study suggests that social structure and interactions among rats may influence transmission. The prevalence and distribution of L. interrogans in rats was also highly variable even over a short geographic distance. These factors should be considered in future risk management efforts. Urban Norway rats are the source of a number of zoonotic pathogens responsible for significant human illness. Leptospira interrogans is one of these pathogens, and although infection in rats is asymptomatic, humans infected through exposure to the bacterium in rat urine can develop fever, renal failure, and pulmonary hemorrhage. Previous studies of L. interrogans in urban rats have been of limited value because they have not taken into account the complex ecology of the rat populations under study. In this study, we found that the prevalence and distribution of L. interrogans varied greatly between blocks in an inner-city neighborhood (reflecting that rats live in tight-knit colonies with small home ranges) and was not related to rat population size. This suggests that the L. interrogans ‘load’ in a block does not depend on the number of resident rats, but rather on some characteristic intrinsic to that block. Additionally, increased weight, body fat, and bite wounds were found to increase the probability of L. interrogans infection, suggesting that the position of a rat within the colony's social hierarchy may also influence transmission. These factors should be considered before undertaking rat control programs.