Urban ecology of Rattus norvegicus and zoonotic leptospirosis in Salvador, Brazil

Leptospirosis is an emerging zoonosis in urban slums, where conditions support large populations of Rattus norvegicus, the reservoir host. Salvador, Brazil, experiences annual epidemics of leptospirosis; however, the lack of information regarding the population ecology of tropical R. norvegicus populations is a major barrier to control. The goal of this study was to describe the ecology of R. norvegicus in Salvador, Brazil, and to identify factors associated with host abundance, distribution, and prevalence of leptospiral carriage in urban slum communities. Overall, 108 rats were captured, necropsied, and screened for leptospiral carriage. Demographic features of the sample population were compared to a matched sample from Baltimore, Maryland. Environmental surveys of households located in trap areas were used to characterize features of the rat habitat associated with abundance. The results indicate that despite reaching sexual maturity at the same age, adult tropical rats of both sexes were smaller than temperate rats on multiple measures of body size. The total prevalence of leptospiral carriage was 63.1%, varying by trap site from 50% to 100%. Prevalence increased by mass (an index of age), but did not differ between sexes, implicating horizontal transmission and environmental exposure as the major modes of intra-species transmission. Areas that had undergone rodenticide poisoning within three months prior to trapping had higher proportions of large animals, and higher infection prevalence compared to un-manipulated sites. Of the environmental features examined, only presence and severity of rodent signs and a summary index of rodent exposure were associated with rat capture at the household level. The findings suggest that the demographics and distribution of tropical R. norvegicus structure the prevalence of leptospiral carriage, the risks of rodent exposure, and the patterning of disease risk. Further studies elucidating these complex relationships are prerequisite to implementing sound spatially and temporally-specific control programs aimed at reducing reservoir host abundance and the incidence of human disease.