Indirect Association of Monsoon Rains with the Patterns of Dog-Mediated Rabies in Sri Lanka

In many rabies endemic countries including Sri Lanka, dog populations are the main rabies reservoir and the main source of human infection. Understanding the ecology and spatiotemporal determinants of dog populations could inform targeted dog population control and rabies vaccination campaigns. Studies from India have postulated a positive association between monsoon rains and dog mating frequency, i.e., rainy season mating followed by a peak of births in the subsequent dry season, leading to an increase in the unvaccinated dog population.  To test the hypothesis (positive association between rainfall patterns and reported rabies cases) in Sri Lanka, spatial and temporal patterns of dog and human rabies cases from 2004 through 2021 were analysed and compared with rainfall patterns in relation to Sri Lanka’s three climate zones: wet, intermediate, and dry. Rabies cases were frequent in the highly populated south-west of the country, particularly in the wet zone. The study reveals an association between dog rabies cases and rainfall, with a notable increase in cases during the dry season across all three zones based on the Mann-Whitney U test. Hence, this suggests a seasonal variation in dog rabies incidence, potentially linked to rainy season mating followed by a peak of births in the subsequent dry season, among other potential factors. Hence, this supports the hypothesis of a potential rainfall-driven rise in the dog population. While further investigation is essential, this potential association of rainfall and fluctuations of dog populations could be considered in planning risk-based vaccination programmes, including spay-neuter programs for adult dogs (>1 year old), pre-rainy season vaccination and targeted vaccination of <1 year old pups soon after the end of the rainy season.