Unraveling the role of rat and flea population dynamics on the seasonality of plague epidemics in Madagascar.

Plague continues to pose a public health problem in multiple regions of the world, including Madagascar, where it is characterized by a pronounced seasonal pattern. The drivers of plague seasonality remain poorly understood. Using a deterministic compartmental model, calibrated to rat and flea capture data, serological data collected in active rural foci, and human plague surveillance data, we analyzed the effects of seasonal rat and flea population dynamics on plague transmission. The models that incorporated seasonal fluctuations in rat and flea populations provided better predictive performances than those that did not. We found that a simpler mass-action model also performed well. Driven by these seasonal changes, the effective reproduction number (R_e) between rats peaks at 1.45 [95% credible interval (CI): 1.41, 1.48] in October and falls to 0.6 (95% CI: 0.57, 0.63) in March. We estimated that 0.5% (95% CI: 0.2%, 0.9%) of rats are infected annually, indicating that plague is not the main driver of rat population changes. Using our model, we evaluated intervention strategies and found that targeting both rats and their fleas at the start of the epidemic season (July-September) was the most effective approach for reducing human plague cases. Such an approach contrasts with the reactive strategy currently employed in Madagascar. Our findings highlight the role of flea and rat populations in plague seasonality and identify strategies that could be deployed in Madagascar to better control plague epidemics.