A Non-Stationary Relationship between Global Climate Phenomena and Human Plague Incidence in Madagascar

Abstract

Background

Plague, a zoonosis caused by Yersinia pestis, is found in Asia and the Americas, but predominantly in Africa, with the island of Madagascar reporting almost one third of human cases worldwide. Plague's occurrence is affected by local climate factors which in turn are influenced by large-scale climate phenomena such as the El Niño Southern Oscillation (ENSO). The effects of ENSO on regional climate are often enhanced or reduced by a second large-scale climate phenomenon, the Indian Ocean Dipole (IOD). It is known that ENSO and the IOD interact as drivers of disease. Yet the impacts of these phenomena in driving plague dynamics via their effect on regional climate, and specifically contributing to the foci of transmission on Madagascar, are unknown. Here we present the first analysis of the effects of ENSO and IOD on plague in Madagascar.

Methodology/principal findings

We use a forty-eight year monthly time-series of reported human plague cases from 1960 to 2008. Using wavelet analysis, we show that over the last fifty years there have been complex non-stationary associations between ENSO/IOD and the dynamics of plague in Madagascar. We demonstrate that ENSO and IOD influence temperature in Madagascar and that temperature and plague cycles are associated. The effects on plague appear to be mediated more by temperature, but precipitation also undoubtedly influences plague in Madagascar. Our results confirm a relationship between plague anomalies and an increase in the intensity of ENSO events and precipitation.

Conclusions/significance

This work widens the understanding of how climate factors acting over different temporal scales can combine to drive local disease dynamics. Given the association of increasing ENSO strength and plague anomalies in Madagascar it may in future be possible to forecast plague outbreaks in Madagascar. The study gives insight into the complex and changing relationship between climate factors and plague in Madagascar.

Author Summary

Plague is a vector-borne bacterial infection with rodents and their fleas as its principal hosts. Transmission to humans occurs via the bite of an infected flea. In the highlands of Madagascar, plague is endemic and more than one hundred human cases are reported every year. Global climate is known to affect many infectious diseases and has been shown to affect plague incidence in other areas of the world. The ENSO and the IOD are global climate drivers affecting rainfall and temperature in Madagascar. Our study investigates the effect of global climate drivers on human plague incidence on the island. We found a link between ENSO, IOD, temperature and precipitation and plague incidence throughout the 48-year time-series although it was not constant over time. The correlation between ENSO and plague turned from weakly positive to strongly negative and then to positive, and the association with the IOD became stronger with time. We demonstrate that during periods of high ENSO intensity, plague incidence is likely to increase via ENSO's impact on temperature and precipitation. This shows that climate indices can be a tool to help predict human plague incidence.