Factors influencing the re-emergence of plague in Madagascar

Abstract

Plague is an infectious disease found worldwide and has been responsible for pandemics throughout history. Yersinia pestis, the causative bacterium, survives in rodent hosts with flea vectors that also transmit it to humans. It has been endemic in Madagascar for a century but the 1990s saw major outbreaks and in 2006 the WHO described the plague as re-emerging in Madagascar and the world. This review highlights the variety of factors leading to plague re-emergence in Madagascar, including climate events, insecticide resistance, and host and human behaviour. It also addresses areas of concern for future epidemics and ways to mitigate these. Pinpointing and addressing current and future drivers of plague re-emergence in Madagascar will be essential to controlling future outbreaks both in Madagascar and worldwide.

Keywords: Yersinia pestis; climate; drug resistance; infection; insecticide resistance; re-emergence.

Figure 1.. Plague cases from 1954 to 2018.

Number of plague cases from 1954 to 2018; in the whole of Africa (black), in Madagascar (blue), in the Democratic Republic of Congo (DRC) (orange), and in the United Republic of Tanzania (URT) (green) [8–11]. Please note that reported numbers of plague cases in 2017 are inconsistent between sources — here the figures from the WHO 2019 report have been used [11].

Figure 2.. Diagram of plague transmission.

1. Flea life cycle: Fleas lay their eggs in the moist soil of rodent burrows; larvae feed on rodent faeces, and develop to form pupae and later fleas. Adult fleas parasitise rodents and can become infected with Y. pestis. 2. Enzootic cycle: Rodents act as hosts and reservoirs for Y. pestis, vectored by the fleas X. cheopis, X. brasiliensis, and S. fonquerniei. 3. Epizootic cycles occur when fleas infect mammals which are not the natural hosts of Y. pestis. 4. These newly infected mammals may spread plague to new areas and, lacking resistance, die quickly from Y. pestis. 5. Humans may become infected via the bite of a flea originating from an infected rodent or other mammal. 6. Bubonic plague may develop into pneumonic plague by affecting the lungs to induce a cough – this generates respiratory droplets containing Y. pestis which are inhaled by other individuals, resulting in direct transmission between people [1,15].

Figure 3.. Correlations between climatic factors and plague.

The relationship between plague incidence and climate are complex and further explored in the following section. Some years a positive or negative correlation appears between plague and climatic factors; other years have none, and not all years have ENSO or IOD events. Note the strong positive correlations of plague with all climatic factors in the 1990s [17].