Understanding the persistence of plague foci in Madagascar

Abstract

Plague, a zoonosis caused by Yersinia pestis, is still found in Africa, Asia, and the Americas. Madagascar reports almost one third of the cases worldwide. Y. pestis can be encountered in three very different types of foci: urban, rural, and sylvatic. Flea vector and wild rodent host population dynamics are tightly correlated with modulation of climatic conditions, an association that could be crucial for both the maintenance of foci and human plague epidemics. The black rat Rattus rattus, the main host of Y. pestis in Madagascar, is found to exhibit high resistance to plague in endemic areas, opposing the concept of high mortality rates among rats exposed to the infection. Also, endemic fleas could play an essential role in maintenance of the foci. This review discusses recent advances in the understanding of the role of these factors as well as human behavior in the persistence of plague in Madagascar.

Figure 1. Madagascar plague mapping from 2007 to 2011.

Dashed line: limits of the main plague foci (central and northern foci). Green area: districts that have notified plague cases. Most plague cases were reported from the district of Tsiroanomandidy during this period. (Sources: OCHA, Institut Pasteur de Madagascar).

Figure 2. Main vectors and rodent reservoirs in Madagascar.

Fleas involved in plague transmission in Madagascar: Synopsyllus fonquerniei female (1) and Synopsyllus fonquerniei male (3) are found on outdoor rats, whereas Xenopsylla cheopis female (2) and Xenopsylla cheopis male (4) live on indoor rats. Rat species involved in plague transmission in Madagascar: Rattus rattus (5) and Rattus norvegicus (6).

Figure 3. Plague transmission cycle.

A) Plague cycle in the rural area of Madagascar. Rural plague foci of the highlands are organized into three habitats: houses (arrow), sisal hedges (arrowhead), and rice fields (star). The black rat, R. rattus (3), is the main rodent involved in transmission associated with X. cheopis (1) and the endemic flea S. fonquerniei (2). (Photo of plague foci: S. Rahelinirina). B) Plague cycle in the urban areas of Madagascar. Urban plague occurs mainly in the cities of Antananarivo (Isotry Market, left) (7) and Mahajanga (Abattoir suburb, right) (6). R. norvegicus (4) and X. cheopis (1) are involved in each focus. The Asian shrew (S. murinus) (5) has long been suspected to play a major role in the epidemiological cycle of plague in Mahajanga. C) Plague cycle in the forest area. A sylvatic transmission occurs in Madagascar with R. rattus (3) and endemic micromammals (such as Setifer setosus) (8) as reservoirs. S. fonquerniei (2) is the major vector of the disease in this area. The role of other endemic fleas (9) is not yet determined. (Photo of forest of Ampahitra: S. Telfer; Setifer setosus: V. Soarimalala).