Phylogeography and Molecular Epidemiology of Yersinia pestis in Madagascar

Plague, caused by the bacterium Yersinia pestis, has been a problem in Madagascar since it was introduced in 1898. It mainly affects the central highlands, but also has caused several large outbreaks in the port city of Mahajanga, after it was reintroduced there in the 1990s. Despite its prevalence, the genetic diversity and related geographic distribution of different genetic groups of Y. pestis in Madagascar has been difficult to study due to the great genetic similarity among isolates. We subtyped a set of Malagasy isolates and identified two major genetic groups that were subsequently divided into 11 and 4 subgroups, respectively. Y. pestis appears to be maintained in several geographically separate subpopulations. There is also evidence for multiple long distance transfers of Y. pestis, likely human mediated. Such transfers have resulted in the reintroduction and establishment of plague in the port city of Mahajanga where there is evidence for multiple transfers both from and to the central highlands. The maintenance and spread of Y. pestis in Madagascar is a dynamic and highly active process that relies on the natural cycle between the primary host, the black rat, and its flea vectors as well as human activity

Epidémies de peste urbaine à Majunga, côte ouest de Madagascar : résumé

In August 1991, an epidemic of urban plague broke out in Majunga, a port on the western coast of Madagascar. As the first resurgence of the disease in this area in 70 years, the episode was a dramatic reminder of the ubiquity of the disease and layed low the falsely reassuring idea that plague was confined to well known centers of high altitude. After the epidemic had claimed 12 victims, 21 patients suffering from bubonic plague and 7 cases of pneumonic plague, it was brought under control by treating exposed subjects and introducing antivectorial measures. Transmission to man continued in the shape of sporadic cases of bubonic plague until April 1992 and a total number of 41 probable or confirmed bacteriological cases (not counting those deceased) were recorded out of 2020 suspected cases. The history of the disease related almost all these cases to the highly populated and unsanitary neighbourhood of Marolaka, situated on the edge of the Betsiboka estuary, and which has kept semi-rural characteristics. Its murine population had practically disappeared, decimated by an epizootic in early July. Out of the two captured sets of two #Rattus rattus (41 fleas per animal) and two #R. norvegicus each, three were carriers of the bacillus. Numerous shrew-mice (#Suncus murinus) occupied the zone and three tested positive to the direct bacteriological test. For the insectivore, the mean #Xenopsylla cheopis index 4 (n=16), rising to 26 in positive animals. The discovery of a plague infested rat carrying #X. cheopis and the persistence of #P. irritans after insecticide treatments, led us to doubt the efficiency of the antivectorial fight that had been undertaken. Risk of endemization was referred to. After a period of quiet of 3 years, isolated cases were detected in March and May 1995, followed by a new epidemic between July 1995 and March 1996. 617 clinical suspects of bubonic plague were notified... (D'après résumé d'auteur