---

---

---

---

Science Current Events and Science News RSS Feeds

---

---

Email a Friend Printer Friendly

The black rat's role in spreading human plague in Madagascar

April 26, 2001

Human plague first hit the Madagascan coasts in 1898, then became established permanently in the Hautes Terres, the high central region of the island, during the 1920s. Over the following decade, there were thousands of victims. Subsequently it has become less prevalent, first thanks to mass vaccination, then by way of antibiotics and preventive insecticide spraying. Since the 1980s, however, there was a resurgence of the number of cases and a coastal focus of the disease reappeared in the port town of Mahajanga, after 60 years free of outbreak.
A rodent specialist from IRD, who currently pursues his research at the Centre for Population Biology and Management at Montpellier, did research between 1996 and 2000 on the rodents that could be involved in transmitting the disease in Madagascar. The work was carried out as part of the RAMSE programme (1) which was conducted by the Malagasy Ministry of Health, the Institut Pasteur of Madagascar and IRD.
Plague is caused by a bacterium, Yersinia pestis, spread by fleas (the vector) which live on wild rodents which are themselves resistant (known as reservoirs). The disease is spread by other rodents, more susceptible to the plague, which liberate their fleas when they die. The latter, in their turn, infect humans. In Madagascar, the classical reservoir species (Sciuridae such as central Asian marmots and North American prairie dogs, or Gerbillidae species) do not exist and the endemic rodents are disappearing. However, the black rat, Rattus rattus, has invaded all habitats. The specialist's research confirmed that, although highly susceptible to the disease, it was the only reservoir. This species of rat made up 95 to 98% of all those caught in rural areas, whether in the fields or in houses. Chromosomal and genetic analyses showed the existence of only one species of black rat in Madagascar. Its high reproduction rates (measured both in the laboratory and in the field) and its strong ability to recolonize areas compensate for the mortality caused by plague.
The study undertaken has also explained why human epidemics are seasonal, most intense between November and April. A monthly monitoring of rodents and their fleas, over two successive years, has in fact brought evidence that, in the cultivated areas, the rodent populations fall abruptly in September and are at their lowest point in October and November. That is the very moment when rat fleas are most abundant. With not enough rat hosts, the fleas then seek other mammals on which to feed and therefore resort to humans thereby infecting them.
In Madagascar the black rat harbours two species of flea: Xenopsylla cheopis, an "imported" species characteristic of house rats and Synopsyllus fonquerniei, which are endemic to the island and feed on rats which live outside. The foci of plague in rural areas are restricted to areas above 800 m altitude, whereas the black rat is present throughout the island. The study has shown that below that altitude, X. cheopis is less abundant and especially that the endemic flea S. fonquerniei is absent. As the bacterium can live at low altitudes and black rats are the same whatever the altitude, it seems that the factor limiting the spread of plague is the presence of this endemic flea (2).
The rodent specialist also examined the reappearance of the plague in the port of Mahajanga in 1991. Since 1995, epidemics have been breaking out each year whereas there are few black rats in that town. The study has revealed that a species of shrew from Asia, Suncus murinus, plays a dominant role in this: they represent the majority (75 to 85%) of the small mammals captured. They carry flea vectors of the disease (X. cheopis) and strains of Yersinia pestis have been identified on them. Furthermore, as on the high plateau, a correlation has been established between the variations in abundance of S. murinus and of their fleas and the season when human cases of plague are recorded.
Complementary investigations are currently being conducted. Some researchers, at the Institut Pasteur of Madagascar, are studying the possible existence of wild reservoirs and especially new vectors in the forest areas where new strains of Y. pestis have been observed. Others, at the CBGP at Montpellier are analysing the genetic structure of populations of black rats in their different habitats.

(1) Recherches appliqués '  Madagascar sur la Santé et l'Environnement.
(2) For information on the influence of th environment and living standards on plague transmission in rural areas, see the science bulletin Fiche d'actualité N° 132.

Institut de Recherche pour le Développement, Paris (IRD)