Susceptibility to Plague of the Rodents in Antananarivo, Madagascar

Conclusions

R. novegicus has gradually replaced R. rattus in Antananarivo city after its introduction in the 50′s. In 2001, all the rats trapped were R. norvegicus suggesting that this species is nowadays the main reservoir of plague in Antananarivo city. Experimentally, we have demonstrated the natural resistance to Y. pestis of the two populations of rats in Antananarivo (R. norvegicus and R. rattus), as compared to the white laboratory rats and to the rats from non endemic areas. The longitudinal surveillance of the rodents during 4 years, has showed their high seroprevalence for the anti-F1 IgG antibody and a high flea index. The survey has also evidenced Y. pestis in pools of fleas confirming the transmission of the plague bacillus among these rodents. It is likely that many of the rodents get infected but survive and further develop anti-F1 antibodies or remain carriers of the plague bacillus. Indeed rat mortality phenomenon is not observed in the capital, conversely to what is seen in the rural villages or in the harbor of Mahajanga. This natural resistance may explain the maintenance of plague in this city. The absence of epizootic and subsequently the lack of free fleas probably explain the sporadic transmission of plague to human.