Correlates of environmental factors and human plague: an ecological study in Vietnam.

Background: Human plague caused by Yersinia pestis remains a public health threat in endemic countries, because the disease is associated with increased risk of mortality and severe economic and social consequences. During the past 10 years, outbreaks of plague have occasionally occurred in Vietnam's Central Highlands region. The present study sought to describe and analyse the occurrence of plague and its association with ecological factors.
Methods: The study included all 510 communes of the Central Highlands region (with a total population of approximately 4 million) where 95% of incidence of plague cases in Vietnam had been reported from 1997 through 2002. Plague was clinically ascertained by using a standard protocol by WHO. Data on domestic fleas and rodents were obtained by using traps and periodic surveillance in accordance with the WHO guidelines. Temperature, duration of sunshine, rainfall and humidity were recorded as monthly averages by local meteorological stations. The association between these ecological factors and plague was assessed by using the Poisson regression model.
Results: From 1997 through 2002, 472 cases of plague were reported, of whom 24 (5.1%) died. The incidence of plague peaked during the dry season, with approximately 63% of cases occurring from February through April. The risk of plague occurrence was associated with an increased monthly flea index (RR and 95% CI: 1.93; 1.61-2.33 for months with the flea index >1) and increased rodent density (RR 1.23; 1.15-1.32 per each 3% increase in density). Moreover, the risk of plague increased during the dry season (RR 2.07; 1.64-2.62), when rainfall fell <10 mm (RR 1.44; 1.17-1.77).
Conclusions: These data suggest that the flea index, rodent density and rainfall could be used as ecological indicators of plague risk in Vietnam. The data also suggest that the occurrence of plague in Vietnam's Central Highlands is likely resulted from multiple causes that remain to be delineated.