Mitigating the future impact of Cholera Epidemics

Cholera epidemics have been analysed using epidemiological models that depend on humanto-environment-to-human spread of toxigenic Vibrio cholerae bacteria (Koelle et al. 2004, Koelle et al. 2005). Within such a framework the vulnerable human population is the key driving variable, and mitigation of cholera outbreaks lies in both the human-to-environment domain (sanitation) and in the environment-to-human domain (water supply, immunisation). Previous international research on cholera identified a potential environmental (i.e. aquatic) reservoir of the bacteria (Colwell & Huq 1994). CSIR research in central Mozambique demonstrated the presence of cholera bacteria in the rivers at the onset of the rainy season while no cholera cases were reported among the local population. This demonstrates that cholera epidemics are not linked to the mere presence of the bacterium, but also to its virulence. The research then focused on the seasonal recurrence of epidemics in Beira between 1999 and 2005 and tested the possibility that environmental factors such as temperature, rainfall, humidity, sea-surface temperatures and ocean chlorophyll (plankton production) might determine Vibrio cholerae virulence. Rainfall emerged as the key driver of both inter-annual and intra-annual variability in the number of reported cholera cases. The Beira results suggest that an endemic reservoir of Vibrio cholerae bacteria is amplified by environmental variables independently of the human-toenvironment link. Under these circumstances, water supply and not sanitation is the key to mitigating an epidemic. The cholera epidemic that occurred in KwaZulu-Natal, South Africa, between 2000 and 2001 was also analysed using the approach developed in the Beira study. The results demonstrate that the scale of the epidemic in KwaZulu-Natal was also controlled by rainfall, but cholera epidemics in KwaZuluNatal are not an annual occurrence, despite a fairly regular rainfall regime. Further testing is required to determine if the Vibrio cholerae bacteria is endemic in KwaZuluNatal as it is presumed to be in Beira. If it is not endemic then mitigation must focus on preventing human-toenvironment spread and the emphasis must be on sanitation. However, at the onset of any future epidemic the mitigation measures should emphasise the environment-to-human link via water supply and immunisation.