Estimating the reproductive numbers for the 2008-2009 cholera outbreaks in Zimbabwe

Abstract

Cholera remains an important global cause of morbidity and mortality, capable of causing periodic epidemic disease. Beginning in August 2008, a major cholera epidemic occurred in Zimbabwe, with 98,585 reported cases and 4,287 deaths. The dynamics of such outbreaks, particularly in nonestuarine regions, are not well understood. We explored the utility of mathematical models in understanding transmission dynamics of cholera and in assessing the magnitude of interventions necessary to control epidemic disease. Weekly data on reported cholera cases were obtained from the Zimbabwe Ministry of Health and Child Welfare (MoHCW) for the period from November 13, 2008 to July 31, 2009. A mathematical model was formulated and fitted to cumulative cholera cases to estimate the basic reproductive numbers R(0) and the partial reproductive numbers from all 10 provinces for the 2008-2009 Zimbabwe cholera epidemic. Estimated basic reproductive numbers were highly heterogeneous, ranging from a low value of just above unity to 2.72. Partial reproductive numbers were also highly heterogeneous, suggesting that the transmission routes varied by province; human-to-human transmission accounted for 41-95% of all transmission. Our models suggest that the underlying patterns of cholera transmission varied widely from province to province, with a corresponding variation in the amenability of outbreaks in different provinces to control measures such as immunization. These data underscore the heterogeneity of cholera transmission dynamics, potentially linked to differences in environment, socio-economic conditions, and cultural practices. The lack of traditional estuarine reservoirs combined with these estimates of R(0) suggest that mass vaccination against cholera deployed strategically in Zimbabwe and surrounding regions could prevent future cholera epidemics and eventually eliminate cholera from the region.

Fig. 1.

Cholera cases and deaths in Zimbabwe from 1992 to 2009 (4). The y axis for the graph is logged.

Fig. 2.

Map of Zimbabwe, provinces, and neighboring countries. The darker-colored regions show one of the cholera-affected districts (Manica) in Mozambique in 2006 (26) and some of the cholera-affected provinces (Southern and Lusaka) in Zambia in 2010 (27) that are on the border with Zimbabwe.

Fig. 3.

Model flow diagram.

Fig. 4.

Cholera model fitting for the cumulative cholera cases where the solid red line represents the model fit and the circles mark the reported data for the cumulative number of cholera cases in the provinces using parameter values in Table S1 and population sizes in Table 1.

Fig. 5.

Cholera model fitting for the cholera cases. The solid line represents the model fit, and the dashed line and the circles mark the reported data for cholera cases in the provinces using parameter values in Table S1 and population sizes in Table 1.