Researchers Report New Mathematical Models of Cholera Epidemic
Two young researchers at Old Dominion University report in this week's Proceedings of the National Academy of Sciences (PNAS) that they have developed biomathematical models designed to prevent cholera from getting a new foothold in today's urbanized world.
Jin Wang, assistant professor of mathematics and statistics, and Holly Gaff, assistant professor of biological sciences, are among six authors of the article, which centers on their analysis of a cholera epidemic in Zimbabwe.
The other authors include Shu Liao, who earned a Ph.D. in mathematics from ODU in 2010 and was a student of Wang. Zingoga Mukandavire (first author), David L. Smith and J. Glenn Morris Jr. of the University of Florida are also on the research team that produced the article, "Estimating the Reproductive Numbers for the 2008-2009 Cholera Outbreaks in Zimbabwe."
Between August 2008 and July 2009 nearly 100,000 cases of cholera were reported in the landlocked, central African country of Zimbabwe, and 4,287 deaths were recorded. Of particular interest to the researchers was Zimbabwe's lack of large bodies of water, which can serve as a source of contamination.
Cholera is caused by Vibrio cholera bacteria that infect the small intestines of humans. Victims can quickly become dehydrated from vomiting and diarrhea, and death can result less than 24 hours after the first symptom appears. Transmission is typically through contaminated drinking water or food.
But Wang and Gaff, and the other authors of the PNAS article, found from studying records of the 12 months of outbreaks in Zimbabwe that cholera transmission dynamics there were "potentially linked to differences in environment, socio-economic conditions and cultural practices."
A large number - and perhaps most - of the people who contracted the disease during the year did so by way of human contact, according to the researchers' analysis. Contact with victims, or with their feces, clothes and bedding can spread the cholera bacteria. Furthermore, some people can be carriers of the bacteria without actually succumbing to the disease.
At the start of the outbreaks, the article reports, most victims likely were infected by contaminated drinking water. It was the funeral feasts for some of these victims, however, that seemed to hasten the epidemic. Outbreak investigations done by the World Health Organization and others produced telling data. In one town an initial victim died on Nov. 18 and nine more cases were reported by Nov. 22. All nine had attended the initial victim's funeral.
"In Zimbabwe and Africa in general, funeral feasts associated with the culture of eating together with fingers from the same bowl and an obligation to shake hands in comforting the bereaved have been a major cause of cholera spread during the outbreak," the article states.
"Although multiple models for cholera have been created, most of the earlier models focused on endemic cholera and interactions between environmental variables and disease occurrence, building on data from areas (such as Bangladesh) where there is close contact between infected populations and estuarine (or riverine) environment," according to the article.
The researchers believe that landlocked Zimbabwe "presents a very different setting for cholera, presaging the future of cholera epidemics in a rapidly urbanizing world." They add that data from the country's recent epidemic helped them to construct cholera transmission models that provide insight into the nature of the epidemic in Zimbabwe and, on a broader scale, to the control of cholera globally.
A key goal of the researchers was to develop a way to estimate the "reproductive numbers" for the 2008-09 outbreaks. This means that they used data collected during the epidemic to break down how many secondary infections each primary infection caused. A reproductive number of 1, for example, means that each case results in one more case, and in such circumstances there is no full-scale outbreak.
Their crunching of the data indicates that at various times during the year, and in various provinces of Zimbabwe, the reproductive numbers ranged from around 1 to almost 3. "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," the article states.
The good news, the authors added, is that the overall outcome of their analysis suggests that mass vaccination against cholera deployed strategically in Zimbabwe and surrounding regions could prevent future epidemics and eventually eliminate cholera from this central African region.
Wang and Gaff have both been on the ODU faculty for four years. One of Wang's research focuses is mathematical biology. Gaff, who is also affiliated with ODU's Virginia Modeling, Analysis and Simulation Center (VMASC), has research specialties in mathematical modeling and simulation of infectious diseases, including tick-borne diseases, and the ecology of infectious diseases. Her recent work modeling the spread of cholera in Zimbabwe and elsewhere led to her appointment to the U.S. Centers for Disease Control and Prevention modeling team that is studying current cholera epidemic in Haiti.