Keeping chickens and especially dogs out of bedrooms could help reduce the risk of deadly Chagas disease infection in rural areas of Central and South America, according to a new report in the 27 July issue of the international journal, Science.
The study is the first mathematical model of Chagas disease infection to use data from individual households on a community-wide basis, says lead author Joel E. Cohen of Rockefeller University and Columbia University.
Chagas disease, or American trypanosomiasis, is a chronic, frequently fatal infection caused by the parasite Trypanosoma cruzi, an American cousin to the African sleeping sickness parasite. T. cruzi is transmitted through the feces of blood-feeding bugs called triatomines, commonly known as "kissing" bugs, "cone-nosed" bugs, vinchuca, or barbeiro.
According to the World Health Organization, 16-18 million people from Mexico to Argentina are affected by Chagas disease, and another 100 million, or 25 percent of the region's population, are at risk of infection. Infection is lifelong, and can result in fatal heart disease.
Despite national-level insecticide spraying programs and blood screening in Latin America, Chagas disease remains a " serious obstacle to health and economic development," especially for the rural poor, say study authors Cohen and Ricardo E. Gürtler of Universidad de Buenos Aires.
To better understand how the T. cruzi parasite is transmitted in rural areas, Cohen and Gürtler created a mathematical model based on household data from three rural villages in northwest Argentina. The model looks at seasonal fluctuations in household bug and parasite populations, and their relationship to the numbers of humans, chickens, and dogs living within a household.
Data collected by the field research team show that humans, chickens, and dogs in the study villages all sleep indoors during the spring, with the chickens kept inside to prevent theft or predation. Since chickens provide most of the blood meals for the bugs, the indoor bug population begins to grow in the spring, and reaches its peak in the summer.
Although chickens can't become infected with T. cruzi, they provide a significant food source for the bugs. Thus, they can contribute to the overall population of the parasite by increasing the number of bugs available to feed on infected household members such as dogs and humans.
After chickens, the blood-feeding bugs prefer to take their meals from dogs--selecting them roughly twice as often as humans--and infected dogs are much more infectious to the bugs than infected humans. With this in mind, the mathematical model predicts that having two infected domestic dogs in an average household of five humans is probably "the worst thing householders can do" in terms of increasing the T. cruzi population, say the authors.
The study notes that removing infected dogs from a household is enough to nearly wipe out the transmission of the parasite, barring the reintroduction of any infected dogs, children, or bugs.
The Science researchers suggest that their findings can be used in conjunction with insecticide programs and appropriate construction materials--which decrease the number of domestic bugs--to slow down the Chagas infection rate. "National-level spraying programs in Latin America deserve lots of credit, but budgets in these countries aren't always sufficient to keep up the spraying. It might take ten years to get around to spraying all the rural villages, but it only takes three to five years for bugs to fully recolonize these homes," says Cohen.
Cohen hopes that governments, anthropologists, and health educators can help spread the message that keeping domestic animals out of indoor sleeping areas can reduce the prevalence of the Chagas parasite in bugs and the risk of infection in humans.
The Science researchers also suggest that their community-wide, household model could be used to evaluate how the spread of other diseases, such as malaria or leishmaniasis, may be affected by close contact with domesticated animals.
This research was supported in part by the Rockefeller Foundation, NSF, Universidad de Buenos Aires, the Fulbright and Thalmann programs, and CONICET of Argentina.