Where sleeping does lie affects human risk of Chagas disease infection, report Cohen and Gürtler.
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Removing domestic animals from the bedroom may significantly reduce transmission
By characterizing the discrete population dynamics of an individual household, and by collecting data for many individual households to serve as an empirical base, a Rockefeller University researcher and his Argentinean colleague have identified improved control tactics for a vexing public health problem.
A mathematical model called a system of nonlinear difference equations indicates that the chronic, frequently fatal Chagas disease infection could be avoided or greatly reduced in humans in the Gran Chaco region of South America by barring domestic animals from bedrooms. Chagas disease, or American trypanosomiasis, is caused by infection with the protozoan parasite Trypanosoma cruzi and occurs predominantly in rural Latin America. Some cases have also been reported in the southern U.S.
The research provides an immediate and practical defense against the disease, complementing the more commonly implemented insecticide spraying and housing improvement controls.
"The mathematical model is a way of putting together everything we know about household transmission of infection, to see how all the pieces of the puzzle affect the bottom line, human infection," says Joel E. Cohen, head of the Laboratory of Populations at The Rockefeller University and Columbia University’s Earth Institute.
"The model makes it possible to do, inside of a computer, experiments that we could not ethically do in a real household, such as hypothetically varying the number of infected dogs without trying to reduce the number of infected bugs. Our simple mathematics gives insight attainable no other way."
Reporting in the July 27 edition of Science, Cohen and Ricardo E. Gürtler of the University of Buenos Aires predicted how four populations—bugs, chickens, dogs and people—interact with T. cruzi each season in an individual household.
The model was developed in close connection with data collected over ten years in three rural villages—Amama, Trinidad and Mercedes—in the Province of Santiago del Estero, Northwest Argentina.
T. cruzi is spread through the feces of domestic blood-feeding bugs called triatomines, also known as "kissing bugs" (with local names such as "vinchuca," "chipo," or "barbeiro").
Domestic triatomine bug populations start to grow in Argentina during the spring. Part of the bug population growth is fueled by blood meals the bugs take from chickens, which during the spring are kept indoors to brood protected from theft or predation.
Chickens cannot be infected with T. cruzi whereas dogs and humans can be. But the infected dogs are much more infectious to the bugs than the infected humans, and therefore fuel population growth of both bugs and T. cruzi.
By the time the chickens roost outdoors in the summer, the already large domestic bug population shifts to feed preferably on dogs and secondarily on humans, which rapidly spreads the prevalence of T. cruzi among bugs. This in turn increases the infection rate among dogs and humans.
Because triatomine bugs are more likely to feed on chickens and dogs than on humans, summer infection rates grow for humans as people share domestic sleeping quarters with infected dogs.
The mathematical model provides thirteen predictions about the transmission of T. cruzi. Two of them greatly affect human health:
- keeping dogs in the bedroom is "the worst thing householders can do," with respect to Chagas disease;
- and exclusion of infected dogs from sleeping areas will virtually eliminate transmission of the parasite, at least in semi-arid settings like those studied in the Gran Chaco region.
"Because of budget constraints in spraying programs in Latin America, 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.
The World Health Organization estimates that 16-18 million people are infected with T. cruzi, and another 100 million people in Central and South America are at risk of infection. The predictions made by Cohen and Gürtler could help control the disease, and provide a mathematical approach for evaluating the possible effects of domestic animals on other infectious diseases such malaria, leishmaniasis and Japanese encephalitis virus.
This research was supported in part by the Rockefeller Foundation, The National Science Foundation, Universidad de Buenos Aires, the Fulbright and Thalmann programs, and CONICET of Argentina.
John D. Rockefeller founded Rockefeller University in 1901 as The Rockefeller Institute for Medical Research. Rockefeller scientists made significant achievements, including the discovery that DNA is the carrier of genetic information. The University has ties to 21 Nobel laureates, six of whom are on campus. At present, 34 faculty are elected members of the U.S. National Academy of Sciences, including the president, Arnold J. Levine, Ph.D., and Joel E. Cohen, the senior author of this new analysis of Chagas disease. Celebrating its centennial anniversary in 2001, Rockefeller – the nation’s first biomedical research center–continues to lead the field in both scientific inquiry and the development of tomorrow’s scientists.
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Science