Spring 2001 Issue
A Troublesome Insect and a Deadly Disease
Researchers at a CDC campus in suburban Atlanta have constructed a model house that duplicates the microenvironment of a typical rural dwelling in Latin America. They are using the structure to study the behavior of a large, unpleasant-looking tropical insect called the “kissing bug,” which favors as its habitat the cracks and crevasses of poorly constructed buildings. Unlike many species of the bug, which are harmless, this Latin American cousin effectively transmits Trypanosoma cruzi (T. cruzi), the protozoan parasite that causes Chagas’ disease, a debilitating chronic ailment that afflicts 16-18 million people in Latin America and claims nearly 50,000 lives a year.
At CDC, Ben Beard, Ph.D., chief of Vector Genetics of CDC’s Division of Parasitic Diseases, and other scientists are working to identify ways to prevent, detect and treat Chagas’ disease, an infection that can affect the heart, intestines and nervous system. Beard’s focus is on prevention, and he is using the model of the kissing bug’s rural tropical dwelling to test a promising new method for killing the parasite once it gets inside the insect. It seems the insect has a symbiotic dependence on certain bacteria that live in its gut and share living space with T. cruzi. Beard and colleagues have developed an altered form of the bacteria that produces a substance that kills the parasite. They introduce the altered bacteria into the habitat and then study the insects to determine such factors as how they take up and respond to the altered bacteria and what bacteria levels are needed to kill the parasite. Results have been promising, and Beard hopes that someday this method can be used to prevent insect transmission of the disease.
“But just because something works in the laboratory doesn’t mean it will work in the wild,” Beard says. He adds that once this method of parasite control has been fine tuned, he and his colleagues hope to test the altered bacteria in a natural insect population, possibly at a CDC field station in Guatemala.
Guatemala is the site of another CDC research project related to Chagas’ disease. Scientists there want to learn the extent to which school children living in areas where the disease is endemic suffer from the heart abnormalities the parasite causes. These abnormalities are detectable via electrocardiogram (EKG). In order to conduct their study, the researchers needed funds to purchase an EKG machine and to hire people trained to use it. Last year, Emory Black of the Exposition Foundation in Atlanta learned of this project, and through the CDC Foundation, his organization made a grant to fund the study.
Black says he had first-hand experience with Chagas’ disease at EARTH University in Costa Rica. “Scientists there were doing fascinating work,” says Black, a member of the CDC Foundation board of visitors. The scientists had discovered a plant in nearby tropical rain forests that contained a substance that blocked a key enzyme in the parasite. They were mapping the molecular structure of the parasite’s enzymes using enzyme crystals grown in the zero-gravity environment of the space shuttle. This experience prompted Black’s interest in CDC’s work in Guatemala.
“Chagas’ disease is a major problem for Latin America, and I hope this contribution will help raise awareness about the disease and the importance of investing resources to slow and even stop its deadly effects,” he says.
Daniel Colley, Ph.D., director of the Division of Parasitic Diseases of CDC’s National Center for Infectious Diseases, says that new, more accurate methods for detecting and treating Chagas’ disease are desperately needed. A major problem hindering disease control is that many people infected with the parasite are totally unaware of it. To understand how this is possible, Colley says it is necessary to understand something of the more intimate habits of the kissing bug.
“First of all, the Latin American kissing bug feeds at night while people are sleeping. A person may be unaware that they have been bitten. Second,” Colley explains, “the insect is a post-prandial defecator.” This means that it defecates during and immediately after biting and feeding. The parasite lives in the insect’s feces. The bite itches, the victim scratches, and the feces, along with the parasite, get rubbed into the bite, eyes or mouth.
Colley says some people experience symptoms days to weeks after they are bitten, such as swelling around an eye or cold-like symptoms, but many have no symptoms at all and may not even know they have been bitten. An infected person can carry the infection for 10 to 15 years and exhibit no signs at all. The majority never experience ill effects. However, about 30 percent develop severe abnormalities of the heart and gastrointestinal system, often resulting in premature death.
“Even if we knew who was infected and who wasn’t,” Colley says, “we don’t have an effective drug to treat the disease. What we have works in the disease’s early stages, but we don’t really know how effective it is.”
More testing is needed to assess the effectiveness of the drugs currently used, and new ones that work better need to be developed and tested. Colley points out that developing and testing new drugs is very expensive, and it is hard to interest drug companies in spending money on drugs for which they do not see a market. However, even if all transmission of Chagas’ disease stopped today, 16-18 million people would still be infected for the rest of their lives and in need of an effective drug.
Colley says another factor that is important for the control of Chagas’ disease is assuring the safety of blood supplies, both in Latin America and other parts of the world. He says currently no test exists that is 100 percent accurate for screening donated blood for the presence of the T. cruzi parasite.
“Right now we don’t have a fail-safe way of telling how big the problem is, and we need one,” says Colley.
- Rosemarie Perrin