The New Yorker | November 20, 2015 | By Jennie Erin Smith
If Thomas Cropper, a public-health veterinarian at Lackland Air Force Base, in San Antonio, Texas, thought about Chagas disease at all, he thought about it as a Central and South American problem. Named after the Brazilian physician who described it, in 1909, Chagas is a classic—one might say egregious—example of a neglected tropical disease. It is caused by the protozoan parasite Trypanosoma cruzi, which is delivered to its host by kissing bugs, known formally as triatomines. The bugs are bloodsuckers—their nickname comes from their penchant for biting near the eyes or mouth—and they can swell to the size of grapes as they feed, causing them to defecate and leave the parasite behind to make its way into the host’s bloodstream. A gross and not particularly efficient mode of transmission, it’s still good enough to have kept Chagas going since pre-Columbian times. According to the World Health Organization’s shifting estimates, between six and seven million people in Latin America are currently infected. If you’re infected but don’t have symptoms, you’re likely to find out only after donating blood. If you do have symptoms, you’re probably in trouble. About a third of Chagas patients develop a chronic form that leads to heart damage and failure.
Cropper specializes mainly in zoonoses—diseases transmissible from animals to humans—and in keeping service members away from them. Not long after he arrived at Lackland, in 2008, he learned that a military working dog had developed heart problems while deployed in Kuwait. The dog was returned to Lackland, the home of the Department of Defense’s canine school, and confirmed positive for Chagas. A study of all the working dogs on base, in fact, found that about eight per cent had antibodies against T. cruzi. Many never developed symptoms at all, but young pups would sometimes drop dead without warning. Cropper began asking about Chagas and what the risks might be for humans. “What else are these stupid bugs feeding on?” he wondered. This turned out to be a somewhat urgent question, because Lackland is where about thirty-five thousand Air Force and National Guard recruits are trained every year, often outdoors. The base encompasses a vast, cactus-strewn wilderness with populations of wood rats, armadillos, skunks, opossums, and other easy targets for a kissing bug.
Cropper called in an entomologist, Walter Roachell, and a microbiologist, Candelaria Daniels, from the Army Public Health Command at Joint Base San Antonio. Roachell found five species of kissing bug at Lackland, some in the nests dug by wood rats in the bottoms of cacti. A training instructor who accompanied Roachell claimed that he’d never before seen one of the bugs, even though certain species are distinctive—pretty, even—with their folded wings encircled by what could be described as a striped skirt. “I looked down and pointed out one crawling between his feet,” Roachell told me. Daniels, meanwhile, sought to determine the bugs’ infection rates. More than half of them, her analysis revealed, carried T. cruzi. They were eating a lot of things—wood rats, armadillos, even rattlesnakes. And, more alarmingly, nearly thirty per cent tested positive for human blood. “They were indeed feeding on people,” she said. Cropper had vegetation cut back, insecticide sprayed, and treated bed nets installed where trainees slept, and similar measures were taken around the kennels. At one point, a routine blood donation by an airman, who had neither a history of travel to Latin America nor a mother likely to have transmitted the disease to him in utero, turned up positive. He remembered suffering a swelling around his eye—one early sign of a Chagas infection—which he blamed on dirt. It “may have been feces from the kissing bug,” Cropper said, but no one was really sure.
Even among vector-borne tropical diseases, Chagas remains a redheaded stepchild. It’s slower to kill than dengue or malaria, and therefore easier to ignore. Chagas scientists are used to incremental progress, low recognition, and poor funding; they sometimes must scream for attention. (In 2012, one group raised eyebrows after calling Chagas “the new H.I.V.”) Benznidazole, the principal drug that is used to treat the disease, was developed more than forty years ago, and another, nifurtimox, is even older. Both kill the parasite, although a large clinical trial, published last month in the New England Journal of Medicine, found that benznidazole could not halt heart deterioration already in progress.
Military science has tightly defined parameters; when it comes to parasitology, only research with practical applications gets funded. In San Antonio, the spectre of risk to personnel—and expensively trained working dogs—meant that a lot of money and attention could be directed toward Chagas. When I visited the lab at Public Health Command Central, Roachell and a microbiologist were designing a pesticide-impregnated mammal food, not unlike the anti-tick treatments that are given orally to dogs, to kill the feeding kissing bugs and interrupt the transmission cycle. In addition, Daniels is working with Cropper and a physician at Lackland on what, to the military, may be the most important of the Chagas studies under way: screening in recent trainees. Three thousand service members are being asked to volunteer blood samples. The researchers have no baseline infection rates to refer to within that group, but so far, with a little more than a tenth of the samples in, no one has tested positive, a sign that the preventive measures have helped. In dogs, only two per cent now have antibodies.
Military scientists aren’t the only ones working with Chagas in Texas, even if they may be the best funded. In the past few years, state health officials, entomologists, and academic veterinarians have all launched local studies. This has created the popular impression of Chagas as a new threat, with some articles in the press linking it to climate change or illegal immigration. But what is striking to researchers is how everyone missed Chagas for so long, when the literature shows that Texas—along with much of the rest of the Southwest—has been an endemic Chagas region since people began looking. Local transmission has been documented since 1955 in people, and since 1972 in dogs. Roachell even learned that an exhaustive study on kissing bugs and T. cruzi had been conducted at Lackland and published in 1970. That study reported that the bugs hung out in wood-rat nests and were heavily infected with the parasite—not so different from what he and Daniels found.
Still, the idea of Chagas as a foreign illness persisted for half a century. When blood banks nationwide started screening for the disease, in 2007, it was generally assumed that immigrants and their children were the only people at risk. Our native kissing bugs weren’t as effective as vectors as the species in Latin America, it was believed; housing was better here and could not harbor the bugs in the way that mud walls and thatched roofs were known to; the strain of parasite in local bugs caused a milder form of the disease, one that didn’t involve the heart. All myths, it turns out. Two years ago, Sarah Hamer, a veterinarian and assistant professor at Texas A&M University, began asking Texans to scoop up kissing bugs in plastic baggies and mail them to her lab. People sent more than twenty-five hundred samples, often from immediately around their homes. More than half were carriers of T. cruzi. Blood meals from human and canine hosts were not uncommon. A couple of bugs, collected at a zoo, had been feeding on tigers.
That same year, 2013, Chagas was added to the list of infectious diseases that had to be reported to the Texas Department of State Health Services. This allowed for closer scrutiny of human cases. Thirty-nine were picked up through last year, half of them locally acquired. “That’s one out of every sixty-five hundred blood donors screened,” Melissa Garcia, a research associate at Baylor College of Medicine, in Houston, told me. “Pretty high for a rare disease.” In a follow-up study, Garcia found that forty-one per cent of blood donors in southeastern Texas who had acquired Chagas had cardiac abnormalities consistent with the disease, suggesting that local parasite strains are no more benign than tropical ones. “A lot of theories that have sounded comforting to us in the past will not bear out,” she predicted.
This year, Cropper prodded Paula Stigler Granados, a public-health specialist with the University of Texas Health Science Center at Houston, to apply for a grant from the U.S. Centers for Disease Control and Prevention to keep all the researchers talking to one another, and to get the word out about Chagas. Stigler Granados, who used to live in Chile and is no stranger to the illness, now has five years and a little more than half a million dollars with which to canvass Texas’s military hospitals, cardiologists, obstetricians, primary-care clinics, hunters, campers, and veterinarians. Health-care providers are her main concern, for now. “Chagas has a different face than we originally thought it did,” she said. “What we’re finding is, a person goes and gives blood, gets a positive result for Chagas, and the doc tells them, ‘It’s usually a false positive. We don’t have Chagas here.’ ”