Chagas Disease in Texas: New Surveillance Data

NEW ORLEANS — Researchers reported high rates of Trypanosoma cruzi, the parasite responsible for Chagas disease, in a pair of studies on free-running and shelter dogs in Texas, suggesting that the pathogen is more prevalent in the local environment than commonly thought.

But there was good news for Texas hunters in another study presented here: although survey data indicated frequent potential exposure to the insect that transmits T. cruzi, not one of more than 1,000 hunters tested had been infected.

Results of the three studies were reported during a poster session at the American Society of Tropical Medicine and Hygiene annual meeting.

Dogs and T. cruzi

Chagas disease affects some 8 million people worldwide, including about 300,000 in the U.S. Yet it tends to fly under the radar in this country, perhaps because it is still generally considered a tropical disease.

Similarly, although T. cruzi is known to infect dogs and cause serious illness, it is not among the parasitic organisms that U.S. veterinarians routinely screen for.

To get a better handle on rates of infection in dogs, researchers at Texas A&M University in College Station under Sarah Hamer, DVM, PhD, conducted two studies: one in shelter dogs at seven locations across the state, and another in pet dogs from poor semi-rural communities called “colonias” in the lower Rio Grande valley.

In the shelter dog study, presented by Carolyn Hodo, DVM, PhD, the researchers tested 608 dogs for T. cruzi and four other vector-borne pathogens (including the Dilofilaria immitis heartworm). Rates of T. cruzi ranged from 5.5% in Fort Worth to 29.5% in San Antonio; the overall mean was 18.1%.

Hodo and colleagues noted that these percentages were comparable to those seen for D. immitis, which is routinely screened in the U.S., suggesting that it would make sense to do the same for T. cruzi. Moreover, whereas dog heartworm is not a human health concern, Chagas disease certainly is. Hodo said dogs are “useful as sentinels for vector-borne disease.”

In this case, she told MedPage Today, it certainly applies, since the vector for T. cruzi — so-called kissing bugs — requires many blood meals and are just as likely to feed on humans as on dogs. Large numbers of infected dogs are a signal that T. cruzi is also highly prevalent in the environment.

Even higher prevalences were seen in the colonia study, presented by Italo Zecca, MPH, in which 231 dogs from seven communities were tested. Seroprevalence for T. cruzi ranged from 20.0% to 55.9%, with an overall average of 35.5%. Whereas many of the shelter dogs in Hodo’s study came from people’s homes where they would have been primarily kept indoors, most dogs in colonias live outdoors.

However, the raw numbers in both studies should be interpreted cautiously, the researchers noted. In the colonia study, PCR molecular tests were positive in only 3.9% of the dogs. Zecca and colleagues indicated that positive PCR results “may signal acute infection or dogs that are more likely to be infectious to vectors.” But specificity for T. cruzi serological tests is known to fall well short of 100%.

Risk to Hunters

That issue also cropped up in the study of hunters, reported by Sarah Gunter, PhD, of Baylor College of Medicine in Houston. Serological testing of 1,093 individuals identifying themselves as hunters yielded positive results in 23 — but none were confirmed with the CDC’s “gold standard” testing for Chagas, Gunter said.

That was “good news,” she said, given that survey responses from the 1,093 participants indicated frequent exposures to situations and environments where infection could occur. Most respondents knew nothing about Chagas disease and few took steps to reduce their risk.

Participants were asked a number of questions related to likely contact with kissing bugs or blood from potentially infected animals.

• Majorities reported the following:
• Staying overnight in the open or in tents or cabins
• Seeing kissing bugs while hunting
• Never/rarely using insect repellent
• Never/rarely wearing gloves when field dressing animals

More than 90% reported contact with deer and 72% with feral hogs, both of which are known reservoirs for T. cruzi. Only 40% said they had heard of Chagas disease.

Gunter told MedPage Today that it should be a public health priority to educate hunters about Chagas disease and what they can do to minimize risk. She noted that the state’s Parks and Wildlife Department (which facilitated the current study) conducts training programs in which such education could be easily included. This training is mandatory for all hunters born after Sept. 2, 1971.

2018-10-31T18:00:00-0400