TUESDAY, March 20 (HealthDay News) -- Insects that are part of a family of bugs that transmit Chagas disease are well-established and feeding on human blood in certain regions of the United States, a new study finds.

What remains a mystery, however, is why the insects infect millions in Central and South America with Chagas disease, a serious condition that can lead to life-threatening digestive and heart problems, yet few such cases have been documented in the United States.

So far, researchers have documented only seven cases of Chagas in the United States.

"The basic message is that the bug is out there, it's feeding on humans, and it carries the parasite, so there may be greater potential for humans to have the disease in the United States than previously thought," Lori Stevens, a biologist at the University of Vermont, said in a university news release. "Very likely, with climate change they will shift further north and the range of some species will extend."

In the study, which was published online March 14 in the journal Emerging Infectious Diseases, researchers collected specimens from the Reduviid family of insects in Arizona and California.

About 38 percent of the black, wingless insects, also known as "kissing bugs" because they bite sleeping humans around the mouth, contained human blood. Researchers said this was an unexpected finding because the 11 species of kissing bugs found in the United States were not known to feed on people.

More than half of the insects contained Trypanosoma cruzi, the parasite that causes Chagas disease.

Researchers speculate that the prevalence of the disease could be greater than previously thought.

"We think the actual transmission is higher than the seven cases we have identified," study co-author Patricia Dorn, an expert on Chagas disease at Loyola University in New Orleans, said in the news release. "But even with these findings, we think the transmission of Chagas -- of the T. cruzi parasite -- is still very low in the U.S."

The initial symptoms of Chagas disease can be vague, including fever and swelling around one eye and at the site of the bug bite. The disease can go into remission and reemerge years later as a more serious illness, resulting in potentially deadly digestive and heart problems.

The bug's saliva also can lead to severe allergic reactions.

Researchers theorized the bug hasn't become more widespread in the United States because most homes in the country are made with concrete basements, screened doors and windows, and tighter construction. In contrast, kissing bugs have thrived among the thatched roofs, stick-and-mud construction and dirt floors found in rural and poor regions of Central and South America.

Another theory is that unlike the kissing bugs found in other countries, the species found in the United States do not defecate while they are feeding on their host. As a result, there is less opportunity for the parasite that causes Chagas disease to enter the bloodstream.

One way to prevent the spread of Chagas disease is to use screens or close windows at night. Campers should close their tents or sleeping bags.

The study's authors said they hoped to examine a larger sample of kissing bugs from more areas in the United States.

"Chagas isn't going to spread fast, but it could spread," Stevens said. "Finding out how prevalent it is now would be a good idea."

More information

The U.S. Centers for Disease Control and Prevention provides more information on Chagas disease.