An explosion of wild pigs in the U.S. could be exposing people to dangerous parasites, a new study says.

The wild pig, Sus scrofa, was first introduced to the U.S. from Europe as livestock in the 1500s, but over the years many animals have escaped captivity.

Today there's an estimated four million wild pigs spread across 39 states, with large populations in California, Texas, and the Southeast, according to the study.

Because they're so hardy and can eat almost anything, feral pigs have been living high on the hog, producing several litters of piglets a year, said study co-author Chris DePerno, an ecologist at North Carolina State University in Raleigh.

Mixed into this group are escaped domestic pigs, Sus scrofa domesticus. (See pictures of unusual domesticated animals, including a Vietnamese potbellied pig.)

Within two generations out of captivity, domestic pigs usually lose their pink hue and turn striped and coarse-haired, DePerno said, allowing them to blend in with feral populations.

Though "Tasty," Wild Pork May Be Dangerous

In the new research, DePerno and colleagues found evidence of exposure to the parasites Toxoplasma gondii and Trichinella in the blood of 83 wild pigs killed in North Carolina between 2007 and 2009.

The results are very similar to those of studies conducted in other parts of the country—including Texas and South Carolina—that show feral pigs could be disease vectors, DePerno said.

However, this is the first time scientists have found these particular parasite species in wild pigs, he said.

Although T. gondii and Trichinella have been eliminated in domestic swine, more people these days are hunting wild pigs for food, DePerno said.

"They're pretty tasty—more flavorful than domestic pigs."

People would most likely be infected by eating parasite-ridden meat. Once transmitted, both parasitic species can invade muscle tissue and organs and cause flulike symptoms, DePerno said.

More than 60 million men, women, and children in the U.S. already carry the Toxoplasma parasite, but very few show symptoms, because a healthy immune system usually keeps the disease in check.

Still, toxoplasmosis—the disease caused by T. gondii—is a leading cause of death due to foodborne illness in the United States, according to the U.S. Centers for Disease Control and Prevention (CDC). The organism is especially toxic to pregnant women and those with weak immune systems.

Trichinella infections can also range from mild to severe, with the worst cases causing potentially fatal heart and breathing problems, according to the CDC. Even in moderate cases, fatigue, weakness, and diarrhea can last for months.

Education, More Data to Combat Pig-Borne Illnesses

As the feral pig population expands, so will the incidence of wild-pig hunts, the study authors surmise.

The team therefore recommends education programs for hunters, to help them understand the risks of exposure to parasites both from cleaning pig carcasses and eating wild pork.

There's also the possibility that people could get the parasites from regular pork if domestic swine come into contact with infected feral pigs.

For instance, free-range pigs may bump into their wild brethren and pick up diseases—a possible downside to the popularity of free-range pork, he said. (Also see "Swine Flu Virus Hiding Out in Pigs, May Reemerge.")

"The take-home message," DePerno said, "is that more places need to do this kind of research to determine disease prevalence, especially related to domestic pigs, pets, and human health."