A comparison of rats living in the wild and the lab lends support to the idea that an overly hygienic environment can lead to allergies and autoimmune diseases.

According to the "hygiene hypothesis," exposure early in life to infections from household dust, germy siblings or surfaces may reduce the risk of developing disease in adulthood.

William Parker, a professor of experimental surgery at Duke University in Durham, N.C., and his team compared lab rodents to more than 50 rats and mice captured and killed in cities and farms.

"Laboratory rodents live in a virtually germ- and parasite-free environment, and they receive extensive medical care — conditions that are comparable to what humans living in Westernized, hygienic societies experience," Parker said in a release.

 "On the other hand, rodents living in the wild are exposed to a wide variety of microbes and parasites, much like humans living in societies without modern health care and where hygiene is harder to maintain."

Industrialized societies that emphasize hygiene have higher rates of allergy, asthma and autoimmune diseases such as rheumatoid arthritis compared to the developing world.

Stimulating the immune cells of wild rodents made no difference, but lab rats overreacted, the team reports in the Scandinavian Journal of Immunology.

The wild rodents also showed as much as four times higher levels of immunoglobulins related to allergy and autoimmune disease, but didn't get sick.

Since wild animals are likely exposed to more parasites, the antibodies would likely bind to parasite threats.

But in lab animals, the same antibodies would bind to harmless environmental allergens instead of parasites, leading to allergies, he said.

Looking at differences between animals in the wild and the lab may help scientists figure out what environmental exposures may be protective.

One flaw of the research is the team didn't know if the wild animals were exposed to any unusual diseases that could influence the results.