"Earlier it was thought that exposure to pets early in childhood was a risk factor for developing allergic disease," coauthor Dr. Merja Nermes, of the University of Turku in Finland, told Reuters Health by email. "Later epidemiologic studies have given contradictory results and even suggested that early exposure to pets may be protective against allergies, though the mechanisms of this protective effect have remained elusive."

One-third of infants in households with pets had animal-specific bifidobacteria in their fecal samples, compared to 14%of controls. It's not clear where the infants without furry pets at home acquired the bacteria, the authors wrote.

The theory is that when pet microbes are found in the infant intestinal biome this may strengthen the immune system. And here is what the researchers found.

Fecal samples collected from diapers when the babies were one month of age were tested for the DNA of two types of Bifidobacteria that are found specifically in animal guts: B. thermophilum and B. pseudolongum.

When the babies were six months old they had skin prick tests to assess allergies to cow's milk, egg white, flours, cod, soybeans, birch, grasses, cat, dog, potato, banana, and other allergens. Nineteen infants had reactions to at least one of the allergens tested. None of these infants had B. thermophilum in their fecal samples.

Past research has linked growing up on a farm or exposure to dog dander indoors with protection against airway allergens, the study team wrote. Other studies have found increased "richness and diversity" in the gut microbes of kids exposed to household pets.

"When infants and furry pets live in close contact in the same household, transfer of microbiota between pets and infants occurs," Nermes said.

"If a family with a pregnant mother or an infant wants to have a pet, the family can be encouraged to have one, because the development of allergic disease cannot be prevented by avoiding pets," she said.

- Published online September 3 in the Journal of Allergy and Clinical Immunology – Medscape Online