Studies suggest that infants who grow up with dogs in their home are less likely to develop asthma. Researchers may now have found one reason why. Pets, dogs in particular, may protect infants from the effects of a common virus, respiratory syncytial virus (RSV).

Infants with severe RSV infections have an increased likelihood of developing childhood asthma.

The researchers think that exposure to certain microbes in early infancy changes the early composition of an infant's intestinal flora and this sets the tone for how the developing immune system will respond later in childhood.

The researchers fed mice house dust from homes with dogs. They then exposed these mice to RSV and found that the mice did not show symptoms of infection--airway inflammation and increased mucus production. They also compared the intestinal bacteria of dust-fed mice to mice that hadn't been fed dust and found differences in the types of bacteria living in the GI tract. These differences were seen whether or not the non-dust fed mice had been exposed to RSV.

Previous studies by these researchers had shown that the microbial composition of house dust from homes with a dog or cat living in them was different from that of the dust from a pet-free home.

The researchers think that exposure to certain microbes in early infancy changes the early composition of an infant's intestinal flora and this sets the tone for how the developing immune system will respond later in childhood. Asthma is an inappropriately strong immune response to substances in the environment (allergens). Early exposure to and colonization by beneficial bacteria may lead to a generally calmer immune response to the many potential allergens commonly present in the environment.

Evidence that this hypothesis is true comes from the observation that people who grow up on farms are less likely to develop Allergies or asthma than their urban counterparts are. This has been thought to come from exposure to livestock and their associated microorganisms.

Future studies will try to determine if the protective effect of dog ownership does come from one or more specific species of bacteria and if so, will seek to identify these species.

The research was presented at the 2012 (112th) General Meeting of the American Society for Microbiology (ASM) on June 19 in San Francisco. It has not yet appeared in a peer-reviewed journal.