26.9 THIS IS HOW WE DO IT: Does contact with dogs make kids healthier?

Infants get sick a lot. On average, a baby experiences three to six episodes of respiratory tract infection during the first year of life. There is a lot of variation, however, from one household to another. Numerous factors affect the frequency and severity of childhood illnesses, such as attending day care, having older siblings, and a parental history of asthma or smoking.

Recently, another factor was identified. Studies show that children exposed to animals—including pets—early in childhood seem to get sick less frequently and to experience fewer infections, allergies, and asthma attacks. These observations have even given rise to a name for this phenomenon: the “hygiene hypothesis,” which states that reduced exposure to infectious agents—resulting from improved hygiene—suppresses development of the immune system in children. And this, in turn, leaves them more vulnerable to illness later in life.

Could a young child’s exposure to infectious agents really be a good thing? As we saw in the previous section, exposure to pathogens (and the antigens that mark them) causes our immune system to be more effective at fighting later infections. Our immune system learns and remembers. Without early challenges, we may be depriving it of important “learning” opportunities.

This is an issue almost everyone is interested in, and one with significant public health implications, so it’s worth exploring more. Of course, it would not be ethical to deliberately expose infants to pathogens in a randomized, controlled, double-blind type of study. So how could this problem be investigated?

In 2012, some researchers reported a study in which they examined the effect of contact with dogs and cats on the frequency of respiratory tract illnesses during the first year of life. Using a “prospective birth cohort study,” they observed 397 children until the age of one year, beginning when their mother was pregnant. They collected information on the children through weekly diaries kept by the parents, including information about any contact with dogs or cats and any respiratory symptoms or infections.

Several features of prospective cohort studies make them a valuable tool for scientific thinking. Defining before the study period begins the groups—for example, infants having frequent or infrequent contact with pets—and the outcomes that will be measured can reduce potential sources of bias. And collecting data regularly throughout the study increases accuracy and reduces the likelihood of “recall error” (such as might happen if parents were simply asked about their child’s illnesses over the past year).

The results were dramatic. Children having dogs at home experienced 31% fewer respiratory infections and 44% fewer ear infections, and received 29% fewer antibiotic prescriptions. (The researchers found that cat ownership, too, provided a protective effect, but it was much weaker than for dog ownership.)

In their study, the researchers noted only the amount of animal exposure and the incidence of respiratory tract infections, so they could not offer a definitive explanation for their findings. They speculated, however, that animal contacts were responsible for strengthening the immune system. Specifically, they proposed that (1) animals bring dirt into the home, (2) the more dirt brought in, the greater the diversity of bacteria in the home, and (3) exposure to greater bacterial diversity has a positive effect on maturation of the child’s immune system.

In some of the homes with dogs, the dog was only occasionally inside (less than 6 hours per day), while in others, the dog was inside “often” (6–16 hours per day) or “mostly” (more than 16 hours per day). Which condition do you think produced the greatest health benefit? As it turned out, the greatest benefit occurred when the dog was inside only occasionally. The researchers suggested that this was because dogs living mostly indoors probably brought in less dirt (and microbes) than the dogs spending most of their time outdoors.

So, how clean should an infant’s environment be? This is a hugely important question and one that the researchers weren’t sure they could answer yet. Still, their finding adds to a growing body of evidence supporting the hygiene hypothesis and suggesting that there are benefits to an environment in which a developing child’s immune system is challenged.