Do kids need more dirt?

by Sari Harrar

A new study says that for some kids, a dog or cat at home lowers risk for colds and ear infections. Researchers weren’t sure why -- but suspect that pets, especially dogs, expose young children to micro-organisms that help “teach” their immune systems how to respond. It brings up a question that’s been bugging parents and experts for more than a decade: Is a too-clean environment unhealthy for kids? Do they just need more dirt in their lives?

The dog study looked at nearly 400 little kids from Finland. Those who had lived with dogs during their first year were about one-third more likely to be healthy during their first year, compared to babies who didn't have a pet in the home. Babies with dogs in the home were 44 percent less likely to develop an ear infection, and 29 percent less likely to need antibiotics than their petless peers. Exposure to cats helped somewhat, too.

What’s so magical about pets? It may be their dander or the microbes they haul into the house. According to the hygiene hypothesis, kids may not be getting enough of either. University of Montreal experts blame spotless homes and less outdoor playtime for a 30 percent rise in allergies and asthma in the developing world between the early 1980s and the late 1990s.  Having fewer siblings lowers kids’ exposure to bacteria and viruses, too. Antibacterial soaps and household cleaners knock out microorganisms, while air-tight homes keep’em at bay, too. As a result, the theory goes, young children’s immune systems miss out on exposure to germs needed to “teach” them how to react.

The hygiene hypothesis got its start in 1989, when a researcher named D.P. Strachen brought attention to a connection between family size and rates of allergy and asthma. Kids from bigger families were at lower risk. Since then, other studies -- like the dog study -- have filled in missing pieces. In one, researchers at the University of Arizona compared asthma and wheezing in 1,035 kids with their family size and whether or not they went to daycare as babies. More brothers and sisters, as well as going to daycare in the first six months of babyhood, protected kids in the long run. Wheezing was more frequent at age 2 in these kids, but dropped significantly by age 6.

Earlier this year, a mouse study looked at why. Compared to mice raised under ‘normal’ conditions, mice raised in germ-free environments had  high levels of white blood cells called invariant natural killer T cells (iNKT) in their lungs and intestines.These cells release proteins that cause inflammation and attract more inflammatory white blood cells -- bad news, because inflammation plays a role in many autoimmune diseases including asthma and colitis.

So, how can parents “harness” the healthy lessons of the hygiene hypothesis? Nobody’s suggesting families give up clean water, antibiotics, vaccines or even household cleaners. But maybe it’s OK to relax about the dog hair on the steps, the germy kids at daycare, the baby crawling across a less-than-spotless floor and messy backyard fun -- from mucking around in the sandbox and making mud pies to picking dandelions and rolling down hills.