Pets have hidden benefits Nick David/Getty

Pets really do seem to prevent allergies: the more cats or dogs you live with as an infant, the lower your chance of developing asthma, hay fever or eczema.

Some studies have found that having a pet early in life protects from allergies later in childhood. Bill Hesselmar at the University of Gothenburg, Sweden, and colleagues wondered if having more than one pet would increase the benefit.

They looked at data from two previous studies. The larger of the two included data from 1029 children aged seven to eight. The incidence of allergies was 49 per cent in children who had spent their first 12 months of life in a home with no pets. This fell to 43 per cent in children who as babies had lived with one pet, and 24 per cent for children who had lived with three pets. Two of the children had lived with five pets – neither of them had allergies.


The second study tracked 249 children from birth. After eight or nine years, the rate of allergies was 48 per cent for children who had had no exposure to pets in their first year, 35 per cent for children with exposure to one pet and 21 per cent for children who had lived with two or more pets.

This shows that there is a dose-dependent relationship: more exposure to pets means more protection. That could explain why some previous studies did not find a link. “A dog or a cat that is seldom inside the house, or seldom in close contact with the child, may not be protective,” says Hesselmar. It also rules out the possibility that the link can be explained by selection bias: the result of families with allergies choosing not to have a pet.

Previous studies have found that children who grow up on a farm with livestock have a lower risk of allergies. Hesselmar thinks having multiple pets is like living on a “mini-farm”, with lots of exposure to allergens.

Allergies have been on the rise since the mid-20th century, but we still don’t really know what causes them. Hesselmar thinks that pets carry microbes that stimulate the human immune system so that children don’t become allergic. Spending time with other children and being outdoors in early life also seem to have a protective effect.

Journal reference: PLoS One, DOI: 10.1371/journal.pone.0208472