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Q. What is the evidence behind reports that having a pet reduces the risk of asthma?
A. The current literature does not allow one to give didactic advice on pet ownership to families who wish to reduce the risk of asthma in their children. The literature is confusing and practically any association between pet ownership and asthma can be supported by one or another study. Definitions of pets vary and many studies are cross-sectional, relying on recall of past events.
Also, families with a history of asthma often avoid keeping furry pets to try to reduce the risk in their children. Studies often report different findings for allergic sensitisation than for wheeze.
The results will also reflect the population studied there is evidence from Sweden that high-risk children can become sensitised from allergen exposure in the community whether or not they keep pets at home, and will wheeze. Ownership of pets may therefore be more relevant in lower-risk children.
The Children's Respiratory Study from Arizona reported a reduced rate of frequent wheeze in those with dogs, with no effect for cats and no effect on allergic sensitisation.
Another prospective study of American children found increasing numbers of pets (cats and dogs) were associated with a reduced risk of allergic sensitisation, but had no effect on asthma diagnosis.
Other similar studies from the Isle of Wight and Norway have reported no association between pet ownership and asthma diagnosis or wheeze. But there are other studies that have reported an increase in asthma among previous pet owners.
The European Community Respiratory Health Survey found that among those without cats, sensitisation to cat was more common in countries with higher rates of cat ownership.
There is some evidence that children exposed to high levels of cat allergen may develop IgG to cat rather than IgE, and this IgG response is not associated with asthma. There are several studies that will give clearer answers to the question of whether pet ownership increases, reduces or has no effect on asthma development.
Angela Simpson, honorary consultant in respiratory medicine, North West Lung Centre, Wythenshawe Hospital, Manchester