To determine urban-rural differences in asthma prevalence and identify home environment factors which may explain these differences.

In 2002/03, a survey was sent to 12556 households of Manitoba children born in 1995, asking parents if they or their 7-year old children had asthma, and whether smokers or pets were present in the birth home. Survey responses were linked to health care records within the first year of life. The likelihood (odds ratio, OR) of asthma, adjusted for family history of atopy, was determined according to urban/rural location, and birth exposure to tobacco smoke, pets and lower respiratory tract infections (LRIs).

3564 (28.4%) of the surveys were returned. 8% of children in northern rural areas, 10% in southern rural areas and 14% in urban areas had asthma. Early life exposures to tobacco smoke (51%), cats (26%), dogs (42%) and LRIs (31%) were highest, and family history of atopy was lowest (34%) among children in the rural north. Asthma was significantly more likely in children with atopic family history (OR=3.61) and children exposed to tobacco smoke (OR=1.44), LRIs (OR=1.88) and cats (OR=1.47). Persistent dog exposure was protective (OR=0.72). Independent of these associations, northern rural children were 40% less likely and those in southern rural areas were 20% less likely to have asthma than urban children.

Despite increased exposure to environmental risk factors, children in northern and southern rural areas were less likely to develop asthma. This effect may be related to genetic predisposition and/or environmental exposures not measured.