University of Michigan
Department of Pediatrics
Evidence-Based Pediatrics Web Site

Question

• Does exposure to dogs and cats in early years of life decrease risk of allergic sensitization and asthma?

Clinical Bottom Lines

1. Exposure to dogs and cats in the first year of life may reduce subsequent risk of allergic sensitization to multiple allergens during childhood, including non-pet allergens. (1)
2. Children exposed to two or more dogs or cats in the first year had less atopy than children exposed to one pet.
3. Recommending pet exposure to families may increase the number of pet-related injuries.
   

Summary of Key Evidence

1. 474 children were followed from birth to age 6 to 7 years in a prospective birth cohort study. At age 6 to 7 years, patients underwent skin prick testing, serum allergen-specific IgE, and pulmonary function tests with methacholine airway responsiveness. (1)
   
2. The prevalence of any skin prick positivity at age 6 to 7 years was 33.6% with no dog or cat exposure in the first year of life, 34.3% with exposure to 1 dog or cat and 15.4% with exposure to 2 or more dogs or cats (P=.005).
3. The prevalence of any positive allergen-specific IgE test result was 38.5% with no dog or cat exposure, 41.2% with exposure to 1 dog or cat, and 17.9% with exposure to 2 or more dogs or cats (P=.003).
4. When boys and girl were considered separately, exposure to a single dog or cat in the first year of life was associated with an increased prevalence of atopy and seroatopy in girls while both outcomes declined in boys exposed to a single dog or cat.
5. A pattern of decreasing total serum IgE with increased exposure to dogs and cats was seen, but only among boys and among children with a parental history of asthma, not among girls or among children without a parental history of asthma.
6. 7% of the children in this study had current asthma. The prevalence of current asthma was lower in boys who had been exposed to 2 or more dogs or cats in infancy compared with no exposure (5.1% vs 11.8%), but was not statistically significant (p=.43) and no difference was seen for girls.
7. When relationship between maternal and paternal history of asthma, allergies and hayfever and presence of dogs or cats in the household, no significant associations were found. Adjustments were made for cord serum IgE concentration, sex, older siblings, parental smoking, parental asthma, bedroom dust mite concentration at 2 years and current pet ownership.
   

Additional Comments

• A systematic review of the literature through 1999, (Apelberg et al) concluded that previous exposure to dogs and cats increased the risk of asthma and wheezing in children older than 6 years. (6)
• In a birth cohort of 2531 children followed to age 4 years , Nafstad et al found that being exposed to pets in early life reduced the risk of asthma (OR, 0.7; 95% CI 0.4-1.0) and allergic rhinitis (OR, 0.6; 95% CI 0.4-1.0). (4)
• In a birth cohort of 1246 children followed to the age of 13, Remes et al reported that children who had 1 or more dogs in the home at birth were significantly less likely to develop frequent wheeze than children without early dog exposure. Neither early exposure to dogs or to cats was associated with skin prick positivity or total serum IgE concentrations. (5)
• In a birth cohort of 448 children with at least one parent with a history of atopy, Celedon et al found that among children with no maternal history of asthma, early exposure to a cat allergens was associated with a reduced risk of wheezing between the ages of 1 and 5 years. Among children whose mothers did have a history of asthma, however, such exposures were associated with an increased risk of wheezing at or after the age of 3 years. There was no association between wheezing and exposure to dog or dog allergens. (2)
• One hypothesis is that high levels of allergen exposure induce a modified T helper cell type 2 (TH2) response with production of allergen specific IgG and IgG4 antibodies without allergic sensitization.
• A second hypothesis is that bacterial endotoxin exposure (associated with household pets) shifts the developing immune system away from a TH2 type pattern of response, which favors development of allergic sensitization, toward a TH1-type response which promotes cellular immunity.
  

Citation

1. Ownby DR, Johnson CC, Peterson EL. Exposure to dogs and cats in the first year of life and risk of allergic sensitization at 6 to 7 years of age. JAMA. 2002;288:963-972.
2. Celedon JC, Litonjua AA, Ryan L, Platts-Mills T, Wiess ST, Gold DR. Exposure to cat allergen, maternal history of asthma, and wheezing in first 5 years of life. Lancet. 2002;360:781-782.
   
3. Nafstad P, Magnus P, Gaarder PI, Jaakkola JJK. Exposure to pets and atopy-related diseases in the first 4 years of life. Allergy. 2001;56:307-312.
   
4. Remes ST, Castro-Rodriguez JA, Holberg CJ, Martinez FD, Wright AL. Dog exposure in infancy decreases the subsequent risk of frequent wheeze but not atopy. J Allergy Clin Immunol. 2001;108:509-515..
   
5. Apelberg BJ, Aoki Y, Jaakkola JJK. Systematic review: exposure to pets and risk of asthma and asthma-like symptoms. J Allergy Clin Immunol. 2001;107:455-460.
   
6. Platts-Mills TA. Paradoxical effect of comestic animals on asthma and allergic sensitization. JAMA. 2002;288:1012-1014.
   

CAT Author: Jocelyn Schiller, MD

CAT Appraisers: John Frohna, MD

Date appraised: November 25, 2002