Subgroup differences in the associations between dog exposure during the first year of life and early life allergic outcomes

Background: The effect of dog exposure on the risk of children developing allergic disease remains controversial. Many analyses have not considered that associations may vary within population subgroups. Objective: To examine whether associations between living with a dog in the first year of life and allergic outcomes vary within subgroups selected a priori (race, gender and delivery mode). Methods: Black (n = 496) and White (n = 196) children enrolled in the WHEALS birth cohort study had a clinical examination at age 2 years to assess eczema and allergen-specific IgE (sIgE) and perform skin prick testing (SPT). Whether the child lived with an indoor dog in the first year of life was assessed through interview, as was doctor diagnosis of asthma at ages 3–6 years. Results: Living with a dog was associated with decreased odds of having ≥ 1 positive SPT (OR = 0.56, 95% CI: 0.34, 0.91) and having eczema (OR = 0.34, 95% CI: 0.20, 0.60). The association with SPT was stronger in those children born via caesarean section (c-section) vs. vaginally (OR = 0.29, 95% CI: 0.12, 0.74 vs. OR = 0.76, 95% CI: 0.43, 1.37, respectively, interaction P = 0.087) and in those who were firstborn vs. not (OR = 0.27, 95% CI: 0.11, 0.67 vs. OR = 0.82, 95% CI: 0.45, 1.47, respectively, interaction P = 0.044). The association with eczema was stronger in children born vaginally compared with those born via caesarean section (OR = 0.17, 95% CI: 0.06, 0.43 vs. OR = 0.65, 95% CI: 0.31, 1.35, respectively, interaction P = 0.025) and was stronger in Black vs. White children (OR = 0.30, 95% CI: 0.15, 0.61 vs. OR = 0.78, 95% CI: 0.29, 2.11, respectively, interaction P = 0.12). Dog keeping was not significantly inversely associated with having ≥ 1 elevated sIgE and only approached statistical significance with asthma. Conclusions and Clinical Relevance: Results likely vary between studies due to variability of specific exposure–outcome associations in subgroups defined by other factors as well as the relative distributions of those subgroups. Important allergic disorder associations will be missed without subgroup analyses.