High-level exposure throughout the first year of life had the greatest protective effect, with those genetically predisposed to poorer lung function benefiting the most.
Early exposure to dog allergen was significantly associated with a reduced risk of childhood asthma and higher lung function at age five years, particularly for those at genetic risk of poorer lung function, a new study has found.
While higher dog allergen (Can f1) exposure during infancy was significantly associated with reduced asthma risk at age five (OR 0.52), no significant associations were observed for cat allergen, dust mite allergens, endotoxins or polysaccharides found in bacterial cell walls ((1-3)-β-d-glucan).
The protective effect was stronger in children with exposure to high Can f1 with low exposure to all other allergens (OR 0.29).
Notably, co-exposure to high levels of dog allergen and endotoxin was not associated with a reduced risk, suggesting that the protective association with dogs may not be attributable to increased microbial exposure.
More than 1000 children from the Canadian CHILD Cohort Study – a multi-centre longitudinal, prospective, general population birth cohort study which enrolled 3454 pregnant women between 2008 and 2012 – were included in the subcohort analysis.
Half were male, 70% were white and the mean age of spirometry measurement was 5.1 years. Skin prick tests for common allergens were performed at one, three and five years of age, and 6.6% of the cohort developed asthma by age five.
The researchers collected dust samples from the children’s homes when they were three months old, from their mattress, bedroom floor and the floor in the most-used living area (as identified by the parents).
They adjusted for confounders such as sex, annual family income, parental history of asthma and hours spent away from home per week at age of allergen collection. They also applied previously published cut-offs for high and low exposure to the allergens (2.11ng/mg for Can f1).
No established exposure level parameters exist for endotoxins and (1-3)-β-d-glucan, so high exposure was categorised as the 75th percentile of the cohort.
High level exposure to Can f1 occurred in 30% of the infant population, 13% of which was exclusive (i.e. no other high-level allergen exposure). High exposures were also noted for cat allergen (29%), dust mite allergens (14%) and endotoxin/glucan (24%). High level co-exposure of Can f1 and endotoxin was found in 11% of the group.
“Young infants spend most of their time indoors, positioning early-life indoor allergen and endotoxin exposure as one of the most important modifiable factors in the development of asthma,” the authors wrote.
While dog ownership at any single time point (assessed at three, six and 12 months) was associated with a lower risk of asthma at age five years (OR 0.56), continuous dog ownership in the first year of life was associated with a greater risk reduction (OR 0.33).
“We observed that parental-report of dog ownership was protective against adverse outcomes, but contrary to our objective measures of Can f1 exposure, dog ownership was close to, but did not reach, formal significance for asthma (p=0.09) or lung function (p=0.10),” the authors wrote.
“However, in an exploratory analysis using continuous dog ownership… we observed a statistically significant association with reduced risk of asthma at age five [years].
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“This distinction in exposure definition may partially explain divergent findings among published studies, noting that the amount of allergen may be affected by factors such as the area where the pet spends most of their time (living room, child’s bedroom, etc.), the frequency and effectiveness of room cleaning, presence or absence of carpeting, etc.”
The researchers also created a polygenic risk score (PGS) for each participant, based on a genome-wide association study of over 400,000 individuals in the UK Biobank and SpiroMeta Consortium.
Children with low PGSs were genetically predisposed to poorer lung function. However, researchers found that dog allergen exposure in infancy could potentially offset this somewhat; those with low PGSs who were exposed to high levels of Can f1 had higher lung function measures at age five than those with low exposure.
Similarly, children with low PGSs who were co-exposed to high levels of Can f1 and endotoxin demonstrated higher spirometric measures, suggesting a potential gene-environment interaction influencing lung function.
Overall, children with greater early-life exposure to dog allergen had significantly higher FEV1 z-scores (β=0.23) at age five years than those with low exposure, irrespective of asthma status.
The findings raise the possibility that early-life exposure to dog allergen could influence longer-term respiratory health trajectories.
Impaired lung function in childhood is a key predictor of abnormal patterns of lung growth and a higher risk of developing airflow obstruction and COPD in early adulthood, the authors noted.
“[However,] little is known about effective measures to prevent lung function decline,” they wrote.
“The current analysis suggests that the protective effect of Can f1 on asthma was derived from improved lung function.”
