BACKGROUND: Early-life exposure to household pets has the capacity to reduce 
risk for overweight and allergic disease, especially following caesarean 
delivery. Since there is some evidence that pets also alter the gut microbial 
composition of infants, changes to the gut microbiome are putative pathways by 
which pet exposure can reduce these risks to health. To investigate the impact 
of pre- and postnatal pet exposure on infant gut microbiota following various 
birth scenarios, this study employed a large subsample of 746 infants from the 
Canadian Healthy Infant Longitudinal Development Study (CHILD) cohort, whose 
mothers were enrolled during pregnancy between 2009 and 2012. Participating 
mothers were asked to report on household pet ownership at recruitment during 
the second or third trimester and 3 months postpartum. Infant gut microbiota 
were profiled with 16S rRNA sequencing from faecal samples collected at the mean 
age of 3.3 months. Two categories of pet exposure (i) only during pregnancy and 
(ii) pre- and postnatally were compared to no pet exposure under different birth 
scenarios.
RESULTS: Over half of studied infants were exposed to at least one furry pet in 
the prenatal and/or postnatal periods, of which 8% were exposed in pregnancy 
alone and 46.8% had exposure during both time periods. As a common effect in all 
birth scenarios, pre- and postnatal pet exposure enriched the abundance of 
Oscillospira and/or Ruminococcus (P < 0.05) with more than a twofold greater 
likelihood of high abundance. Among vaginally born infants with maternal 
intrapartum antibiotic prophylaxis exposure, Streptococcaceae were substantially 
and significantly reduced by pet exposure (P < 0.001, FDRp = 0.03), reflecting 
an 80% decreased likelihood of high abundance (OR 0.20, 95%CI, 0.06-0.70) for 
pet exposure during pregnancy alone and a 69% reduced likelihood (OR 0.31, 
95%CI, 0.16-0.58) for exposure in the pre- and postnatal time periods. All of 
these associations were independent of maternal asthma/allergy status, 
siblingship, breastfeeding exclusivity and other home characteristics.
CONCLUSIONS: The impact of pet ownership varies under different birth scenarios; 
however, in common, exposure to pets increased the abundance of two bacteria, 
Ruminococcus and Oscillospira, which have been negatively associated with 
childhood atopy and obesity.