Mechanistic studies support the potential for mercury (Hg) to alter immunity, including
via in utero exposure. As yet, there are few prospective studies of in utero Hg
exposure and subsequent immune-related outcomes, especially in infancy. We investigated
the association of biomarkers of prenatal Hg exposure and maternal silver-mercury
dental amalgams with the occurrence of infant allergy, respiratory infection, and
respiratory symptoms in the first year of life. The New Hampshire Birth Cohort Study
(NHBCS) ascertained information on infant allergies, infections and symptoms through
telephone interviews at 4, 8 and 12 months postpartum and measured total Hg in maternal
toenails collected at ~28-30 weeks gestation. Information on maternal fish consumption
and presence of dental amalgams was obtained from a questionnaire administered at
study enrollment at 24-28 weeks. A total of 1,321 NHBCS mother-infant pairs had at
least one Hg exposure measure (toenail Hg or information on dental amalgams) and information
on dietary fish intake. Generalized linear models and generalized estimating equation
models with Poisson regression adjusted for potential confounders (maternal age, level
of education, parity, smoking, alternative Healthy Eating Index-2010, infant sex,
gestational age, feeding mode, and day care attendance) were used to assess the association
between infant outcomes and prenatal toenail Hg levels. We subsetted this analysis
on mothers who consumed fish (n=706) as a measure of in utero methylmercury (MeHg)
exposure. Associations between infant outcomes and dental amalgams as a measure of
in utero inorganic Hg exposure were assessed among mothers who did not consume fish
(n= 218). Among women who ate fish during pregnancy, higher maternal toenail Hg concentrations
were associated with an increased risk of lower respiratory infections and respiratory
symptoms requiring a doctor visit among infants age 9 to 12 months (relative risk
(RR) 1.4 (95% CI: 1.1, 1.9) and 1.2 (95% CI: 1.0, 1.4) respectively), whereas a reduced
risk of lower respiratory infections was observed among infants 0 to 4 months of age
(RR = 0.7 (95% CI: 0.5, 1.0). We found little to no evidence of associations of toenail
Hg with upper respiratory infections, allergy or eczema at any age to one year. Among
infants of mothers who did not consume fish, we found an elevated risk of upper respiratory
infections requiring a doctor visit in relation to having dental amalgams during pregnancy
(RR = 1.5 (95% CI: 1.1, 2.1)). Overall, weaker associations were observed with lower
respiratory infections, respiratory symptoms, and medically confirmed allergies, and
there was no association with eczema. Our analyses of a US birth cohort, along with
prior mechanistic work, raise the possibility that gestational Hg exposure through
fish/seafood consumption and dental amalgams may alter respiratory infections and
respiratory symptoms in infants.