Parents who suck on their child's pacifier to clean it may be inadvertently reducing that child's risk of developing allergies, researchers found.

At age 18 months, children born to parents who said they cleaned their child's pacifier with their mouths were less likely than those born to parents who cleaned the pacifier in other ways to have asthma (odds ratio 0.12, 95% CI 0.01 to 0.99) and eczema (OR 0.37, 95% CI 0.15 to 0.91), according to Bill Hesselmar, MD, PhD, of Queen Silvia Children's Hospital in Gothenburg, Sweden, and colleagues.

At age 36 months, the association remained for eczema (HR 0.51, 95% CI 0.26 to 0.98), but not for asthma, the researchers reported online in Pediatrics.

The findings suggest that the transfer of oral microbes from the parent to the infant could be responsible for modifying the allergy risk, and indeed, the make-up of the bacteria in the infants' saliva distinguished between those with parents who did and did not suck on their child's pacifier.

"Exposure of the infant to parental saliva might accelerate development of a complex oral/pharyngeal microbiota that, similar to a complex gut microbiota, might beneficially affect tolerogenic handling of antigens by the oral/pharyngeal lymphoid tissues," Hesselmar and colleagues wrote. "Moreover, oral bacteria are swallowed and hence also affect the composition of the microbiota in the small intestine, which may in turn regulate tolerance development in the gut."

The study joins others in suggesting that changing an infant's microbiota may influence the development of allergic disorders, according to Amal Assa'ad, MD, of Cincinnati Children's Hospital Medical Center, and tells clinicians "that we have to let nature play out a little bit and not be too clean and not be forming artificial barriers in the connection between the mother and the infant and the parents and the infant."

"We have to at some point reach a balance where we're making sure we're not predisposing [infants] to infections at the same time [we're making] sure we're giving them what they were naturally expected to get from the parents ... so we end up with a balanced body that doesn't veer towards allergies and doesn't veer towards serious infections and harm," said Assa'ad, who was not involved in the study.

It has been shown previously that infants with less diversity among their gut microbes have a greater risk of developing allergies, which suggests that exposure to commensal microbes could have beneficial effects on the immune system.

Hesselmar and colleagues explored whether exposure coming from saliva transferred on a pacifier was related to allergy development among 184 full-term infants born to women enrolled in the AllergyFlora study. For the study, researchers mostly approached families with at least one allergic parent, a situation present for 80% of the participants.

When the children were 6-months-old, the parents were interviewed about pacifier use and cleaning practices and other information.

Overall, 74% of the infants used a pacifier in the first 6 months of life.

For those infants, 83% of parents reported using tap water to clean the pacifiers, 54% reported boiling, and 48% reported using their mouths (parents could select more than one option).

By 18 months, 25% of the children had eczema, 5% had asthma, 15% had sensitization to food antigens, and 2% had sensitization to inhaled antigens.

Pacifier use itself was not associated with the risk of any of those outcomes, but parental sucking on the pacifier was related to a lower likelihood eczema and asthma. Sensitization was not related to pacifier cleaning practices.

Both delivery mode -- cesarean or vaginal -- and maternal education were related to the likelihood of a parent sucking on the pacifier, and after adjustment for both variables the relationship with eczema remained significant (OR 0.27, 95% CI 0.086 to 0.819).

Vaginal delivery and parental pacifier sucking were independently associated with a reduced likelihood of developing eczema, with the prevalence of the condition lowest among infants covered by both characteristics (20%) and highest among those covered by neither (54%).

"Thus, vaginal delivery, which is a source for transfer of a complex microbiota from mother to infant and parent and infant sharing of a pacifier might both lead to microbial stimulation, with beneficial effects on allergy development," Hesselmar and colleagues wrote.

A possible concern with transferring a pacifier from a parent's mouth to a child's is the spread of respiratory infection, although there was no difference in the rate of such infections based on pacifier cleaning practices in the current study.

In addition, there is the possibility that cariogenic bacteria can be transferred: "However, caries seems to be unrelated to pacifier use and may even be negatively associated with 'close' salivary contact between infant and parent," the researchers wrote.

They acknowledged that the study was limited by the small sample size and by the difficulty of diagnosing asthma in early childhood, and called for replication in larger studies and in older children.