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For years, health officials have told parents not to share utensils with their babies or clean their pacifiers by putting them in their mouths, arguing that the practice spreads harmful germs between parent and child. But new research may turn that thinking on its head.

In a study published Monday in the journal Pediatrics, scientists report that infants whose parents sucked on their pacifiers to clean them developed fewer allergies than children whose parents typically rinsed or boiled them. They also had lower rates of eczema, fewer signs of asthma and smaller amounts of a type of white blood cell that rises in response to allergies and other disorders.

The findings add to growing evidence that some degree of exposure to germs at an early age benefits children, and that microbial deprivation might backfire, preventing the immune system from developing a tolerance to trivial threats.

The study, carried out in Sweden, could not prove that the pacifiers laden with parents’ saliva were the direct cause of the reduced allergies. The practice may be a marker for parents who are generally more relaxed about shielding their children from dirt and germs, said Dr. William Schaffner, an infectious diseases expert at Vanderbilt University who was not involved in the research.

“It’s a very interesting study that adds to this idea that a certain kind of interaction with the microbial environment is actually a good thing for infants and children,” he said. “I wonder if the parents that cleaned the pacifiers orally were just more accepting of the old saying that you’ve got to eat a peck of dirt. Maybe they just had a less ‘disinfected’ environment in their homes.”

Studies show that the microbial world in which a child is reared plays a role in allergy development, seemingly from birth. Babies delivered vaginally accumulate markedly different bacteria on their skin and in their guts than babies delivered by Caesarean section, and that in turn has been linked in studies to a lower risk of hay fever, asthma and food allergies. But whether a mother who puts a child’s pacifier in her mouth or feeds the child with her own spoon might be providing similar protection is something that had not been closely studied, said Dr. Bill Hesselmar, the lead author of the study.

In fact, health officials routinely discourage such habits, saying they promote tooth decay by transferring cavity-causing bacteria from a parent’s mouth to the child’s. In February, the New York City health department started a subway ad campaign warning parents of the risk. “Don’t share utensils or bites of food with your baby,” the ads say. “Use water, not your mouth, to clean off a pacifier.”

In the new study, doctors at the University of Gothenburg and elsewhere followed a group of about 180 children from birth. The children were examined regularly by a pediatric allergist, and their parents were instructed to keep diaries recording details about food introduction, weaning and other significant events.

By the age of 18 months, about a quarter of the children had eczema, and 5 percent had asthma. Those whose parents reported at least occasionally cleaning their children’s pacifiers by sucking them were significantly less likely to develop the conditions — particularly eczema — and blood tests showed that they had lower levels of a type of immune cell associated with allergies. Analyses of the children’s saliva also showed patterns that suggested the practice had altered the kinds of microbes in their mouths.

The researchers then looked to see if the method of childbirth provided any additional protection.

It did. The children who were delivered through Caesarean section and whose pacifiers were rinsed or boiled had the highest prevalence of eczema, nearly 55 percent. The group with the lowest prevalence of eczema, about 20 percent, were born traditionally and had parents who cleaned their pacifiers in their mouths.

But are these parents also transmitting harmful infections to their children?

The bacterium that causes dental cavities, Streptococcus mutans, is highly contagious. Studies show that children can be infected at a very young age, and that the strain they pick up is usually one that they get from their mothers. That is why health authorities tell parents to do things that can lower the rate of transmission to their children, like not sharing utensils or putting their mouths on pacifiers.

But Dr. Joel Berg, president of the American Academy of Pediatric Dentistry, said those efforts are misguided, since parents are bound to spread germs simply by kissing their children and being around them. “This notion of not feeding your baby with your spoon or your fork is absurd because if the mom is in close proximity to the baby you can’t prevent that transmission,” he said. “There’s no evidence that you can avoid it. It’s impossible unless you wear a mask or you don’t touch the child, which isn’t realistic.”

Dr. Berg, who does salivary research at the University of Washington, said the new findings underscore something he has been telling his patients for years, that “saliva is your friend.” It contains enzymes, proteins, electrolytes and other beneficial substances, some of which can perhaps be passed from parent to child.

“I think, like any new study, this is going to be challenged and questioned,” he said. “But what it points out pretty clearly is that we are yet to fully discover the many and varied benefits of saliva.”