Children with a dog in the home at age one had a significantly reduced risk of eczema at the age of four, but children who had a cat were more likely to have the ailment at the same age, the study found. Dog ownership also conferred protection against becoming allergic to cats.
"It's speculative, but possible that the protective effect is due to a sort of natural immunotherapy where children who are exposed to dogs become tolerant over time, in the same way that people on allergy shots develop tolerance to allergens," said study author Dr Tolly Epstein, an assistant professor in the division of immunology, allergy and rheumatology at the University of Cincinnati Medical School.
Or, dog allergens may have other effects on the immune system that are not yet understood, she added.
The study was done using newborns in the Cincinnati area whose parents had allergies or eczema, making the children more likely to develop the condition. Skin tests were done to see which infants were allergic to dogs and cats, regardless of whether their families had either.
Eczema is an itchy skin inflammation and may be associated with allergies (atopic eczema) or not (non-atopic eczema), Epstein explained. Between 15% and 30% of children have had eczema, which can come and go, or disappear by a certain age. Both genetics and environment are thought to play key roles in the development of the condition.
Epstein said eczema rates have risen dramatically in the past 30 years, and researchers want to understand the causes.
The study found that children who tested positive for a dog allergy and did not live with a dog had four times the risk of getting eczema than those who tested positive and did own a dog by age four. People can test positive for an allergy but not have any symptoms, according to experts.
The higher the dog allergen levels were in the homes, the lower the risk was for the child developing eczema by age four, according to the study.
Most of the 636 children in the study were white. Among the 131 black children, few had dogs as pets, but those whose families got a cat by the time they were age one were 12 times more likely to have eczema at age four. However, due to the small numbers, the results were not significant.
The study also looked at the association between eczema and eggs, milk and nuts, some of the most common food allergies in infants. Some experts recommend delaying common allergic foods as a strategy to protect children against allergies, but the study findings did not support that.
"We tend to be so focused on food allergies with young children, but the study showed aero-allergens [airborne allergens such as pet dander or auto emissions] may be more important than has been previously understood," Epstein said.
The study found that delaying the introduction of eggs into infants' diets may have no impact on their risk for getting eczema in later years, with some indication that it benefited babies when introduced early. However, those findings were not statistically significant. Findings relating to nuts also were inconclusive.
Findings not definitive
Diet guidelines for infants recommend no solid foods until six months, with serial introduction after that to monitor the effect, said Dr Wanda Phipatanakul, an associate physician at Children's Hospital in Boston.
Phipatanakul said the study, published in an upcoming print issue of the Journal of Pediatrics, was carefully done, but was not definitive. Other research had shown conflicting results on the impact of cats and dogs, she said.
"The jury is still out," said Phipatanakul, who is also an assistant professor of pediatrics at Harvard Medical School. "I don't think anyone, including the authors, is saying to go get a few dogs, or don't get a cat to reduce your risk."
People with allergies should avoid what causes their allergies, "or you will keep getting symptoms," she said.
"They did a lot of advanced analyses and looked at it [the data] in detail," Phipatanakul said, adding that she encourages more research to help doctors "learn more and employ better interventions or strategies."
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