Debating Early Allergen Exposure for the Prevention of Asthma

A large audience of allergists and asthma specialists attended a debate-like session at the 2018 AAAAI Annual Meeting to explore whether exposure to allergens early in life helps prevent future asthma: Miles Weinberger, MD, FAAAAI, a pediatric pulmonologist in Encinitas, California, offered argued in favor of this thinking, and Thomas A.E. Platts-Mills, MD, PhD, FAAAAI, of the University of Virginia, argued against it.

Dr. Weinberger presented a significant amount of literature to support the hypothesis. In a 2018 study published in the Journal of Allergy and Clinical Immunology by Valovirta et al, the findings suggest that treatment with the SQ grass sublingual immunotherapy tablet reduced risk of asthma symptoms and need for asthma medication. They also found that the treatment had a positive, long-term clinical effect on rhinoconjunctivitis symptoms and medication.

Another article by Stokholm et al published in 2017 in the Journal of Allergy and Clinical Immunology examined infants born to mothers with a history of asthma and looked at household cat and dog history, following patients for 12 years. They found that cat and/or dog exposure from birth was associated with a lower prevalence of asthma among children with a certain genotype. In addition, cat—but not dog—allergen levels were inversely associated with asthma development in children with another specific genotype.

One of the studies cited by Dr. Weinberger was published by Dr. Platts-Mills et al in 2001 in the Lancet: They found that exposure to cat allergen could produce an antibody response without sensitization or risk of asthma. The authors said the response is a form of tolerance and could explain observations that animals in a household can decrease asthma risk.

A study by Perzanowski et al published in 2002 in the American Journal of Respiratory and Critical Care Medicine found that living with a cat was inversely related to having a positive skin test for a cat allergy and incidence of physician-diagnosed asthma. The effect on asthma was most pronounced among children with a family history of asthma. In addition, the evidence also suggested development of an immune response by many of the children exposed to cats at home. Weaker protective trends were found with dog ownership.

Dr. Weinberger also presented studies to support his argument relating to microbes: A 2011 by Ege et al in the New England Journal of Medicine found that children living on farms were exposed to a wider range of microbes than were children in the reference group, explaining the inverse relation between asthma and growing up on a farm. Similarly, a 2016 study by Stein et al published in the New England Journal of Medicine found that dust in Amish people’s households had endotoxin levels 6.8 times higher than dust from Hutterite households, leading to an immune response that lowers asthma risk in Amish children.

“It all depends on what you mean by the word ‘prevents’” Dr. Platts-Mills said, beginning his arguement. “It also depends on what you mean by ‘allergen.’” For example, “There is zero evidence that early exposure to dust mite prevents asthma,” he said.

After a brief review of the major indoor allergens, Dr. Platts-Mills admitted there is “beautiful data” on cat allergens, but he said those findings cannot be applied to other indoor allergens, including dogs. “Finding on one study that early exposure to one allergen in one population cannot be a blanket statement,” he said. Furthermore, he added, there are very few studies showing any protective effect from early exposure to dog.

He cited literature that should caution against the hypothesis that early allergen exposure can reduce asthma risk. One example was published in 1990 by Sporik et al in the New England Journal of Medicine: It found that exposure during early childhood to dust mite allergens was an important determinant of subsequent development of asthma. Another study published by Soto-Quiros et al in the Journal of Allergy and Clinical Immunology in 2012 showed that high titers of immunoglobulin E antibody to dust mite allergen were common and significantly increased the risk for acute wheezing provoked by rhinovirus among asthmatic children.

Overall, Dr. Platts-Mills rested his argument on the paucity of research studies that have shown each allergen’s protective effects against asthma. He reviewed numbers of studies for each of the major allergens, including:

  • Only 2 studies out of about 500 examining dust have shown protective effects
  • Studies of roach: 1 of 60
  • Studies of cat: 90 of 120
  • Studies of dog: 52 of 100
  • Studies of mouse: 0 of 30
  • No studies have produced quant data on indoor exposure in early life
  • Many studies focus on high exposure of allergens but not necessarily during early life

Ultimately, Dr. Platts-Mills said allergists need much more proof before they can say with confidence that early exposure to allergens can prevent asthma.

Presentation 2551: Early Allergen Exposure Prevents Asthma