MANCHESTER, UK—Both allergens and viruses have been known to worsen asthma symptoms. New work demonstrates that allergen exposure and viral infections act synergistically to trigger exacerbations in adults.[1]

Respiratory virus and allergen exposure are each implicated in asthma. “But people tend to look into single factors and look at them in isolation,” Adnan Custovic, MD, PhD, Professor of Allergy at North West Lung Centre in Manchester, United Kingdom, told RESPIRATORY REVIEWS. “What we decided to do is to put all of these things together.”

Said Dr. Custovic, “We selected all who had been admitted into the hospital for asthma over a period of a year, and then we matched them with people with stable asthma, as well as with non-asthmatic inpatients.” Into these three groups, the researchers recruited a total of 178 patients, controlling for sex, age, and smoking status. They tested for viral RNA in nasal lavage samples using polymerase chain reaction, performed skin-prick tests, measured serum immunoglobulin E (IgE) levels in each patient, and surveyed patients’ homes to assess antigen exposure.

Although total IgE levels were higher in asthmatic inpatients than in those with stable asthma (253.7 vs 123.7 kAU/L), both asthmatic groups had higher levels than did non-asthmatic patients (63.2 kAU/L). Asthma patients, whether stable or hospitalized, had similar frequencies of positive skin-prick tests and similar titers of antibodies against mite, cat, and dog; both measures of sensitization were lower in the non-asthmatic controls.

BED ALLERGENS, VIRUSES CAN EXACERBATE ASTHMA

Bedrooms of patients admitted for asthma contained significantly higher quantities of mite, cat, and dog antigens than did the bedrooms of stable asthma patients; home exposures were similar for stable asthmatic and non-asthmatic controls. Interestingly, those admitted with asthma were significantly more likely to be both sensitized and exposed to a particular antigen (34 of 55; 66%) than were stable asthma patients (20 of 54; 37%) or non-asthmatic controls (eight of 53; 15%).

Asthma patients were also more likely to carry a virus: Picornavirus or coronavirus was cultured from 26.2% (16 of 61) of hospitalized and 17.5% (10 of 57) of stable asthma patients, but from only 8.5% (five of 59) of non-asthmatic controls.

Risk analysis revealed that when examined in isolation, viral infection’s impact was insignificant, whereas sensitization and exposure to an allergen significantly increased an asthma patient’s risk for hospitalization. Controlling for sensitization, allergen exposure, and viral infection, the researchers calculated an odds ratio (OR) of hospitalization for sensitized and exposed asthma patients of 2.3. Adding together the risk linked to sensitization/exposure with the risk attributable to viral infection alone might be expected to yield the risk for patients with both factors. “But when these factors coincide, the risk [associated with] these two factors together is considerably higher than the simple summing up of the two risks,” Dr. Custovic remarked: Despite virus’ statistical insignificance as a risk factor, viral infection combined with sensitization/exposure raised the OR to 8.4.

Linking risk factors with outcome does not necessarily suggest an effective intervention, Dr. Custovic cautioned, but reducing risk factors seems a reasonable approach. “There’s a lot of work to be done in devising effective strategies to reduce personal exposure and to control viruses,” he argued. “It’s only then that we can really use these to make asthmatics better.”

—Mimi Zucker, PhD