Increased exposure to cat allergen is associated with greater bronchial responsiveness (BR) in people with certain common allergies, even if they are not specifically allergic to cats. This suggests that reduced exposure to cats may be beneficial for allergic individuals, regardless of their specific allergies.
"This was an unexpected finding," said Susan Chinn, D.Sc., lead author of the study. "We presupposed that we would find increased responsiveness only in those individuals who were exposed to cat allergen and whose blood tests showed that they were allergic to cats. But our study suggests that all allergic individuals have signs of asthmatic responses if exposed to cat allergen, even if blood tests show that they are not allergic to cats."
Dr. Chinn, of the Imperial College in London, and 12 other researchers reported their findings in the first issue for July 2007 of the American Journal of Respiratory and Critical Care Medicine, published by the American Thoracic Society.
The study examined cross-sectional data from 1,884 participants in 20 centers in the European Community Respiratory Health Survey (ECRHS) follow-up, which included measurements of house dust mite and cat allergen in mattress dust samples, and data on IgE sensitization to four major allergens—cat, house dust mite, Cladosporidium (a common mold) and timothy grass.
The researchers used the "objective measure of choice in epidemiological studies on asthma"—BR in response to a methacholine challenge— to analyze the interaction between exposure to house dust mite and cat allergen and prior allergic sensitization. Because the study included complete data on nearly 2,000 individuals across 20 centers in Europe, researchers were able to exclude potentially confounding effects.
"Our primary results showed no correlation between levels of house mite dust and BR among individuals with sensitization to any of the four tested allergens," said Dr. Chinn. "But even moderate exposure to cat allergen resulted in significantly greater responsiveness."
Among people with any sensitization, the difference in BR between those who had low versus high exposure to cat allergen was almost as great a difference as that between non-asthmatic and asthmatic individuals in the United Kingdom centers of the ECRHS.
This study supports and clarifies previous research that has found that asthma is strongly related to indoor allergens and that patients with specific sensitizations exhibit greater BR in response to exposure to the allergens to which they are sensitized. However, the interaction between sensitization of any kind and current exposure was unexpected.
More than one in four of the individuals included in the ECRHS were sensitized to at least one of the allergens tested, indicating that avoidance of cat exposure would be beneficial to a much wider population than previously expected. Furthermore, cat allergen levels were ubiquitous in cat-owning communities, and their results showed effects of cat allergen exposure at lower levels than generally regarded necessary to produce a measurable result.
"Based on the current research, it appears that many individuals could benefit from reduced cat ownership and exposure," says Dr. Chinn. "However, because the findings were unexpected, it is important that results are replicated in other studies before firm recommendations are made."
The researchers could not rule out the possibility that cat allergen exposure or cat ownership could be a proxy for exposure to endotoxin, known to be an immune stimulant marginally associated with asthmatic symptoms, which is found in higher concentrations in cat owners' homes.