BALTIMORE—Does exposure to rats increase the risk of asthma among inner-city residents? Until recently, this question has received surprisingly little attention. However, new data suggest that rat allergen is common in inner-city homes and that sensitization to it significantly increases asthma morbidity.[1]

Researchers from Johns Hopkins University made these discoveries after reanalyzing house dust samples that were collected during the National Cooperative Inner City Asthma Study (NCICAS). Rat allergen was present in about one third of the homes; children who were both exposed and sensitized to rat allergen had a markedly higher rate of unscheduled medical visits, hospitalizations, and days with slowed activity than did other children.

NCICAS

The NCICAS was a large, multicenter evaluation of the home environments of 1,528 children with asthma from eight inner-city areas. Although rat allergen exposure and sensitization was not formally evaluated in NCICAS, skin tests performed at baseline showed that 19% of children were sensitized to rat allergen.

During the original investigation, only half of the participants had home visits during which dust samples were collected; samples were taken from different rooms in the homes. Furthermore, valid skin test results, complete home characteristics data, and information on the adverse sequelae of asthma were available for only some children. As a consequence, the final population in the Johns Hopkins reanalysis consisted of 480 children.

A monoclonal-based ELISA was used to measure levels of rat allergen (Rat n 1) in dust samples. In the NCICAS samples, Rat n 1 was present in 21% of bedrooms, 27% of living rooms, and 19% of kitchens. For all homes with analyzable dust, 33% had detectable Rat n 1 in any room. There was no correlation between rat and mouse allergens in any samples.

Forty-two percent of the children in this study had a family history of asthma, 58% lived in homes with at least one smoker, and 21% were sensitized to rat allergen. Evidence of infestation correlated positively with detectable levels of allergen for both mice and rats. The prevalence of sensitization to rat allergen did not differ between children with detectable rat allergen in the bedroom or living room and those without.

HEALTH EFFECTS OF SENSITIZATION, EXPOSURE

The children with complete data on asthma morbidity were grouped according to their sensitivity and exposure to rat allergen. Those with both sensitivity and exposure to rat allergen had a hospitalization rate that was almost six times higher than that among children neither exposed nor sensitized. Children who were exposed or sensitized (but not both) had intermediate hospitalization rates.

The number of unscheduled doctor visits and days of reduced activity due to asthma were more than 50% higher in the sensitized, exposed group than in the children who were neither exposed nor sensitized. Again, the other children had intermediate rates for these two variables.

EXPOSURE IS COMMON

This study suggests that exposure to rat allergen is common, although not as common as exposure to mouse allergen, which was detected in 95% of homes. In fact, the ranges of rat allergen in the study homes were more than a hundredfold lower than those of mouse allergen in the same homes. The authors proposed that the difference in allergen levels was due to the nesting habits of mice and rats—with rats typically living outdoors and foraging indoors, while mice live indoors near a food source.

“We do not know how rat allergen gets inside homes,” commented Peyton A. Eggleston, MD, Professor of Pediatrics at Johns Hopkins University. “But we know that laboratory rat pelt is contaminated with allergen—making it likely that rats passively contaminate homes when they come in foraging for food. Alternatively, allergen may be brought in by families from contaminated areas outside their homes.”

Despite higher levels of mouse allergen exposure, sensitization to rat allergen was more common. Exposure and sensitization to rat allergen also had a significant impact on asthma morbidity. This association remained significant after the investigators controlled for cockroach sensitization and exposure, and for other environmental and psychosocial factors. Dr. Eggleston, who is also Professor of Environmental Health Sciences at Johns Hopkins’ Bloomberg School of Public Health, noted that the reasons for increased sensitization to rat allergen rather than mouse are unknown. However, he added, “sensitization and exposure [to rat allergen] in homes should be confirmed in other studies.”

Rat allergen reduction measures have the potential to significantly reduce asthma morbidity in asthma patients who live in the inner city and could also reduce the frequency of health care use in this group. The present study is one of the first reports of rodent allergen in homes, said Dr. Eggleston. To have the greatest impact on public health, he concluded, “We need interventional trials to determine how best to reduce exposure and what health benefits current recommendations might produce.”

—Gale Jurasek

Reference
1. Perry T, Matsui E, Merriman B, et al. The prevalence of rat allergen in inner-city homes and its relationship to sensitization and asthma morbidity. J Allergy Clin Immunol. 2003;112:346-352.

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