certain rodents, such as hamsters, gerbils, and guinea pigs, may contribute to household allergen levels because they have become increasingly popular household pets. At least 35,000 individuals are exposed to rodent allergens because they work in scientific investigations or breed and care for rodents. In the United States, approximately 11–15 percent of laboratory workers are allergic to rodents (Slovak, 1987); many who become allergic are forced to seek alternative employment. Fifty-five percent of them are sensitive to two or more species; the majority (37–75 percent) are sensitive to rats, mice, and rabbits, whereas 24–33 percent are sensitive to guinea pigs. Patients with atopy may develop symptoms more rapidly than those without it. Moreover, patients with a family history of allergy and positive skin tests to other environmental allergens may be at risk for the development of asthma (Sjostedt and Willers, 1989). Hypersensitivity pneumonitis resulting from exposure to rodent proteins is rare.

Two major allergens have been found in rat urine, Rat n I (α-2-euglobulin) and prealbumin. Skin testing for both allergens shows an equal degree of frequency in sensitized patients. Euglobulin is a major component of pubertal male rat urine but is barely detectable in the urine of females (Knysak, 1989).

In rat rooms and during disturbance of rat litter, a large portion of the urinary allergen is airborne; particles are approximately 7 μm in diameter. Air sampling measurements during certain laboratory activities have indicated that feeding or cleaning of cages results in levels of 20 ng of Rat n I/m3; injections or handling of the animals results in levels of 13 ng/m3. Lower levels (3.1 ng/m3) are found during surgery or during sacrifice of the animal (Eggleston et al., 1989).

The male rat is capable of releasing high levels (up to 20 ng/min) of airborne urinary allergens (Swanson et al., 1990). Personal sampling devices on 12 sensitized subjects working with rats revealed allergen concentrations that ranged from less than 1.5 to 310 ng/m3; even higher levels were noted during cage cleaning. These subjects experienced nasal symptoms with exposure, and researchers obtained direct evidence for an allergic reaction by finding histamine in their nasal lavage fluids. Five of the 12 subjects also had lower respiratory tract responses with decreases in pulmonary function (Eggleston et al., 1990).

Two major allergens, Mus m I and Ag 3, have been identified as related to mice. Mus m I is a urinary prealbumin; it has been molecularly cloned, and its amino acid sequence was found to share 80 percent homology with Rat n I. Research has noted biochemical differences between urinary proteins from different inbred strains of mice, which may explain why some mouse laboratory workers become sensitized to specific strains. The Ag 3 allergen is found primarily in hair follicles and the stratum corneum, implying that it is found predominantly on the hair and skin of mice. Mus m I has

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