Cat allergy is especially apt to cause asthma, and the presence of IgE antibody to cat allergen is a significant risk factor for acute attacks of asthma in patients who seek treatment in emergency rooms (Luczynska et al., 1990). Individual sensitivity to these animals may be exquisite, so that even brief exposure to cats can precipitate severe asthma episodes.

The most important cat allergens are Fel d I and cat albumin. Fel d I is the major cat allergen (Ohman et al., 1973), and it is distributed throughout all breeds of cats. Approximately 80 percent or more of cat-allergic patients have IgE antibody to Fel d I (Duffort et al., 1991). In recent years, the complete amino acid sequence of Fel d I has been determined, the genes encoding the protein have been identified, and several allergenic epitopes have been mapped (Morgenstern et al., 1991).

Commercially available cat extract reagents for skin testing have variable quantities of Fel d I and serum albumin. They are prepared from dander (hair with epithelial scrapings) or whole pelts. The concentration of Fel d I is 10 times greater at the root than at the tip of the hair (C. Charpin et al., 1991). It is produced primarily in the sebaceous gland and to a lesser extent in the basal squamous epithelial cells of the skin. It appears to be stored mainly on the surface of the epidermis and the fur. Because saliva also contains Fel d I, licking and grooming may spread the allergen on the hair. The mean daily production of Fel d I from cats is approximately 3-7 µg (Dabrowski et al., 1990); however, there is day-to-day and diurnal variability in Fel d I shedding from individual cats. Washing the cat provides only temporary relief: the original concentration of Fel d I reappears within 1 month.

Cat-allergic patients develop symptoms rapidly upon entering a house with a cat, which suggests that the allergen is constantly airborne. Indoor air sampling measurements in fact have confirmed this hypothesis. In addition, studies have determined the quantity of Fel d I that will produce a 20 percent decrease in FEV1 (forced expiratory volume at 1 second) in cat-sensitive patients who undergo bronchoprovocation testing. A room containing two live cats has a concentration of Fel d I in this range (Van Metre et al., 1986); however, many cat-allergic individuals will be symptomatic in the presence of even one cat. In a 1988 study by Wood and colleagues, this allergen was found in virtually 100 percent of homes sampled for its presence in settled household dust (Wood et al., 1988). The median level of Fel d I in the dust samples was 90 ng per gram of dust (range: 2–130,000 ng Fed d I/g of dust).

Although there are significant differences in allergen content in homes with and without pets, many homes that normally are without a cat nonetheless contain surprisingly high levels of Fel d I in dust (Wood et al., 1988). These levels vary from less than 2 to 7,500 ng/g of Fel d I/g; it is thought that the allergen is transported into houses on the clothes of inhabitants.

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