Cat antigen in homes with and without cats may induce allergic symptoms

Allergic reactions to cats or dogs is extremely common, occurring in 3-10% of the general population and 15-40% of atopic individuals. Exposure to antigens is extremely common, since over half of all homes in the United States contain at least one cat or dog, and up to one-third of individuals allergic to cats live in a home with one or more cats. In homes with cats, levels of Fel d 1 (major cat allergen) are generally very high, and it has been previously assumed that such levels are capable of inducing symptoms in most patients allergic to cats. Interestingly, cat allergen has also been found in dust samples from homes without cats as well as virtually every other building that has been assessed including newly built homes, shopping malls, and even hospitals. Allergen levels in these sites are typically 100 to 1000 times lower than those found in homes with cats, but again the clinical significance of low-level exposure is not known. The authors in this study endeavored to measure airborne Fel d 1 (rather than levels in settled dust) in homes with and without cat and then attempted to correlate such levels with clinical level of reactivity and sensitization. Measurable levels of airborne Fel d 1 were seen in all 37 homes tested with cats (range 1.8-578 ng/m³: median 45.9 ng/m³) and in 10 of 40 homes without cats (2.8-88.5 ng/m³: median 17 ng/m³). Fel d 1 was present in the settled dust of 38 of 40 homes without cats (range 39-3750 ng/g: median 258 ng/g). Over the course of the study, repeated samples obtained weekly from 12 homes without cats yielded measurable airborne Fel d 1 in at least 1 of the 4 samples from all homes. In previous studies this research group has described a "cat room facility" which they have used for previous cat challenges in patients. This room is a 13.7 m³ space containing two cats and is furnished with a bed, upholstered chair, and small carpet. The cats are free to move about this room except during challenge periods, when they are placed in a wire cage and the room is carefully monitored for a standard ventilation and air turnover rate. From prior research it is known that low levels of airborne Fel d 1 (<500 ng/m³) when used for challenges in the "cat room facility" can induce upper and lower respiratory symptoms in cat allergic subjects. This study confirms that such levels of cat exposure are possible in many homes without cats, and accordingly, such an environment could induce symptoms in at least some patients who are sensitive to cats. The authors conclude that the assessment of cat exposure cannot be based solely on the presence or absence of a cat in the home, because measurable airborne Fel d 1 is clearly frequently found in homes without cats. It also appears that any measurable airborne Fel d 1 might be clinically significant; and although cat avoidance should still be recommended as first line treatment, complete avoidance clearly is not possible. The need for medications and immunotherapy should be carefully considered.