Clinical efficacy of specific immunotherapy to cat dander: A double-blind placebo-controlled trial

Allergy to cat dander is extremely common, affecting 10-15% of patients in the United Kingdom who have allergic rhinitis or asthma. Fifty to seventy-five percent of homes may have a family cat. Because of the popularity of cats as family pets, it can be extremely difficult to impossible for a cat-allergic patient to avoid cat allergen. The allergenic particles are small and can remain airborne for up to 24 hours. Furthermore, they stick to clothing and can be transported to areas such as schools or work environments in high enough quantities to trigger symptoms in sensitive patients. Immunotherapy for cat dander is available, but there are few formal studies evaluating its efficacy. Available data suggest that desensitization can be achieved as measured by bronchial provocation tests and other end-organ reactions. In this study, the efficacy of an "induction" course of cat immunotherapy was assessed, using "natural exposure" as the principal endpoint, and sensitivity to objective threshold tests as subsidiary outcome measures. Twenty-eight adults patients with severe cat-induced rhinitis, poorly controlled by medication and environmental control, were stratified into two groups according to cat sensitivity. Patients within each grade were then randomly allocated to random or placebo treatment. Patients were treated with a depot preparation of standardized cat allergen (Alutard SQ, ALK, Denmark) or placebo consisting of progressive dilutions of histamine. Injections were given two times per week progressing to a maintenance dose of 100,000 SQ-U over a 6-12 week period. Titration skin prick tests were done before randomization, after reaching half the maintenance dose, and on reaching the maintenance dose. Testing was performed to D. pteronyssinus as well as to cat. Intradermal skin tests were also performed and wheal diameter was measured at 15 minutes and a late-phase reaction diameter was measured at 24 hours. Conjunctival provocation tests were performed, although the timing of this test was not mentioned in the article. Cat sensitivity was assessed utilizing a somewhat unique measure of a standardized cat challenge. Subjects visited a house in which three cats had lived for over 8 years and had documented high levels of cat allergen. Baseline PEFR and symptom scores were measured before entering the house, and then periodically while in the house. Subjects sat in the living room with all doors and windows closed, while the cats moved about the room. Subjects remained in the house until one or more of their symptoms were graded severe, or they experienced distress. No other time limit to the exposure was mentioned, presumably all patients at some point developed severe symptoms. Upon leaving the house, subjects were given a salbutamol inhaler, 10 mg cetirizine and 50 mg prednisolone to use prn. The cat challenge was performed pretreatment and at some unspecified point posttreatment, presumably after the patients had reached maintenance doses of immunotherapy. In comparing the groups, the active treatment group actually was somewhat less sensitive than the placebo group, but since each group was compared to itself at baseline, this should not have had significant impact. In the placebo group, there was no significant change in skin test parameters, cat visit scores, or conjunctival provocation. In the active treatment group, there was a marked reduction in cat visit symptom score, an attenuation in the fall in PEFR induced by cat exposure, an increase in conjunctival threshold, and an increase in skin test size. This study, in somewhat unique fashion, demonstrates that cat immunotherapy is effective in reducing responses to cat exposure and various laboratory parameters in severe cat allergic adults. This effect appears to occur quickly in the course of immunotherapy, although the authors were lax in describing the timing of some of their outcome measures. It would have been interesting to report if there were any differences between cat owners and noncat owners in response. Also, the groups were stratified at entry into two levels of severity, but the authors do not mention any differences between the groups.