Comparing the effectiveness of four antihistamines with olopatadine in healthy Thoroughbred horses.

We compared the antihistamine effect of four new antihistamines with olopatadine, which is used to treat equine allergic diseases. Six healthy Thoroughbred horses received oral doses of olopatadine (50 mg), levocetirizine (50 mg), bilastine (200 mg), rupatadine (100 mg), and desloratadine (50 mg) at >2-week intervals. The effects were investigated by measurement of the wheal area induced by histamine intradermal injection, and inhibition rate was compared with positive and negative controls. The maximum wheal inhibition rate (mean ± SD) of olopatadine (85.3% ± 7.7%) was significantly higher than those of bilastine (51.1% ± 11.8%), rupatadine (55.3% ± 6.6%), and desloratadine (59.8% ± 6.4%), but not levocetirizine (85.4% ± 8.0%). Levocetirizine is expected to have a high antihistamine effect on horses.