The antihistamine cetirizine has no apparent benefit in treating insect bite hypersensitivity (IBH), reports a group of Swiss researchers led by Lena Olsén from the Division of Pathology, Pharmacology, and Toxicology at the Swedish University of Agricultural Sciences.

Also referred to as sweet itch or summer eczema, IBH is a seasonally recurrent skin disorder caused by a hypersensitivity to the saliva of biting insects such as the biting midge of the genus Culicoides. IBH is very itchy, causing affected horses to rub and scratch. That ultimately led to crusts, papules, and hair loss on the skin.

Treatment typically entails insect avoidance via overnight stabling, use of blankets, and insect repellents. Since complete avoidance is challenging, some owners also elect to use corticosteroids and antihistamines to control the condition.

In humans, cetirizine is a potent and effective antihistamine. However, the role of antihistamines in horses with IBH has not been adequately explored. To better define the impact of the antihistamine cetirizine in IBH, Olsén and colleagues conducted a double-blinded, placebo-controlled field study in 89 horses from 32 farms. Forty-five horses were treated with cetirizine (0.4 mg/kg) orally twice daily for three weeks, while the remaining horses received a placebo