Cetirizine Not Effective in Horses with Sweet Itch

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.

Key findings of the study were:

  • There was no difference in clinical signs of dermatitis between the treatment and control group after three weeks.
  • Horses that were blanketed and stabled overnight had a significant reduction in dermatitis severity compared to horses that were blanketed only, stabled only, or neither blanketed nor stabled.

"The findings indicate that cetirizine was of no apparent benefit in treating IBH at the dose rate tested," concluded the researchers.

Instead, owners are recommended to minimize exposure to midges at peak activity periods (i.e., sunset and sunrise).

The study, "Pharmacokinetics and effects of cetirizine in horses with insect bite hypersensitivity," is scheduled to be published in an upcoming edition of the Veterinary Journal.

The abstract is available on PubMed.

About the Author

Stacey Oke, DVM, MSc

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

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