Some horses respond to insect bites with inflammation and itching; others react with cellular-level defense mechanisms. | Photo: iStock

For years, people have assumed this meant that horses affected by insect bite hypersensitivity (or IBH) just “react” to fly bites while others don’t. But a new international study is revealing that, actually, all horses probably react: It’s just that some of them react in a way that causes them to become tolerant.

In a pioneering study, researchers from the Netherlands have determined that insect bites can cause some horses to respond with inflammation and itching while others react with cellular-level defense mechanisms. Dietmar M.W. Zaiss, PhD, now affiliated with the Institute of Immunology and Infection Research at the University of Edinburgh, Scotland, said these itch-free horses aren’t just tolerant.

Tolerant means the horse’s body doesn’t have any reaction to the bites at all. But, at least in the early stages of exposure to the bites, horses’ bodies are reacting. They’re actively fighting off the allergen “as if it were an intracellular pathogen,” Zaiss