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Even a seemingly classic case of pigeon fever, with external abscesses that cause a horse’s chest to bulge like a pigeon’s, requires a bacterial culture for definitive diagnosis. “I always tell clients that an abscess is just an abscess until you culture it,” says Nathan Slovis, DVM, DACVIM, of Hagyard Equine Medical Institute in Lexington, Kentucky. “You don’t know what organism is causing it; you can’t just assume, until it’s cultured. It might be strangles, which could be much more serious, or a contaminated puncture wound.”

With internal infection, blood tests such as complete blood counts (CBCs) and serum biochemical profiles will show evidence of infection or chronic inflammation. In the absence of external abscesses, elevations in serum