Even though the weather in California is beautiful, you’ve got a horse that’s not acting quite himself. This 2-year-old colt has a fever, has been depressed, has gone off feed, and isn’t tearing the barn down as usual. You know he’s got a respiratory bug, and it’s not getting any better. The veterinarian says the lungs are a bit congested; probably a bacterial infection. While it is worrisome to have a sick horse, you are relieved—modern antibiotics to the rescue

“So,” you say, “just give him penicillin for a week or so and he’ll be right as rain.”

Feeling smug with your knowledge that Streptococcus strains are the No. 1 pathogen in equine respiratory disease, and that the No. 1 antibiotic for treating strep is penicillin, you start to walk away.

The veterinarian makes you pause with his next question