[TOMT] [BOOK] [1990s-2000s] A kids' chapter book about a girl/woman (?) who diagnoses a horse's mysterious illness as being related a bug or parasite in the hay. Part of a series about rescuing animals, I think
I am looking for a particular book I read as a kid, as I want to read it to my kids now. I hated reading but it was one of the only books I liked, about a horse with a mysterious illness that ended up being some sort of bug or parasite it was ingesting in the hay. I feel like it was probably part of a series with other animal rescue/veterinarian books. Does anyone know what I'm talking about?
I'm reading the summary to Mustang in the Mist which is part of the animal ark series, and it seems similar (the horse was affected by "grass sickness" caused by a bacteria) but I really feel like the plot of the book included a specific insect or parasite in hay.