Protozoa And Diarrhea

My 18-year-old mare developed a case of excessive protozoa. She started getting soft manure and then diarrhea. I first dewormed her and had a fecal count done, which was negative. When the diarrhea persisted, my trainer suggested I have her tested for excessive protozoa, which a fecal smear confirmed.

She was started on metronidazole, which firmed up her manure in two days, but we had to discontinue it as she went off her feed and became colicky. Of course the diarrhea came back (since we didn’t finish the metronidazole), so she was then put on two doses of ponazuril. It has been two weeks and this drug appears to have worked. I’ve started her on a probiotic but don’t know if this is really necessary or not.

My veterinarian says we will never know why she developed this condition, and yet the other horses at the barn, and using the same pasture, were never affected. My mare does tend to be overly sensitive. Can you shed more light on how to avoid it in the future’

Horse Journal Response:

Protozoa are normally found in the equine large intestine and on fecal examinations, although the manure has to be kept moist and preferably examined fresh to see them well. Even though no disease-producing forms of protozoa have been recognized in horses (e.g. Giardia or Cryptosporidium), it has been known for a long time that horses with unexplained chronic diarrhea often respond to drugs with antiprotozoal activity. The one most commonly used in the past was iodochlorhydroxyquin (Rheaform). Today, it’s usually metronidazole. The double-dose ponazuril was a good idea. It’s used that way for protozoal problems in other species.

Although protozoa are normal in equine manure, what is often seen in these protozoal diarrheas is one species that’s present in high numbers. The protozoa themselves aren’t the root cause of the problem, though. The imbalance may have been started by something like colon inflammation (e.g. bute-induced colitis), diet change, parasite die-off or feed intolerance.

With a sensitive horse, something like a change in the simple carbohydrate level of the pasture might start it. Manure eating can be a sign of incomplete digestion of feeds, and it’s an instinctive way to recycle nutrients.

Once the imbalance has been established, it may be difficult to reverse. Probiotics are a good idea in your case. Be careful to keep grain feeds small and frequent, introduce new loads of hay slowly, and always graze her at the same time of day.

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