Foal-heat diarrhea occurs in most foals around 10 days of life and corresponds roughly to the mare’s first estrus cycle. It’s usually short-lived, resolves spontaneously and has no effect on the foal’s appetite, activity level or hydration. For an unfortunate few, however, it can become serious.
Why it occurs isn’t clear. An interesting theory is that the foal’s rummaging around on the ground and experimenting with solid foods, or even things he really shouldn’t be eating, is responsible.
A 1986 study lends some credibility to this since researchers found the manure of these foals resembled small intestinal contents, with high concentrations of electrolytes that explained the greater water content. These researchers believe this is a transition phase for the foal, where the large bowel isn’t yet functioning as it does in an adult horse and found that over time the consistency, pH and electrolyte/water content of the foal’s manure changes to a more adult-like picture.
Whatever the cause, it’s important to recognize that there’s no need to panic if your one- to two-week-old foal shows some diarrhea as long as he remains bright, active and nurses well. After a few days to a week, it should start to clear up. If it doesn’t, or if the foal appears ill or weak in any way, call your veterinarian.
Early-life diarrhea may also have a pathogenic cause. The parasite strongyloides has evolved an ingenious survival mechanism where it will lie dormant in the mare’s tissues until a trigger, probably hormone shifts, wakes up the slumbering larvae and directs them to the mammary gland. After the colostrum is gone, and the mare’s true milk comes in, within 24 hours of foaling, the larvae start to appear in her milk and the foal is infected when nursing. Shedding of larvae in milk can continue for a month or longer.
Studies show no direct connection between the presence or absence of strongyloides in foals and how severe their foal-heat diarrhea is. However, heavy infestations may contribute to the problem. This is one aspect you can help minimize by deworming the mare with ivermectin within the first 24 hours after foaling.