Foal Heat Diarrhea (FHD) is an expected event in healthy normal foals at 1-2 weeks of age. The cause is not completely understood but is thought to relate to the foal beginning to eat solid feed. It may reflect the changing bacterial population and function of the intestine. The diarrhea corresponds in timing to the mare's first heat cycle after foaling (the "Foal Heat" ), hence its name. It is unlikely that the mare's estrus hormones have anything to do with the foal developing diarrhea.

FHD appears as a watery diarrhea in an otherwise normal and healthy seeming foal. Foals with FHD remain vigorous and continue to nurse. Only rarely do foals become clinically dehydrated and require treatment.

In contrast, if a foal of this age has diarrhea and loses its appetite or seems depressed or lethargic, you should consult your vet immediately because other illnesses that cause diarrhea in foals are much more serious and life-threatening, and it is not always easy to differentiate.

DIAGNOSIS is usually presumptive - meaning we assume that an otherwise healthy-seeming 1-2 week old foal with diarrhea "just has foal heat diarrhea".

TREATMENT with anti-diarrheal medications like bismuth can be done, but is usually unnecessary and may not change the course of the diarrhea. Many horse owners like to apply Vaseline or Desitin or other soothing ointment to the rear end, to help lessen scalding the skin from the diarrhea.

FHD usually stops in its own at about 2 weeks of age, but can last for weeks in some cases.