Foal heat diarrhea is a condition involving transient diarrhea in young foals, lasting for a few days up to two weeks. Foals with heat diarrhea do not have any systemic illness. [1][2]

While it is not known exactly what causes foal heat diarrhea, researchers believe this condition may occur because the flora of the foal’s gastrointestinal tract is developing. [2]

It may also result if the foal consumes feed other than its mother’s milk or adult horse feces (a normal behaviour in young foals). [3]

Common signs of foal heat diarrhea include watery feces, skin irritation, and hair loss where diarrhea has made contact with the skin. [3]

Foal heat diarrhea should be distinguished from other causes of diarrhea in foals, which involve infection and require veterinary treatment.

There is no specific treatment for foal heat diarrhea and most cases resolve without complications or the need for medical intervention. [3]

Therapeutic strategies for foals with diarrhea include providing electrolytes and applying creams and ointments to areas of affected skin. [4]

What is Foal Heat Diarrhea?

Also referred to as foal heat scours, this mild diarrhea occurs in horses between 5 and 15 days old. The condition usually lasts 3 days to 14 days. [5]

Diarrhea refers to an increased frequency of defecation with increased water content in feces. Foal heat diarrhea is transitory and occurs in the absence of infectious illness due to viruses, bacteria, or protozoa.

Foal heat diarrhea was previously believed to be caused by changes in the composition of the mare’s milk during her heat (estrous) cycle following birth. [1]

However, researchers now recognize that the condition affects foals regardless of the mare’s milk composition. [6] Orphaned foals and those raised separated from their mother can develop scours. [2]

A study that analyzed mares’ milk composition post-foaling and during the foal heat period concluded that it was not a causative factor in the development of foal heat diarrhea. [1]


Diarrhea is one of the most common diseases in young foals and may occur in over half of all foals until the age of weaning. [7][8][9]

Foal heat diarrhea is estimated to affect between 49 and 80% of foals. [7][8]

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The exact cause of foal heat diarrhea is unknown. The current leading theory is that the condition is caused by changes within the foal’s gastrointestinal flora due to diet. [3]

Newborn foals are born with an immature gut microbiome. [16] Foals aged 2 to 30 days have significantly less diverse microbial populations in their gastrointestinal tract compared to older animals. [16]

A newborn’s microbiome is primarily comprised of bacteria from the Firmicutes phylum. [16] The population of microbes changes throughout the first month of life and becomes similar to that of the mother by the time they are 60 days old. [16]

Scours might be triggered by the introduction of new bacteria to the gastrointestinal system by consuming feed particles and the mare’s manure (a behaviour known as coprophagy). [3]

Secretory Diarrhea

A study that analyzed the fecal material of young foals found that changes in their normal bacterial flora resulted in secretory diarrhea, which typically resolved in a few days without requiring specific treatment. [6]

Secretory diarrhea is caused by secretions from either the small or large intestines and changes in the absorption of water and electrolytes in the gastrointestinal system.

One study concluded that foal heat diarrhea is most likely caused by hypersecretion in the cells of the small intestine in response to diet. [6] These