Diarrhea in Newborn Foals: What Is Normal and When To Call a Vet

By Dr Duncan Houston

Diarrhea in a newborn foal is one of those signs that can be completely harmless or genuinely urgent, depending on the foal in front of you.

A bright, active foal with mild loose manure at 5 to 14 days old may simply be going through normal early gut changes. A dull, weak, dehydrated, feverish foal with watery diarrhea is a very different situation and should be treated as urgent.

The key is not just looking at the manure. The key is looking at the whole foal: nursing, attitude, hydration, temperature, strength, and whether the diarrhea is mild, worsening, bloody, foul-smelling, or accompanied by other signs.

Quick Answer

Mild diarrhea in a bright, nursing foal between about 4 and 14 days old is often “foal heat diarrhea,” a common self-limiting condition linked to early gut changes rather than the mare’s heat cycle. It usually does not need specific treatment if the foal remains alert, nurses normally, has normal vital signs, and does not become dehydrated. Diarrhea becomes concerning if the foal is weak, dull, feverish, not nursing, dehydrated, colicky, bloated, or passing blood or foul-smelling watery manure. (MSD Veterinary Manual)

Why Foal Diarrhea Needs Careful Judgement

Foals are not small adult horses. They can deteriorate quickly.

A healthy foal may look lively one moment and become weak, dehydrated, or septic faster than owners expect. That is why diarrhea in foals needs a different level of caution from mild loose manure in a healthy adult horse.

In practice, what matters most is whether the foal is otherwise normal. A foal that is bright, nursing, moving well, and passing mild loose manure is usually less concerning. A foal that is off the udder, lying down more than usual, weak, feverish, or dehydrated needs veterinary attention quickly.

What Causes Diarrhea in Newborn Foals?

Foal diarrhea can happen for several reasons.

Common causes include:

• Normal early gut maturation

• Foal heat diarrhea

• Changes in gut microbiota

• Overfeeding

• Improper milk replacer or unsuitable orphan feeding

• Ingesting indigestible material such as dirt, sand, roughage, or bedding

• Parasites

• Bacterial infection

• Viral infection

• Sepsis

• Antibiotic-associated gut disruption

• Diet or management changes

• Stress or systemic illness

MSD Veterinary Manual notes that foal diarrhea can result from overfeeding, improper nutrition, and ingestion of indigestible material such as roughage, sand, dirt, or rocks. It also lists parasites including Strongyloides westeri, Parascaris equorum, and Cryptosporidium species as reported associations. (MSD Veterinary Manual)

The mistake is assuming all foal diarrhea is foal heat diarrhea. Some is. Some definitely is not.

What Is Foal Heat Diarrhea?

Foal heat diarrhea is a mild, self-limiting diarrhea that often appears in young foals.

Despite the name, it is not thought to be caused by the mare’s heat cycle. It can also occur in orphan foals, which makes the old hormone theory unlikely. Current explanations include normal changes in the foal’s intestinal microbial flora, early nibbling of hay or grain, and coprophagy, which means eating small amounts of manure. (MSD Veterinary Manual)

Typical foal heat diarrhea looks like this:

A foal with true foal heat diarrhea should not be depressed, weak, off milk, feverish, dehydrated, or painful.

Why Does Foal Heat Diarrhea Happen?

The exact cause is not fully proven, but the best explanation is gut transition.

In the first weeks of life, the foal’s digestive tract is adapting rapidly. The foal moves from relying almost entirely on milk to gradually exploring the environment, nibbling hay or grass, and developing a more complex gut microbiome. UC Davis notes that foal gut bacteria change with age and diet, and that microbes associated with the mare, colostrum, and milk are among the first to colonise the foal’s gastrointestinal tract. (Center for Equine Health)

That shift can produce mild loose manure without the foal being truly sick.

Is It Normal for Foals To Eat Manure?

Yes, in small amounts, this can be normal.

Foals often nibble or eat small amounts of manure, usually from the mare. This behaviour is called coprophagy. It looks unpleasant to us, but it may help expose the foal to microbes involved in gut development. MSD Veterinary Manual notes that coprophagy may have a role in the gut changes associated with foal heat diarrhea. (MSD Veterinary Manual)

This is normal only when the foal is otherwise well. If the foal is dull, weak, not nursing, bloated, or has severe diarrhea, do not write it off as normal gut development.

Severity Guide: How Worried Should You Be?

The key checkpoint is simple: mild diarrhea in a bright nursing foal can be monitored; diarrhea in a sick foal is urgent.

When Is Foal Diarrhea an Emergency?

Call your vet urgently if your foal has any of these signs:

• Refuses to nurse

• Nurses weakly or less often

• Mare’s udder is unusually full because the foal is not drinking

• Lethargy or weakness

• Fever or abnormally low temperature

• Sunken eyes

• Dry or tacky gums

• Cold ears or limbs

• Rapid heart rate

• Bloating or colic signs

• Watery diarrhea

• Bloody diarrhea

• Black manure after the normal meconium period

• Foul-smelling diarrhea

• Severe skin scalding

• Collapse

• Seizures

• Abnormal gums, including pale, dark red, purple, or very injected gums

Sepsis is one of the most serious concerns in neonatal foals. MSD Veterinary Manual notes that early sepsis signs can be vague, including depression, lethargy, more lying down than usual, and reduced nursing. Signs can progress to loss of suckle reflex, abnormal mucous membranes, rapid heart rate, weakness, shock, diarrhea, and other organ involvement. (MSD Veterinary Manual)

A sick foal is not a “wait and see” patient. Foals are tiny overachievers in many ways, but they do not have much spare reserve.

What Else Can Look Like Foal Heat Diarrhea?

Several important conditions can cause diarrhea in young foals.

Overfeeding

This is common in orphan foals, bottle-fed foals, or foals reunited with the mare after separation. Too much milk or the wrong feeding schedule can overwhelm digestion. MSD lists overfeeding as a cause of diarrhea in foals. (MSD Veterinary Manual)

Improper milk replacer

Foals should not be fed unsuitable replacers such as calf milk replacer unless a veterinarian specifically guides the plan. Improper nutrition is a recognised cause of foal diarrhea. (MSD Veterinary Manual)

Eating sand, dirt, bedding, or roughage too early

Foals explore with their mouths. Dirt, sand, roughage, rocks, and bedding can irritate the gut or contribute to digestive upset. (MSD Veterinary Manual)

Parasites

Some parasites can be associated with foal diarrhea, including Strongyloides westeri, Parascaris equorum, and Cryptosporidium species. Diagnosis and treatment should be vet-led, not guessed. (MSD Veterinary Manual)

Bacterial infection

Bacterial diarrhea can be serious, especially in neonatal foals. It may be part of systemic infection or sepsis.

Viral infection

Viral diarrhea can affect foals and may spread between animals depending on the cause.

Sepsis

This is the big rule-out in a dull, weak, febrile, or poorly nursing foal. Inadequate colostrum transfer is a major risk factor for neonatal infection and sepsis. (MSD Veterinary Manual)

Do Not Forget Colostrum and IgG

Any newborn foal with diarrhea, weakness, poor nursing, or dullness should make you think about passive transfer.

Foals rely on colostrum for early antibody protection. A 50 kg foal needs good-quality colostrum in the first 12 to 18 hours, and oral colostrum has only minimal benefit after about 18 to 24 hours because the intestine can no longer absorb large antibody molecules effectively. Complete failure of passive transfer is defined as IgG below 400 mg/dL at 24 hours, and partial failure is 400 to 800 mg/dL. (MSD Veterinary Manual)

This is why IgG testing matters. A foal can appear to have nursed and still have poor passive transfer. Test, do not guess.

How Vets Diagnose Foal Diarrhea

Your vet will start with the whole foal, not just the manure.

They may assess:

• Nursing behaviour

• Hydration

• Temperature

• Heart rate

• Respiratory rate

• Gum colour

• Capillary refill time

• Attitude and strength

• Abdominal comfort

• Navel health

• Joint swelling

• Mare’s udder

• Manure character

• Whether the foal passed meconium normally

• Whether other foals are affected

Depending on the case, testing may include:

• IgG testing

• Complete blood count

• Blood chemistry

• Electrolytes

• Blood culture if sepsis is suspected

• Fecal testing

• Parasite testing

• Testing for specific infectious causes

• Ultrasound

• Fluid and hydration assessment

For suspected sepsis, MSD lists diagnosis as involving history, clinical signs, laboratory testing such as CBC, serum chemistry, serum IgG, microbial culture, lactate or blood gas testing, and sometimes advanced diagnostics such as ultrasound or CT. (MSD Veterinary Manual)

What Should You Do Right Now?

1. Look at the whole foal

Ask:

• Is the foal bright?

• Is the foal nursing normally?

• Is the foal standing and moving well?

• Is the foal lying down more than normal?

• Is the mare’s udder full?

• Are the gums moist and pink?

• Is the foal dehydrated?

• Is the manure mildly loose or truly watery?

The manure matters, but the foal’s attitude matters more.

2. Check the foal’s age

Mild loose manure in a bright foal around 4 to 14 days old is often consistent with foal heat diarrhea. Mild diarrhea outside that window can still be benign, but it deserves more attention, especially in a very young foal. (MSD Veterinary Manual)

3. Monitor nursing closely

A normal foal nurses frequently. If the foal is missing feeds, weak at the udder, falling asleep while trying to nurse, or not emptying the mare’s udder, call your vet.

4. Protect the skin

Even mild diarrhea can scald the skin around the tail, thighs, and perineum.

Gently clean the area and apply a vet-approved barrier product if needed. MSD notes that a protectant around the perineum can help prevent scalding in foals with mild self-limiting diarrhea. (MSD Veterinary Manual)

5. Do not give random medication

Do not give leftover antibiotics, anti-diarrheal medications, dewormers, probiotics, human medication, or electrolyte products without veterinary advice.

A sick foal may need fluids, plasma, antimicrobials, glucose support, or intensive care. Giving the wrong product can waste valuable time.

6. Call early if you are unsure

Foals are not the place for heroic optimism. If the foal seems off, call.

What Not To Do

Do not assume every case is foal heat diarrhea.

Do not wait if the foal is dull, weak, feverish, dehydrated, or not nursing.

Do not stop the foal from nursing unless your vet tells you to.

Do not give adult horse medications to a foal without veterinary instruction.

Do not scrub scalded skin aggressively.

Do not change milk replacer, feeding volume, or frequency without a vet-guided plan.

Do not ignore the mare. A foal with poor nursing may leave the mare with a full udder, and mare health can affect foal care too.

Common Mistakes Owners Make

Calling it foal heat diarrhea too quickly

Foal heat diarrhea should not make the foal sick. If the foal is unwell, the label no longer fits.

Missing poor nursing

A foal can look like it is standing near the udder but not actually drinking enough.

Forgetting IgG testing

A healthy-looking foal can still have failure of passive transfer. IgG testing is one of the most valuable checks in the first day of life.

Over-cleaning the foaling area with harsh products

Cleanliness matters, but harsh residues and irritation are not helpful. Clean and dry beats chemical warfare.

Giving antibiotics unnecessarily

Antibiotics can be life-saving when needed, especially in sepsis, but unnecessary use can disrupt the gut and should be vet-guided.

Waiting until the foal is collapsed

The earlier a sick foal is treated, the better the chance of a good outcome.

How To Reduce the Risk of Foal Diarrhea Problems

You cannot prevent every case, but good management reduces risk.

Helpful steps include:

• Use a clean, safe foaling area

• Keep bedding dry and appropriate

• Ensure the mare is healthy before foaling

• Monitor the foal’s first stand and first nursing

• Arrange an early newborn foal exam

• Test IgG within the first day

• Watch mare udder fill and foal nursing frequency

• Keep the mare’s tail and udder reasonably clean

• Avoid unnecessary early antibiotics

• Use vet-guided parasite control

• Avoid unsuitable milk replacers

• Keep orphan foal feeding carefully measured

• Monitor manure daily

• Protect the skin early if diarrhea starts

• Call your vet promptly if the foal seems even slightly abnormal

The best prevention is not panic. It is preparation.

Will My Foal Be Okay?

Many foals with mild foal heat diarrhea do very well and need little more than monitoring and skin protection.

The outcome changes when diarrhea is severe, watery, foul-smelling, bloody, or associated with weakness, dehydration, fever, poor nursing, or sepsis. In those cases, early veterinary care is the difference-maker. MSD notes that neonatal sepsis is a major cause of foal death, but early goal-directed therapy, including fluids, broad-spectrum antimicrobials, plasma when needed, and supportive care, is central to treatment. (MSD Veterinary Manual)

So yes, your foal may be completely fine. But the way to know is to judge the foal, not just the stool.

FAQs

Is diarrhea normal in newborn foals?

Mild diarrhea can be normal in foals around 4 to 14 days old if the foal remains bright, active, nursing normally, and has normal vital signs. Diarrhea is not normal if the foal is weak, dull, feverish, dehydrated, not nursing, or passing blood. (MSD Veterinary Manual)

How long does foal heat diarrhea last?

Foal heat diarrhea is usually short-lived and self-limiting. If diarrhea lasts more than 24 to 48 hours, worsens, becomes watery, smells foul, or the foal seems unwell, call your vet.

Should I treat foal heat diarrhea?

Specific treatment is usually not needed if the foal is bright, nursing, and otherwise normal. Gentle cleaning and a skin protectant around the perineum can help prevent scalding. (MSD Veterinary Manual)

Can foal diarrhea be caused by infection?

Yes. Diarrhea can be caused by bacterial, viral, or parasitic disease, and in neonatal foals it may also be linked with sepsis. A sick foal with diarrhea should be assessed urgently.

When should I call the vet?

Call your vet if the foal is not nursing, seems dull or weak, has fever, dehydration, colic signs, bloating, blood or mucus in the manure, foul-smelling watery diarrhea, or diarrhea that persists or worsens.

Final Thoughts

Diarrhea in a newborn foal is not automatically a disaster, but it is never something to ignore.

Foal heat diarrhea is common and often harmless when the foal is bright, active, nursing well, and otherwise normal. The danger is assuming that every loose stool belongs in that category. Sick foals need fast help.

The practical rule is simple: watch the foal more than the manure. If the foal is bright, hydrated, nursing, and acting normally, monitor closely and protect the skin. If the foal is dull, weak, feverish, dehydrated, painful, bloated, bloody, or off the udder, call your vet immediately.

Foal diarrhea is common. A sick foal is urgent.

If you are unsure whether your foal’s diarrhea is normal, foal heat diarrhea, dehydration, infection, poor nursing, or something urgent, ASK A VET™ can help you work through the signs and decide what to do next.