Threadworms in Horses: Strongyloides Infection in Foals

Quick Answer
  • Threadworms in horses are usually caused by Strongyloides westeri, a small-intestinal parasite seen most often in young foals.
  • Many foals have mild or no signs, but heavier infections can be linked with diarrhea, poor growth, dullness, and occasionally skin irritation from larvae entering through the skin.
  • Foals are commonly exposed through the mare's milk, and less often through skin penetration or ingestion from a contaminated environment.
  • Diagnosis usually involves a fecal flotation or fecal egg count, sometimes paired with coproculture if your vet needs more detail.
  • Treatment often includes a dewormer such as ivermectin or oxibendazole chosen by your vet, plus supportive care if the foal is dehydrated or weak.
Estimated cost: $40–$350

What Is Threadworms in Horses?

Threadworms in horses usually refers to Strongyloides westeri, an intestinal parasite that most often affects young foals. Adult worms live in the small intestine, where they may contribute to irritation of the gut lining. In many cases, infection is mild. In some foals, though, higher parasite burdens are associated with diarrhea, poor thrift, and reduced energy.

This parasite behaves differently from some other equine worms. Foals can become infected very early in life, including through lactogenic transmission, meaning larvae passed from the mare to the foal in milk. Because of that, signs may show up in very young foals rather than only after pasture exposure.

Threadworms are not always the only explanation for diarrhea in a foal. Foal diarrhea can also be caused by diet changes, rotavirus, bacterial disease, sand, or more serious neonatal illness. That is why a parasite finding on a fecal test should always be interpreted by your vet in the context of the foal's age, exam findings, and overall health.

Symptoms of Threadworms in Horses

  • Mild to moderate diarrhea in a young foal
  • Poor weight gain or an unthrifty appearance
  • Low energy or reduced nursing enthusiasm
  • Skin irritation or dermatitis on the lower legs from larvae penetrating the skin
  • Occasional colic signs or abdominal discomfort with heavier parasite burdens
  • Respiratory irritation during larval migration has been described, but it is not the most typical presentation
  • Dehydration, weakness, fever, or a foal that will not nurse

Mild diarrhea in a bright, nursing foal may still deserve a call to your vet, especially if the foal is very young. Threadworms are one possible cause, but they are not the only one. A foal can look only mildly affected at first and still become dehydrated quickly.

See your vet immediately if your foal is weak, stops nursing, develops a fever, shows repeated colic signs, has worsening diarrhea, or seems depressed. Those signs raise concern for more serious disease than an uncomplicated parasite infection.

What Causes Threadworms in Horses?

Strongyloides westeri is the parasite behind threadworm infection in foals. The most important route of infection is thought to be through the mare's milk. Larvae can also infect foals by penetrating the skin or by being swallowed from a contaminated environment, especially where manure control is poor and many young horses are housed together.

Young foals are affected most often because they have not yet built up the same level of immunity seen in older horses. Adult horses usually do not develop important intestinal disease from this parasite, even if they are exposed.

Farm management also matters. Heavy environmental contamination, crowding, wet bedding, and inconsistent manure removal can all increase exposure pressure. Even so, finding threadworm eggs on a fecal test does not automatically mean they are the only cause of a foal's symptoms. Your vet may also look for bacterial, viral, nutritional, or management-related causes of diarrhea.

How Is Threadworms in Horses Diagnosed?

Diagnosis usually starts with a physical exam and a discussion of the foal's age, nursing behavior, manure changes, and farm parasite history. Your vet will often recommend a fecal flotation or fecal egg count to look for Strongyloides eggs. These eggs can be delicate, so laboratory technique matters.

If your vet suspects threadworms are contributing to disease, test results are interpreted alongside the foal's clinical signs. Merck notes that disease has been associated with high egg counts, and coproculture may be used to identify third-stage larvae when more confirmation is needed.

Because foal diarrhea has many causes, diagnosis may go beyond parasite testing. Depending on the case, your vet may recommend bloodwork, a hydration assessment, fecal testing for infectious disease, or repeat exams over time. That broader approach helps separate a manageable parasite problem from a foal that needs more intensive care.

Treatment Options for Threadworms in Horses

Spectrum of Care means you have options. Here are treatment tiers at different price points.

Budget-Conscious Care

$40–$150
Best for: Bright foals with mild diarrhea or poor thrift, no fever, and no signs of dehydration or systemic illness.
  • Targeted exam or tele-triage guidance from your vet if the foal is bright and nursing well
  • Fecal flotation or fecal egg count when available
  • Single-dose deworming selected by your vet, often ivermectin or oxibendazole
  • Home monitoring of nursing, hydration, manure, and attitude
  • Basic environmental cleanup and manure control
Expected outcome: Often good when the parasite burden is mild and the foal remains hydrated and active.
Consider: This approach keeps care focused and practical, but it may miss another cause of diarrhea if the foal is sicker than expected or does not improve quickly.
Most Common

Recommended Standard Treatment

$150–$350
Best for: Most foals with suspected threadworms, especially when diarrhea is ongoing, weight gain is poor, or the diagnosis is not fully clear.
  • Complete farm call or clinic exam by your vet
  • Fecal flotation or quantitative fecal egg count
  • Deworming plan for the foal based on age, weight, and farm history
  • Supportive care such as oral fluids, nursing support, and follow-up recheck
  • Discussion of mare and herd parasite control around foaling
Expected outcome: Good in uncomplicated cases, especially when treatment is paired with monitoring and farm-level prevention.
Consider: Costs more than a minimal approach, but it gives better confirmation, safer dosing, and a clearer plan if the foal does not respond as expected.

Advanced / Critical Care

$500–$2,500
Best for: Foals with severe diarrhea, dehydration, weakness, fever, repeated colic signs, or concern that threadworms are only part of a larger neonatal illness.
  • Urgent or emergency examination
  • CBC, chemistry, and hydration assessment
  • IV or nasogastric fluid support when needed
  • Expanded infectious diarrhea workup and repeat fecal testing
  • Hospitalization or intensive neonatal monitoring for weak, septic-appearing, or non-nursing foals
Expected outcome: Variable and depends more on the foal's overall condition and any concurrent disease than on the parasite alone.
Consider: This tier offers the most monitoring and support, but it requires a higher cost range and may involve transport, hospitalization, and more diagnostics than every family wants or needs.

Cost estimates as of 2026-03. Actual costs vary by location, clinic, and individual case.

Questions to Ask Your Vet About Threadworms in Horses

Bring these questions to your vet appointment to get the most out of your visit.

  1. Does my foal's age and symptom pattern fit Strongyloides westeri, or should we be more concerned about another cause of diarrhea?
  2. Which fecal test do you recommend for this foal, and how reliable is it for threadworms?
  3. Does my foal need treatment now, or is monitoring reasonable while we wait for test results?
  4. Which dewormer is most appropriate for this foal's age and weight?
  5. Should the mare also be treated or should we change our around-foaling parasite plan?
  6. What signs would mean this is no longer a mild case and my foal needs urgent care?
  7. When should we repeat a fecal test or recheck the foal after treatment?
  8. What barn and pasture changes would lower parasite exposure for future foals?

How to Prevent Threadworms in Horses

Prevention starts before the foal is born. Merck and AAEP guidance support working with your vet on a mare-and-foal parasite plan rather than using fixed-interval deworming without testing. Around foaling, many programs include treating mares with ivermectin or a benzimidazole 1 to 3 days before foaling, and some references also discuss ivermectin shortly after foaling to reduce lactogenic transmission risk.

Good farm hygiene matters too. Prompt manure removal, clean foaling areas, dry bedding, and avoiding overcrowding can reduce environmental contamination. These steps will not eliminate all parasite exposure, but they can lower the burden on young foals.

For the foal, prevention is usually part of a broader parasite-control program. Merck notes that routine antiparasitic treatment often begins around 2 months of age and may be repeated every 6 to 8 weeks during the first year, but the exact schedule should be tailored to the farm, the foal's age, and local parasite patterns. Fecal egg counts are useful for herd planning, though AAEP notes they should not be used alone to diagnose parasite disease.

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