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Key takeaways

Tall fescue (Festuca arundinacea) toxicosis occurs when horses ingest large amounts of fescue grass contaminated with the fungus Acremonium coenophialum.

  • Pregnant and breeding age mares are the most affected by this toxicosis
  • Ingestion of infected fescue causes several reproductive issues, including prolonged gestation, decreased milk production, abortions, stillbirths, and difficulty delivering
  • Foals born to affected mothers are born larger than average but have lower muscle mass and are at greater risk of infections
  • Diagnosis is based on history of exposure, clinical signs, and identification of the fungus in forage
  • Treatment options are limited, making prevention the best option in dealing with fescue toxicosis
  • Prognosis is guarded and is dependent on the timing of treatment.
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A closer look: Fescue Toxicosis in Horses

Fescue toxicosis is uncommon in mares, but requires urgent medical attention due to poor prognosis.

Fescue toxicosis affects both the pregnant mare and her newborn foal.

Symptoms in pregnant mares include:

  • Prolonged gestation (30 or more days beyond the due date)
  • Difficulty giving birth (dystocia)
  • Little or no lactation
  • Thickened placenta

Symptoms in foals include:

  • Weakness
  • Excessive weight and size
  • Reduced muscle mass
  • Failure of passive transfer: decreased lactation by the mare provides foals with less colostrum (the first milk), resulting in lower antibody levels thus increasing the risk of infections

Early medical attention and prevention are the best ways to ensure a safe pregnancy and a healthy foal.

Risk factors

Pregnant mares consuming fungus infected tall fescue grass or hay are at risk of developing fescue toxicosis.

Especially when ingested in the later stages of pregnancy, the mare is at risk of stillbirth, prolonged gestation, and may experience little or no lactation.

Between 30 to 70% of foals born to infected mares die at or prior to birth.

Possible causes

The tall fescue plant is not intrinsically toxic. When not infected by fungi, it is a good source of nutrition.

Fescue toxicosis is caused by the ingestion of the Acremonium coenophialum fungus commonly found in tall fescue grass. Fungal toxin concentrations are highest during the summer, and may increase during periods of drought, excessive rain, or other plant stress. The toxin is able to survive the haying process, making fescue hay a potential source of exposure.

Main symptoms

Testing and diagnosis

Diagnosis of fescue toxicosis is primarily based on physical examination and the identification of fungus in forage through feed analysis. Further diagnostic tools include:

  • Blood tests
  • Diagnostic imaging: ultrasonography is used to identify increases in the thickness of the placenta

Steps to Recovery

There is no definitive treatment for fescue toxicosis. Treatment options include:

  • Domperidone: Domperidone is able to counteract the toxin’s effects on lactation. Best administered 10-20 days before birth.
  • Milk replacement for the foal to ensure adequate nutrition

Domperidone is not widely available and is expensive; as such, prevention is the best course of action.

Prognosis is guarded and depends on the severity of the toxicosis and the timing of ingestion.

If diagnosed early, removing infected fescue and administering domperidone increases the chances of a healthy pregnancy.

If treatment is delayed, both the mare and the foal may die from pregnancy, birth, or post-birth complications.


Fescue toxicosis is not contagious among adult horses, but affected pregnant horses pass on complications to their offspring.

The most effective way of preventing fescue toxicosis is ensuring that pregnant mares do not feed on infested grass or hay, especially during the late phase of the pregnancy. Strategies include:

  • Eradication of the infected grass
  • Feeding the mare an alternative forage source, such as alfalfa or timothy
  • Removing mares in late gestation from infected pastures

Is Fescue Toxicosis in Horses common?

Fescue toxicosis affects pregnant mares and their foals.

Tall fescue is found throughout the United States, but fescue toxicosis cases are primarily reported in temperate southern States.

Typical Treatment

  • Domperidone
  • Milk replacement


Michelle Anderson - Writing for The Horse
Cynthia Gaskill, DVM, PhD - Writing for The Horse
Michelle Anderson - Writing for The Horse
University of Kentucky College of Agriculture, Food, and Environment - Writing for The Horse
Graham Munroe BVSc PhD DESM CertEO DipECVS FRCVS - Writing for Vetlexicon
Michelle S. Mostrom , DVM, MS, PhD, DABVT, DABT - Writing for Merck Veterinary Manual

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