Fescue toxicosis in horses results from ingestion of fescue grass contaminated with the fungus Neotyphodium coenophialum. The fungus produces ergot alkaloid toxins which cause changes in hormone concentrations and blood flow in pregnant mares grazing contaminated pastures or hay.

Nearly all fescue grass contains the fungus, making fescue a particularly high-risk feed for pregnant mares. Affected mares most commonly develop agalactia (lack of milk production), premature placental separation, and prolonged gestation.

These conditions have serious effects on the development of the growing foal, and may result in abortion, stillbirth, or weak-born foals. Dystocia is also common, due to the relatively large size of the foal and malpositioning within the uterus, and can be fatal to both the mare and foal.

Domperidone is used to treat fescue toxicosis in mares by counteracting the effects of ergot alkaloids. While it can induce lactation in mares and prevent some symptoms, it does not treat dysmaturity in the foal.

Prevention of toxicosis involves removing late gestation mares from fescue pastures and providing them with fescue-free hay or grain. Some breeders may also opt to manage or re-seed their pastures to decrease fescue grass concentrations.

Fescue Toxicosis in Horses

Fescue grasses are a group of cool-season perennial grasses belonging to the genus Festuca. They are widely used for livestock pasture and hay, especially in temperate regions. [1]

One of the most common types of fescue used for pasture is tall fescue (Festuca arundinacea), which is known for its hardiness and ability to tolerate a range of conditions, including drought and poor soil quality.

However, tall fescue grass is commonly infected by Neotyphodium coenophialum, a fungal endophyte that produces toxins called ergot alkaloids. [1] Estimates suggest that over 95% of tall fescue pastures have fungal contamination. [2]

The fungus has a symbiotic relationship with fescue, allowing the plant to be more resilient to environmental extremes. [3] But, this fungus also produces detrimental effects in livestock that consume toxic alkaloids in the grass.

Ergot Alkaloids

Ergot alkaloids are a complex group of chemicals produced by fungi infecting grasses. An alkaloid is a naturally occurring organic compound, often derived from plants, that typically contains nitrogen atoms and has a pharmacological effect.

Horses most commonly consume ergot alkaloids by ingesting contaminated hay, pasture, or seed grains. [4] Drought, excessive rain and fertilization all increase the risk of fungal growth and alkaloid contamination on fescue grass. [2]

Ergovaline is the most abundant ergot alkaloid in tall fescue and fescue toxicosis is typically associated with this fungal toxin. [7]

Ergot alkaloids affect horses by binding to cellular receptors and either increasing or decreasing their function. [1] The toxin primarily targets and activates dopamine receptors in horses. This inhibits the release of prolactin, a hormone responsible for milk production in lactating mares.

Alkaloids can also affect blood vessels, restricting blood flow that can lead to damage in affected tissues. [1]

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Pathophysiology

Fescue toxicosis results in three main syndromes in pregnant mares:

  • Agalactia
  • Prolonged gestation
  • Premature placental separation

These syndromes primarily arise through reduced levels of prolactin, the main hormone that stimulates milk production after birth. [1]

Clinical signs in affected mares include lack of milk production, continuing pregnancy well past their anticipated due date, and difficulty foaling. Foals born to affected mares are dysmature, showing symptoms such as a silky hair coat, floppy ears, and poor muscle tone.

Affected foals have a high risk of “dummy foal syndrome” and failure of passive transfer, due to the toxin’s effect on their dam. Most foals require intensive care or hospitalization and have a very poor prognosis.

Agalactia

Agalactia, or lack of milk production, is a hallmark sign of fescue toxicosis. Approximately 90% of mares with fescue toxicosis develop agalactia. [3]

Ergot alkaloids mimic the effects of dopamine on prolactin-secreting cells in the pituitary gland