Toxic forages such as fescues are the most common causes of poisoning in horses. Toxins are produced in tall fescue forage or in the seeds contaminated with an endophytic fungus.
The alkaloid-producing fungus proliferates most during the lush stages of growth during springtime and also when autumn rains follow a dry summer and closely associates with the growing flower and seed head. It is through contamination of the seed that infection by fescue toxins are spread.
The consequences of fescue toxicity in horses are almost exclusively related to poor reproduction. Mares that consume large amounts of endophyte-infested tall fescue can suffer from a number of reproductive problems, and the range of fetal deaths in the last third of pregnancy for herds affected by toxicity varies from 0 to 100%.
The normal gestation length in horses is approximately 11 months. However, mares that graze on endophyte-infected fescue can have pregnancies that last up to 13 months or more.
This becomes a problem because the fetus continues to grow during the extra time, which can result in a difficult birth. In addition, foals born after prolonged gestation often appear to be immature despite their extra fetal development.
The characteristics of a foal born after prolonged gestation include a large frame size and poorly developed muscles. Also, some foals may have overgrown hoofs, while others may have premature and irregular eruption of baby teeth. These foals can also exhibit hyperthyroidism, resulting in poor suckling reflex, incoordination and low body temperature.
The newborn foal can suffer greatly from the effects of fescue toxicosis. Many foals born to mares pastured on endophyte-infected fescue have reduced immunity, which makes them highly susceptible to infections.
Normally, foals receive antibodies from the mare in the form of colostrum, consumed almost immediately after birth. However, mares affected with fescue toxicosis produce a limited milk supply or their milk may be nonexistent. When this happens, the newborn foal is not able to consume enough colostrum to have an adequate transfer of antibodies from the mother to the foal, increasing the risk for contraction of illnesses and infections.
If a mare does not supply an adequate amount of milk or colostrum to the newborn foal, the foal should be given colostrum from a donor mare, or colostrum that is stored in a colostrum bank. If neither of these options is available, the foal should be given colostrum from another species of animal, followed by intravenous antibodies and a diet of supplemental milk. Cow or goat milk is often used, although additional sugar (in the form of dextrose) is added to more closely resemble milk produced by mares.
- Reduced feed intake
- Prolonged gestation
- Premature separation of the chorion
- Thickened placenta
- Retained placenta
- Suppression of lactation
- Lower pregnancy rates
- Early embryonic death
- Poor immunity in foals
- Abnormal foal maturation
- Weakness of foal
Alkaloids produced by these organisms known as endophytes appear to be the culprits that cause fescue toxicity. These alkaloids help chemically defend the fescue plants from insect pests and also have a positve influence on plant hormones and/or growth regulators. Unfortunately, when eaten by a horse, they interfere with body temperature regulation and feed intake. In addition, these alkaloids can constrict blood vessels and reduce blood flow to the extremities. All of which can affect the pregnant mare.
Prevention of fescue toxicity includes good pasture maintenance by mowing grass pastures to keep them in a vegetative state rather than letting them develop seed heads. This will minimize the opportunity for airborne endophyte toxins from invading pasture plants.
Be careful what you feed to mares. Ensure that all pasture/hay seed mixes containing tall fescue and perennial rye grass are guaranteed endophyte free. A sample of the seed can be sent to a laboratory for testing prior to seeding.
Samples of hay and pasture can be tested for the presence of fescue toxins. The testing for alkaloids is dependent on the laboratory having adequate control standards available for comparison. Choose a laboratory which uses HPLC methodology, since thin layer chromatography (TLC) is not sensitive enough to identify ergot alkaloid levels less than 1 ppm. Pregnant mares are known to have foaling problems at half of this level.
Cooperate with an experienced veterinarian who can assis you in using a foaling record form and examining each placenta in cases of mares where fescue toxicity is suspected.
Submit all heavy placentas and dead newborn foals to the laboratory for necropsy.
The best form of treating fescue toxicosis in horses is prevention. However, this is not always possible. Nonpregnant horses can safely graze on endophye-infected fescue pastures, as can broodmares during the second trimester of pregnancy. If a pregnant horse must graze on endophyte-infected fescue, there are pharmaceuticals available to help reduce the adverse effects.
Some of the available drugs include:
A veterinarian should be consulted any time fescue toxicosis in a pregnant horse is suspected.
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