Fescue Toxicosis in Horses

Is your mare having trouble with foaling? Think your foaling dates are almost a month off? Is your mare producing enough milk to care for her foal? Did you get what looks like a full term foal only to find it stillborn? This unfortunate situation may be a result of your pasture.

Tall fescue (Festuca elatior or F. arundinace) is among the most common cool season pasture grass grown in North America and in other countries having a temperate climate. Almost all of the pasture planted before 1980 is infected with Neotyphodium coenophialum, a microscopic fungus or endophyte. The fungus does not effect the growth of the fescue and, in fact, the fungus-infected grass has a growth advantage because it is more drought and stress-resistant than non-infected fescue.

The fungus produces toxic ergot alkaloids including ergovaline, peramine, and ergine.

The most potent of the ergot alkaloids is ergovaline. The concentration of ergovaline varies with the seasons. Ergovaline reaches peak concentrations in the seed heads during the summer months and will decrease during the early fall, followed by a rebound in levels with the fall regrowth. Drought, and reversely rainy conditions, will increase the ergovaline concentrations contained within the plant. Fertilization of fescue with nitrogen and phosphorous-based fertilizers or chicken litter will also increase the concentration of ergot alkaloids. Due to the previously mentioned conditions, the concentration of ergovaline, and therefore the resulting toxicities, would be expected to vary from season to season as well as from year to year. Ergovaline is also present in hay grown from affected pastures.

The variety of fescue also determines the extent to which the fescue is affected by the fungus. Kentucky-31 is the variety of fescue most affected by the endophytes with >95% of the pastures testing positive for the fungus. Kenhy, Mo-96, and Kenmont varieties of fescue are intermediate in their sensitivity to N. coenophialum, with the Fawn variety being the least affected by the fungus. In the U.S. alone, the fungus is estimated to cost producers more than 1 billion dollars of losses in livestock production yearly.

Ergovaline is a potent vasoconstrictor and a strong prolactin inhibitor. Prolactin is a hormone produced by the pituitary gland that is responsible for the growth of the mammary gland following parturition (the act of giving birth) and the production of milk. A complete lack of prolactin production results in no milk production (agalactia) in the horse or the pig following fescue poisoning.

Ergot alkaloids have been linked to prolonged gestation, abortions and fetal death in pregnant mares. Mares in late gestation (over 300 days pregnant) are the most sensitive to the effects of ergovaline. Abortions tend to occur late in gestation. The length of gestation (the period a mare carries her foal) tends to be 20 to 27 days longer than normal. Dystocia or problems with delivery will occur due to the large size of the fetus. The large fetus may result in vaginal tearing during delivery thereby causing rebreeding problems or even death of the mare. Additional clinical signs of poisoning include retained, thickened placentas with secondary endometritis (infection of the lining of the uterus); laminitis; and septicemia. Mares will have a profound drop in milk production and typically do not have normal udder development. Affected mares may not show signs of impending parturition (giving birth). Mare mortality is increased.

Foals that are not born stillborn are weak. Although larger and ganglier, these foals are immature in their development. Foals, once born, have unerupted incisors and overgrown hooves.

Reproductive efficiency in mares will be significantly decreased in non-pregnant mares. These mares will have a prolonged luteal phase to their reproductive cycle. The number of cycles bred per pregnancy will increase, as will the possibility of early embryonic death.

Pregnant mares should only be maintained on endophyte-free pastures especially when in late gestation. Domperidone or Equidone®, an oral gel, has been used with limited success. Domperidone is given once daily for 10 to 14 days before the expected date of parturition. Once foaling has occurred, domperidone is used twice daily if needed to stimulate milk production.

Removal of a mare in the last month of pregnancy from a fescue pasture and the avoidance of fescue hay during this same period will help prevent a foaling problem.

References:

Gupta, Ramesh Editor. Veterinary Toxicology. Elsevier Inc. 2007. Pp. 907-913.

Radostits, Otto and Clive Gay et al. Veterinary Medicine. 10^th Edition. Saunders. 2007. P. 1901-1902.

Stegelmeier, Bryan DVM. “Third of four articles: Myotoxic Plants: Identifying Poisoning Signs, Sequelae.” DVM. October 2007. Pp. 16E-17E.