Endophyte Fescue Toxicity
by Jim Brendemuehl
Much of the fescue grass grown in the United States and Canada is
infected with an endophytic fungus, which produces several types of alkaloids
that are toxic to animals. Fescue toxicity may occur in sheep and cattle, as
well as in horses, although the syndromes are different. In sheep and cattle we
see "fescue foot" and "fescue tail," in which the hooves and
tail literally slough off due to avascular necrosis [death from lack of blood
vessels]the alkaloids cause constriction of the blood vessels to the feet
and tail. Additionally, a "summer slump" syndrome in growing cattle is
characterized by poor condition and lack of weight gains.
In horses the major abnormalities associated with consuming
endophyte-infected fescue are reproductive, with the most common problems
occurring in late gestation. The five classic signs of fescue toxicity in the
pregnant mare are:
- Agalactia--failure to have normal mammary gland development and milk
- Prolonged gestationthe mare carries 30 to 60 days longer than the
normal gestation length of 330 to 350 days, depending on breed.
- Dystociadifficult delivery resulting from a combination of the foal
being abnormally large (30 to 60 day's of extra growth) and failure of the
mare's pelvic ligaments and muscles to relax, as they would in a normal
- Placental abnormalitiespremature placental separation at birth, with
the placenta being extremely thickened and edematous [swollen with fluids].
Horsemen call this condition "red bag," describing the presentation at
delivery of the red velvety chorioallantois [outside] portion of the placenta
instead of the normal glistening white amnion [inside portion of the placenta].
A result of the placenta's premature separation is the disruption of the foal's
oxygen supply before delivery.
- Neonatal death and dysmaturityA final piece of the syndrome is a high
incidence of fetal or neonatal [newborn] death and dysmaturity, also known as "dummy"
foal syndrome. Several factors contribute to this phenomenon, including
disruption of the foal's oxygen supply due to the premature placental
separation, as well as abnormalities in the foal's endocrine, or hormone,
systems--the foal's hormone maturation is markedly delayed, resulting in the
foal being maladjusted (dysmature). Such foals typically do not have normal
suckle reflexes, are unable to stand, have poor thermoregulation [ability to
control body heat], and are depressed and unresponsive. Among the foals that
survive delivery the mortality rate is quite high, even with intensive
management and care.
Although most of the toxicity seen in mares is associated with the last
several months of gestation, problems are also seen earlier in gestation. Mares
grazing infected fescue early in the season (January through April) typically
have a delayed onset of cyclicity [coming into heat]. Mares grazing
endophyte-infected fescue during the first 30 days of gestation have lower
pregnancy rates and higher embryonic loss rates than mares grazing
endophyte-free fescue. After the first 30 days of gestation and up until the
last 30 to 60 days of gestation no greater incidence of abortions has been
Management for pregnant mares and fescue should include the following
- Test all pastures annually for the presence of endophyte. While the fungus
is spread only through seed and can't blow from one infected plant to another,
infected seeds may be carried by birds and in the fecal material of other
animals. Additionally, some so-called endophyte-free seeds are derived from
infected seed that has been stored, which results in the fungus becoming
dormant; after several years the fungus can reemerge, resulting in toxic
- If the fescue is determined to be endophyte infected, pregnant mares should
not be allowed to graze the infected fields or eat infected fescue hay within
two months of their due date. The level of toxic alkaloids in hay is not reduced
- If for some reason pregnant mares cannot be removed from infected fescue
pastures, they should be treated with the drug domperidone, available through
your veterinarian. Although domperidone can be useful in cases where mares are
not removed from infected fescue pastures, it is not a magic bullet. Make every
effort to prevent pregnant mares from consuming infected fescue grass or hay
during the last 60 days of gestation or while they are nursing.
Supplementing the diet of a pregnant mare grazing infected fescue with grain
or legume (alfalfa) hay to dilute the toxins has not proven to be effective in
preventing problems, as it has been in cattle. Mares show toxicity when
consuming levels of less than 50 parts per billion (ppb), making them much more
sensitive than cows to the alkaloids.
Jim Brendemuehl, DVM, PhD, is an equine
Extension veterinarian at the University of Illinois Veterinary Teaching
Hospital in Urbana, where one of his special interests is fescue toxicity in
mares. He is one of this site's consulting virtual veterinarians available
online to respond to your concerns about draft horse reproduction. This article