Fescue Toxicosis

Dr. Kimberley Carr

With foaling season upon us it is important to discuss fescue grass and the negative impact it can have on late term gestation in broodmares. Fescue is commonly found in Oklahoma pastures and is known for being a durable, drought-resistant grass due to a symbiotic relationship with an endophytic fungus called Acremonium coenophialum (or Neotyphdium coenophialum). Research has found that over 85% of Oklahoma’s fescue grass has been infected by this endophyte. Unfortunately, there are no visual differences between regular fescue and infected fescue so horse owners cannot know if their mares are safe to continue grazing their pastures without further testing. Furthermore, there are no treatment regimens that will successfully remove the endophyte from the infected grass, so mare owners are best off if they remove the mares from infected pastures during the late stage of gestation.

            The endophyte, Acremonium coenophialum, survives by living inside intercellular spaces on blades of fescue grass. Since it does not actually enter the host plant’s cells, it is unable to detect the ongoing infection and continues to grow and reproduce seeds that are also infected by endophytes. These infectious endophytes are responsible for producing various alkaloid toxins that benefit the grass in many ways. For example, peramine is an alklaoid toxin that has been proven to prevent the grass from insect damage. Experimental removal of these endophytes has produced “non-infectious” fescue that was shown to be more susceptible to root disease, insects and drought. The primary alkaloid toxin that horse owners should be concerned with is called ergovaline. Studies have shown that ergovaline can be found in every part of infected fescue grass but concentrates most in the seed heads.  

            When late term broodmares graze on infected fescue, they inevitably consume the ergovaline toxin, which can have negatives effects on hormone production and the progression of their pregnancy. Ergovaline will affect the mare’s anterior pituitary gland and cause a decrease in the secretion of prolactin, a hormone that is necessary for mammary gland development and milk production. Often, the only clinical sign of fescue toxicosis is the lack of an udder as the birthing date approaches. Researchers are also studying the effects ergovaline has on pituitary gland development in the fetus. It is possible this toxin also inhibits developments of the fetal anterior pituitary gland, which may help explain why foals from affected mares are stillborn or suffer from neonatal maladjustment syndrome (“dummy foal” syndrome).

            Unfortunately, broodmares affected by fescue toxicity do not show many clinical signs before parturition begins so it is important that mare owners are carefully monitoring their udder development, particularly in the last 21 days of gestation. These mares will also commonly carry past their due dates because the toxin impacts the normal hormone signaling between the mare and the foal, which slows down the progression of the pregnancy. Some of these mares will abort or have stillborn foals, while others will struggle with dystocia because the longer gestational time will lead to foals that are enlarged or malpositioned and cannot pass through the birthing canal without assistance. These mares can also be more likely to a have thickened, edematous placenta and a higher rate of placental retention.  

            The best treatment for fescue toxicosis in mares is to avoid consumption of infected fescue during the late stages of her pregnancy. It is recommended that owners remove their mares from possible sources of infected fescue 90-120 days before their due dates. However, if the mare is allowed to graze on fescue, there are things that can be done to try and minimize the effects of the toxicity. Agalactia is the most common clinical sign of fescue toxicosis in mares so if owners notice that the mare is not developing an udder within the last two-three weeks before the due date, it is recommended that they start her on domperidone. Domperidone will counteract the effects ergovaline has on the pituitary gland and allow for serum prolactin levels to increase, which stimulates udder development and milk production. Mares that are not caught in time or not treated with domperidone will have no milk supply for their foal once it is born. Also, these mares will fail to produce any colostrum, which means the foals will be unable to obtain any antibodies from their mother and will need to be supplemented with colostrum from another mare or be administered intravenous plasma from a donor horse in order to have any immune function for the first several months of their life.

It is also very important these mares be monitored for prolonged gestation, which can be a warning sign for a possible dystocia. In these cases, mare owners should be prepared for the mare to have difficulties with parturition due to increased fetal size or fetal malpositioning. Any owner concerned about fescue toxicosis should speak with their veterinarian and have an emergency plan in place for foaling. It is often best these mares foal out in a hospital where immediate intervention can be provided if a dystocia does occur. Affected mares can also have thick, edematous fetal membranes, which can lead to abortion or stillborn foals due to placental insufficiencies at the end of the pregnancy. However, foals born alive can offer suffer from neonatal maladjustment syndrome, which can have varying degrees of severity. Some of these foals may be unable to stand while others may not have a strong suckle reflex or the ability to find and latch on to the mare’s teat.  These foals will require a significant amount of intervention and supportive care in order to survive and begin to function normally.

Owners should be aware of fescue and the many negative impacts it can have on their mare’s pregnancies and foals. They are encouraged to seek veterinary advice as soon as possible if there are any questions or concerns about possible fescue toxicity in their mares. Early intervention can be the key to saving the mare and foal as well as a lot of money and potential heartache during foaling season. 

Read more in the June 2020 issue of Oklahoma Farm & Ranch.