Fescue Toxicosis in Cattle

Filed Under: Cows, Poisoning

Tall fescue (Festuca elatior or F. arundinace) is among the most common cool season pasture grasses 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. "Endophyte" describes the location of the fungal growth within the grass as endo=within and phyte=plant. The fungus does not affect the growth of the fescue and, in fact, the fungus-infected grass has a growth advantage because it is more drought, parasite, insect, and stress resistant than non-infected fescue. The toxins produced actually make the grass more resistant to herbivores by poisoning them. Pastures containing mixed varieties of fescue will eventually be replaced by the endophyte-contaminated varieties due to the advantageous symbiotic relationship between the grass and the fungus.

N. coenophialum produces multiple toxins including: ergot alkaloids, such as the highly toxic ergovaline; the primary mycotoxins responsible for fescue poisoning in ruminants; and the minor alkaloids, peramine and loline. Peramine and loline alkaloids are primarily deterrents to insects that would ordinarily feed on the grass.

The concentration of ergovaline varies with the seasons. Ergovaline reaches peak concentrations in the grass’s 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 the 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 present in hay grown in 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 and is responsible for the growth of the mammary gland following parturition (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 poisoning. Ruminants, however, have prolactin produced from the placental tissues at parturition which is partially able to overcome the lack of pituitary prolactin, thereby resulting in a decrease rather than a complete cessation of milk production.

Low progesterone production is also a consequence of low prolactin levels. The imbalance of reproductive hormones will lead to early pregnancy problems in cattle. Ergovaline may also inhibit implantation of an ovum in the uterus, thereby resulting in early pregnancy losses.

Diminished prolactin levels also causes deregulation of the thermoregulatory centers of the brain causing hyperthermia or hypothermia. This deregulation is clinically apparent when the environmental temperatures are outside the thermo-neutral range of the animal species affected.

Two major syndromes associated with ergot alkaloids in cattle and sheep are called fescue summer toxicosis and fescue foot, and there is one minor syndrome known as fat necrosis, or lipomatosis. Interestingly enough, fescue summer toxicosis only occurs during the summer months while fescue foot only occurs in the winter.

Fescue summer toxicosis, also known as summer slump or epidemic hyperthermia, is a period of low milk production or a failure to grow and thrive on what seems to look like a nutritious pasture. The condition is worse when temperatures rise above 87ºF or 31ºC. The temperature of affected cattle may rise as high as 104.5ºF or 40.5ºC (hyperthermia). Animals may appear dyspnic, they may hypersalivate, be anorexic (lack of an appetite), and have rough hair coats.

Calves may have a delayed onset of puberty, and reduced conception rates may be seen in adults. The decrease in conception is thought to be due to early embryonic death and not associated with late term abortions or stillborn births that are seen in horses. Cattle may compulsively seek out water and shade. Hyperthermia may not resolve until 6 weeks after the cattle are removed from the offending pasture. The mortality rate with summer slump is negligible.

The low milk production is a result of low prolactin levels, even when premilking stimuli is applied. The prolactin levels may be significantly boosted by the administration of metoclopramide which is a dopamine antagonist. Hyperthermia is thought to be a result of vasoconstriction to cutaneous areas, which reduces heat loss.

Fescue Foot is associated with vasoconstriction of blood vessels which results in an inadequate blood supply to the extremities. Fescue foot problems are more likely to occur when environmental temperatures are less than 8ºC (46.4ºF) and when dietary ergovaline concentrations exceed 400 ppb. Fescue foot is not as common as is summer slump. Clinical signs usually develop within 10 to 14 days of being turned onto the pasture during cold weather. Affected cattle show various degrees of lameness followed in 2 or more weeks by gangrene and sloughing of the extremities, especially the digits (hooves) and to a lesser extent the tail. Vasoconstriction tends to be more severe in the rear legs, which are often visually swollen as compared to the forelegs. Vasoconstriction in the back legs occurs between the coronary band and the fetlock area of the hooves. The area proximal to the vasoconstriction is often congested, which is seen as swelling. Frostbite may be a complicating factor due to vasoconstriction from the cold. The incidence in a herd may be up to 10%. Cattle permanently pastured on a fescue field do not appear to be affected.

Lipomatosis is a syndrome affecting mature cattle. The syndrome is actually a necrosis of the abdominal fat stores in affected animals. The clinical signs of disease depend on the fat deposit affected, and in many cases, the condition is not noticed clinically, and discovery is often made at necropsy due to an unrelated cause of death or at slaughter of the affected animals. When mesenteric fat is involved there will be a scant amount of feces produced, and the cow may appear bloated or an intestinal obstruction may occur. When perirenal fat (fat surrounding the kidneys) is affected no clinical signs of disease are noted. Fat in the pelvic canal may become necrotic and hardened resulting in dystocia problems (problems delivering a calf). Fat necrosis is most common in cattle having the lowest concentrations of serum cholesterol.

The disease may be controlled by growing endophyte-resistant varieties of fescue, and by rotating cattle through different pasture grasses and clover varieties. The endophyte actually invades the seed heads of the affected grass, thereby ensuring its transfer to the next generation of grass. The parental plants may be treated with the fungicide benomyl, which results in pastures with a lower rate of fungal infection by killing the fungus within the seed head, making the grass less toxic to the cattle that graze on it. The oral administration of thiabendazole just before cattle are turned out on affected pasture helps prevent the onset of clinical signs. The recommended dose of thiabendazole is 5 g/45.5 kg BW weekly.

Ammoniation of hay is a process by which the ergovaline content may be decreased until it is safe to feed. Although the process is effective, ammoniation is a time intensive and costly procedure. The hay is enclosed in an air-tight tent and anhydrous ammonia gas is pumped into the tent for a specified amount of time. The ergovaline content of stored fescue hay is decreased by midwinter.

Supplementing livestock with various levels of concentrates will decrease the overall dose of ergovaline obtained from the fescue. Various feed additives are available to decrease the ergovaline affects with varying degrees of success. FEB-200® is purported to bind ergovaline in the gastrointestinal tract, thereby preventing its absorption and limiting its effects. Tasco®, a supplement made from seaweed is proposed to decrease the immunosuppressive effects seen in cattle as a consequence of ergovaline consumption. Additional supplements and food additives are under development.

There are strains of fescue in development that produce the peramine of tall fescue, but minimal amounts or no ergot alkaloids. MaxQ® and ARK Plus® show promise as varieties of fescue that can be grown without adversely affecting cattle, horses, or sheep, and still have the insect and drought resistance of Kentucky-31 fescue.

References:

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

Guyton, Arthur. Textbook of Medical Physiology. W.B. Saunders Co. 5th Edition. 1976. Pp. 992.

Radostits, Otto and Clive Gay, et al. Veterinary Medicine. 10th 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.

Topics: plants, poisoning

Symptoms: hypersalivation, rough coat

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