With winter weather and less grass and hay available in pastures, many horse owners turn to grain and hay purchased from various sources to feed their horses. It pays to get all horse feed from reputable sources that use quality control from harvest to the final product sold to horse owners to prevent incapacitating illness or even death.
Also, it is beneficial to review the clinical signs and causes of feed-associated poisonings in horses. Since long-term poisonings often have vague clinical signs, they are often misdiagnosed or confused with other conditions.
The importance of knowing the safety and palatability of the feed given horses was brought home rather forcefully in 1997 when eight horses at a training facility died within a 36 hour period following acute onset of ataxia, paralysis and tachycardia. One horse exhibited signs of colic and sweating. Supportive therapy including fluids and antibiotics was administered but all eight horses died within 36 hours of onset of the signs.
The horses were fed only alfalfa hay. Water was from a well that was also used for the household. Examination of four of the horses revealed blood tinged foam accumulated around the nostrils with smaller amounts in the trachea of some.
Gas chromatography of stomach contents identified monensin in three of the horses. The source of the monensin was never determined.
Monensin is a drug in the ionophores group used as growth-promoting agents in ruminants and as coccidiostats in poultry and other birds. There are several drugs in this group: monensin, lasolocid, laidlomycin, narasin and salinomycin.
Monensin is marketed under the trade name Rumensin and is commercially produced in large quantities for addition to premixed and pelleted or bulk feeds fed to ruminants.
Horses are extremely susceptible to ionophore poisoning. Poisoning by the ionophores is usually the result of mixing errors at the mill resulting in higher than acceptable levels in feed for ruminants or inadvertent addition to food intended for horses.
Horses that work in cattle feed lots have been poisoned by consuming cattle feed that has been correctly mixed for cattle, but is toxic to horses. The pharmacologic action of the drugs is the inhibition of sodium and potassium ion transport across cell membranes in the horse's system.
This leads to mitochondrial failure, failure of calcium ion retrieval from the cytosol and, ultimately, myofibrillar hypercontraction and necrosis. The clinical signs begin 12-24 hours after consumption of an acutely toxic dose, but may be delayed for days or weeks in the case of chronic low level cases of toxicity.
Signs of acute intoxication in horses may include some or all of the following depending on dose and the individual:
- intermittent sweating
- muscle weakness
- respiratory distress
Depending on dose and individual susceptibility, death can occur in less than 24 hours. Animals surviving the acute intoxication and those with chronic intoxication may exhibit signs of progressive congestive heart failure, poor growth, and poor weight gain due to the toxic effects on the myocardium.
Sudden deaths in the weeks or even months following intoxication have been reported. Ionophores can be detected in stomach contents using thin-layer chromatography. Suspect feeds can be tested by the same method.
Treatment of ionophore poisoning is primarily supportive. Activated charcoal to block absorption is a priority and saline cathartics to encourage elimination may be helpful. Aggressive fluid therapy to correct hypovolemia and support cardiac and renal function are suggested.
Because most ionophore poisonings involve contaminated or poorly mixed feeds, it is important that horse owners deal with reputable feed companies with quality assurance programs in place and enforced in all facilities that produce equine nutritional products.
A fungus-produced ionophore that can cause toxicosis in pastured horses if weather conditions are favorable appears to also exist. Most of these cases have been in cool, wet weather, and research is ongoing to determine the exact source of this particular toxin.
Additional Sources of Toxins in Horse Feed
Red maple leaves are highly toxic to horses. Death is common in cases of ingestion of red maple leaves due to massive destruction of red blood cells. Signs that a horse has ingested red maple leaves include breathing difficulties, jaundice, dark brown urine, increased heart and respiratory rates and lethargy.
Fifty to seventy-five percent of horses that ingest a large quantity of maple leaves (three pounds or more) either die or need to be euthanized.
Black Walnut and choke cherry are other deadly plants for horses and any horse owner should make sure that horses do not have access to these plants and that hay or other feed is not contaminated with leaves from these plants.
Prussic acid poisoning caused by the cyanide that is produced in plants under certain weather conditons include black cherry, choke cherry, pin cherry and grasses such as Johnson grass, sudan grass common sorghum and arrow grass. Frost and drought conditions can cause these plants containing prussic acid to become more toxic. If large amounts are eaten by a hungry horse, death can occur within minutes.
Excess salivation, breathing difficulties, muscle tremors and rapid heart rate signal the onset of prussic poisoning.
Making sure that all horse feed is toxin free and of the best quality possible should be one of the horse owner's highest priorities.