Major mycotoxins: Risks for equine industry

Mycotoxins in horses cause immunosuppression, organ damage, colic, poor growth, reduced reproductive performance, and even death. Horses are susceptible to a wide range of mycotoxins including aflatoxins, trichothecenes such as deoxynivalenol, and T-2 toxin, ergot alkaloids, ochratoxin, fumonisins, zearalenone, slaframine, and tremorgenic. However, factors such as age, general health, and immune status affect horse susceptibility to mycotoxins.

Aflatoxins

Aflatoxins are produced by Aspergillus flavus and Aspergillus parasiticus and they are carcinogenic, teratogenic, hepatotoxic, and nephrotoxic substances. Aspergillus flavus and Aspergillus parasiticus usually form on corn, peanuts, and cottonseeds, and other cereal grains and occasionally on hay and straw. Storing grains at a high moisture level, and failing to clean harvesting equipment, augers, and storage bins before each use enhance the risk for aflatoxin development.

• Affects: Aflatoxicosis is rare in horses. It leads to immune system suppression, reduced vaccine and therapy efficiency, decreased host resistance to environmental and microbial stressors, and enhanced susceptibility to diseases. In addition, aflatoxins reduce water and feed intake, lower performance, and cause feed refusal, weight loss, lethargy, bloody diarrhoea, liver complications such as icterus, kidney damage, and even death.

Furthermore, aflatoxin can be transferred in utero from the mare to the foal affecting the biological and immunological responsiveness of neonatal foals.

Trichothecenes

Trichothecene mycotoxins are the most potent small-molecule inhibitors of protein synthesis, and they include T2 toxin, HT2 toxin, diacetoxyscirpenol, monoacetoxyscirpenol, neosolaniol, 8- acetoxyneosolaniol, 4-deacetylneosolaniol, nivalenol, 4-acetoxynivalenol, deoxynivalenol (DON) or vomitoxin, and 3‐acetyldeoxynivalenol.

• Affects: Trichothecenes interfere with DNA replication and protein synthesis; thus, negatively affecting many vital processes and organs. T-2 toxin suppresses immune system, changes the leukocyte count, decreases antibody formation, reduces horse resistance to environmental and microbial stressors, increases susceptibility to diseases, reduces growth performance, and leads to feed refusal, haemorrhage, bloody diarrhoea, oral and dermal lesions, and tail necrosis.

DON

The DON is the most common trichothecene produced by Fusarium species in cereal grains in high concentration. The DON is a relatively thermostable compound which is soluble in water.

• Affects: Prolonged dietary exposure of horses to DON leads to impaired feed intake, decreased weight gain, altered immune function, diarrhoea, colic, decreased performance, immunosuppression, impaired gut health, decreased resistance to environmental and microbial stressors, and increased susceptibility to diseases.

Information related to the effects of DON on equines and the limiting values or safe concentrations of DON for horses is scarce and inconsistent. Research studies show a large discrepancy between the DON levels causing effects in horses. While some studies imply that horses are not very sensitive to DON, others showed reduced feed consumption at lower levels in corn and wheat (14.9 and 11.8 mg/kg total DON, respectively).

Ergot alkaloids

Ergotism Claviceps purpurea and Claviceps paspali exist in the seeds of bluegrass, bromegrass, ryegrass, and in cereal grains such as rye, wheat, and barley and they produce a mycotoxin-containing sclerotia also known as ergot.

• Affects: Ergot alkaloids are vasoconstrictive, immunosuppressive, and neurotoxic, and they instigate ear, tail and hoof necrosis, and lameness. In addition, ergot alkaloids in mares cause reproductive failure, stillbirths, reduced pregnancy rate, abortions, prolonged gestation, poor mammary gland development, prolonged uterine involution, placental and foetal abnormalities, dystocia, retained placenta, obstructed labour, low prolactin, and colostrum production, agalactia and staggers. In foals, ergot alkaloids trigger respiratory failure, skin, umbilicus, bone, and muscle abnormalities, blindness, testicular atrophy, and decreased immunoglobulin concentration.

Ochratoxins

Ochratoxins are produced by Penicillium and Aspergillus species and they are nephrotoxic, hepatotoxic, and carcinogenic.

• Affects: Ochratoxins increase water uptake and excretion, and cause kidney and liver damage, bladder dysfunction, diarrhoea, and immune suppression. In addition, ochratoxins cross the placental barrier in mares and affect the foetus.

Fumonisins

Fumonisins are isolated from Fusarium moniliforme and Fusarium verticillioides cultures and they are identified in 6 different forms (A1, A2, B1, B2, B3, and B4).

• Affects: Fumonisins detrimentally affect central nervous system, liver, heart, and immune system. Fumonisins intoxication results in the development of equine leukoencephalomalacia (also known as blind staggers or mouldy corn disease), liquefaction of the brain, depression, reduced feed intake, lethargy, ataxia, anorexia, incoordination, hyper excitability, sweating, blindness, and death. Older horses are more susceptible than younger horses to fumonisins intoxication.

Zearalenone

Zearalenone is produced rapidly during wet conditions especially in late summer and early autumn by Fusarium fungi.

• Affects: Zearalenone is quickly absorbed from the gastrointestinal tract after ingestion, and it causes severe reproductive disorders, abortions, stillbirths, oedematous vulvas, prolapsed vaginas, oversized uteri, and internal haemorrhage in mares and impaired semen quality and severe flaccidity of the genitalia in stallions. In addition, zearalenone intoxication leads to respiratory failure, blindness, and eventually death in horses.

Slaframine

Slaframine is an alkaloid mycotoxin formed by the fungus Rhizoctonia leguminicola under wet and humid conditions usually in the spring or fall.

• Affects: Slaframine toxicity causes excessive salivation, increased drinking, weight loss, diarrhea, colic, dehydration, and frequent urination.

Tremorgenic

Tremorgenic mycotoxins are formed by Claviceps paspali in paspalum grass seedheads, such as Dallis and Bahia grasses. This mycotoxin develops on the lower outer leaf sheath and seeds

• Affects: It can cause mild excitability, muscle tremors, muscle spasms, jerky gait, exaggerated leg movements and occasionally seizures.

Concluding remarks

Mycotoxins are naturally occurring toxic substances which are produced by moulds and fungi in horse feed. Mycotoxins cause a variety of negative effects in horses including colic, weight loss, reduced feed and water intake, immune suppression, impaired performance, failure in reproductive performance, lethargy, bloody diarrhoea, and liver, kidney, and central nervous system damages. The risk of equine mycotoxin intoxication increases during the spring and summer months when the weather is damp with high humidity. Although further research is required to investigate the stability of toxins in the horse diet to prevent negative impacts on horse health and welfare.

Source: Mycotoxins in the Equine Diet by Amy Parker, M.S.