horse

 

 

Horses and ponies can be exposed to mycotoxins in pasture, as well as in mouldy forage and hard feed.

The majority of horses are kept out at pasture for at least part of their daily routine, with many being kept on grass for the majority of their lives. This leads them to potential exposure to toxins from fungi that live on actively growing pasture, as well as from preserved forages (hay or baleage) used during cold seasons and prepared compound feeds.

Research into mycotoxins has shown they present a real threat to the performance and health of horses. Although the majority of this work has been extrapolated from other agricultural species, such as pigs and cows, significant new research is now being carried out in horses.

Horses are not typically classified as agricultural animals and are used mainly for athletic work, competition and/or breeding. Many horses are afforded ‘companion animal’ status, including ponies, donkeys and those that have been retired from racing, for example. Due to their status in human society, horses have a longer lifespan than agricultural animals, some living to be 30 years old or more and their exposure to toxins can be long term as a result. Horses used for competitions, such as racing, eventing, dressage, polo, long distance riding and show jumping are high value animals and economic losses from poor performance, health problems and even loss of use due to mycotoxins can be significant. These horses are extremely fit and, therefore, subject to higher levels of stress, not only from the high work level but also travelling and competing. This has a negative effect on the immune system, which consequently may be more susceptible to the effects of mycotoxins. The high value of breeding stock of both sexes makes the negative aspects impact of mycotoxicoses on fertility and successful foaling an important economic consideration within the equine industry.

 

Problems commonly associated with mycotoxicoses include:

  1. Reduced growth rates
  2. Decreased appetite
  3. Poor performance
  4. Colic
  5. Respiratory problems
  6. Hypersensitivity
  7. Reproductive and fertility problems
  8. Increased susceptibility to disease
  9. Brain lesions
  10. Neurological disorders
  11. Organ damage - especially liver, kidneys

In order to effectively recognise mycotoxicoses, horses should be carefully monitored for symptoms, particularly poor performance and respiratory problems in working horses or problems with fertility and pregnancy and poor growth in breeding and young stock. These vague symptoms should be investigated for possible mycotoxin involvement. Symptoms are often very general and can vary greatly, making proper diagnosis difficult. Careful recognition of symptoms and post-mortem analyses combined with adequate feed analyses, provide the most accurate means of a mycotoxicoses diagnosis.

The degree of exposure to mycotoxins i.e. length of time and the concentration of toxins in the feed will affect the symptoms shown. Long-term exposure to low levels of mycotoxins may produce quite vague symptoms. In addition, contaminated feed is likely to contain more than one fungal species and, therefore, multiple mycotoxins, which can exacerbate toxicity, as it is known that multiple contaminations can act synergistically. Clinical observations suggest that once a horse has initially succumbed to mycotoxicoses, it develops a heightened sensitivity to further exposure to the toxin, meaning that it shows a strong reaction even when exposed to very low levels of toxin. This may explain why some horses are so much more sensitive to toxins e.g. Lolitrem in ryegrass staggers than others kept under the same conditions and feeding regime.

Other factors will also affect the response of horses to mycotoxin exposure including breed, age, workload, stress level, nutritional status and immune status of the horse. Horses in hard work or lactating mares are likely to have much higher intakes of cereal-based concentrate feeds and this should also be taken into account, when assessing possible mycotoxin exposure.

 

 


Pasture and Storage Forages

As with other grazing mammals, horses and ponies are at risk from toxin ingestion from fungi that live on pasture plants.

Although hay is the preserved forage traditionally used for horse feed during winter months or whenever grass is not available, the popularity of using small bales of high dry matter silage (haylage or baleage) has increased in recent years. This is because silage is easier and less weather-dependent to make and, if well made, has a higher nutritional value and is more palatable than dried forage. Commercial inoculants are available to optimise the ensiling process and minimise incorrect microbial growth. For any preserved and stored forages, such as hay and baleage, great care must be taken not to feed mouldy bales to horses, as this can expose them unnecessarily to toxins from certain types of fungal growth.

 

 

Compound Feed

Unlike agricultural species, horses are fed according to the workload they perform. Some horses and ponies may never receive any compound feed and others may be offered such feeds several times a day. Historically, compound and other supplemental horse feeds have been based on grain, which can be contaminated with fungal toxins from the field or within storage.

A major problem with mycotoxicoses in horses is the fact that many leisure horse owners do not check the quality of their feedstuffs. For the leisure horse owner, price rather than quality is often the main consideration in purchasing feed. For those with only low numbers or single animals, feed may be left in poor storage conditions for considerable length of time – facilitating the growth of moulds and the production of toxins.

Any mouldy or suspect feed that has been opened for a long period of time or stored in poor conditions should never be fed to horses, as these feedstuffs carry a high risk of mycotoxin contamination.

 

 

Bedding

In temperate and colder areas of the world, horses are often kept indoors for part of the year. This exposes them to sources of mycotoxins from bedding materials, such as straw, which is potentially available for them to consume. In addition, other equids, such as donkeys and mules, may be fed on straw as this more closely matches their need for a very high fibre diet. Care must be taken in selecting straw that is not mouldy to protect animals against ingesting potential toxins.

 

 

Aflatoxin

Aflatoxins are difuranocoumarin derivatives produced by many strains of Aspergillus parasiticus and Aspergillus flavus. Aflatoxins are found in cereals, such as maize, which may be contaminated whilst growing in the field or if poorly stored. Crops become contaminated with aflatoxin when environmental conditions (warmth and humidity) are favourable to fungal growth and toxin production.

Aflatoxin ingestion can have severe consequences for horses and ponies. Consumption of 130 ppb in compound equine feed formulated with maize has been observed to cause death (Vesonder et al. 1991). In order to prevent exposure to aflatoxin, it is advisable to ensure horses are not fed potentially contaminated feedstuffs. High risk feedstuffs include by-products from third-world or developing countries (palm kernel meal, copra) moist feeds (brewing by-products) and hulls of peanuts, which are especially associated with aflatoxin.

Aflatoxins are hepatotoxic and carcinogenic. They cause fatty changes in the liver leading to hepatocyte degeneration, necrosis, and altered liver function. Suppression of liver protein synthesis is a main factor resulting in growth suppression in young animals and the toxin is known to interfere with vitamin D metabolism, contributing to loss of bone strength. By reducing bile salt production, aflatoxin negatively affects lipid and pigment absorption. Additionally the metabolism of other minerals including iron, phosphorus and copper are also affected by aflatoxin.

Various reports of aflatoxicosis in horses are available, although, as for other mycotoxins in this species, most are clinical reports rather than controlled research investigations. Feeding contaminated corn on a farm in Arkansas resulted in the death of three horses and many others suffered health problems. Investigations showed severe hepatic necrosis suggesting involvement of aflatoxin – and feed analyses showed the presence of numerous colonies of Aspergillus flavus and chemical analyses showed the presence of aflatoxin B1 B2 and M1. Ponies fed diets containing 2ppm aflatoxin per day showed higher levels of liver enzymes including GGT (gamma glutamyl transferase) indicating liver damage. There are reports of much lower concentrations of 0.3 ppm causing death in horses. Reports of aflatoxicosis in horses in Thailand fed peanut hulls have been published.

The maximum aflatoxin levels suggested for mature horses should not exceed 50 ppb and breeding stock and hardworking horses should be fed aflatoxin free diets. Other tolerance levels cited suggest 20 ppb (0.02ppm) highlighting the wide range of levels associated with clinical symptoms.

 

Clinical signs of aflatoxicosis include:

  • Loss of weight and/or appetite
  • Loss of body condition
  • Poor feed utilisation (losing weight even though appetite appears unaffected)
  • Immune suppression and increased disease susceptibility
  • Reduced fertility
  • Liver necrosis
  • Death

 

 


Endophyte alkaloid toxins

 

Ryegrass staggers

In parts of the world where insect damage is prevalent in pastures, high endophyte ryegrass varieties are sown. These plants live in symbiosis with the Lolitrem fungus, which releases toxins that affect gait, behaviour and co-ordination in horse, a condition known as ‘ryegrass staggers’. This endophyte is closely associated with leaves and proliferates in hot dry conditions and the toxins accumulate at the base of the plant. When these conditions are followed by periods of rain, the ryegrass undergoes rapid growth, spreading the toxin up the plant stem and dramatically increasing the exposure of grazing animals to the toxin. Hence, staggers is most commonly observed immediately after a period of drought when grass is actively growing once again. Horses and ponies kept on very short grass also face higher levels of exposure, as they are consuming those parts of the plant most contaminated with toxins.

 

Symptoms of ryegrass staggers include:

  • Ataxia
  • Headshaking 
  • Loss of co-ordination
  • Over-reaction to external stimuli
  • Collapse

Fortunately the neurological effects appear temporary and when horses are removed from the pasture the horse usually returns to normal after a week or so.

These endophytes may be passed from one generation of plants to the next via the seeds. It is important to make sure that purchased seed is endophyte-free prior to seeding pastures for horses.

 

Fescue toxins

Certain fescue grasses, especially tall varieties, contain an endophyte called Festuca arundinacea. This fungus grows between the cells inside the plant (see below), so is completely invisible by external examination.


Courtesy of Mary Smith; Vet Med 647

Fescue endophyte

 

Horses that consume these fescues show symptoms that include:

  • Poor appetite
  • Loss of weight and condition
  • Reduced performance
  • Rough coat
  • Uncontrolled body temperature fluctuations
  • Reduced prolactin (the milk stimulating hormone in mares)
  • Loss of appetite

The major influence of this toxin is seen in pregnant mares, as it is associated with an increased risk of abortion, poor cycling, and embryonic mortality as well as ‘red bag delivery’. The latter is where a toughened placenta grows across the cervix, making it impossible for the mare to expel the foal naturally. Another fescue intra-cellular endophyte Neotyphodium coenophialum causes dystocia in mares and deaths of perinatal foals.

Stallions exposed to fescue toxins may have reduced semen volume and fertility problems.

 

 


In general, both fescue and ryegrass endophytes have negative effects on the reproductive system, as mares are sensitive to alkaloid concentrations as low as 300-500 ppb.

Common symptoms of ergot alkaloid poisoning in pregnant mares include:

  • Extended gestation to 12 months
  • Dystocia
  • Red bag delivery
  • Thick oedematous placentas
  • Weak/dead foals with aspiration pneumonia
  • Difficulty in foals breaking through toughened placenta
  • Lack of mammary development or lactation
  • Stillbirths

Pregnant mares should be removed from pasture contaminated with endophytes by day 300 of gestation. Two weeks prior to the expected delivery date, mares can be treated with domperidone to reduce the effects of any suspected ergot alkaloid toxicity. Laboratory tests for ergovaline (the ergot alkaloid of fescue toxicity) and lolitrem B (the ergot alkaloid of perennial ryegrass) are commonly available for veterinary investigation.

 

Claviceps

Paspalum grass harbours the fungus Claviceps paspali, whose spores are visible on the grass seed heads as sooty, blackened spikes. However, Claviceps may also live on a variety of pasture grasses and hay, including ryegrass and bluegrass and commonly produce ergotamine, ergostine, ergocristine, ergocryptine and ergocornine. These toxins are chemically related to LSD and cause a disease known as ergotism, where the animal loses co-ordination, may hallucinate and develops a condition called dry gangrene in its extremities, i.e. its hooves and tail develop major lesions and may die off completely in bad cases.

There have been clinical cases cited involving ergot alkaloids by Claviceps. In Brazil late gestation mares were fed oats containing ryegrass seed and suffered from foetal loss similar to those found with fescue toxicity. The ryegrass seed was the only source of ergot alkaloids. In another case pregnant mares were bedded on rye straw bedding and consumed the bedding. Of the first eight mares to foal, seven had stillbirths. Investigations indicated the presence of two ergot alkaloids in the bedding (one of which was ergocornine) at levels of 450 ppb.


Slaframine

White clover can be a source of mycotoxins if it is allowed to become damp or mouldy when harvested for hay or silage. Any black, slimy patches seen in clover indicate that this type of mould is growing, but it may not always be visible, so take care when harvesting clover hay. White clover appears to contain two types of toxin. The first is a substance that mimics reproductive hormones (an oestrogen analogue), which prevents mares becoming pregnant. This has not been identified, but may be similar to zearalenone. Secondly, the slaframine toxin causes a disease called ‘slobbers’ where the horse drools continuously, tears run down its face and it experiences uncontrolled diarrhoea and urination.

 

Slobber1

 

 

 

Fumonisins

Horses and ponies are very sensitive to, and face considerable risk from, these Fusarium mycotoxins, which are found in grain and ready-mixed feeds (especially those containing maize). This toxin causes a disease called equine leukoencephalomalacia (ELEM) – which basically means the brain develops lesions or holes. This results in muscle tremors, poor co-ordination, loss of the swallowing reflex and depression – rather like an equine form of Parkinson’s disease or dementia.


 


Brain section of horse with ELEM (courtesy of Mary Smith, Vet Med 647)

Fumonisin B1 at a concentration of 10 ppm has been found to be associated with leukoencephalomalacia. The clinical signs associated with the neurological form include apathy, drowsiness, pharyngeal paralysis, blindness, circling, staggering, seizures and eventual recumbency. Death usually follows within 2-3 days. Outbreaks of ELEM typically occur sporadically from late autumn to early spring. Maize or corn screenings can be heavily contaminated and so should not be fed to horses at all.

Concentrations of 5 ppm FB1, FB2, FB3 or higher can cause colic and even death in horses and feed containing more than this level should not be fed to horses.

 

 

 

Trichothecenes

 

Trichothecenes: T-2 toxin, diaceptoxyscripenol (DAS), deoxynivalenol (DON), HT-2 toxin

Trichothecenes are produced by several fungi including Fusarium, Stachybotrys, Trichoderma, Myrothecium and Trichothecium, however, Fusarium is the main species implicated in the production of trichothecenes. Trichothecenes are found commonly in Europe and approximately 180 toxins are known to be produced by Fusarium species in cereals, straw and forage. Stachybotryotoxicosis was first described as an often fatal disease associated with mouldy hay and straw in Russia.


Clinical signs of trichothecenes toxicity in horses include:

  1. Oral lesions
  2. Reduced feed intake
  3. Weight loss
  4. Immune suppression
  5. Poor performance
  6. Colic

DON (deoxynivalenol) is also known as vomitoxin, as it induces vomiting in dogs and pigs. Horses do not have the ability to vomit, but will refuse feed containing DON and hence lose condition. DON is commonly found in maize, wheat bran and wheat by-products that are commonly used in horse diets. Wheat straw used for horse bedding may also be contaminated. DON seems to be produced following alternating hot and cold temperatures. There is little research concerning DON’s effects on horses but research carried out by North Carolina State University found that DON was present in 100% of compound feed samples fed to horse that later suffered from colic (0.2 – 8.3 ppm) and in 70% of the feeds not associated with colicky horses (0 – 2.5 ppm). T2 toxin concentrations greater than 0.5ppm and zearalenone greater than 0.7ppm were found in 31 and 44% of the colic cases, respectively with neither being present in the control group. The relationship between mycotoxins and colic remains under scrutiny.

In Germany in 2002, a large group of Warmbloods (n=104) exhibited sudden weight loss and elevated liver enzyme levels. Straw bedding was identified as the culprit and found to be contaminated with DON at levels of 0.5-2.7 ppm. Following removal of the bedding, the condition of all horses improved. There has been only one trial involving feeding T2 toxin to breeding horses and when fed at a level to mimic a concentration of 1 ppm in the feed, no effect on the ovarian activity of mares was seen. Until further research becomes available for horses, it is suggested that the maximum tolerable level of DON in feed should be 2 ppm.

 

 

 

Zearalenone

As with other mammals, feeding horses with diets contaminated with zearalenone most typically leads to reproductive problems because the toxin mimics important reproductive hormones. In vitro tests have shown that zearalenone and its related compounds can interact with hormone receptors in equine cells and induce proliferation and apoptosis of granulose cells, which may cause reproductive problems. However, no in vivo data is available on this toxin as regards its effects in equine species.

 

Table 1. Maximum tolerance levels of mycotoxins in the total ration dry matter for horses

Mycotoxins

Maximum Tolerable Level (ppm)

Aflatoxin

0.02 ppm

T2 Toxin

No effect on ovarian activity when fed at 1 ppm

DON

2 ppm of total diet or 5ppm in cereal feed

Zearalenone

No effect on ovarian activity when fed at 1 ppm

Fumonisins

< 5 ppm

Ergovaline

0.3-0.5 ppm

 

 

 

Zearalenone

As with other mammals, feeding horses with diets contaminated with zearalenone most typically leads to reproductive problems because the toxin mimics important reproductive hormones. In vitro tests have shown that zearalenone and its related compounds can interact with hormone receptors in equine cells and induce proliferation and apoptosis of granulose cells, which may cause reproductive problems. However, no in vivo data is available on this toxin as regards its effects in equine species.

 

Table 1. Maximum tolerance levels of mycotoxins in the total ration dry matter for horses

Mycotoxins

Maximum Tolerable Level (ppm)

Aflatoxin

0.02 ppm

T2 Toxin

No effect on ovarian activity when fed at 1 ppm

DON

2 ppm of total diet or 5ppm in cereal feed

Zearalenone

No effect on ovarian activity when fed at 1 ppm

Fumonisins

< 5 ppm

Ergovaline

0.3-0.5 ppm

 

 

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