Ensuring the safety of raw materials and feedstuffs for various animal species has become a significant challenge in animal production over the last decades. Today, the livestock industry struggles with different factors that threaten animal health and, consequently, human health. One of the most persistent problems is contamination by mycotoxins which affects both the production systems and livestock management (Jedziniak et al., 2019). Contamination by mycotoxins, which are secondary metabolites from fungi found in animal food, is a problem that is found globally extended. Its origin is directly related to the composition of the food used in animal nutrition, in the environmental conditions and in the handling processes of the food product itself.
In the case of horses, their diet is based specifically on pasture, hay and cereals in a nutritional concentrate. This forage can be naturally contaminated by mycotoxins produced by fungi before, during and after the harvest and during the storage, supposing a continued exposition to these toxins by horses. Therefore, forage contamination by mycotoxins can present a worldwide challenge associated with important health and yield problems in the equine industry. Moreover, in comparison with other livestock animals like ruminants, monogastric animals, where horses are included, are more susceptible to being intoxicated by mycotoxins because of the lack of ruminant microbiota, which acts as a barrier against mycotoxins. Additionally, the main mycotoxin absorption pathway is found in the small intestine, and not in the large intestine. Therefore, horses are more susceptible due to the absorption of nutrients is before microbial degradation by fermentation (Braga et al., 2020).
Even though there is a lack of information and studies about the effects of mycotoxins on horses, it is described that the main species of mycotoxins that more affect horses are fumonisins B1 (FB1), which have been highly related to outbreaks in this species. However, equine mycotoxicosis has also been observed to be induced by aflatoxin B1 (AFB1), deoxynivalenol (DON), T-2 toxin, ochratoxin A (OTA), zearalenone (ZEN), slaframine, tremorgenic and ergot alkaloids (EAs). In fact, it was demonstrated that horses are relatively more resistant to aflatoxins in comparison with other monogastric animals (Mobashar, 2022).
Maximum exposure limits for mycotoxins for the equine industry
According to equine nutritionist Dr. Kyle Newman from Venture Laboratories in the United States, maximum mycotoxin concentrations in adult horses have been determined (Table 1). Nevertheless, it is necessary to take into account different factors that would be excluded from these limits, such as breeding horses, juveniles, or synergies between different mycotoxins. In accordance with Dr. Mobashar’s review based on different studies, tolerance levels for different mycotoxins in the equine feed ration could be determined (Table 2) (Mobashar, 2022). On the other hand, the European Union regulation 2006/576/EC just specifies the maximum allowable quantity limit for fumonisins in 5 mg/kg in animal food. Notwithstanding, more research is still necessary to stably the maximum levels of other mycotoxins that can affect equine health.
Table 1. Maximum mycotoxin exposure limits for equine mycotoxins according to Dr. Kyle Newman.
Table 2. Tolerance levels of mycotoxins in horse feed according to different studies (Mobashar, 2022).
Effects of mycotoxins in horses
Aflatoxins
Aflatoxins, which are mycotoxins produced by Aspergillus genera with carcinogenic effects, have been related to negative effects on equine health. Most of the forage for horses contains maize, oats, and other cereals, that are highly susceptible to contamination by aflatoxins. Equine aflatoxicosis by AFB1 has been related to non-specifical clinical signs, such as cough, inappetence, depression, fever, tremor and ataxia, and related to hepatic lesions, such as centrilobular necrosis, cholestasis and bile duct proliferation, which are monitored by changes in transaminase plasmatic levels (AKT and ALT), and alkaline phosphatase (ALP) (Caloni & Cortinovis, 2011). In ponies, was also observed hepatic microscopic lesions induced by AFB1-induced aflatoxicosis, such as centrilobular fatty change, hepatic cell necrosis and periportal fibrosis, accompanied by elevated levels of transaminases, total plasma bilirubin and icteric index, and clinical signs such as depression, inappetence and weakness. In addition, it was described that AFB1 induces an increased immune response by increasing leukocyte concentrations, especially neutrophils (Braga et al., 2020). On the other hand, AFB1 also can affect the olfactory and respiratory tissues of the horse through its inhalation and may induce chronic obstructive pulmonary disease (COPD) (Larsson et al., 2003).
Fumonisins
Fumonisins are mycotoxins generated by other fungi species from Fusarium genera, and it was described that equine health is susceptible to these mycotoxins. The most common are fumonisins B1, B2 and B3. In fact, FB1 concentration levels in food between 0,02 and 0,12 µg/g can cause outbreaks in equine health. Thus, the intake of contaminated food by FB1 in horses was associated with Equine leukoencephalomalacia (LEM), linked to clinical signs of sweating, inability to swallow, muscle weakness, incoordination and ataxia, hypermetria, cirrhosis, pressure in the head, tonic-clonic convulsions, paresis, hyperexcitability or depression, blindness, dilated pupils and absence of pupillary reflex to light (Bertero et al., 2018). In fact, horses are the only species that suffer from LEM associated with FB1 mycotoxicosis. Concerning this neurologic syndrome, it was seen that FB1 intoxication induces cardiovascular dysfunction defined by a reduction of cardiac expense, negative chronotropic and inotropic effects and a reduction in arterial pulse pressure. It is believed that because FB1 boosts sphingosine levels that affect calcium channels, or because the development of LEM damages cerebral vessels, the result is a reduction in cardiac activity (Bertero et al., 2018). Furthermore, it was observed that fumonisins interfere with the metabolism of sphingolipids, which are important contributors to the formation of cellular and basal membranes, and their physiological functions (Van der Westhuizen et al., 2001).
Trichothecenes
Deoxynivalenol (DON) is a trichothecene, like T-2 toxin, described as a refusal toxin because it induces clinical signs like loss of appetite (Trenholm et al., 1994). In fact, in a study of horses fed with forage contaminated by DON, food intake was reduced and AST levels were increased significantly indicating possible hepatic damage induced by DON mycotoxicosis (Raymond et al., 2003a). Moreover, DON, has also been described as one of the main aerial threats for horses because it has negative effects on the intestinal epithelium integrity and morphology (Cleemput et al.,2019). However, it has been seen that horses have a greater tolerance to DON than other monogastric species, such as swine (Schulz et al., 2015). Finally, the presence of DON in blood was also associated with the development of colics in horses and with the increase in the AST, ALT and ASP levels (Dänicke et al., 2021).
On the other hand, the T-2 toxin, which is considered less common but more toxic than DON, has not been widely studied. In mares, T-2 toxin can induce buccal lesions, but it was not observed intestinal lesions or affections in ovary activity like in other animals (Juhasz et al. 1997).
Ochratoxins
Ochratoxin is a potent nephrotoxic and teratogenic mycotoxin. Ochratoxins are mostly produced in starchy cereal grains, such as maize and wheat with a moisture content of 15.5-16%. It is known that ochratoxin A enters the circulatory system via the portal vein and lymphatic vessels binding to plasma proteins, especially to albumin. There is a lack of information about ochratoxin absorption in horses, but the bibliography hypothesizes that probably, like in other monogastric animals, it is absorbed in the most proximal part of the small intestine (Di Paolo et al., 1993). However, further studies are necessary to be able to determine all the ochratoxin effects on equine health.
Zearalenone
Zearalenone (ZEN), which is a secondary metabolite of Fusarium genera, is an estrogenic mycotoxin that binds to estrogen receptors causing hyperestrogenism, which is characterized by uterus enlargement and rectal and vaginal prolapse, among other reproductive effects. Nevertheless, a study of mares showed how the administration of 1 ppm of ZEN induces lesions in the skin around the mouth, but it was not enough to see adverse effects in reproductive parameters (Juhasz et al., 2001). On the other hand, on a commercial farm of horses, ZEN mycotoxicosis from approximately 2,7 ppm in the equine food induced heavy estrogenic symptoms after a feeding period of 30 days (Gimeno et al., 1983). However, a low ZEN sensitivity in horses was described due to the production of a less active ZEN metabolite (ß-ZOL) versus α-ZOL metabolite, and increased metabolism and subsequent excretion of ZEN (Vance et al., 2019). In vitro studies, demonstrated ZEN cellular alterations in granular cells isolated from equine ovaries and the derivates α-ZOL and ß-ZOL affect the chromatin’s structure from equine sperm (Minervini et al., 2010). Therefore, these studies suggest the role of ZEN in the disruption of equine reproduction. Curiously, in donkeys was seen that ZEN and DON could be associated with the stimulation of cellular apoptosis in endometrial epithelial cells (Song et al. 2021), which could suggest a similar effect in horses.
Conclusions
In summary, mycotoxins represent a significant challenge in the equine industry because horses are more susceptible to intoxication due to their diet based on pasture, hay and cereals. Aflatoxins, fumonisins, trichothecenes, ochratoxins and zearalenone are some of the mycotoxins that can affect equine health, causing a range of health problems, such as hepatic, neurological, respiratory and reproductive diseases. It is important to stably maximum exposure limits for these mycotoxins to protect the equine health, although is still necessary further research to completely understand their effects on horses.
