Discussion
There is very little published data investigating mycotoxin level in forage in the UK. It is well documented that the source of mycotoxin feed contamination is more likely to originate from processed grains or feed than grass or hay that undergoes little or no storage or processing (Krizova et al. 2021). Mycotoxins are often not homogenously dispersed in the feed and this problem is even more apparent when sampling grass across a field (Skladanka et al. 2013). Mycotoxins may therefore stay analytically undetected, even with perfect sampling procedures (Binder 2007). However, the risk of mycotoxin-contaminated forage has been documented (Raymond 2000; Durham 2022), and confirmed in this study, where mycotoxins were identified in 96% of forage samples.
This paper made no attempt to draw causation between mycotoxin ingestion and increased liver enzyme concentrations. There are many and complex reasons for increased liver enzyme concentrations including ingestion of mycotoxins (Raymond et al. 2003; Durham 2022). We included samples from horses with increased liver enzyme concentrations to identify mycotoxins that may have some clinical relevance for horses and to guide further research, not to draw associations between the mycotoxin exposure and liver disease where a control group would be necessary.
The most commonly detected mycotoxin group was emerging mycotoxins, found in 75% (39/52) of samples. However, when identified, emerging mycotoxins were found at concentrations that are not considered a risk to equine health. This contrasts with type B trichothecenes, which although identified in 46% (24/52) samples, was more commonly found at significant concentrations. In 8/24 samples type B trichothecenes were identified at medium or high-risk concentrations. Type B trichothecenes are produced by fusarium moulds and are frequently identified in forage in Europe (Pereira et al. 2019). They can cause significant gastrointestinal disease in humans and pigs from both acute and chronic exposure (Pinton and Oswald, 2014). Feed refusal and gastrointestinal erosions have been noted in pigs after chronic exposure to deoxynivalenol (DON), which is a type of Type B trichothecenes (Pinton and Oswald, 2014). DON was found more commonly in colic cases compared to the control group in one study (Dänicke et al. 2021). In a study by Raymon et. al in 2003, the impact of fusarium mycotoxins fed to horses (DON (14,000 ug/kg), fusaric acid (6400 mg/kg) and zearalenone (2000 ug/kg)) was demonstrated by a significant reduction in feed consumption and GGT significantly increased compared to control day 7-14 (Raymond et al. 2003). They concluded that exercised horses are also susceptible to fusarium mycotoxicosis as indicated by appetite suppression and weight loss when feeding contaminated feed with fusarium mycotoxins for 21 days (Raymond et al. 2005). Whilst these studies demonstrated clinical effects of significant fusarium exposure in horses, no histology was performed, and study duration was limited to 21 days. More research is needed to establish subclinical effects as well as the effects of longer exposure and lower doses.
The lack of a control group was a significant limitation of the study. Previous studies identifying mycotoxin exposure of horses with colic and liver disease, also identified mycotoxins in control populations (Dänicke et al. 2021; Durham 2022). Whilst we cannot conclude in this study if the increased liver enzyme concentrations were related to the mycotoxin exposure, it has demonstrated the frequency at which mycotoxins are identified in UK forage. Despite being found in control populations in other studies, there is insufficient data to conclude that mycotoxins are not potentially significant to equine health. Exposure to high levels has been demonstrated to cause acute disease, but no long-term cohort studies have been performed in horses to assess long term consequences (Raymond et al. 2003; Raymond et al. 2005).
No studies have quantified the cumulative risk of multiple mycotoxins on horse health. Moulds can produce multiple mycotoxins and there is evidence of the synergistic effects of fusarium mycotoxins (Alassane-Kpembi et al. 2017). Adverse performance risks associated with multiple mycotoxin in feed can be evaluated in farm animals to calculate a risk equivalent quantity (REQ) (Yiannikouris, 2013). In this study, 40/52 samples had two or more groups of mycotoxins detected and 25/52 samples had a medium or greater REQ. This suggests that the number of mycotoxins identified should be considered in addition to the type and concentration of mycotoxin detected. However, further work is needed to establish both the effects of individual and multiple mycotoxins on horses.