Effects and symptoms of mycotoxins in cattle
Ruminants have some capacity to protect themselves against the harmful effects of mycotoxins due to detoxifying action of certain ruminal microorganisms. However, modern dairy cows have a much faster passage rate of feed through the rumen, leaving less time for microorganisms to detoxify. This, combined with higher feed intake means that our cows’ natural defences cannot protect them as well as often assumed. Further, a cow’s beneficial ruminal microbiota can be impaired by some mycotoxins as well as under challenging metabolic conditions e.g. sub-acute ruminal acidosis (SARA).
Mycotoxins in cattle feed
Mycotoxins are found in almost every agricultural commodity worldwide. Whether these toxins have been produced by fungi infecting crops in the field or by fungi contaminating feed in storage, they pose a challenge to livestock.
A wide array of grains and forages can be contaminated with mycotoxins. To date, more than 700 mycotoxins have been identified. The BIOMIN Mycotoxin Survey provides a regular update on the occurrence of mycotoxins in raw commodities and finished feed based on thousands of samples collected from across the globe.
Ruminants can also face a challenge from various silage mold mycotoxins. These have been shown to reduce rumen function, to cause scours and to reduce milk production as well as to provoke other specific symptoms according to the type of mycotoxin.
Mycotoxins symptoms in cattle
The harmful effects of mycotoxins do not begin with animal metabolism but with the ruminal microflora which affects the efficiency and productivity of ruminal fermentations. In fact, clinical symptoms may not manifest in most practical situations, but performance will be subsequently compromised, resulting in decreased yield, poor reproduction and increased lameness or mastitis.
Table 1. Major mycotoxins and the dangers for cows. Source: BIOMIN
|Mycotoxin||Recommended risk threshold (ppb)||Effects|
Main consequences of mycotoxins in dairy cows in relation to mammary health and milk production
Reduced milk production results from several factors, including a decrease in intake or feed refusal that is commonly reported with certain mycotoxins such as DON. Mycotoxins can alter rumen function by changing the microbial populations or the breakdown of nutrients, consequently reducing nutrient absorption and impairing metabolism which ultimately leads to reduced availability of the precursors needed for milk synthesis.
Potential mammary-related negative effects of mycotoxins in dairy cows:
|1. Reduced milk production|
|2. Toxic contaminants in milk, especially Aflatoxin M1|
|3. Increased risk of mastitis|
|4. Altered milk composition|
Some of the most important mycotoxin groups for ruminants are:
- Tricothecenes, including well-known mycotoxins such as DON (deoxynivalenol or vomitoxin)
- Ergot alkaloids
Other mycotoxins are able to have a less obvious, but still economically significant impact on the performance of dairy cows.
Aflatoxin carryover to milk and milk products
Because of the milk safety problem of aflatoxin M1, aflatoxins receive perhaps the most attention of all mycotoxins in dairy production. Aflatoxins are partially degraded in the rumen and the remainder biologically converted in the liver to Aflatoxin M1 which is still bioavailable, toxic and unfortunately can be carried over into the milk produced.
Most countries established strict limits for aflatoxins in milk, but it is important to note that also other mycotoxins can negatively affect profitability and animal health and welfare.
One of the most commonly occurring mycotoxins in livestock feeds is deoxynivalenol (DON), better known as ‘vomitoxin’. The name ‘vomitoxin’ originates from the toxin causing vomiting in swine. Deoxynivalenol is a member of the trichothecene family of mycotoxins, specifically Type B trichothecenes. Several species of Fusarium molds are capable of producing trichothecenes. Additionally, some Fusarium mold species can produce the mycotoxins zearalenone and fumonisins. It is not uncommon to detect more than one toxin in a feed sample since molds can produce more than one type of mycotoxin and since more than one mold can infect a plant.
The presence of DON in the feed has been correlated with significantly reduced milk production (see Figure 1). DON (a type B trichothecene) has also been shown to affect rumen microbial processes such as reducing the availability of nitrogen (microbial N). Effects can be seen even if there is degradation of DON over time in the rumen.
In dairy cows, the damage is more likely to be subclinical or indirect such as increased permeability of the intestinal wall (lowering the defence against pathogens and reducing nutrient uptake), reduced productivity, higher somatic cell count issues, and risk of mastitis and metritis. Calves can struggle with scours and respiratory diseases due to the effects of trichothecenes on the immune system.
Deoxynivalenol inhibits protein and nucleic acid (DNA and RNA) synthesis. The negative effects of DON are mainly seen in the gastrointestinal tract and immune system, but the toxin can cause lesions and necrosis of the skin and mucosa as well. The cells lining the intestines are continuously being renewed and are especially sensitive to the effects of DON.
The intestinal epithelium serves two main purposes: 1) to absorb nutrients and 2) to act as a barrier to prevent harmful substances from entering the bloodstream. Both of these functions can be disrupted by DON, leading to reduced nutrient uptake and increased passage of toxins and pathogens into circulation. This can limit animal growth or production capacity as the required nutrients are not absorbed and utilized.
Additionally, other organs may be exposed to pathogens or toxins which enter the bloodstream, increasing the possibility for disease. Disruption of the intestinal mucosa can also lead to diarrhea. A large portion of the immune system is located in the gastrointestinal tract. Immune function can be impaired by disruption of the gut mucosa.
DON can impair production of the white blood cells, which help fight infection. Deoxynivalenol can also weaken the immune system by negatively impacting cytokine and antibody production. The animal’s natural immune response to vaccinations may also be reduced, leaving them susceptible to disease despite vaccination. All of these factors can lead to immune dysfunction in cattle, increasing vulnerability to infections.
There can be an almost complete detoxification of ochratoxin A (OTA, an occasional storage mycotoxin), though a portion of OTA is known to be able to escape detoxification through rumen bypass. Up to 10% of the OTA challenge has been reported to pass unchanged through the goat rumen (and sheep are likely to be similar).
This happens to a lesser extent in cows since they have a larger rumen with a longer transit time. With high feed intake and stress factors, however, more rumen bypass can occur reducing mycotoxin breakdown.
Zearalenone and reproduction issues
Ruminants are susceptible to the estrogenic effects provoked by zearalenone (ZEN) and related compounds. The molecule fits to the oestrogen receptors triggering the wrong hormonal responses and upsetting the reproductive system and disrupting reproductive performance. In the rumen, ZEN is largely biologically converted to alpha-zearalenol (α-ZOL), which is a much more potent form that fits more easily to the oestrogen receptors than ZEN itself. ZEN is also known to increase (worsen) the effect of DON on the gut wall.
Ruminants have a well-known susceptibility to the effects of ergot alkaloids. Ergots can be formed by fungi growing on cereals or by endophyte fungi within tall fescue grass. One of their main effects is vasoconstriction (restricting blood flow) which has an impact on hoof health, heat stress, mastitis and reproduction.
Acidosis impacts mycotoxin degradation
A well-known problem within ruminants is subclinical or acute acidosis (SARA/ARA). This syndrome of low rumen pH often occurs in high-producing dairy farms, especially when diets are grain-rich or stress situations impair the ruminal flora and lead to dysbiosis.
It is assumed that during acidosis the numbers of protozoa decline and as one of the most important mycotoxin-degrading agents, this leads to a decreased degradation and therefore, higher levels of mycotoxins can pass to the intestine and exert toxic effects.
Mycotoxin risk management in cattle
Reducing animal exposure to mycotoxins in feed is key. Identifying contamination can help to reduce exposure.
Robust mycotoxin risk management comprises three steps:
Regular analysis of feed components and silage can help to uncover potential threats to animals. A highly contaminated sample does not mean the entire crop is bad and a ‘clean’ sample does not guarantee that all of the feed is mycotoxin-free.
Good silage management is essential to avoid further growth of molds and thereby prevent the production of mycotoxins. Regular application of a mycotoxin binder and of deactivators is advisable. A proper mycotoxin risk management is essential to avoid unpredictable losses and maintain a high producing dairy herd.