Mycotoxins: a simple explanation for a complex topic (Part 2)
The basics about Fusariotoxins
Pig feed plays a key-role in maximizing the animal’s genetic potential, representing 60 to 80% of total production cost. There are many substances that have to be considered as contaminants in animal feedstuffs. Mycotoxins are naturally occurring compounds in feedstuffs which can have deleterious health effects to animals and humans due to the consumption of animal products. They are a problematic issue in animal feed industry, since about 25% of world’s food crops are contaminated with these toxic substances. Fusarium sp. mycotoxins are the most prevalent mycotoxin-producing fungi occurring in Europe due to their preference for the weather conditions existing in this region.
Fusariotoxins is the common name given to mycotoxins produced by fungi of the Fusarium sp. genus.
These mycotoxins are produced mainly on the field before harvest by a large and complex mould family with species adapted to a wide range of habitats and, although having a special affinity for moderate climates, they contaminate crops all over the world. This fact makes fusariotoxins probably the most economically significant grain mycotoxins worldwide. The most important mycotoxins being produced by Fusarium sp. are trichothecenes (ex.: DON, T-2 toxin), zearalenone and fumonisins. The Fusarium species of most concern are those that produce mycotoxins in wheat, maize, rice, barley, oats and other cereal grains that are used in animal and human diets.
Table 1: Most important mycotoxins produced by Fusarium spp. and their occurrence in Europe
|Producing Fungi ||Occurrence in Europe ||Mycotoxins produced |
|Northern / Central ||Southern |
|Fusarium graminearum ||+++ ||+++ ||Trichothecenes, Zearalenone |
|Fusarium avenaceum ||+++ ||++ ||Trichothecenes |
|Fusarium culmorum ||+++ ||++ ||Trichothecenes, Zearalenone |
|Fusarium poae ||++ ||+/- ||Trichothecenes |
|Fusarium crookwellense ||+ ||+/- ||Trichothecenes, Zearalenone |
|Fusarium subglutinans ||+/- ||- ||Fumonisins |
Feeding cereals to pigs
Feed must provide the correct nutrient requirements for a maximum efficiency regarding reproduction and growth while generating a food. Over the past few years there have been scares related to food safety such as BSE and dioxins leading to the prohibition with animal protein sources, such as meat meal, feather meal, poultry meal, hoof meal and blood meal and although feeding of fish meal in small percentages is allowed, the importance of plant protein sources has increased enormously. No matter what the origin of the feed is - bought feed mixture or on-farm mixed feed - rations should provide animals a correct quantity of digestible energy (DE), proteins, minerals and vitamins.
Cereal grains account from 55 to 70% of the total feed mixture, representing the main source of energy for pigs. Wheat, barley and maize are typically included in pig feed due to their high DE value, therefore being a major source of mycotoxins contamination.
Although many Fusarium species can infect cereal grains, Fusarium graminearum is the major causal agent of head scab of small grains and of red ear rot in maize. There are two typical routes of entry of Fusarium graminearum infection in cereals:
- The spores are already on the field at the time of silks’ emergence thus infecting the silk channel;
- Birds, insects or extreme weather conditions are able to cause damage to the kernels before their hardening thus providing an opportunity for fungal damage.
Figure 1: The typical entry routes of Fusarium graminearum infection in cereals
There are many factors that make the process of mycotoxin and mycotoxicoses prevention and counteraction more difficult, amongst them:
- On the field, fungi contamination is very hard to prevent;
- Mycotoxins are very stable substances thus their elimination is a very complex issue;
- Mycotoxins can be bound with certain plant nutrients which makes them undetectable to most of the analytic procedures (“masked” mycotoxins);
- Different mycotoxins are often produced by the same fungi leading to synergistic effects in the animals;
- There are many animal factors influencing animal’s response to mycotoxins (interactant factors).
Avoiding field contamination
There are many factors that can influence the growth of Fusarium fungi and the occurrence of fusariotoxins on the field. Some prevention measures can be drawn based on the knowledge of the predisposing factors.
- Land management/Crop rotation – cropping systems in which maize is rotated with wheat or in which wheat is grown each year in the same field appear to increase the disease epidemic. Thus, avoiding this type of rotation can have a positive impact on the prevention of the development of these fungi.
- Tillage procedures - Many Fusarium fungi species are soil borne and survive in the crop residues as saprophytes, breaking down plant residues. The spore form as they multiply is very resistant to temperature and to adverse weather conditions therefore assuring their survival in the crop residues. It is consequently advisable to eliminate residues from the field through deep tillage.
- Plant stress – stress factors such as high temperatures, drought, poor fertilization, and high competition for nutrients are some of the aspects known to increase the mycotoxin production in the field. The choice of the convenient variety of seeds for a certain location, irrigation in critical periods and a balanced fertilization are some of the measures that can be used to avoid mycotoxin contamination during plant growing.
- Crop damage – the mechanical, insect or bird damage of grains provide a good opportunity for fungal installation and development, thus their prevention is of major importance.
- Harvesting – an adequate harvesting date is extremely important to avoid mycotoxin contamination as unstable weather conditions such as late rain will exponentially increase its occurrence.
These measures however are limited in their ability to avoid mycotoxins. Moreover, after harvest, other mycotoxin producing fungi will develop and produce mycotoxins depending on the storage conditions.
Due to their chemical structure and low molecular weight, mycotoxins are chemically stable; they resist high temperatures and several manufacturing processes. For example, zearalenone (ZON) is stable during storage, milling and cooking. Deoxynivalenol (DON) and T-2 toxin are stable at 120°C and relatively stable at 180°C. Feed manufacturing or pelleting at temperatures 81 to 85oC show no detoxification ability, thus maintaining the threat. That is the reason why it is important to prevent the occurrence of toxins in raw material.
Some nutrients found in feed such as glucose and amino acids can be comparable to magnets, enabling mycotoxins to be attracted and bound to them, resulting in stable and more complex molecules that are no longer detectable or identified by routine mycotoxin analyses. So-called “masked mycotoxins” and their toxic precursors are released during digestion hydrolysis thus maintaining their toxic effects to animals.
Because each plant can be contaminated with more than one fungus and each fungus species is able to produce more than one mycotoxin, the probabilities of co-occurrence of mycotoxins in a feed commodity are quite high. The interaction between mycotoxins often leads to synergistic effects, when the negative effects of one mycotoxin are amplified by the presence of another mycotoxin. This is especially true in the case of Fusariotoxins once there are different mycotoxins that can be produced by the same fungus, namely ZON and DON that often occur together in the presence of Fusarium graminearum and Fusarium culmorum.
As can be seen from the figures below, symptoms of mycotoxins in animals depend on many animal-related, mycotoxin-related and environment-related factors which make the detection, diagnosis and treatment of the related problems very difficult.
Figure 2: Interactant Factors
The animal production battle
Successful pig production results from an accurate equilibrium between several parameters. The knowledge about the factors that can disturb that balance gives the producer an advantage regarding their prevention and counteraction. Unfavorable climate conditions such as late summer rains and rapid shifts between dry and humid conditions are an open door for the development of Fusarium fungi on the field. The production of Fusariotoxins by this type of fungi is becoming an increasingly important issue in agriculture as the conditions for its occurrence are more favorable. Mycotoxins must therefore be faced by animal producers as a growing concern in the livestock production.