The mycotoxin DON has been recognised as an important contaminant of feed for farm animals. Even in low concentrations it can affect animal health – probably best known by causing anorexia. Mycotoxin risk management is therefore essential.
By Dr Paula Kovalsky, product manager, mycotoxin risk management, Biomin, Austria
Deoxynivalenol (DON) is the main type B trichothecene produced by several strains of the plant pathogen Fusarium. The fungus can infect nearly all types of plants and DON is frequently detected in grain commodities intended for animal feed. In temperate regions of the world, DON is one of the most prevalent fungal toxins.
Mycotoxin surveys show that DON can be present at high concentrations around the globe. Out of a total of 1,729 samples tested in 2013, the highest levels for DON were observed in Chinese wheat samples (average concentrations above 1,800 ppb) and in European maize samples (980 ppb), levels that have a strong impact on pigs. As pigs' diets may contain a high percentage of maize and wheat, the exposure to this mycotoxin can be rather frequent.
Shifts in crop phenology
There are several factors that influence the production of DON in the plant. The host is most susceptible to fungal infection at anthesis, which is the period in which the flower is fully open, and also shortly thereafter. A warm and moist environment accompanied by frequent precipitation favours fungal growth, infection and the development of disease.
For instance, prediction models show that by the year 2040 in Canada, the time point of flowering of maize and wheat will occur earlier than usual. Increased wetness before flowering is known to enhance fungal infection and mycotoxin contamination.
Climate changes also seem to have an effect on the movement of population species of fungi that currently occur mainly in southern Europe, such as F. verticilloides in maize, to northern countries, consequently resulting in the formation of other mycotoxins. It has been observed that distinct populations of Fusarium are displacing other species due to differences in aggressiveness. In the US there are population shifts occurring, which lead to differences in the levels of DON produced.
In terms of resistance towards Fusarium, there are vast differences not only among plants but also among varieties. Maize and small grains are excellent hosts for Fusarium. The rising production of maize and grains and the drastic increase in their residues on the soil surface provide favourable conditions for the growth of fungus.
DON can be modified by the plant to so-called masked mycotoxins that are not detected by routine analytical methods. Yet, these compounds can account for a considerable amount of total toxin content in the feed which may often be overlooked. Masked mycotoxins, such as D3G, are produced by the plant as a defense mechanism to deactivate toxic substances. However, this glucose conjugate can be metabolised in the gut of the animal and the toxic form (DON) is thereafter released.
Gut and brain
The toxicity of DON is derived from its epoxide group which is responsible for the binding of the toxin to ribosomes. This mechanism, known as ribotoxic stress, leads to protein synthesis inhibition. The toxicity of DON is also due to its ability to cross biological barriers such as the intestinal and blood-brain barriers. The ingestion of the toxin is associated with alterations of the intestinal, nervous and immune systems.
Among farm animal species, pigs show the highest sensitivity to DON. There are species-specific differences in the bioavailability of DON and pigs are highest in the rank, followed by broilers and ruminants. Bioavailability depends highly on the localisation of the gut microbiota prior to the small intestine.
DON is mainly absorbed in the small intestine by crossing the intestinal epithelium. In pigs, up to 89% of the ingested toxin is absorbed after oral exposure. Figure 1 shows the metabolism of DON in pigs. Compared to other farm animals the microbial biomass of swine in the stomach is relatively low which explains the higher sensitivity towards DON.
After ingestion of DON-contaminated feed, DON targets the intestinal epithelial cells. These are exposed to the entire contamination of the feed regardless of the amount of DON that is absorbed. This consequently enables the absorption of other substances, including toxins and other harmful compounds.
The intestinal absorption of nutrients such as glucose and amino acids is inhibited by DON due to the alteration of an essential transport system which is also responsible for the daily water uptake in the gut. Such modifications are thought to be the reason for the resulting diarrhoea after DON ingestion.
DON indirectly affects the immune system of the animal through:
* Changes in the permeability of the intestinal epithelial cells;
* Promotion of bacterial translocation;
* Modification of the production of the intestinal mucus.
Dosage dependent effects
At low doses, DON acts as a pro-inflammatory substance leading to intestinal inflammation. Low concentrations of DON alter the immune functions of porcine PMNs (polymorphonuclear cells) which are the first line of defense against infection. High doses of DON result in the inhibition of the intestinal immunity which increases susceptibility to intestinal infections by bacteria such as E. coli.
In pigs, 25-30 % of the available DON in the blood is found after 2-60 minutes in the cerebro-spinal fluid, which serves a vital function in the brain-blood flow. The toxin is able to cause several alterations of the brain functions, such as the activity of brain neurons in relation to anorexia and emesis.
The two most common symptoms caused by DON are:
* Anorexia due to increases in the secretion of hormones with anorexic action; insulin and gut satiety hormone peptide PYY;
* Vomiting as a result of acute exposure to doses as low as 50 µg of DON/ kg body weight.
The effects of DON in pigs not only depend on the quantity of mycotoxin uptake, but also on other parameters such as age, physiological state and nutrition. Unfortunately, the ingestion of the toxin does not always lead to visual symptoms, which makes the problem sometimes difficult to detect.
Counteraction of DON
Some intestinal bacteria are capable of partially deactivating DON to DOM-1, see Figure 2. In pigs, DON is hardly ever transformed naturally to the non-toxic DOM-1. The majority of ingested DON is found primarily in bile, blood and urine and to a lower extent in faeces. Also only very small amounts of DOM-1 are recovered in the urine of pigs that have ingested a DON-contaminated diet. This demonstrates an efficient and nearly complete DON absorption (up to 89%) in the upper digestive tract before a microbial de-epoxidation can occur, resulting in the high bioavailability of the toxin in the pig.
Studies have shown that the active strain BBSH 797 (DSM 11798) prevents the absorption of DON via biotransformation in pigs.
In a trial, 24 piglets were randomly assigned to three experimental groups with two replicates per group. The control group received a basal diet. The DON group received a diet containing 2000 ppb of DON. The DON + BBSH group received the same diet as the DON group and contained the DON-deactivating microorganism. Figure 3 shows the DON and DOM-1 concentrations in the serum of the tested piglets. On day 1, all piglets received the basal diet. On days 2 and 3, the piglets in the toxin groups received contaminated feed. Concentrations of DON in the DON group were significantly higher compared to the DON + BBSH group, which showed the efficacy of BBSH 797 to modify the toxin to the non-toxic DOM-1. This was also confirmed by the high concentration of DOM-1 in the serum of the DON + BBSH group on day 3. On day 4, all piglets were fed the basal diet and it can be observed that the concentration of DON and consequently DOM-1 decreased to the same levels as on day 1.
The strain BBSH 797 has been recently registered in the EU as the first live micro-organism with the ability to deactivate DON to DOM-1 due to its de-epoxidase activity. DOM-1 is no longer able to bind to ribosomes as it lacks the distinct toxic epoxide group.