Assessing the impact of chronic exposure to mycotoxins
Sunday, April 3, 2011
Research suggests that ingesting mycotoxins can result in organ damage or immune suppression in pigs, which may result in increased susceptibility to disease and reductions in growth performance and feed intake
by JANICE MURPHY
Feed quality is a constant source of concern, so swine producers must be vigilant in assessing the level to which feed grains are contaminated with toxins.
Mycotoxins are toxic chemical compounds commonly produced by fungi found on cereal crops. Of the over 300 toxins out there, aflatoxin (AF) and deoxynivalenol (DON) are the most prevalent in North America. In 2007, a North Carolina Co-operative Extension survey found that 34 per cent of corn tested contained greater than 20 micrograms of AF per kilogram and over 60 per cent of feed contained DON.
Consuming feed with high levels of mycotoxins can have a severe and lasting effect on swine performance and health. When feed concentrations exceed the limits allowed by the Food and Drug Administration (200 micrograms of AF per kilogram for finisher pigs and 5,000 micrograms of DON per kilogram in corn), these toxins cause weight loss, decreased growth and production, liver and kidney damage, immune suppression, and even death. For both of these particular mycotoxins, swine tend to be the most sensitive livestock species.
The types of symptoms exhibited and the degree to which swine are affected largely depends not only on the type of mycotoxin and concentration in feed but also the age and stage of production of the pig. Young pigs and breeding swine are generally the most susceptible to mycotoxins. It is also likely that combinations of certain mycotoxins produce heightened negative effects on performance above and beyond what would normally be expected from each of the individual mycotoxins, and this therefore must be taken into consideration.
Very little research effort has been expended on investigating the combined effects of low levels of AF and DON in naturally infected corn. Contamination by AF and DON often indicates the presence of other toxins as well, because similar fungi often grow together. Natural infection models are critical, especially at low levels of infection, since it is possible that, although pigs eating diets with reduced AF and DON may not be critically affected, they may suffer chronically from reduced weight gain, feed intake and a weakened immune system.
Recent research at North Carolina State University set out to investigate growth and immune responses of pigs consuming combinations of low levels of AF and DON together from naturally contaminated corn. Sixty gilts were randomly assigned to four treatments:
A - a control diet without detectable AF and DON;
B - a diet with 60 micrograms of AF and 300 micrograms of DON per kilogram;
C - a diet with 120 micrograms of AF and 600 micrograms of DON per kilogram;
D - a diet with 180 micrograms of AF and 900 micrograms of DON per kilogram.
Pigs were allowed free access to feed and water for 33 days. Feed intake and body weight were measured weekly. Blood samples were collected at the end of the experiment to measure the numbers of blood cells, to conduct liver function tests and to measure immunological variables. In addition, liver, kidney, and spleen tissues were collected from 20 pigs (one representative pig per pen) on day 33 for weight, colour and damage evaluation.
After 33 days on the experimental diets, pigs fed diets C and D tended to have decreased weight (although not significantly) compared with pigs fed the control diet – 8.6 and 11.5 per cent, respectively. (See Table 1.) Overall, there was no significant difference between treatments in weight across the entire experiment.
When compared with the control, pigs on treatments C and D tended to have reduced average daily gain (ADG; 0.52 vs. 0.43 and 0.41 kilograms per day, respectively) and average daily feed intake (ADFI; 1.04 vs. 0.92 and 0.88 kilograms per day). These differences, however, were only significant during the first and third weeks for ADG and for the third and fourth weeks of the experiment for ADFI. Feed efficiency was not significantly affected by treatment throughout the duration of the experiment.
Pigs fed diet D showed a significant increase (27.2 per cent) in white blood cell numbers compared with control pigs, and a 26.5 and 39.3 per cent increase over pigs fed diets B and C. All other blood hematological measurements showed no differences among treatments.
Most of the immunological variables showed no difference between treatments, with the exception of serum concentrations of tumor necrosis factor alpha (TNFa). The level of TNFa in pigs fed diet D increased significantly, by 12.1 and 15.5 per cent, compared with pigs fed diets A and C. The researchers wondered if this increase might be an indication that pigs fed the highest level of AF and DON were exhibiting an acute-phase response to inflammation caused by the toxic effects of the mycotoxins.
Results of the blood biochemical analysis for liver function showed pigs fed diets B, C, and D had significantly higher globulin levels compared with pigs on the control diet. Pigs on treatment D also showed a significant increase (58.7 per cent) in alkaline phosphatase in comparison with control pigs, and a 67.5 and 72.7 per cent increase over pigs on treatments B and C. In addition, cholesterol concentrations of pigs on treatment D were significantly greater than pigs fed diets A, B, and C by 74.0, 37.9, and 50.3 per cent, respectively. The researchers suggested that such abnormally high levels of excretion for these liver metabolites were likely due to mild liver damage.
Pigs fed diets B and D had significantly greater liver fibrosis when compared to the control pigs. Histopathological analysis of the liver of pigs fed diet D showed 50 per cent more fibrosis in comparison to pigs fed the control diet. All other measurements of liver damage showed no difference among treatments.
Based on these results, it seems that ingesting mycotoxins can result in organ damage or immune suppression in pigs, which may result in increased susceptibility to disease and reductions in growth performance and feed intake. Altogether, these effects can translate into considerable losses for the swine industry.
Overall, this research determined that diets containing both AF and DON at levels greater than 60 and 300 micrograms per kilogram, respectively, may reduce growth performance and decrease feed intake. Furthermore, diets containing 120 micrograms of AF per kilogram and 600 micrograms of DON per kilogram may result in altered immune health, systemic inflammation, and partial liver damage, causing further reductions in growth performance.
At this point, however, it cannot be clearly explained whether or not these negative effects are only due to the toxic effects of AF and DON or a combination of factors, including the reduction in nutrient intake. This topic warrants further investigation. BP
Janice Murphy is a former swine nutritionist with the Ontario agriculture ministry who now lives and works in Prince Edward Island.
Source:
A. C. Chaytor, M. T. See, J. A. Hansen, A. L. P. de Souza, T. F. Middleton and S. W. Kim. 2011. Effects of chronic exposure of diets with reduced concentrations of aflatoxin and deoxynivalenol on growth and immune status of pigs. J. Anim. Sci. 2011. 89:124–135, doi:10.2527/jas.2010-3005, originally published online Oct 1, 2010.
