Mycotoxins revisited

Mycotoxicoses occur as result of ingestion of pre-formed toxins elaborated by fungi in cereals. Although there are upwards of 200 mycotoxins described, aflatoxins, fusariotoxins, deoxynivalenol and ochratoxins are most commonly encountered and are responsible for losses in the commercial egg industry. Prevention of mycotoxicoses relies on detection and rejection of severely contaminated consignments of cereal ingredients, blending of affected and "clean" consignments of ingredients and supplementing diets with mycotoxin binders of proven efficacy.

Pre-harvest conditions that may influence the level of contamination include drought, especially during the stage of tassel formation in corn, or following unseasonable rain or high humidity at harvest. Post-harvest contamination can occur during storage under conditions of high temperature and humidity and may be exacerbated by rodent or insect damage.

Aflatoxicosis is frequently associated with corn harvested at the beginning of the season when consignments may be delivered to feed mills with moisture content in excess of 15 percent. Drying of "new-season" corn to below 13 percent is recommended as a preventive measure, especially if there is evidence of cracked kernels or fungal growth detected by black light (UV) examination at the time of delivery.

Production Impact

Aflatoxins are elaborated by Aspergillus flavus and A. parasiticus. These toxins damage mitochondrial DNA, with the liver as the principal target organ. Although acute aflatoxicosis is rare, a case reported from North Carolina resulted in 50 percent mortality in a flock of commercial laying hens fed a diet containing a high level of toxin, assayed at 100 ppm.

Generally, aflatoxicosis produces subtle effects in mature commercial laying flocks that may not be readily diagnosed. Changes include deterioration in shell strength, elevation in the prevalence of blood spots and blood filled eggs, non-specific decline in egg production of up to 5 percent and immunosuppression.

Chronic low-level ingestion of toxins results in increased susceptibility to respiratory and enteric infections and can stimulate vertical transmission of Salmonella Enteritidis in flocks predisposed by intestinal colonization. Affected flocks are frequently refractory to vaccination, resulting in susceptibility to both IB and ILT.

The tricothecenes including T-2 toxin, deoxynivalenol (DON) and diacetoxyscirpenol (DAS) are all elaborated by Fusarium spp. Levels in excess of 3 ppm T-2 toxin will markedly reduce production in egg laying flocks and result in feed refusal due to the corrosive action of the toxin on the lining of the oral cavity and pharynx. With low-level chronic feed intake, hens develop an ulcerative stomatitis involving the palate, angle of the beak, and tongue. Clotting of blood is affected and fusariotoxicosis may increase blood spots. Interference with lipid metabolism and a resultant decrease in plasma carotenoids may result in pale yolks.

DON is produced by Fusarium roseum. At levels above 5 ppm, flocks may show feed refusal and even regurgitation, which is associated with decreased egg production and lowered feed conversion efficiency.

Zearalenone produced by Fusarium roseum produces cystic changes in the oviduct attributed to an estrogenic effect. This toxicity has been identified in broiler breeders, which show a reduction in egg production and prolapse of the terminal oviduct. The condition may occur infrequently in commercial hens cages or in aviaries and floor systems.

Ochratoxins are produced by Penicillium viridicatum and Aspergillus ochraceous. These compounds are extremely pathogenic at levels exceeding 0.3 ppm with the kidney as the target organ. Laying hens show reduced egg production and characteristic yellow-brown staining of eggshells due to the deposits of urate in the cloaca.

Oosporein elaborated by Chaetomium spp occurs very infrequently in the United States and is usually associated with consumption of contaminated milo. At levels exceeding 1 ppm, hens will show excessive excretion of urates with staining of shells and an increase in mortality due to visceral gout may occur. Deposits of urate on the pericardium and peritoneum should be differentiated from water deprivation, viral nephritis and nephropathogenic strains of infectious bronchitis virus.

It is emphasized that environmental conditions before and after harvest that contribute to elaboration of mycotoxins may result in multiple toxicities. Accordingly, flocks may be affected by low levels of aflatoxins, fusariotoxins, DON or DAS, which are synergistic in their effect. Mycotoxicosis should always be suspected in poorly defined and non-specific clinical conditions involving decreased egg production, deterioration in shell quality, the appearance of bloodspots and immunosuppression, denoted by low antibody titers.

Detection of Mycotoxins

Fungal proliferation generally occurs in discrete areas in a consignment of grain. These "hotspots" are scattered in a railcar or barge load and accordingly inherent sampling errors can result in widely different assay results. Twenty to 30 heavily contaminated kernels in one ton of corn can produce an aflatoxin level of 50 ppb resulting in condemnation of the entire consignment. Given the inherent bias in sampling, the results of assays should be carefully interpreted based on physical inspection of grain consignments, moisture content, season, and source.

Samples of grain suspected to be contaminated with mycotoxins can be submitted to a state diagnostic laboratory for a mycotoxin screen using high performance liquid chromatography (HPLC) assay. Individual states set their own policies on submission of samples for either diagnostic or quality control purposes. Costs vary but are generally lower then the fee charged by commercial laboratories. Generally, mycotoxin screens conducted by commercial laboratories to detect and quantify the major contaminants will cost between $250 and $500 per sample and results are usually available after five to 10 working days.

In order to address the needs of commercial mills producing feed for company farms or for external sales, rapid qualitative and semi-quantitative test kits are available to assay for specific mycotoxins. Since small quantities of grain are used for the assay, it is reiterated that sampling error will influence evaluation of the mycotoxin status of a consignment to a greater degree than the sensitivity and specificity of the test.

Reducing the Production Impact

Management procedures that can be applied to reduce the impact of mycotoxins include identifying and segregating contaminated feed during storage for subsequent mixing with unaffected grains. Lightly contaminated products are frequently diverted to non-susceptible species. Feed silos on farms should be sealed to prevent entry of rain water. Interlinked two-bin installations are preferred as these will permit emptying and cleaning on rotation to prevent mold proliferation after delivery. It is noted that anti-caking additives including organic acids (propionic, formic, and acetic acids) inhibit mold proliferation by reducing pH values of feed but will not inactivate pre-formed toxins produced before harvest or subsequently during handling transport and storage.

Treatment of ingredients with chloramine, ammonia or extraction with solvents can theoretically reduce aflatoxin levels, but these procedures are impractical for commercial applications.

During the past 10 years, there has been considerable research into mycotoxin binders, which inhibit absorbtion by sequestering toxins in the intestinal tract. In many developing countries bentonite clays are added to feed to bind aflatoxins. These compounds vary widely in their efficacy and depending on their source there is a danger of contaminating feed with naturally occurring dioxins. Hydrated sodium calcium aluminum silicate and other zeolites are commercially available and have shown variable activity, principally against aflatoxins. Natural products are variable in composition and commercially available synthetic compounds have narrow specificity.

Studies conducted in France and the United States have demonstrated that extracts of the cell wall of Saccharomyces cerevisiae sequester mycotoxins across a wide range of pH values. Affinity binding values for aflatoxins and DON range from 90 to 95 percent and 36 percent and 15 percent, respectively, for zearalenone and ochratoxin. The structure of specific fractions of beta-D-glucans extracted from yeast determines the intensity of binding. This property has been extensively examined with respect to the stereochemistry of the interaction between the binder and mycotoxin to select the most efficient binders.

Research into the mechanisms of sequestering has been stimulated by increased use of DDGS. This ingredient may be contaminated with aflatoxin present in the corn substrate. The toxin is concentrated during extraction of ethanol resulting in levels of aflatoxin in DDGS which may induce clinical signs and a measurable decline in production parameters. Since current FDA restrictions do not allow label claims for "mycotoxin binding," the commercial product is marketed as MTB 100 in the United States as an anti-caking additive. Extensive use of the product in Asia and Latin America provides a positive return over the cost of supplementation, especially with premium egg products and when market prices are high for generic eggs.

Conclusion

Suppression of mycotoxicoses requires an integrated approach to detection and control. Reducing the impact of contamination depends on detection and suppression of pre-formed toxins by the addition of binders in addition to practical measures on farms to prevent proliferation of fungi.