BOLETIM TÉCNICO No. 9 - www.micotoxinas.com.br
Mycotoxin Concerns in Dairy Cattle
L.W. Whitlow, Ph.D., Animal Science Department and W.M. Hagler, Jr., Ph.D., Poultry Science Department North Carolina State University, Raleigh, North Carolina
Mycotoxins are poisons produced by molds, a type of fungus. Molds occur universally in various feedstuffs including roughages and concentrates, and can produce mycotoxins under certain conditions. Production of mycotoxins can occur in the field before harvest, or post-harvest, during storage, processing, or feeding. Mold growth and the occurrence of mycotoxins are often related to:
Extremes in weather conditions, which can cause plant stress or hydration of feedstuffs.
Poor storage practices, which affect feedstuff quality.
Conditions for mold growth and mycotoxin formation include:
Physical damage to the commodity.
Presence of the fungal spores.
The Aspergillus, Fusarium, and Penicillium molds are thought to be the ones of most importance in producing mycotoxins detrimental to cattle. The mycotoxins of greatest concern include: aflatoxin, which is produced by an Aspergillus mold; deoxynivalenol (DON), zearalenone, T-2 Toxin, and fumonisin, which are produced by Fusarium molds; and ochratoxin produced by Penicillium molds. Several other mycotoxins, produced by these and other molds, are known to be prevalent at times, including derivatives of those listed previously. A lack of observation and lack of simple analytical techniques has probably made it difficult to understand the prevalence of these and other mycotoxins and their impact on animal production.
Fusarium commonly affects corn, wheat, and barley, and also is found in oats, rye, and triticale. Fusarium causes field diseases such as head blight (scab) in small grains, and ear and stalk rots in corn, which are characterized by yield loss, quality loss, and mycotoxin contamination. In wheat, excess moisture at flowering and afterward is associated with increased incidence of mycotoxin formation.
In corn, Fusarium diseases are more commonly associated with warm conditions at silking and with insect damage and wet conditions late in the growing season. Aspergillus flavus and aflatoxin in corn are favored by the heat and drought stress associated with warmer climates. Aflatoxin seems to be enhanced by insect damage before and after harvest.
The requirements for mold growth vary by type of mold. General conditions include:
Temperatures from 23°F to 140°F.
Approximately 70% humidity.
Oxygen (only 0.5% is needed).
A fairly wide range of pH (molds do not grow well at extreme low or high pH levels).
A suitable organic substrate.
The Aspergillus species grow at lower moisture levels and are associated with storage damage. Aflatoxin production is associated with higher temperatures. The Fusarium species generally require higher moisture levels and are able to grow at much lower temperatures. It is critical to remember that conditions most suitable for mold growth do not necessarily indicate the optimum conditions for mycotoxin formation. For example, the Fusarium molds associated with alimentary toxic aleukia (ATA) have been reported to grow prolifically at temperatures of 77°F to 86°F without producing much mycotoxin. However, at near freezing temperatures large quantities of mycotoxins are produced without much mold growth.
Mycotoxins can increase disease incidence and reduce production efficiency in cattle. They exert their effect through three primary mechanisms:
Alteration in nutrient content, absorption, and metabolism.
Changes in the endocrine and neuroendocrine function.
Suppression of the immune system (CAST, 1989).
The resulting symptoms are often nonspecific, making a diagnosis difficult. The difficulty of diagnosis is increased due to limited research, occurrence of multiple mycotoxins, nonuniform distribution, interactions with other factors, and problems of sampling and analysis.
Field work suggests that while a definitive diagnosis cannot be made directly from symptoms or specific tissue damage, certain observations can be helpful:
Mycotoxins should be considered as a possible primary factor resulting in production losses and increased incidence of disease.
Documented symptoms in ruminants or other species can be utilized as a general guide to symptoms observed in the field.
Systemic effects as well as specific damage to target tissues can be used as a guide to possible causes.
Postmortem examinations may indicate no more than gut irritation, edema, or generalized tissue inflammation.
Rule out other possible causes, such as infectious agents or other toxins.
Analyze feeds for common mycotoxins.
Observe for responses to simple treatments, such as dilution or removal of the contaminated feed.
Diagnosis may be impossible as the clinical picture may be very complex.
Dairy herds thought to suffer from a mycotoxicosis typically have a loss in milk production. Fresh cows perform poorly and generally have an increased incidence of disease. Usually there is intermittent diarrhea, sometimes with bloody or dark manure. Cows may not respond well to typical veterinary therapy. Symptoms may be nonspecific and wide ranging and may include:
Reduced feed intake.
Rough hair coat.
Increased abortions or embryonic mortalities.
Silent heats or irregular estrus cycles.
Expression of estrus in pregnant cows.
Decreased conception rates.
There may also be a higher incidence of disease, particularly in fresh cows, such as displaced abomasum, ketosis, retained placenta, metritis, mastitis, and fatty livers. There may only be a few or there may be many of these symptoms evident. More than one mycotoxin may be present, and there may be interactions with other agents.
Aflatoxin, produced primarily by Aspergillus flavus, is a mycotoxin of major concern, because it is carcinogenic and is commonly found in peanuts and corn in the southern U.S. Aflatoxin can be found in more northern areas in some years. For example, the 1988 crop year was very dry, resulting in aflatoxin contamination in corn in several northern states. Cottonseed from the southwestern U.S. is susceptible to aflatoxins while the cottonseed products produced in the southeastern U.S. seem to escape aflatoxin contamination.
Major efforts are directed at eliminating food residues. FDA limits aflatoxin (the only mycotoxin so regulated) to no more than:
200 ppb in breeding cattle rations.
300 ppb for finishing beef cattle rations.
20 ppb in lactating dairy cattle rations.
0.5 ppb in milk.
Regulatory pressures and a widespread awareness have helped minimize aflatoxin problems. The General Accounting Office (GAO) concluded in 1991 that industry, federal, and state programs are effective in detecting and controlling aflatoxin, and that it is doubtful that additional programs or limits would reduce the risk of aflatoxin in the food supply. This concern also minimizes the likelihood that aflatoxin will result in significant production or health effects on dairy herds.
Milk levels of aflatoxin will be about 1.7% the concentration found in the total ration dry matter. Aflatoxin residues can be found in tissues, and thus, cattle should not be fed aflatoxin-contaminated diets for 21 days prior to slaughter.
Aflatoxin can reduce performance and impair health, but detrimental levels are generally far higher than the 25 ppb to 50 ppb level which can cause illegal milk residues. Although no level of aflatoxin is considered safe, the degree of toxicity is related to level of toxin, duration of feeding, and the amount of other stresses affecting the animal. Levels above 300 ppb to 700 ppb are considered toxic to cattle (CAST, 1989). There is some suggestion that dairy cattle may be affected at levels as low as 100 ppb, especially if fed for an extended period and if other stressors are present. Impure sources of aflatoxin produced by culture are more detrimental than equal amounts of pure aflatoxin. Several studies suggest naturally-contaminated feeds often contain other toxins.
Deoxynivalenol (DON) or Vomitoxin
DON is a Fusarium-produced mycotoxin and is one of the more commonly detected mycotoxins. Incidence may be as high as 50% to 80% of feeds. Surveys have shown DON to be the primary mycotoxin associated with swine health problems including feed refusals, diarrhea, vomiting, reproductive failure, and deaths. In cattle, DON has been associated with reduced feed intake and milk production. It is believed that DON is but one causative agent that may be present in these studies and that DON may serve as a marker for feeds containing other mycotoxins.
As with other mycotoxins, DON itself does not show the same level of toxicity associated with naturally-contaminated feeds.
Some workers have suggested other specific mycotoxins that may be present with DON in naturally- contaminated feeds which enhance the toxicity. Many such interactions are possible since Fusarium molds produce many mycotoxins, and it is well documented that several mycotoxins may be found in the same feed and that Fusarium species isolated from the field can produce an array of mycotoxins.
It is presently thought that DON serves as a marker which indicates exposure of feed to a situation conducive to mold growth and mycotoxin formation. A positive DON analysis suggests the possible presence of other mycotoxins or factors more toxic than DON itself. A dietary level of 300 ppb to 500 ppb DON may cause problems when fed to cattle.
T-2 toxin, a Fusarium-produced mycotoxin occurs in a fairly low proportion of feed samples (3% to 5%). T-2 has been associated with reduced feed consumption, loss in yield, gastroenteritis, intestinal hemorrhages, and death. Controlled experimentation has demonstrated that T-2 is associated with feed refusal and gastrointestinal lesions, but failed to show a hemorrhagic syndrome.
While data with cattle are limited, the effects of T-2 in laboratory animals are well documented. T-2 is known to suppress immunity and interfere with protein synthesis. It is toxic to the intestine, lymphoid tissues, liver, kidney, spleen, and bone marrow. A calf given T-2 via a stomach tube developed severe depression, hindquarter ataxia, knuckling of the rear feet, listlessness, and anorexia.
Experience suggests that T-2 is a severe gastrointestinal tract irritant, which can cause hemorrhage and necrosis of the intestinal tract. Diarrhea is usually present but may not be hemorrhagic. With high levels of T-2, there can be congestion and irritation to the stomach, intestines, liver, lungs, and heart. T-2 may not occur alone and thus, naturally-contaminated feeds may contain other similar toxins.
Cattle data are not sufficient to establish a tolerable level of T-2; but, a practical recommendation is to avoid more than 100 ppb of T-2 toxin in the diet.
Zearalenone is a Fusarium-produced mycotoxin which elicits an estrogenic response in monogastrics. As with other mycotoxins, its occurrence is dependent on seasonal weather conditions, with zearalenone being more prevalent in wet and cool seasons. Zearalenone has been found in 10% to 20% of feeds in some surveys. Several reports have related zearalenone to an estrogenic response in ruminants. Large doses are associated with abortions in cattle. Other cattle responses may include vaginitis, vaginal secretions, poor reproductive performance, and mammary gland enlargement of virgin heifers.
Controlled studies with zearalenone at high levels have failed to reproduce the degree of toxicity that has been associated with zearalenone-contaminated feeds in field observations. In the field, dietary levels of about 750 ppb zearalenone and 500 ppb DON resulted in poor consumption, depressed milk production, diarrhea, and total reproductive failure, while controlled research with levels of 13 ppm to 22 ppm showed only mild reproductive symptoms. Obser-vations suggest that zearalenone may be associated with poor feed intake, a loss of milk production, poor conception, and increased reproductive tract infections.
Establishment of a tolerable level of zearalenone for cattle is difficult based on a meager amount of data. As with DON, zearalenone may serve as a marker for toxic feed. Zearalenone above 200 ppb to 300 ppb in the diet may be of concern.
Many other mycotoxins may affect ruminants but are thought to occur less frequently or be less potent. Diacetoxy-scirpenol, HT-2, and neosolaniol may occur along with T-2 toxin and cause similar symptoms. Fumonisin is less potent than other mycotoxins, but in large amounts can affect cattle. Ochratoxin has been reported to affect cattle, but it is rapidly degraded in the rumen. Thus, it is thought to be of little consequence except for preruminants.
Tremorgens, such as fumigaclavine A and B, are thought to be common in silages of the southeastern U.S. They can cause anorexia, diarrhea, unthriftiness, and irritability. Others mycotoxins such as rubratoxin, citrinin, patulin, cyclopiazonic acid, sterigmatocystin, and ergot alkaloids may be of some importance.
Analytical techniques for mycotoxins are improving. Several commercial laboratories are available and provide screens for a large array of mycotoxins. Cost of analyzes has been a constraint but can be insignificant compared with the economic consequences of production and health losses related to mycotoxin contamination. Newer immunoassays have reduced the cost for analyzes.
Collection of representative feed samples is a problem primarily because molds can produce vary large amounts of mycotoxins in small areas making the mycotoxin level highly variable within the lot of feed. Samplings of horizontal silos show mycotoxins to be highly variable throughout the silage, however, the silo face appears to have higher levels. Because mycotoxins can form in the collected sample, it should be preserved and delivered to the lab quickly. Samples can be dried, frozen, or treated with a mold inhibitor before shipping.
Acceptable levels of mycotoxins should be conservatively low due to nonuniform distribution, uncertainties in sampling and analysis, and the potential for more than one source in the diet.
Prevention and Treatment
Prevention of mycotoxin formation is essential since there are few ways to completely overcome problems once mycotoxins are present. Ammoniation of grains can destroy some mycotoxins, but there is no practical method to detoxify affected forages. Following accepted silage-making practices aimed at preventing deterioration primarily through elimination of oxygen is a very important mycotoxin-preventative management practice. Some additives may be beneficial in reducing myco-toxins because they are effective in reducing mold growth. Ammonia, propionic acid, and microbial or enzymatic silage additives are shown to be at least partially effective at inhibiting mold growth.
Silo size should be matched to herd size to ensure daily removal of silage at a rate faster than deterioration. Feed bunks should be cleaned regularly. Care should be taken to ensure that high moisture grains are stored at proper moisture content and in a well-maintained structure. Grains or other dry feed, such as hay, should be stored at a moisture content (<14%) below which molds do not readily grow. Aeration of grain bins is important to reduce moisture migration and to keep the feedstuffs in good condition.
Obviously, moldy feed should be avoided. If unacceptably high levels of mycotoxins occur, dilution or removal of the contaminated feed is preferable. However, it is often a problem to completely replace some feeds in the ration, particularly the forage ingredients. Increasing nutrients such as protein, energy, and antioxidants may be advisable. Animals show marginal responses to increased protein. It has been suggested that cattle may respond to certain vitamins and minerals, especially those with antioxidant activity. Acidic diets may intensify the effects of mycotoxins.
Favorable research results have been seen when adsorbent materials, such as clays (bentonites), are added to mycotoxin-contaminated diets of rats, poultry, swine, and cattle. Adsorbent materials bind some mycotoxins, reducing their availability to the animal. Research as shown that adsorbents, added to diets containing aflatoxin, significantly reduce aflatoxin residues in milk. However, no adsorbent material is approved by the FDA for the prevention or treatment of mycotoxicosis. Several of these adsorbent materials are recognized feed additives which are used as flow agents, pellet binders, etc. Much additional research is needed on prevention and treatment of mycotoxins.
Areas of Needed Information
CAST has published a list of major needs for research (CAST, 1989). Included in their list are surveillance of feeds for mycotoxin presence and quantity, assessment of control methods, development of resistant plants, improvement of sampling and analysis, improved understanding of effects on animals (particularly on immunosuppression), toxicological evaluation of newly discovered mycotoxins, and assessment of economic effects.
CAST, Council for Agricultural Science and Technology. 1989. Mycotoxins: Economic and Health Risks. Task Force Report No. 116.