BOLETIM TÉCNICO No. 21 - www.micotoxinas.com.br
AFLATOXICOSIS AND RUMINANTS
CASSEL, E.K, S.M. BARAO, AND D.K CARMEL
Aflatoxicosis is a disease caused by the
consumption of aflatoxins. Aflatoxins are secondary mold metabolites produced by some
strains of Aspergillus flavus and Aspergillus parasiticus. The four most
common aflatoxins are B1, B2, G1, and G2. Contaminated grains and grain byproducts are the
most common sources of aflatoxin. Corn silage may also be a source of aflatoxins, because
the ensiling process does not destroy toxins already present in silage.
Aflatoxins are metabolized in
ruminants by the liver and are excreted in the bile. Aflatoxin B1 is the most potent
mycotoxin (toxic substance produced by a mold) to affect cattle. B1 increases the apparent
protein requirement of cattle and is a potent carcinogen (cancer causing agent). When
significant quantities of B1 are consumed, the metabolite M1 appears in milk within 12
hours. Research suggests M1 is not as carcinogenic or mutagenic as B1, but it does appear
to be as toxic as its parent compound.
Symptoms
Beef and dairy cattle are more susceptible
to aflatoxicosis than sheep or horses, although other mycotoxicoses occur in these
species, such as facial eczema in sheep and leukoencephalomalacia in horses. Young animals
of all species are more susceptible than mature animals to the effects of aflatoxin.
Pregnant and growing animals are less susceptible than young animals, but more susceptible
than mature animals.
Feed refusal, reduced growth rate and
decreased feed efficiency are the predominant signs of chronic aflatoxin poisoning. In
addition, listlessness, weight loss, rough hair coat and mild diarrhea may occur. Anemia
along with bruises and subcutaneous hemorrhage are also symptoms of aflatoxicosis. The
disease may also impair reproductive efficiency, including abnormal estrous cycles (too
short and too long) and abortions. Other symptoms include impaired immune system response,
increased susceptibility to disease, and rectal prolapse.
Pathology
Clinical laboratory findings vary with the
animal species, level of aflatoxin in the ration, and the duration of feeding. There are
no consistent diagnostic changes in hematocrit, hemoglobin, and differential cell counts
in animals fed aflatoxin. Leukocytosis may occur in animals with secondary bacterial
infections. Serum bilirubin levels may be elevated and typically serum protein levels are
decreased.
Lesions observed at necropsy related to
either acute or chronic liver disease are dependent upon the level of aflatoxin and the
duration of feeding. A majority of acute liver damage observed has been the result of
experimentally high doses, while chronic liver damage is a more common field observation.
The liver is usually pale tan, yellow or orange. Hepatic fibrosis and edema of the
gallbladder may also be observed.
The diagnosis of aflatoxicosis is often
difficult because of the variation in clinical signs, gross pathological conditions and
the presence of infectious diseases due to the suppression of the immune system. On the
farm, more than one mold or toxin may be present in the contaminated feed, which often
makes definitive diagnosis of aflatoxicosis difficult.
The prognosis of aflatoxicosis depends upon
the severity of liver damage. Once overt symptoms are noticed than prognosis is poor.
Treatment should be directed at the severely affected animals in the herd and further
poisoning prevented.
Aflatoxicosis is typically a herd rather
than an individual cow problem. If aflatoxicosis is suspected, the ration should be
analyzed immediately. If aflatoxins are present, the source should be eliminated
immediately. Levels of protein in the ration and vitamins A,D,E,K and B should be
increased as the toxin binds vitamins and affects protein synthesis. Good management
practices to alleviate stress are essential to reduce the risk of secondary infections.
Secondary infections must receive immediate attention and treatment.
Aflatoxicosis can only be prevented by
feeding rations free of aflatoxin. Preventing aflatoxin contamination is outlined in fact
Sheet 444, but since preventing contamination is not always possible, here are a few keys
facts to remember when dealing with contaminated feeds in animal rations:
- The
recommended feeding level is 0 parts per billion (ppb).
- The
level of aflatoxin an animal can tolerate will depend upon the age and sex of the animal,
its health status, and overall management level of the farm.
- To
avoid contamination of milk, lactating dairy cattle should not receive more than 20 ppb in
the total ration.
- Calves should not receive milk from cows fed in
excess of 20 ppb.
- Beef cattle should not receive more than 400 ppb
in the total ration. Weanlings should not receive more than 100 ppb in their total ration.
- Poultry
and swine are more sensitive to aflatoxin contamination and should not receive more than
20 ppb aflatoxin in the total ration.
These are only suggested guidelines. This
does not suggest that feeding at these levels or below will reduce or eliminate the
potential for aflatoxicosis. There are no clear-cut safe feeding levels. To feed at a
level other than 0 ppb is a risk assumed by the person making the decision to do so.
Conclusion
Aflatoxins are highly toxic to livestock,
poultry and people. Even fed at nonfatal levels, aflatoxin can seriously impair animal
health and productivity. For lactating dairy cattle, do not exceed 20 ppb aflatoxin in
rations to avoid exceeding the food and Drug Administration level of 0.5 ppb in milk.
Aflatoxin is just one of many mycotoxins that can adversely affect animal health and
productivity.