Fungal toxins
Mycotoxins are fungal metabolites which may
contaminate foods, animal feeds, and are toxic to humans or their domestic
animals. They are proposed to have carcinogenic properties. As common
adulterants of foods or animal feeds, they are important food contaminants. Mycotoxicosis results
from the ingestion of toxin in a mold-contaminated food.
The
fungi include the molds, yeasts, mildews, blights, rusts, and mushrooms. Many
fungi are useful. Some are edible, e.g., mushrooms and single-cell protein from
yeast. Others are widely used in industrial and food fermentations; e.g., Aspergillus
oryzae is used in the production of soy sauce, miso, and sake, and
molds take part in the ripening of certain cheese. The metabolite of Penicillium
chrysogenum, penicillin, has contributed immensely to human well-being.
Some mushrooms are harmful or poisonous to humans, but in contrast, molds have
generally been regarded as harmless.
The two predominant genera of fungi in stored
products are probably Penicillium and Aspergillus, members of which produce
mycotoxins.
Ergot
The first documented case of mycotoxicosis was that
of rye ergot. Claviceps purpurea parasitizes rye
and other grains and produces many lysergic acid derivatives which are
responsible for the syndrome. Consumption of the infested grain or flour made
from it over a period of time can result in gangrenous ergotism.
Outbreaks of ergotism were quite common during the
Middle Ages. More recent outbreaks of ergotism have been reported in the Soviet
Union (1926-1927), England (1928), and France (1951).
Claviceps purpurea is a parasite of grasses including cereals. As
part of its life cycle, the tissues of infected grains are replaced by sclerotium
which is fungal mycelium. The sclerotium helps to survive the adverse
conditions of the winter and germinate later. It is also known as an ergot.
Ergotism is the name for severe pathological
syndromes affecting humans or other animals that have ingested plant material
containing ergot alkaloid, such as ergot-contaminated grains. The common name
for ergotism is "St. Anthony's fire", in reference to the
severe burning sensations in the limbs.
Ergotism, is due to ergot alkaloids
which is toxic and cause a constriction of the peripheral blood capillaries
leading, to fingers and toes becoming gangrenous and necrotic.Ergots contain
alkaloid metabolites which may be incorporated into the flour, and the bread,
made from the harvested grain. The ergot sclerotium contains high
concentrations (up to 2% of dry mass) of the alkaloid ergotamine, and ergoline
group. Ergot alkaloids have a wide range of biological activities including
effects on circulation and neurotransmission. Ergot contains lysergic acid
which is a precursor for the synthesis of LSD which is a potent synthetic
hallucinogenic drug.
There are two types of ergotism. The first is
characterized by muscle spasms, fever and hallucinations and the
victims may appear dazed, be unable to speak, or have other forms of paralysis
or tremors, and suffer from hallucinations. This is caused by stimulation of
the central nervous system by some of the alkaloids. The second type of
ergotism is marked by violent burning, and shooting pain of the
poorly vascularized distal organs, such as the fingers and toes. It is caused
by effects of ergot alkaloids on the vascular system due to vasoconstriction,
sometimes leading to gangrene and loss of limbs due to severely restricted
blood circulation.
Ergot metabolites also have profound effects on the
central nervous system stimulating smooth muscle activity. The neurotropic
activities of the ergot alkaloids may also cause hallucinations, convulsions,
and even death. Other symptoms include strong uterine contractions, nausea,
seizures, high fever, vomiting, loss of muscle strength and unconsciousness.
Ergot alkaloids has been used in pharmaceutical
preparations, to treat migraine headaches, and to induce uterine contractions
and control bleeding after childbirth. Since the Middle Ages,
controlled doses of ergot were used to induce abortions and to stop maternal
bleeding after childbirth.
The causative agents of most ergot poisonings are
the ergot alkaloid class of fungal metabolites. The fungi of the genera
Penicillium and Aspergillus also produce ergot alkaloids, particularly some
isolates of the human pathogen Aspergillus fumigatus.
Aflatoxin
Aflatoxicosis
is a fungal toxicosis that may affect all species of animals. Aflatoxins are produced by certain strains of Aspergillus
flavus and A. parasiticus. These fungi grow on
carbohydrate-rich feeds such as peanuts, cottonseed, corn, sorghum and cereal
grains when they are stored in hot conditions without adequate drying and
aeration. Optimal
conditions for the production of aflatoxin would be an aw of 0.85 and a
temperature of 25 to 40oC.
The number and types of
aflatoxins produced vary with the strain. For example, A. flavus strains
produce B1 and its related metabolities, while A. parasiticus produces
both B1 and G1 and the related metabolites.
In 1959 there was the deaths of
several thousand turkey poults and other poultry on farms in East Anglia due
to poisoning of the groundnut meal used as a protein supplement in the pelleted
feed. The contaminant, which was called aflatoxin, fluoresces intensely under
ultra-violet light and was shown to be produced by the mould Aspergillus
flavus growing on the groundnuts.
Acute aflatoxicosis can
be caused by ingestion of high doses of aflatoxin over a short period of time. Aflatoxin
toxicity may result in nausea, vomiting, abdominal pain, convulsions, and other
signs of acute liver injury. Long-term exposure also leads to various
complications like growth retardation, cirrhosis, and hepatocellular carcinoma.
The two major aflatoxins have been designated B1
and G1 because they fluoresce blue (B1) and green (G1) when exposed to
long-wave ultraviolet light. Aflatoxins B2 and G2 are the dihydroderivatives of
B1 and G1. Aflatoxins M1, M2, and P1 are the hydroxylated derivatives of B1 and
B2 which are excreted in the urine, faeces, and milk as metabolic products of
B1 and B2 following their consumption by mammals.
Aflatoxin B1, the most toxic of the aflatoxins, is
toxic to various animals. Many of the other aflatoxins have been shown to be
toxic or carcinogenic to different species of fish, mammals, and poultry. When
cows eat feed containing aflatoxin, aflatoxin M1 and M2 is excreted in the
milk. Although M1 and M2 are less toxic than the parent compounds B1 and B2, M1
retains its toxic and carcinogenic ability in many animals. M1 has also been
detected in the urine of Philippine women who had consumed peanut butter containing
aflatoxin.
Many commodities will support the growth of
toxigenic strains, including various dairy products, bakery products, fruit
juices, cereals, and forage crops. Aflatoxins have been reported
from a wide range of foods and animal feeds. Initially, it was considered that
aflatoxin contamination was a problem of poor storage of commodities after
harvest allowing the growth of storage fungi such as Aspergilli with consequent
formation of mycotoxins. High humidity and warm temperatures can give rise to
the highest levels of aflatoxin in food.
Aflatoxins can be produced in the growing crop
before harvest also. Aflatoxigenic species of Aspergillus can establish an
endophytic relationship with the healthy plant and produce low, but
significant, amounts of aflatoxin when the plant is stressed, such as occurs
during a drought.
It is assumed that a correlation is there between
aflatoxin and liver cancer and liver damage in different parts of the world.
Very young children may be exposed to aflatoxins even before they are weaned
because mothers, consuming aflatoxin in their food, may secrete aflatoxin M1 in
their milk.
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