Page 1055 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Aflatoxins Chapter | 69 987

VetBooks.ir cereal grains, egg products, cheese and many other com- occurs in storage-transport because of the infinite number
of microcosms that can be formed around each seed.
modities, feedstuffs and foodstuffs have been shown to
A small percentage of the kernels in a lot of wheat,
contain AFs. Cottonseed and other oilseeds can be a
source of AFs in animal diets. Preharvest contamination barley, corn, cottonseed, peanuts, and other feedstuffs and
of cottonseed with AFs occurs. In storage, growth of afla- foodstuffs may be contaminated with very high concentra-
toxigenic fungi may occur in cottonseeds when the aver- tions of AFs. Sampling error can occur even with a
age moisture level is greater than 7% 8%. The lipids and well-designed sampling protocol. Accurate sampling is
protein in cottonseeds enhances production of AFs. essential and can be overlooked when using AF test kits
Peanut hay, peanuts, and peanut byproducts are an impor- in the field. Interpretations of analytical findings must be
tant source of AFs. done in context of the sampling procedure, and aflatoxi-
In stored peanuts, the growth of aflatoxigenic fungi can cosis cannot be ruled out based on negative chemical
occur when moisture exceeds 8% and ambient temperature analyses of feedstuffs. Lumps of feedstuffs can be very

is above 25 C. A. flavus can grow in soybeans at a w of 0.77. high in AFs and these lumps can be consumed.
Distiller’s byproducts can be a source of AFs. Fermentation
does not destroy AFs. On a dry matter basis, the concentra- PHARMACOKINETICS/TOXICOKINETICS
tion of AFs in the stillage, compared to AFs in the feed-
stock, is increased by a factor of 3 4 due to the loss of Absorption
starch. Approximately 40% of the AFs are in the syrup (dis-
tiller’s solubles) fraction and 60% are in the solids fraction. AFs are efficiently absorbed by passive diffusion from the
AFs generally are not found in the alcohol fraction. gastrointestinal tract and are primarily transferred from
the intestine into the hepatic portal blood. The rate of
absorption of AFB 1 in rats is dependent on concentration
providing evidence that AFs are absorbed by passive dif-
CHEMISTRY AND SAMPLING
fusion. Young animals absorb AFs more efficiently than
AND ANALYSIS
older animals. For example, rats at 2.5 weeks of age
AFs have a difuranocoumarin chemical structure absorbed AFB 1 at a rate B15 times greater than rats 4 5
(Fig. 69.1). Approximately 18 AFs have been chemically weeks of age. Rats absorb AFB 1 most efficiently from the
characterized. AFs are in two chemical groups, the difuro- duodenum and jejunum. Lactation can affect AF absorp-
coumarocyclopentenone series (includes AFB 1 , AFB 2 , tion. The rate of AFB 1 absorption from the duodenum of
AFB 2A , AFM 1 , AFM 2 , AFM 2A , and aflatoxicol) and the rats in diestrus was greater than the rate of absorption of
difurocoumarolactone series (includes AFG 1 and AFG 2 ). AFB 1 from the duodenum in rats in mid-lactation. Cattle
The “B” Group fluoresce blue in long wavelength ultravi- were given a single oral dose of AFs from rice culture
olet light and the “G” Group fluoresce green. The primary (42% AFB 1 and 27% AFB 2 ) in gelatin capsules (Cook
concern in foodstuffs and feedstuffs are AFB 1 , AFB 2 , et al., 1986). AFB 1 and AFM 1 were observed in venous
AFG 1 , and AFG 2 . Analytical results generally are the sum blood 30 min after dosing and reached maximal levels
of the concentrations of these four toxins. AFB 1 is the 4 8 h after dosing. The maximal levels of AFM 1
most potent AF, a carcinogen, and this chemical form is occurred before AFB 1 and the maximal blood levels of
generally the most abundant in feedstuffs and foodstuffs. AFB 1 generally were higher than AFM 1 . These findings
The order of toxicity is AFB 1 . AFG 1 . AFB 2 . AFG 2 . suggest that AFs are rapidly absorbed from the rumen.
Hydroxylated aflatoxin metabolites are excreted in milk,
and the important metabolites are AFM 1 and AFM 2 , from Metabolism and Excretion
AFB 1 and AFB 2 , respectively. Although AFM 1 and
AFM 2 are commonly associated with milk and other edi- Biotransformation of AFs in the liver is important in the
ble animal products, aflatoxigenic fungi can also release toxicology of AFs. The biotransformation of AFs can also
these metabolites into the substrate. occur in the kidney and intestinal tract. Research has
The analytical methods for AFs have recently been focused on the biotransformation of AFB 1 . Except for the
reviewed and will not be discussed in this chapter (Xie AFB 1 -8,9-epoxide, the biotransformation products are less
et al., 2016). In assaying for AFs, the assumption is the toxic than AFB 1 . Cytochrome P450s (CYP enzymes)
analytical results can be applied to the matrix samples. A have a key role in the biotransformation of AFB 1 to
representative sample may be difficult to obtain because AFB 1 -8,9-epoxide. The formation of AFB 1 -8,9-epoxide is
the distribution of AFs in the suspect feedstuff or food- the most significant biotransformation pathway because
stuff can be highly heterogeneous inclusive of feeds and the AFB 1 -8,9-epoxide forms adducts with DNA, RNA,
foods “thoroughly” mixed during manufacturing and proteins. Conjugation of AFB 1 -8,9-epoxide with glu-
(Shephard, 2016). Heterogenous distribution of AFs often tathione (GSH) is an important detoxification pathway.