of DHP may also appear in the faeces and
urine, while mimosine may itself undergo
decarboxylation within the tissues of the
ruminant to yield mimosinamine which is
then excreted in the urine. Ruminants in
Australia, the USA and Kenya are known to
lack the requisite bacteria involved in the
detoxification of the two DHP isomers. In
other regions where Leucaena is indigenous
(Central America) or is naturalized (Hawaii
and Indonesia), ruminants possess the full
complement of bacteria required for DHP
degradation, which accounts for the absence
of Leucaena toxicity in these countries.
Phyto-oestrogensPhyto-oestrogens are actively metabolized
in the rumen with the result that activity of
these substances in the plant depends
upon the extent of microbial transforma-
tions in the rumen (Adams, 1995).
Genistein, for example, is degraded to non-
oestrogenic compounds. Formononetin,
however, is demethylated and reduced to
the more oestrogenic compound, equol.
Rumen microbes may require up to 10 days
to adapt to phyto-oestrogens, so genistein
may initially evoke oestrogenic effects.
AflatoxinsThe aflatoxins are actively metabolized in
the rumen and in the tissues of animals,
with serious implications for human health
(Smith, 1997). In dairy animals, ruminal
transformation of AFB 1 to AFM 1 results in
the latter being secreted in the milk. AFM 1
has been ascribed with carcinogenic
properties. Carcinogenicity of AFB 1 , how-
ever, is greater, arising from the formation
of a reactive epoxide which then permits
covalent binding to cellular components
such as DNA to yield genotoxic adducts.
ZearalenoneThe ovine metabolism of ZEN has been
proposed to involve the synthesis of at
least five metabolites including zearalanone,
-zearalenol, -zearalenol, -zearalanol
and -zearalanol (Miles et al., 1996). It
should be noted that -zearalanol is used
as a growth promoter with the name
zeranol. High levels of some of these forms
may be excreted in the urine as
glucuronides by grazing sheep.Toxicology
Acute toxicityThe classical assessment of toxicity of any
compound generally centres on considera-
tion of LD 50 data. In the case of ANFs, such
indicators of acute toxicity are of limited
practical relevance in field cases of toxicity
(Table 18.3). This is not to imply that ANFs
do not induce lethality. Thus, it has been
demonstrated that the dietary administra-
tion of concanavalin A can result in high
mortality in quail (D’Mello, 1995). In
calves, a total oral sporedesmin dose of
3 mg kg^1 body weight can cause 100%
mortality within 3–5 days (Flaoyen and
Froslie, 1997). For mycotoxins, much
emphasis has been directed at the acquisi-
tion of LD 50 data (Table 18.4) following an
incident in 1960 in the UK when 100,000
turkey poults died from acute necrosis of
the liver and hyperplasia of the bile duct
(‘turkey X disease’), attributed to the
consumption of groundnuts infected with
A. flavus. This event marked a defining
point in the history of mycotoxicoses,
leading to the discovery of the aflatoxins.
However, it is now recognized that LD 50
values are subject to wide variation
depending, for example, upon age, sex and
size of animals. There are also species
differences in sensitivity to a particular
mycotoxin (Table 18.4). More significantly,
it is acknowledged that features of chronic
toxicity are of greater relevance in practical
husbandry of farm animals. Thus, ZEN is
associated with relatively low intrinsic
toxicity as judged by LD 50 values, but
chronic investigations demonstrate that its
oestrogenic property towards mammals is
an over-riding feature. Similarly, in humanAnti-nutritional Factors and Mycotoxins 389