relatively low nutritional value of the seeds
of Lathyrus and Vicia species remains to be
established (D’Mello, 1995). On the other
hand, at least two mycotoxins have been
implicated definitively in field cases of
toxicosis. The lolitrem alkaloids cause
perennial ryegrass staggers in ruminants, a
disorder characterized by neurological
symptoms including incoordination,
staggering, shaking of the head and collapse.
Hepatic damage in sheep caused by
phomopsin A can result in hyper-
ammonaemia, which in turn may precipi-
tate degenerative changes in the central
nervous system. Spongy transformation of
the brain in sheep affected by chronic
lupinosis in the field has been described.
Identical lesions in sheep were reproduced
in experimental forms of acute and chronic
lupinosis. A clear association was pro-
posed between the extent of liver injury
and the incidence of brain lesions.
Evidence of spongy transformation inclu-
ded vacuolation of the white matrix of the
brain stem. The vacuolation occurred with
the highest frequency and severity in the
cerebellar peduncles and around the nuclei
present in the white matter of the cerebel-
lum. Affected sheep may thus appear dis-
orientated.
Immunocompetence
An immunological dimension has been
attributed to the adverse effects of a wide
range of ANFs and mycotoxins. However,
the evidence is plausible for just a few
of these compounds. Although soybean
antigens are associated primarily with
digestive aberrations, there is no doubt
now that immunocompetence is also
severely compromised in sensitized calves
and piglets. The involvement of the
immune system in antigen challenge arose
from observations that the adverse effects
in the calf occurred only after administra-
tion of successive feeds containing defatted
and heated soybean products. This implied
that the initial feeds sensitized the calf to
the antigens, with overt digestive perturba-
tions ensuing after subsequent feeds. Such
a response is consistent with the classical
pattern of antibody synthesis (D’Mello,
1991b). Confirmation of the involvement of
the immune system in soybean hypersensi-
tivity arose from direct observations link-
ing elevated serum IgG concentrations with
the specific antigens, i.e. glycinin and -
conglycinin. Although the evidence for the
involvement of the immune system in calf
and piglet soybean hypersensitivity is now
unquestionable, it is still not clear which of
the immune reactions precipitates the
intestinal lesions. Tissue damage may be
caused by release of lysosomal enzymes or
an array of compounds such as histamine,
kinins, leukotrienes and prostaglandins.
The immunotoxic effects of myco-
toxins have been reviewed extensively
(Pier, 1991; Pestka and Bondy, 1994). Most
of the evidence is based largely on studies
with laboratory animals or on in vitro
measurements with lymphocytes subjected
to mitogenic stimulation. It is apparent
that, for example, oral administration of
AFB 1 to chicks lowers peripheral blood
lymphocyte proliferation to the mitogen
and lectin, concanavalin A. Chicken
macrophages treated with fumonisin B 1
develop morphological abnormalities, with
reduced phagocytic potential (Pestka and
Bondi, 1994). The implications for farm
animal health in practical situations need
to be addressed, particularly when feeds
contain two or more mycotoxins as co-
contaminants.Reproductive dysfunction
The reproductive tract and associated
endocrine systems are particularly sensi-
tive to the action of secondary substances
from plants and fungi and there are well-
defined examples of field cases positively
linked with these compounds. Thus, the
aetiology of ‘clover disease’ in sheep has
been correlated with the intake of the
phyto-oestrogenic isoflavonoids. Although
of less potency than endogenous steroidal
oestrogens, isoflavonoids can exert
significant effects due to their abundance
in leguminous plants. In addition, they are
capable of modulating the synthesis and
action of oestrogen in a variety of pathways
(Whitten et al., 1997). The isoflavones com-
petitively inhibit the binding of oestradiol396 J.P.F. D’Mello