Page 1005 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Cyanogenic Glycoside Containing Plants Chapter | 64 937

VetBooks.ir advantage to utilize toxins from poisonous plants is not neurological effects of the central nervous system involv-
ing the optic and auditory nerves. In the tropics and in
unique to the panda. There are many examples in the lit-
West Africa cassava is an important source of starch in
erature reporting animal, insect or microbial adaptation to
certain toxins providing some level of protection against diets of many people and animals. While low cyanide cul-
predators or a novel nutritional advantage. tivars of cassava have been developed the tubers still
must be prepared properly to avoid the disease. Cassavas
are classified into two main types: sweet cassava and bit-
ACUTE POISONING
ter cassava. Sweet cassava tubers usually contain fewer
As a glycoside, the cyanogenic glycosides in plants are than 50 mg HCN/kg on fresh weight basis, whereas bitter
relatively nontoxic to plants or animals. Cyanogenic cassava may contain up to 400 mg HCN/kg (Table 64.2).
glycosides only become toxic when the free hydrogen Sweet cassava can generally be made safe to eat by peel-
cyanide (HCN) is cleaved from the glycoside through a ing and thoroughly cooking the tubers (Kwok, 2008).
two-step enzymatic process (Cheeke and Shull, 1985). In Bitter cassava requires more extensive preparation.
the plant the glycosides and enzymes are in different plant Usually the tubers are peeled, grated and soaked in water
compartments, thus protecting the plant cells from HCN to allow leaching of CN. The water is discarded then the
toxicity. The glycosides are usually contained within cel- cassava is thoroughly cooked. Likewise, for safe human
lular vacuoles whereas the enzymes (glycosidases, lyases) consumption, bamboo shoots must be boiled before eating
are found in the plant cytosol. When plant cells are dam- (Kwok, 2008).
aged or stressed through crushing, chewing, frost, drought Livestock species ingesting low levels of cyanide
etc., the glycoside comes in direct contact with the (Sudan grass hay, silage or sorghum) over prolonged peri-
enzymes and the HCN levels increase quickly, and if ods develop a similar neurological disease including pos-
ingestion occurs, poisoning may result (Wattenbarger terior paralysis, ataxia, urinary incontinence and cystitis
et al., 1968; Burrows and Tyrl, 2013). Drying or curing (Smith et al., 1963; McKenzie and McMicking, 1977).
over time will reduce cyanide levels and ensiling of This is especially severe in horses and the disease has
cyanide-containing plants is reported to reduce cyanide been called “Equine Sorghum Cystitis-Ataxia Syndrome”
content by more than 50% (Pickrell et al., 1991). It is (Adams et al., 1969; Van Kampen, 1970). Mares exhibit-
important to allow sufficient time for these processes to ing theses clinical signs are sometimes called “dribblers”
reduce HCN levels before feeding to livestock. Cyanide because the mares rear legs and tail are dirty and un-kept
poisoning is a factor of multiple events such as ingestion from constant urine dribbling down both back legs. This
rate, HCN liberation rate from the plants, animal species also happens in male horses with obvious different gross
involved, health status of the animals and detoxification observations. In mares the CN-induced cystitis is further
and excretion rates. As previously mentioned animal spe- complicated by an ascending infection of the urinary tract
cies differ in response to cyanide toxicity and this is espe- and kidneys. While some improvement may occur if the
cially true for acute cyanide toxicity. forage source is removed, the demyelination of the nerve
Acute cyanide poisoning may occur in people also and tracts leaves some permanent irreversible debilitation
reports of poisoning from eating bitter almonds, apricot (Van Kampen, 1970). In pregnant sheep ingestion of for-
pits or seeds from other stone fruits or bamboo shoots can age containing low levels of CN has been associated with
be found in the literature (Burrows and Tyrl, 2013). goiters in lambs (Cheeke and Shull, 1985). Ultimately,
Bamboo shoots are a popular food item with some Asian this thyroid enlargement in the neonate results from one
cultures, and bitter cassava is still an important source of step in the detoxification mechanism producing a thiocya-
food in tropical countries. Both can lead to acute poison- nate intermediate that inhibits the metabolism of iodine
ing if not properly prepared. and elevates the concentration of thyroid stimulating hor-
mone (Tewe and Iyayi, 1989; Soto-Blanco et al., 2008).
Teratogenic effects in foals and piglets have been
CHRONIC POISONING
reported when cyanogenic glycosides were ingested dur-
On a worldwide basis chronic cyanide poisoning in ing certain times of gestation (Burrows and Tyrl, 2013).
humans is a serious health issue especially in tropical Skeletal malformations in pigs were reported in Missouri
regions. Cassava is a primary nutritional staple for more when pregnant sows ate wild black cherry leaves (Selby
than 300 million people throughout the tropics et al., 1971). Piglets were born with limb deformities and
(Balagopalan et al., 1988). In people a permanent chronic other malformations, while foals from mares grazing in
neurological condition associated with cassava ingestion Sudan grass pastures exhibited arthrogryposis and ankylo-
is referred to as “Tropical Ataxic Neuropathy” (Salkowski sis of the joints (Prichard and Voss, 1967). Seaman et al.
and Penney, 1994). The disease is manifest as paralysis of (1981) reported limb deformities in calves from pregnant
the lower extremities resulting in ataxia and other heifers grazing Sudan grass pastures. Keeler (1984)