Page 926 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

880 SECTION | XIV Poisonous Plants

VetBooks.ir was neurotoxic. However, most narrow-leaved species
have now been shown to also contain cardiotoxins
(Ogden et al., 1992).
Historically, naturalists observed as early as the
mid- 1800s that birds avoided eating certain butter-
flies, such as the monarchs, whose larvae fed on milk-
weeds. It was later learned that the feeding larvae
were able to concentrate some of the cardenolides that
were potent emetics and thus created an effective
defense against herbivory. As of the late 1980s, 27
species of milkweed were recognized as common feed
for butterfly larvae, and the most toxic species were
often preferred. Milkweeds are also host to many other
insect species.

O
O

FIGURE 61.9 Narrow leaf (left) and broad leaf (right) milk-weeds
O
(Asclepias spp.) represented.
Sugar OH
represented in Table 61.12. The stems of the plants are 3
erect and range from 4 to 150 cm tall; leaves are opposite O 5
or whorled. Flowers are of various colors, umbellate clus- 6
ters or solitary, and terminal or extra-axillary. Numerous (14)
Cardenolide
seeds are contained in an inflated milkweed-type pod and
are flat and large, with each bearing a tuft of long, silk- The basic structure of the series of identified cardeno-
like hairs for wind distribution. lides is a 23-C steroidal backbone with a five-membered,
singly unsaturated lactone ring at the C-17 position, a
hydroxyl group at C-14, and methyl groups at C-10 and
Distribution and Habitat C-13. Glycosidic linkage usually occurs at the C-3 or C-2
positions and may include glucose, rhamnose, or theve-
Milkweeds are adapted to a broad range of habitats and
tose as the sugars. Additional functionalities attached to
environmental conditions and are found in most plant
the backbone further influence solubility and binding,
communities. Occupied habitats include prairies, wood-
thus increasing or decreasing toxicity. The cardenolides
lands, open areas, rocky plains, sand dunes, swamps,
1
1
inhibit Na /K -ATPase, and the structure activity rela-
marshes, seeps, canyon bottoms, dry washes, desert lands,
tionship is believed to reside in the unsaturated lactone
and waste lands. Most species are weedy and form small
ring at C-17 and the hydroxyl group at C-14 (Joubert,
to large patches often along roadsides, in disturbed areas,
1989).
or in overgrazed pastures.
Sheep, goats, cattle, horses, and domestic fowl have
been poisoned with milkweeds. Clinical signs usually
Toxicology begin with depression, weakness, and labored breathing
Some species of milkweeds have been used as medi- accompanied by ataxia and loss of balance, progressing to
cines, food, or a source of poison. Most species contain recumbency, seizures, and death. The order of clinical
a mixture of steroidal glycosidic cardenolides (14) that signs is dependent on the type of animal affected, the tox-
are toxic and induce two syndromes one with GI/car- icity of the plant, the time of ingestion, and the amount of
dio-toxic effects and the other with neurological effects. the toxin consumed. Clinical effects appear within a few
The early literature suggested that the broad-leaved hours of ingestion, and death may follow within 1 to a
group was cardiotoxic, whereas the narrow-leaved type few days later in fatal cases.