Page 880 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

838 SECTION | XIV Poisonous Plants

VetBooks.ir (ISOPP 9) will be held in Ho Hot, Inner Mongolia, China.
In addition, current research information on poisonous
plants from throughout the world is available in the online
peer-reviewed journal, the International Journal of
Poisonous Plant Research, by the Poisonous Plant
Research Laboratory, U.S. Department of Agriculture
Agricultural Research Service. International research, both
basic and applied, as well as case reports of plant poison-
ings and short communications are published in a spring
and a fall issue each year.

ASTRAGALUS AND OXYTROPIS SPECIES
FIGURE 61.1 Comparison of Astragalus and Oxytropis. Astragalus mol-
(LOCOWEEDS, NITRO SPP., AND lissimus (purple loco) is on the left, and Oxytropis lambertii is on the right.
SELENIUM SPP.)
Locoweeds diverse and are a principal means of distinguishing
between species (Figure 61.1).
Of all the poisonous plants in the United States, the Astragalus is a very large and complex genus, with
Astragalus and Oxytropis genera cause the most losses to 354 species and 198 varieties of Astragalus (552 taxa) in
the livestock industry in the western states (Graham et al., the United States and Canada (Welsh et al., 2007).
2009; Cook et al., 2009c). The locoweeds are those species Oxytropis is much smaller, with 22 species and 35 varie-
of the Astragalus and Oxytropis genera that contain the ties (57 taxa). Barneby (1964) earlier described and clas-
“loco” toxin (swainsonine) and induce the classic neurolog- sified 368 species and 184 varieties of Astragalus and 35
ical and pathological signs of “locoism.” Swainsonine is species of Oxytropis in North America. Regardless, the
also present in other Astragalus species not usually consid- Astragalus genus is the largest of the Leguminosae fam-
ered locoweeds, such as some selenium and nitro- ily. Species and variety identification are difficult even
containing Astragalus. There are three toxic syndromes for trained botanists. Because of obvious similarities
associated with these species: (1) locoism, caused by the between Oxytropis and Astragalus, some botanists treat
indolizidine alkaloid swainsonine (1) (24 species); (2) them together as Astragalus. Certainly, from a toxicology
nitrotoxins (356 taxa); and (3) species that accumulate sele- perspective, both can contain swainsonine and induce the
nium at high levels (22 species). same condition in livestock; therefore, both genera are
treated the same in this discussion.
Description
Distribution and ecology
These species are members of the Fabaceae
The Astragalus and Oxytropis are worldwide in their dis-
(Leguminosae) or pea family. This family is distinguished
tribution and toxic effects on livestock. Table 61.1 lists
by its papilionaceous flower (butterfly-like) having a sin-
those species in the western United States suspected of
gle large banner petal, two side petals or wings, and two
field cases of poisoning or that have been shown by
lower petals fused together to form a keel. Flowers are
chemical analysis to contain swainsonine (Molyneux
leguminous (pea-like), few or many, in axillary racemes,
et al., 1991).
and they may be blue, purple, yellow, or white in color;
Species of Astragalus and Oxytropis occur in every
fruit is a legume pod of various shapes, sizes, and sur-
major plant community. However, livestock poisoning is
faces among the species containing one or more kidney-
erratic due to the cyclic nature of the locoweed popula-
shaped seeds. The distinguishing feature in Oxytropis is
tions. Locoweeds have different survival strategies that
the porrect beak on the keel petal, whereas the Astragalus
allow perpetuation of the species through long-term cli-
have a blunt keel petal (Figure 61.1). Some Astragalus
matic cycles and short-term weather conditions (Ralphs
species have extensions of the keel forming points, but
et al., 2003). Climate controls the establishment and growth
they are not turned upward as in Oxytropis. In addition,
of these plants by the amount and timing of precipitation.
Oxytropis species are acaulescent (without a stem, all
The following are the three main survival strategies:
leaves basal) with leafless flowering stalks originating
from the crown, whereas leaflets of Astragalus species 1. Annual plants avoid drought by seed dormancy
grow from the multibranched stems with the flowering through dry cycles and germinate in years when suffi-
head on top (Figure 61.1). Leaflets are opposite and pin- cient moisture is available (winter annuals such as
nately compound. Pod shape, size, and chambers are very wootonii and A. emoryanus).