Page 886 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

844 SECTION | XIV Poisonous Plants

VetBooks.ir Nitro-containing Astragalus (milkvetches) system involvement resulting in knuckling of the fetlocks,
goose stepping, clicking of the hooves, “cracker heels” pro-
There are more than 260 species and varieties (356 taxa)
gressing to paralysis, and death. A respiratory syndrome is
in
Astragalus
North
of
America
nitro-containing
also present in the chronic and acute forms, with emphy-
(Barneby, 1964; Welsh et al., 2007). They are frequently
sema-like signs causing the animals to force respiration:
referred to as milkvetches, as are some of the other
“roaring disease.” Sheep manifest the respiratory syndrome
Astragalus species. Nitro-toxins are therefore the most
more than the central nervous syndrome and are more
common toxin in the Astragalus, followed by swainsonine
resistant to poisoning compared to cattle.
(loco) and selenium. Major livestock losses occur in
The toxic principles are β-D-glycosides of 3-nitro-1-
many regions of the western United States. These plants
propanol (NPOH) or 3-nitropropionic acid (NPA). The
are very diverse and concentrated on the deserts, foot-
glycoside conversion occurs more readily in the ruminant
hills, and mountains of the west.
because of the microflora and is apparently the reason for
increased toxicity in ruminants. The glycoside (miserotox-
Description in) is metabolized to the highly toxic NPOH in the gastro-
The description of these plants is the same as that of the intestinal (GI) tract of ruminants (Williams et al., 1970).
locoweeds. The milkvetches emerge from late April to June Thus, NPOH is absorbed in the gut and apparently con-
depending on elevation and snow cover. After seed dispersal verted to NPA by the liver. Further metabolism yields
in July or August, the stems and leaves become dry and less inorganic nitrite and an unidentified metabolite that may
dangerous as toxicity and palatability are reduced. be involved in toxicity. It appears that NPOH is more rap-
idly absorbed from the gut than is NPA; therefore, forage
Distribution containing the alcohol form is the most toxic.
The aliphatic nitro-containing Astragalus are distributed
Prevention and Treatment
throughout North America, with substantial livestock losses
There is no preferred treatment for milkvetch poisoning,
occurring in the 17 western states where cattle or sheep are
concentrated in areas of milkvetch infestation (Table 61.1). although treatment with methylene blue appears to reverse
There are examples of a number of Astragalus spp. such as the methemoglobinemia but does not prevent death in cat-
A. cicer that are good forages and apparently contain low tle. Oxidation of NPOH to NPA was prevented if alcohol
levels of the toxins discussed. Because of the extent of this dehydrogenase was saturated with ethanol or inhibited
group of Astragalus, only a few of those implicated in poi- with 4-methylpyrazole before NPOH was given. This sug-
sonings are mentioned here. For a more comprehensive gests that NPOH is a good substrate for the enzyme alco-
review of the nitro-containing Astragalus,the reader is hol dehydrogenase. This information could be useful in
referred to Williams and Barneby (1977). Examples of these acute cases; however, its value in treatment of poisoning
include A. emoryanus (emory milkvetch) in New Mexico, in the field is unknown.
Texas, and Arizona (A. emoryanus also contains swainso- Livestock losses can be reduced by decreasing the
nine); A. tetrapterus (four-winged milkvetch) in Oregon, density of the Astragalus species with herbicides or
Utah, Nevada, and Arizona; A. pterocarpus (winged milk- avoiding grazing livestock on infested areas when the
vetch) in Nevada; and A. miser var. serotinus, A. miser var. plant is most poisonous. Wasatch milkvetch contains the
oblongifolius,and A. miser var. hylophylus (collectively highest concentration of miserotoxin from bloom to
referred to as timber milkvetch) in western Canada, immature pod stage of growth. Nitro compounds are
Montana, Idaho, Utah, Colorado, and Wyoming. found in all parts of the plant, but the leaves contain the
highest concentration. Once the leaves begin to dry and
lose their green color, the nitro levels drop very rapidly
Toxicology
and the plant is relatively nontoxic. However, the toxins
The nitro-containing Astragalus species cause an acute and in plants pressed green and preserved in herbaria appear
chronic type of poisoning in sheep and cattle. The acute to remain stable for years (Williams and Barneby, 1977).
form results in weakness, increased heart rate, respiratory Herbicide treatment decreases the density of plants and
distress, coma, and death. Although blood methemoglobin also decreases the toxicity of the plants once they start to
is high (induced from nitro-toxin metabolism to nitrites) dry; therefore, spraying milkvetch appears to be the best
and a contributing factor to the respiratory difficulties, method to reduce losses and still utilize infested ranges.
administration of methylene blue in cattle does not prevent
death. Therefore, the methemoglobinemia is apparently not Seleniferous Astragalus
the primary cause of death. The chronic form is the most
frequent form of poisoning observed and follows a course Approximately 22 24 species of Astragalus known to
of general weakness, incoordination, central nervous accumulate selenium (Se) have been identified (Rosenfeld