Page 887 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

Poisonous Plants of the United States Chapter | 61 845

VetBooks.ir and Beath, 1964; Welsh et al., 2007). These are less and duration of exposure. Acute cases of selenium poison-
ing are rare and usually involve animals that have been
numerous and more geographically restricted than the
exposed by one of three methods. First, livestock graze
nitro-containing species. Many of these species are
referred to as Se-indicator plants because they only grow forages that have accumulated selenium from seleniferous
on soils high in bioavailable selenium; therefore, they are soils. Second, selenium toxicosis occurs from environ-
helpful in locating and identifying areas or soils high in mental contamination from agricultural drain water,
selenium. The Astragalus are generally deep rooted plants reclaimed soils from phosphate or ore mining, or from fly
and may bring selenium from deeper soil profiles unavail- ash. Third, acute selenosis can be caused by accidental
able to other plants so that it is available and can subse- overdosing with organic selenium or Bo-Se in the treat-
quently be taken up by grasses and other forbs. It is these ment of white muscle disease or by misformulated feed
facultative accumulators that create most of the subacute mixes. The signs of acute selenium poisoning include
or chronic toxicity problems for livestock. diarrhea, unusual postures, increased temperature and
heart rate, dyspnea, tachypnea, respiratory distress, pros-
Description tration, and death (Tiwary et al., 2006). Gross pathologi-
cal findings are usually limited to pulmonary congestion
The selenium-containing Astragalus species appear simi-
and hemorrhage and pulmonary edema. Histologically,
lar to those of locoweed and nitro-containing species as
multifocal myocardial necrosis and pulmonary alveolar
discussed previously. One identifying feature of the
vasculitis are common (Tiwary et al., 2006).
selenium-containing Astragalus species is an unpleasant
Chronic selenium poisoning is common and referred
garlic-like odor of the volatile selenium compound
to as alkali disease because most areas with high concen-
dimethyl diselenide, especially if picked and allowed to
trations of available selenium are alkaline in nature.
sit in a warm car or window. The Se-indicator plants are
Chronic selenosis occurs from prolonged ingestion of sel-
generally considered to be unpalatable to livestock
eniferous forages containing 5 40 ppm Se. Clinical signs
because of their high selenium content. Some Astragalus
include rough coat, hair or wool loss, poor growth, emaci-
species have been shown to accumulate selenium at con-
ation, abnormal hoof growth and lameness, dermatitis,
centrations of up to 10 mg Se/g (dry weight) while grow-
and depressed reproduction (Rosenfeld and Beath, 1964;
ing on soils containing 2 10 μg Se/g. The majority of
Raisbeck, 2000). In swine, a condition of paralysis (polio-
selenium in accumulator plants is found as organic
myelomalacia or polioencephalomalacia) often occurs
methylselenocysteine and selenocystathionine or as inor-
with cervical or lumbar involvement (Panter et al.,
ganic selenate. Many selenium-containing Astragalus may
1996b). The description of a second chronic syndrome in
also contain other toxins; for example, A. praelongus and
cattle called “blind staggers” has been redefined and is
A. bisulcatus contain swainsonine (the loco toxin), and
now believed to be polioencephalomalacia induced by
A. toanus contains nitro-toxins and swainsonine in
high sulfate water or high sulfate forage sources.
addition to selenium.
Selenium is found in plants in both inorganic and
organic forms. The organic forms are more bioavailable
Distribution
than the inorganic forms, resulting in higher tissue con-
The selenium-containing Astragalus species are limited in
centrations when administered at equivalent doses
their distribution to geographical locations and soil sites
(Tiwary et al., 2006; Davis et al. 2011). Although a dra-
of high selenium (Table 61.1), of which most are alkaline
matic difference in tissue selenium uptake between
in nature, causing selenium to be oxidized and making it
organic (selenomethionine) and plant (A. bisulcatus)
soluble and available to plants. Major seleniferous areas
forms and inorganic (sodium selenate) forms occurs, the
of the west are in North and South Dakota, Montana,
clinical and pathological syndromes are similar that is,
Wyoming, Colorado, and Utah, with minor isolated pock-
poliomyelomalacia in pigs (Panter et al., 1996b) and pul-
ets of seleniferous soils in most of the other western
monary edema and hemorrhage in sheep (Tiwary et al.,
states.
2006, Davis et al. 2011).
The Astragalus species most associated with selenium
poisoning include A. bisulcatus (two-grooved milkvetch),
A. praelongus (stinking milkvetch), A. pattersonii Prevention of Poisoning
(Patterson milkvetch), A. pectinatus (tiny-leaved milk-
There is no treatment for selenium poisoning except
vetch), and A. racemosus (alkali milkvetch).
removal of the source, allowing spontaneous recovery in
chronic cases. Monitoring soils in a particular area and
Toxicity understanding the plant communities can provide the
With selenium poisonings, one may observe acute, sub- management information to avoid poisoning. In areas
acute, or chronic selenosis depending on the daily dose where selenium is a problem, many ranchers have