Locoweed (alsocrazyweed andloco) is acommon name inNorth America for any plant that producesswainsonine, analkaloid harmful tolivestock. Worldwide, swainsonine is produced by a small number ofspecies, most of them in threegenera of theflowering plant familyFabaceae:Oxytropis andAstragalus inNorth America,[1] andSwainsona inAustralia. The term locoweed usually refers only to the North American species ofOxytropis andAstragalus, but this article includes the other species as well. Some references may incorrectly listDatura as locoweed.[2]
Locoweed is relativelypalatable to livestock, and some individual animals will seek it out. Livestock poisoned by chronic ingestion of large amounts of swainsonine develop a medical condition known aslocoism (swainsonine disease, swainsonine toxicosis in North America) andpea struck in Australia.[3] Locoism is reported most often in cattle, sheep, and horses, but has also been reported inelk anddeer. It is the most widespread poisonous plant problem in the westernUnited States.[1][4]
Most of the 2,000 species ofAstragalus, including many that are commonly known as locoweeds, do not produce swainsonine. Some species, including a few that produce swainsonine, accumulateselenium. This has led to confusion between swainsonine poisoning and selenium poisoning due to this genus.[5]
The first technical account (inEnglish) of locoism was published in 1873, in the United States.Linguists have documentedlocoism in use among English speakers by 1889, and bothloco andlocoweed in use by 1844.[6]
Loco, aloanword fromSpanish, is understood by most English-speaking users in the sense ofcrazy, and this appears to have also been the sense understood byvaqueros.[6] In Spanish, however,loco has an older, different sense. InSpain, where the nativeAstragalus species are not known to cause locoism, for centuriesloco has been applied to some of these species in the sense oframbling: common names includeyerba loca (hierba loca; rambling herb) andchocho loco (rambling lupine).[citation needed]
The presence of a toxin in locoweed was demonstrated in 1909. Initially, the toxin was reported to be abarium compound, but that was soon disproved. Swainsonine, first isolated fromSwainsona, was shown to be responsible for pea struck in 1979, and was reported in bothOxytropis andAstragalus in 1982.[7]
Since 1982, swainsonine has been isolated from still more plants, some of which also are reported to cause locoism or medical conditions similar to it. The first report of locoism inSouth America, involvingAstragalus pehuenches, was published in 2000.[8]
Swainsonine is produced by a small number of species, including species in several genera of plants and two genera of fungi.
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Oxytropis is distributed throughout westernNorth America, particularly in theGreat Plains andRocky Mountains. However, most species ofOxytropis have narrow habitat requirements and within those habitats are abundant only in unusually wet years.[1] The species most frequently encountered by livestock areO. lambertii (Lambert locoweed, purple locoweed, woolly locoweed) and especiallyOxytropis sericea (white locoweed, white point locoweed, white point loco). Swainsonine has also been found inO. campestris (inCanada).[9]
Some species ofAstragalus (milkvetch) are also referred to as locoweed. Swainsonine has been found in:[9]
InArgentina, locoism (locoismo) was first reported in 2000. A flock of sheep grazing a pasture withAstragalus pehuenches was poisoned and 220 sheep (73%) died.[8] Although this was the first report of locoism inSouth America,[8] swainsonine had been isolated previously fromA. pehuenches and several other species in Argentina andPeru.[8][10]
In theOld World, native plants causing locoism have not been reported.Astragalus lusitanicus inMorocco was suspected,[11] but has been shown be neither a producer of swainsonine nor an accumulator of selenium. Its toxicity is suspected to be due to a novelalkaloid.[12]
InAustralia, species ofSwainsona (Darling pea) that cause pea struck include:[9][13]
Astragalus andOxytropis are two of 20 genera (and 78 names of genera) in the tribeGalegeae, subtribe Astragalinae. Some authorities includeSwainsona in the subtribe.[14] Formerly,Swainsona was in another subtribe, Coluteinae, that has been combined into Astragalinae.
Swainsonine has also been isolated fromSida carpinifolia andIpomoea carnea, and both species have been reported to cause locoism.[15]
Embellisia, afungus isolated fromOxytropis lambertii, has also been shown to produce swainsonine and to cause locoism in rats.[16]Rhizoctonia leguminicola, a fungalplant pathogen that may occur on red clover (Trifolium pratense), also produces swainsonine. Although intoxication due to this fungus resembles locoism, it has additional signs and symptoms due to the production of other toxins.[17]
Intoxication with swainsonine has several kinds of effect.
Livestock that graze for several weeks on locoweed (and little else) develop alysosomal storage disease similar to geneticmannosidosis.[18] Swainsonine inhibits alysosomalenzyme,alpha-mannosidase.[19] This results in abnormal accumulation of the molecules normally processed by the enzyme, and this accumulation leads tovacuolation of most tissues. Vacuolation is most obvious inneurons andepithelial cells. The vacuolation resolves shortly after poisoning is discontinued, but if the vacuolation is so severe that it destroys cells, it may result in some neurologic damage that is irreversible and permanent.[18] The damage is highly varied. In cattle athigh altitude, complications of locoism can includecongestive heart failure.[20]
Diagnosis of clinical poisoning is generally made by documenting exposure, identifying the neurologic signs, and analyzingblood serum for alpha-mannosidase activity and swainsonine.[18]
Inmule deer, clinical signs of locoism are similar tochronic wasting disease.Histological signs of vacuolation provide adifferential diagnosis.[21]
Sub-clinical intoxication has been investigated in cattle grazing onAstragalus mollissimus. As the estimated intake of swainsonine increased,blood serum alpha-mannosidase activity andalbumin decreased, andalkaline phosphatase andthyroid hormone increased.[22]
BecauseO. sericea is both frequently encountered and relativelypalatable to livestock, it is an important cause of economic losses in livestock production. Keeping livestock away from locoweed-infested pasture in spring and fall when grass and other forbs are not actively growing is recommended. Another suggested remedy is to provide palatable supplemental nutrients if animals are to be kept in infested pasture. These remedies take into account livestock preference for locoweed during seasons when grass is dry and not very nutritious.[23]Conditioned food aversion has been used experimentally to discourage livestock from eating it.[24][25] In horses, a small study has shown promising results usinglithium chloride as theaversive agent.[25]
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