Oxalate poisoning
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Oxalate poisoning in animals generally occurs when quantities of oxalate-containing plants are grazed by livestock that are not accustomed to eating the plants. The most important oxalate-containing plants in North America include halogeton (H. glomeratus), and greasewood (S. vermiculatus) [1,2]. In Australia and Africa oxalate poisoning has been associated with livestock grazing Oxalis spp. Other plants known to contain significant quantities of oxalate are listed below. Plants normally accumulate oxalates in the form of soluble potassium and sodium oxalates. The more toxic potassium acid oxalate predominates in plants with a very acid cell sap (pH 2) (Oxalis spp.); sodium oxalate occurs in plants with a cell sap pH of 6 (Halogeton, Sarcobatus spp.).
Oxalate poisoning most often occurs when unadapted sheep or cattle are allowed to graze large amounts of Halogeton or Sarcobatus as they pass through or are pastured overnight on rangeland containing large stands of these plants. Under normal range conditions sheep are most frequently poisoned by oxalate-containing plants. Ruminants in general tolerate relatively more oxalate in their diet than other animals because they are able to detoxify oxalate in the rumen thereby preventing the absorption of the soluble oxalates. When large quantities of soluble potassium and sodium oxalates are eaten that overwhelm the rumen's ability to metabolize the oxalates, they are absorbed into the bloodstream and form insoluble calcium and magnesium oxalates. It is these insoluble salts that precipitate in the kidneys and cause kidney failure. Factors that predispose an animal to oxalate intoxication include the amount and rate at which the oxalate plant is eaten and the quantity of other feed diluting the oxalate in the rumen. Prior adaptation of rumen microflora to oxalates allows the animal to consume more oxalate because the increased number of oxalate-degrading bacteria in the rumen more effectively metabolize the oxalate. Ruminants allowed to graze small quantities of oxalate-containing plants are able to increase their tolerance for oxalate 30 percent or more over a few days. Once adapted to oxalate, sheep and cattle can make effective use of range forages containing oxalate that would otherwise be toxic.
Toxic Effects of Oxalates
Once absorbed from the gastrointestinal tract, soluble oxalates rapidly combine with serum calcium and magnesium, causing a sudden decrease in available serum calcium and magnesium. In the acute phase of oxalate poisoning the sudden decrease in soluble serum calcium (hypocalcemia) impairs normal cell membrane function, causing animals to develop muscle tremors and weakness, leading to collapse and eventually death. Oxalates also interfere with cellular energy metabolism that contributes to the acute death of affected animals. In chronic oxalate poisoning, insoluble calcium oxalate filtered by the kidneys causes severe damage to the kidney tubules (oxalate nephrosis). If animals do not die from the acute effects of the low blood calcium levels and impaired cellular energy metabolism, death results from kidney failure.Clinical Signs
Within a few hours of consuming toxic levels of oxalate, sheep and cattle develop muscle tremors, tetany, weakness, reluctance to move, depression, and recumbency resulting from hypocalcemia (insufficient calcium) and hypomagnesemia (insufficient magnesium). Coma and death may result within 12 hours. Animals that survive the acute effects of oxalate poisoning frequently succumb to kidney failure. As animals become uremic (increased serum creatinine and urea nitrogen levels), they develop severe depression, stop eating, and after a few days become comatose and die.Horses may over time develop chronic calcium deficiency while grazing certain tropical grasses containing soluble oxalates (listed above). Oxalates in the grasses combine with calcium to form relatively insoluble calcium oxalate, thereby reducing calcium absorption and altering the calcium:phosphorus ratio. This causes mobilization of bone calcium through the action of parathyroid hormone to compensate for the low blood calcium levels. Over time the horse's bones lose sufficient calcium so that they become soft and misshapen (nutritional secondary hyperparathyroidism). Sheep and cattle can be similarly affected, but are better able to metabolize oxalates in the rumen thereby reducing their effect on dietary calcium.
Diagnosis
A diagnosis of oxalate poisoning can be made on the basis of the type of plants being eaten, the clinical signs, hypocalcemia, and the presence of oxalate crystals in the urine. However, oxalate crystals in the urine are usually not present in acute oxalate poisoning. Necropsy lesions seen in oxalate poisoning will depend on the severity of the poisoning. In acute poisoning, the kidneys may be edematous and dark red in color, whereas in chronic poisoning the kidneys may be pale and smaller than normal. Perirenal edema is a characteristic feature of oxalate poisoning in pigs and cattle consuming pigweed (Amaranthus retroflexus). The rumen may often become hemorrhagic due to the presence of large quantities of oxalate in the rumen epithelial lining. The demonstration of calcium oxalate crystals in the kidneys and rumen epithelium histologically is diagnostic of oxalate poisoning.Treatment
Treatment with intravenous calcium gluconate, although theoretically appropriate for correcting hypocalcemia, is not effective in reversing the effects of the oxalate on cellular energy metabolism. Irreversible oxalate nephrosis and the effects of oxalates on cellular energy metabolism are more detrimental to the animal than hypocalcemia. A theoretical approach to treating acute oxalate poisoning would be to administer intravenous calcium gluconate, magnesium sulfate, glucose, and a balanced electrolyte solution to maintain kidney perfusion. Giving limewater (Ca[OHΤ) orally will help to prevent absorption of further soluble oxalate.Prevention of Oxalate Poisoning
Livestock should not be grazed on rangeland on which oxalate-containing plants predominate without precaution, especially if the animals are hungry and have not been adapted to oxalate in their diet. Livestock should be introduced to oxalate plants for at least 4 days by incrementally increasing the time they are allowed to graze the plants. Overstocking and overgrazing will inevitably result in oxalate poisoning if there is not other vegetation for the animals to eat. Cattle and sheep driven through or held overnight in pastures rich in oxalate-containing plants are prone to poisoning, and such circumstances should be avoided. Supplementary dicalcium phosphate in the diet before and during high-risk oxalate exposure is an effective means of reducing fatalities. High levels of dietary calcium bind oxalate in the rumen as insoluble, nonabsorbable calcium oxalate. Calcium may be provided to the animals in a salt mix (75 lb salt, 25 lb dicalcium phosphate) or in pelleted alfalfa at a 5 percent concentration and fed at the rate of 0.5 lb per sheep per day. Livestock diets can also be supplemented with hay to help reduce the total intake of oxalate-containing plants.Plants Containing Oxalates
{| |Amaranth ||(Amaranthus spp.) |- |Five-hooked bassia ||(Bassia hyssopifolia) |- |Beet and chard ||(Beta vulgaris) |- |Fat Hen ||(Chenopodium album) |- |Halogeton ||(Halogeton glomeratus) |- |Kochia, summer cypress ||(Kochia scoparia) |- |Shamrock, soursob, sorrel ||(Oxalis spp.) |- |Purslane ||(Portulaca oleracea) |- |Poke ||(Phytolacca americana) |- |Sorrel, dock ||(Rumex spp.) |- |Rhubarb ||(Rheum spp.) |- |Russian thistle, tumbleweed ||(Salsola spp.) |- |Greasewood ||(Sarcobatus vermiculatus) |- |Buffel grass ||(Cenchrus ciliaris) |- |Elephant grass ||(Panicum spp.) |- |Kikuyu grass ||(Pennisetum clandestinum) |- |Setaria grass ||(Setaria sphacelata) |- |Jack in the pulpit ||(Arisaema spp.) |- |Elephant’s ear ||(Alocasia spp.) |- |Anthurium, flamingo flower ||(Anthurium spp.) |- |Lords and Ladies, Cuckoo-pint ||(Arum spp.) |- |Wild calla, water arum ||(Calla palustris) |- |Caladium ||(Caladium spp.) |- |Dumb cane ||(Dieffenbachia spp.) |- |Pothos, variegated philodendron, taro vine ||(Epipremnum pinnatum) |- |Monstera, cutleaf philodendron, bread fruit vine ||(Monstera spp.) |- |Philodendron ||(Philodendron spp.) |- |Peace lily ||(Spathephyllum spp.) |- |Umbrella tree ||(Schefflera spp.) |- |Calla lily ||(Zantedeschia spp.) |- |Spinach ||(Spinacia oleracea) |-
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