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Anilide or Amide Compounds (propanil, cypromid, clomiprop): |
| These herbicides are plant growth regulators, and some members of this group are more toxic than others. Hemolysis, methemoglobinemia, and immunotoxicity have occurred after experimental exposure to propanil. The half-life in catfish is >15 days. See
organophosphate compounds, below, for discussion of bensulide. |
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Bipyridyl Compounds or Quaternary Ammonium Herbicides (diquat, paraquat): |
| The bipyridyl compounds are nonvolatile desiccant herbicides used at rather low rates of 2 oz/acre (150 mL/hectare). These compounds act rapidly, are inactivated on soil contact, and rapidly decompose in light. They produce toxic effects in the tissues of exposed animals by development of free radicals. Tissues can be irritated after contact (eg, mouth lesions after recent spraying of pastures). Skin irritation and corneal opacity occur on external exposure to these chemicals,
and inhalation is dangerous. Animals, including humans, have died as a result of drinking from contaminated containers. |
| Paraquat and diquat have somewhat different mechanisms of action. Diquat exerts most of its harmful effects in the GI tract. Animals drinking from an old diquat container showed anorexia, gastritis, GI distension, and severe loss of water into the lumen of the GI tract. Signs of renal impairment, CNS excitement, and convulsions occur in severely affected individuals. Lung lesions are uncommon. |
| Paraquat has a biphasic toxic action after ingestion. Immediate effects include excitement, convulsions or depression and incoordination, gastroenteritis with anorexia, and possibly renal involvement and respiratory difficulty. Eye, nasal, and skin irritation can be caused by direct contact, followed within days to 2 wk by pulmonary lesions as a result of lipid-membrane peroxidation and thus destruction of the type I alveolar pneumocytes. This is reflected in progressive
respiratory distress and is evident on necropsy as pulmonary edema, hyaline membrane deposition, and alveolar fibrosis. Toxicity of paraquat is enhanced by deficiency of vitamin E or selenium, oxygen, and low tissue activity of glutathione peroxidase. |
| Due to slow absorption of these chemicals, intensive oral administration of adsorbants in large quantities and cathartics is advised. Bentonite or Fuller’s earth is preferred, but activated charcoal will suffice. In conjunction with supportive therapy, eg, vitamin E and selenium, excretion is accelerated by forced diuresis induced by mannitol and furosemide. Oxygen therapy should not be used, and some suggest only 15% oxygen in inspired air. |
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Carbamate and Thiocarbamate Compounds (terbucarb, asulam, carboxazole, EPTC, pebulate, triallate, vernolate, butylate, thiobencarb): |
| These herbicides are moderately toxic; however, they are used at low concentrations, and poisoning problems would not be expected from normal use. Massive overdosage, as seen with accidental exposure, produces symptoms similar to the insecticide carbamates, with lack of appetite, depression, respiratory difficulty, mouth watering, diarrhea, weakness, and seizures. Thiobencarb has induced toxic neuropathies in neonatal and adult laboratory rats. It appears to increase
permeability of the blood-brain barrier. |
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Aromatic/Benzoic Acid Compounds (chloramben, dicamba): |
| The herbicides in this group have a low order of toxicity to domestic animals, and poisoning after normal use has not been reported. Environmental persistence and toxicity to wildlife is also low for this group. The signs and lesions are similar to those described for the phenoxyacetic compounds (see below). |
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Phenoxyacetic and Phenoxybutyric Compounds (2,4-D [2-4-dichlorophenoxyacetic acid], 2,4,5-T [2,4,5-trichlorophenoxyacetic acid], 2,4-DB, MCPA): |
| These acids and their salts and esters are commonly used to control undesirable plants. As a group, they are essentially nontoxic to animals exposed to properly treated forage. When large doses are fed experimentally, general depression, anorexia, weight loss, tenseness, and muscular weakness (particularly of the hindquarters) are noted. Large doses in cattle may interfere with rumen function. Dogs may develop myotonia, ataxia, posterior weakness, vomiting, diarrhea, and
metabolic acidosis. (The oral LD50 for 2,4-D and 2,4,5-T in dogs is ~100 mg/kg body wt.) Even large doses, up to 2 g/kg, have not been shown to leave residues in the fat of animals. These compounds are plant growth regulators, and treatment may result in increased palatability of some poisonous plants as well as increased nitrate and cyanide content. |
| The use of 2,4,5-T was curtailed because extremely toxic contaminants, collectively called dioxins (TCDD and HCDD), were found in technical grade material (see also
Halogenated Aromatic Poisoning: Introduction). The 2,3,7,8-TCDD is considered carcinogenic, mutagenic, teratogenic, fetotoxic, and able to cause reproductive damage and other toxic effects. Although manufacturing methods have reduced the level of the contaminants, use of this herbicide is very limited worldwide. Its EPA registration was canceled and it is no longer manufactured in the USA. |
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Dinitrophenolic Compounds (dinoseb, binapacryl, DNOC): |
| The old 2-4 dinitrophenol and dinitrocresol compounds were highly toxic to all classes of animals (eg, LD50 of 20-56 mg/kg body wt). Poisoning can occur if animals are sprayed accidentally or have immediate access to forage that has been sprayed, because these compounds are readily absorbed through skin or lungs. Dinitrophenolic herbicides markedly increase oxygen consumption and deplete glycogen reserves. Clinical signs include fever, dyspnea, acidosis,
tachycardia, and convulsions, followed by coma and death with a rapid onset of rigor mortis. Cataracts can occur in animals with chronic dinitrophenol intoxication. Exposure to dinitro compounds may cause yellow staining of the skin, conjunctiva, or hair. An effective antidote is not known. Affected animals should be cooled and sedated to help control hyperthermia. Phenothiazine tranquilizers are contraindicated; however, diazepam can be used to calm the animal. Atropine sulfate,
aspirin, and antipyretics should not be used. Administration (IV) of large doses of carbohydrate solutions and parenteral vitamin A may be useful. If toxic amounts of one of these is ingested and the animal is alert, emetics should be administered in animals that readily vomit (eg, not horses); if the animal is depressed, gastric lavage should be performed. Treatment with activated charcoal should follow. |
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Organophosphate Compounds (glyphosate, bensulide): |
| These are widely used herbicides with low toxicity, although fish and pond life have been killed experimentally. Sprayed forage appears to be preferred by cattle for 5-7 days after application and causes little or no problem. Acute LD50 in rats is >5.6 g/kg. |
| A few dogs and cats show eye, skin, and upper respiratory tract signs when exposed during or subsequent to an application to weeds or grass. Nausea, vomiting, staggering, and hindleg weakness have been seen in dogs and cats that were exposed to fresh chemical on treated foliage. The signs usually disappear when exposure ceases, and minimal symptomatic treatment is needed. Washing the chemical off the skin, evacuating the stomach, and tranquilizing the animal are usually
sufficient. Massive exposure with acute signs due to accidental contact should be handled as an organophosphate poisoning (
Organophosphates: Overview). |
| Bensulide, listed as a plant growth regulator, has an oral LD50 in rats of 271-770 mg/kg, in dogs the lethal dose is ~200 g/kg. The most prominent clinical sign is anorexia, but other signs are similar to 2,4-D poisoning. |
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Triazolopyrimidine Compounds (bromacil, terbacil): |
| These compounds can cause mild toxic signs at levels of 50 mg/kg body wt in sheep, 250 mg/kg in cattle, and 500 mg/kg in poultry when given daily for 8-10 doses. Signs include bloat, incoordination, depression, and anorexia. Application rates of ~5 lb/acre (5.6 kg/hectare) can be hazardous, especially for sheep, but no field cases of toxicity have been reported. |
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Phenyl or Substituted Urea Compounds (diuron, fenuron, linuron, monolinuron): |
| Exposure to toxic amounts of these herbicides is unlikely with recommended application and handling of containers. Signs and lesions are similar to those described for the phenoxyacetic herbicides (see above). The substituted urea herbicides induce hepatic microsomal enzymes and may alter metabolism of other xenobiotic agents. Altered calcium metabolism and bone morphology have been seen in laboratory animals. |
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Polycyclic Alkanoic Acids or Aryloxyphenoxypropionic Compounds (diclofop, fenoxaprop, fenthiaprop, fluazifop, haloxyfop): |
| Members of this group that are approved as herbicides are of moderately low toxicity with acute oral LD50 doses from 950 mg/kg to >4,000 mg/kg. (Haloxyfop-methyl is an exception and has an LD50 in male rats of ~400 mg/kg.) They tend to be more toxic if exposure is dermal. The dermal LD50 of diclofop in rabbits is only 180 mg/kg. |
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Triazinylsulfonylurea or Sulfonylurea Compounds (chlorsulfuron, sulfometuron, ethametsulfuron, chloremuron): |
| Toxicity in this group appears to be quite low. The oral acute LD50 in rats is in the range of 4,000-5,000 mg/kg. The dermal acute LD50 in rabbits is ~2,000 mg/kg. |
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Triazine, Methylthiotriazine, Triazinone Compounds (atrazine, cyanazine, pro-metryn, metribuzin, simazine): |
| Although these herbicides are widely used, incidents of poisoning are uncommon. Occasionally, accidental exposure of animals to large dosages (eg, open containers, spills) can cause toxic effects and even death. (Doses of 500 mg of simazine/kg or 30 mg of atrazine/kg for 36-60 days were lethal to sheep.) Generally, single doses >100-200 mg/kg body wt can be detrimental; repeat administration may reduce the toxic dose to <100 mg/kg body wt. Deaths have been reported in
sheep and horses grazing triazine-treated pastures 1-7 days after spraying. Cumulative effects are not evident. The signs and lesions are similar to those described for the phenoxyacetic compounds (see above). |
| The oral LD50 of metribuzin is 2,200 mg/kg in rats and 500-1,000 mg/kg in birds. No harmful effects were apparent when it was fed to dogs at 100 ppm in the diet. |
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