Herbicides are phytotoxic chemicals that are used routinely for unwanted plants and weed control. Most herbicides are quite selective for specific plants and are not as poisonous for animals. Less selective compounds, such as arsenicals, chlorates, and dinitrophenols, are more toxic to animals. Most toxicity problems in ruminant animals result from exposure to excessive quantities of herbicides because of improper or careless use or disposal of containers. When herbicides are used properly and within limits then problems are rare.Long term disease caused by herbicides is even more difficult to diagnose. It may include a history of herbicide use close to the animal or the animal's feed or water source, or a gradual change in the animal's performance or behaviour over several weeks, months, or even years. Occasionally, it involves manufacture or storage of herbicides nearby. Samples of possible sources (such as contaminated feed and water) for residue analysis, as well as tissues collected at necropsy, are essential. Months or even years may be required to successfully identify a problem of long term exposure.
Inorganic Herbicides
The inorganic herbicides are older compounds that are less expensive and more toxic than newer compounds. They are no longer used much in developed countries.The use of inorganic arsenicals (sodium arsenite and arsenic trioxide), sodium chlorates as herbicides has been reduced greatly because of livestock losses, the long-lasting effects on the environment, and their association with causing cancer.
1. Inorganic arsenicals: Sodium arsenate and chromic copper arsenate are not currently registered with the Environmental Protection Agency. Arsenic derivatives continue to be available in other parts of the world in wood preservatives and insecticides. These compounds can be hazardous to animals when used as recommended.The highly soluble organic arsenicals (methane arsonate, methyl arsonic acid) can concentrate in pools in toxic quantities after a rain has washed them from recently treated plants. Acute poisoning results in high death rate in large animals. Major action seen in gastro intestinal tract. Colic anorexia, staggering,partial paralysis of rear limbs and convulsions are common in subacute toxicity. BAL (British Anti levisite) is specific antidote for arsenicals herbicides
2. Ammonium sulfamateis not currently registered with the Environmental Protection Agency. It is used to kill brush and poison ivy.
3. Boraxhas been used as an herbicide and an insecticide. It is toxic to animals if consumed in moderate to large doses. Poisoning has not been reported when borax was used properly but has occurred when borax powder was scattered in the open for cockroach control. Signs of severe poisoning are diarrhea, rapid onset of weakness and an unwillingness to stand, and perhaps convulsions. An effective antidote is not known. Treatment consists of supportive care. Detergents containing borax should be stored away from where pets can get access to them.
4. Sodium chlorateis seldom used as an herbicide but remains registered. Treated plants and contaminated clothing are highly combustible and constitute fire hazards. Ingestion of treated plants and consumption of feed to which it is mistakenly added as salt can cause chlorate poisoning. Treatment with methylene blue must be repeated frequently. Blood transfusions, fluids, and mineral oil containing 1% sodium thiosulfate may be beneficial in treatment.
B. Organic Herbicides
Many organic herbicides can cause problems in a variety of animals.
1. Anilide or amide compounds(propanil, cyprinid, clomiprop) are plant growth regulators, and some members of this group are more toxic than others. Exposure to these compounds can affect red blood cells and the immune system.
2. Bipyridyl compounds or quaternary ammonium herbicides: It includes diquat and paraquat. These herbicides are used at low rates (2 ounces per acre [150 milliliters per hectare]), act quickly, are inactivated on soil contact, and quickly decompose in light. They produce toxic effects in the tissues of exposed animals. Skin irritation and clouding of the cornea can be seen after external exposure, and breathing in these chemicals is dangerous. Animals, including people, have died as a result of drinking from contaminated containers.Diquat exerts most of its harmful effects in the gastrointestinal tract. Signs of kidney damage, central nervous system excitement, and convulsions occur in severely affected animals. Paraquat has 2 phases to its toxic action after ingestion. Immediate signs include excitement, convulsions, lack of coordination, and inflammation of the gastrointestinal tract, loss of appetite, and possibly kidney involvement and breathing difficulty. Eye, nose, and skin irritation can be caused by direct contact, followed within days to 2 weeks by breathing problems.Treatment includes administration of activated charcoal or other adsorbants in large quantities, medications that cause emptying of the bowels and that increase urine output, and supportive treatment.
3. Carbamate and thiocarbamate compounds(terbucarb, asulam, carboxazole, EPTC, pebulate, triallate, vernolate, butylate, thiobencarb) are moderately toxic. However, they are used at low concentrations, and normal use should not result in poisoning. Massive overdosage in accidental exposure causes lack of appetite, depression, breathing difficulty, diarrhea, weakness, and seizures. antimuscarinic is drug atropine sulphate is specific antidote in acute carbamate toxicosis, maintained for 1-2 days.
4. Aromatic/benzoic acid compounds(chloramben, dicamba) have not caused poisoning after normal use. In overdosage, signs and tissue changes are similar to those described for poisoning by the phenoxyacetic compounds.
5. Phenoxyacetic and phenoxybutyric compounds(2,4-D [2-4-dichlorophenoxyacetic acid], 2,4,5-T [2,4,5-trichlorophenoxyacetic acid], 2,4-DB, MCPA) are commonly used for weed control. As a group, they are essentially nontoxic to animals when forage has been properly treated. When large doses are consumed, depression, loss of appetite, weight loss, tenseness, and muscular weakness (particularly of the hindquarters) are seen. In dogs, the muscles may remain contracted for longer than normal and have difficulty relaxing. Dogs also become uncoordinated and weak, and may have vomiting and diarrhea.The use of 2,4,5-T was limited and its registration cancelled because extremely toxic contaminants, collectively called dioxins (TCDD and HCDD), were found. They are known hepatotoxic chemical. Neuromuscular blockage and ataxia generally seen in cattle. No specific antidote is available forPhenoxyacetic and phenoxybutyric compounds.
6. Dinitrophenolic compounds: It includes dinoseb, binapacryl, and DNOC. The old 2-4 dinitrophenol and dinitrocresol compounds were highly toxic to all animals. 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. Signs include fever, difficulty breathing, metabolic abnormalities, a rapid heartbeat, and convulsions, followed by coma and death. Cataracts can develop in animals with longtermdinitrophenol poisoning. Exposure to dinitro compounds may cause yellow staining of the skin, conjunctiva (of the eye), or hair. An effective antidote is not known. Affected animals should be cooled and sedated to help control fever. Atropine sulfate, aspirin, and fever reducers should not be used. Carbohydrate solutions given intravenously and vitamin A injections may be useful.
. Organophospate compounds(for example glyphosate, bensulide) are widely used herbicides that have low toxicity. Exposure to toxic amounts is unlikely with recommended application and handling of containers.Dogs and cats have shown vomiting, eye, skin, and upper respiratory signs when exposed during or after an application to weeds or grass. Staggering, hypotension, metabolic acidosis and hind leg weakness have been seen cattle and small ruminants that were exposed to fresh chemicals on treated foliage. The signs usually disappear when exposure stops, and minimal symptomatic treatment is needed. Washing the chemical off the skin, emptying the stomach, and tranquilizing the animal are usually sufficient.
8. Substituted Anilines
The most commonly used herbicides of this group are alachlor, acetochlor, butachlor, metolachlor, and propachlor. Low doses in rats and dogs do not produce any adverse effects, but longterm exposure in dogs causes liver toxicity and affects the spleen. Ocular lesions produced by alachlor are considered to be unique to the Long-Evans rat, because the response has not been seen in other strains of rats or in mice or dogs.Compared with other substituted anilines, propachlor is severely irritating to the eye and slightly irritating to the skin in cattle. Propachlor produces skin sensitization in guinea pigs. High doses of propachlor produce erosion, ulceration, and hyperplasia of the mucosa and herniated mucosal glands in the pyloric region of stomach and hypertrophy and necrosis of the liver in rats. In dogs, there is poor diet palatability, which results in poor food consumption and weight loss. There is no suitable antidote. Supportive and symptomatic treatment is recommended.
9. Imidazolinones
Imidazolinone herbicides include imazapyr, imazamethabenz-methyl, imazapic, imazethapyr, imazamox, and imazaquin. These are selective broad-spectrum herbicides. Imidazolinone herbicides caused slight to moderate skeletal myopathy and/or slight anaemia in dogs during 1-yr dietary toxicity studies with three structurally similar imidazolinones (imazapic, imazaquin, and imazethapyr). There is no evidence of any adverse effect on reproductive performance or of fetal abnormalities in rats or rabbits. There is no suitable antidote. Supportive and symptomatic treatment is recommended.
C. Other Herbicides
Bromacil and terbacil are commonly used by methyluracil compounds. Toxic doses of bromacil can be hazardous, especially for sheep, but no field case of toxicity has been reported. The nitrile herbicides, ioxynil and bromoxynil may uncouple and/or inhibit oxidative phosphorylation. Ioxynil, presumably because of its iodine content, causes enlargement of the thyroid gland in the rat.A number of substances are used as defoliants in agriculture. For example, sulfuric acid is used to destroy potato haulms and two closely related trialkylphosphorothioates (DEF and merphos) to defoliate cotton. A notable feature of the latter is that it produces organophosphate-induced delayed neuropathy in hens. Chlomequat is used as a growth regulator on fruit trees. The signs of toxicity in experimental animals indicate that it is a partial cholinergic agonist.
TREATMENT:
Treatment includes administration of antidotes, activated charcoal or other adsorbents in large quantities, medications that cause emptying of the bowels and that increase urine output, and supportive treatment. On dermal exposure, the animal should wash with soap (mild detergent) water and rinsed thoroughly. Excessive seizures and convulsions can be controlled by diazepam or barbiturate sedatives. Oxygen therapy is advised if dyspnoea is prominent.
About the Author :
R.Y. Nanotkar, K. Adilaxmamma, Y. Muralidhar and Bhadraiah
Assistant professor, Professor and Head
Dept. of Veterinary Pharmacology & Toxicology,
College of Veterinary Sciences (C.V.Sc.), S.V.V.U., Tirupati (A.P.)
Corresponding [email protected]
