- Email: [email protected]
Organophosphate and Carbamate Insecticides
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Dennis J. Blodgett, DVM, PhD
• Organophosphate and carbamate insecticides are common pesticides used in agriculture, around the home, and on and around domestic animals. • Both groups of insecticides inhibit AChE activity. • Clinical signs of acute poisoning are generally divided into three major categories. Muscarinic signs include SLUD, miosis, bronchospasm, bradycardia, and emesis. Nicotinic signs include muscle tremors, ataxia, weakness, and paralysis. CNS signs can range from severe depression to hyperactivity and seizures. • Exposures in dogs and cats to lipophilic organophosphates may lead to an “intermediate syndrome,” characterized mostly by a delayed onset of anorexia, depression, generalized weakness, muscle tremors, ventroflexion of the neck, and abnormal behavior. • Blood or brain AChE activity should be assessed and is an excellent screening tool to determine exposure in clinically affected patients or postmortem. • Treatment options include decontamination, use of atropine sulfate, 2-PAM, diazepam, and respiratory support.
SOURCES Organophosphate (OP) insecticides are marketed for control of insects on plants, animals, soils, and around the house. A few OP products are sold as anthelmintics. Many OPs are designed to remain on the surface of objects to which they are applied; others are meant to be absorbed by plants or animals and to become systemic. Labels on the products have the term phosphate, phos, phoro, or phosphor, somewhere in the long chemical name of the ingredients. Most OPs are very insoluble in water and are 941
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often formulated with oily vehicles or organic solvents. Others are sold as dusts, wettable powders, emulsions, or adsorbed to clay particles. The OPs used to combat fleas are sold as sprays, dips, shampoos, flea collars, or flea bombs. As newer classes of insecticides are marketed for flea control, OPs are involved in fewer cases of poisoning associated with intentional applications. Agricultural uses of OPs are varied, but many are used for control of corn rootworms. Many of these corn rootworm insecticides are very toxic. As such, agricultural OPs have begun to replace strychnine as the malicious pesticide of choice for killing dogs in some areas of the United States.1 Baits are often mixed with food items (e.g., meat, bread, carcasses, and tuna fish) in cases of malicious poisoning. Carbamate insecticides were designed for purposes similar to those described for OPs. Carbamates are similar in structure to physostigmine and neostigmine. The use of carbamates in flea products is also declining with the advent of safer classes of insecticides. Carbamates are still used abundantly for household and agricultural applications. Not all pesticides having the word carbamate in their chemical name are acetylcholinesterase (AChE) inhibitors. Care should be taken to obtain the entire chemical name of a product when working up a possible pesticide exposure. Carbamates, as in the case of OPs, have also gained in popularity as malicious pesticides of choice. Onset times are often shorter than OPs, and the poisoned animal may die within yards of the bait.
TOXIC DOSE The toxicity or lethal dose (LD) of the insecticides is as varied as the types of insects they were formulated to control. Little is known about the lethal doses of OPs and carbamates for dogs and cats. The median lethal dose (LD50) in rats ranges from less than 1 mg/kg to more than 4 g/kg. There is also a large species variation in susceptibility. In general cats, fish, and birds seem to be more susceptible. The indiscriminate tastes of dogs make them likely candidates for insecticide toxicoses, however. Younger animals are usually more susceptible to toxicosis than mature animals. Because OPs and carbamates have similar mechanisms of action, multiple exposures to these types of insecticides are usually additive, especially if the longeracting OP was encountered before the shorter-acting carbamate. A general list of carbamates, listed in order from most to least toxic, includes aldicarb (Temik), carbofuran (Furadan), methomyl, propoxur, and carbaryl (Sevin). A general list of OPs, listed from most to least toxic, includes disulfoton (Di-Syston), terbufos (Counter), phorate (Thimet), parathion,
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chlorpyrifos (Dursban), fenthion (Spotton), diazinon, and malathion. Chlorpyrifos is especially toxic for cats, with an oral minimum LD of 10 to 40 mg/kg.2
TOXICOKINETICS Carbamate and OP insecticides may be absorbed through the skin, respiratory tract, or gastrointestinal tract. Most of the OP insecticides must be metabolized in the body by cytochrome P450 enzymes or flavin monooxygenase enzymes to become active.3 Organophosphate insecticides are formulated with either sulfur or an oxygen atom attached via a double bond to the phosphorus atom. Sulfur products are often denoted by a thio in the name of the insecticide (e.g., phosphorothionate). Sulfur products are inactive until cytochrome P450 enzymes in the liver or other organs (e.g., lungs and brain) replace the sulfur with oxygen. In massive exposures, this time lag for activation is not noticeable and may require only 5 to 10 minutes to activate sufficient amounts of OP to cause a toxicosis. OP products that have oxygen attached to the phosphorus are active as soon as they are absorbed into the body. Carbamates do not require liver activation and are active cholinesterase (ChE) inhibitors as absorbed. Typical metabolic routes for OPs and carbamates are the cytochrome P450 routes, hydrolysis (enzymatic and nonenzymatic), and various conjugation mechanisms.4,5 Distribution occurs throughout the body. The OPs tend to be more lipid soluble than carbamates. Therefore, OPs are more likely to cross the blood-brain barrier. However, many carbamates can and do cause dramatic central nervous system (CNS) signs in animals. Metabolites of OPs and carbamates tend to be primarily eliminated in the urine.5
MECHANISM OF TOXICITY The OP and carbamate insecticides interfere with the metabolism or breakdown of acetylcholine (ACh) at cholinergic sites. ACh is the neurotransmitter found between preganglionic and postganglionic neurons of the autonomic nervous system; at the junction of postganglionic parasympathetic neurons in smooth muscle, cardiac muscle, or exocrine glands; at neuromuscular junctions of the somatic nervous system; and at cholinergic synapses in the CNS. Acetylcholine esterase (i.e., AChE, acetylcholinestase, true ChE) is the enzyme responsible for breaking down ACh at these sites. It is also located on the surface of red blood cells. AChE has an anionic and an esteratic site in which ACh is temporarily bound until
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hydrolysis initially releases choline from the anionic site and then acetate from the “acetylated” esteratic site. AChE is inhibited by both OP and carbamate insecticides at these cholinergic sites, so that ACh keeps depolarizing the postsynaptic membrane and cannot be broken down by synaptic AChE, which is inactive because it is “phosphorylated” or “carbamylated.” Depolarization of postsynaptic membranes begins as a stimulatory process that may progress to paralysis because repolarization of the membranes is inadequate. The OP insecticides were designed to emulate the structure of ACh and occupy both the anionic and esteratic sites of AChE. The OP moiety that fills the anionic site is quickly hydrolyzed away, leaving the phosphorus atom with multiple attached groups bound to the esteratic site. In the case of OP insecticides, binding of the phosphorus atom with its attached groups to the esteratic site of AChE is considered “irreversible” binding since the half-life may be hours or days compared with the microsecond binding of ACh.3,4 The OPs with dimethoxy groups tend to have shorter half-lives than OPs with diethoxy groups. At some point, the organic carbon groups (i.e., methyl or ethyl) attached to the phosphorus may be hydrolyzed away and replaced by hydrogen. This makes the bond between the phosphorus atom and the esteratic site unbreakable. At this point, the enzyme has undergone “aging” and will never be functional again. The body will have to synthesize new AChE at the rate of approximately 1% per day.6 The OPs with dimethoxy groups tend to age faster than OPs with diethoxy groups.3 Carbamate insecticides initially occupy both the anionic and esteratic sites of AchE also. The moiety attached to the carbonyl structure is quickly hydrolyzed away, leaving AChE “carbamylated” at the esteratic site. Carbamylation is considered a “reversible” inhibition of AChE and has a half-life of approximately 30 to 40 minutes.4 No aging of AchE is possible with carbamate insecticides. Although the half-life of AChE inhibition by carbamates is shorter than that of OPs, the time is long enough for clinical signs and sometimes death to occur. Other esterase enzymes in the body act for the most part as a cushion or prevention against clinical signs. Most of these enzymes can be inhibited or can hydrolyze OPs and carbamates without producing any noticeable clinical signs. Arylesterases or type A esterases hydrolyze OPs and carbamates without being inhibited in the process. Type B esterases, consisting of AChE, pseudocholinesterase (pChE), carboxylesterase, and neuropathy target esterase (NTE), are inhibited. Pseudocholinesterase, located in serum, liver, pancreas, and nervous tissue,7 and carboxylesterase (aliesterase), located in serum, liver, muscle, and nervous tissue, can both be inhibited without causing clinical signs. Inhibition of NTE correlates with a loss of myelin and axons in the spinal cord approximately 2 to
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3 weeks after exposure to some OP compounds. The resulting syndrome is called delayed neuropathy or organophosphate-induced delayed neurotoxicity (OPIDN). This effect is monitored in a chicken test model before any insecticide can be registered. Therefore, delayed neuropathy is very rare and is reported only occasionally with exposure to industrial OP chemicals (e.g., tri-o-tolyl phosphate, found in hydraulic fluid). Death from either OPs or carbamates is associated with respiratory problems resulting from massive respiratory tract secretions, bronchiolar constriction, intercostal and diaphragm muscle paralysis, and respiratory paralysis from CNS effects in the medulla.
CLINICAL SIGNS Acute syndrome Clinical signs can be grouped under three broad categories: muscarinic, nicotinic, and CNS signs. Muscarinic signs are usually the first to appear, followed by nicotinic and then CNS effects. Usually the progression of signs is hard to differentiate, however. All of the clinical signs listed here are not necessarily seen in every case. Not all OP and carbamate toxicoses look alike, and the same toxicosis may look much different in different species, at different doses, with different routes of exposure, or at different stages in the toxicosis. Major systems in the body that are affected include the pulmonary and gastrointestinal systems and the CNS. Onset of clinical signs depends on the dose, formulation, and route of exposure. Massive oral doses often produce clinical signs within 10 minutes and can cause death easily within 30 minutes. Dermal exposures or ingestion of delayed-release products may take 12 to 24 hours or sometimes longer to produce clinical signs. MUSCARINIC SIGNS. These signs are sometimes attenuated by sympathetic stimulation via ganglia. They include the following: salivation, lacrimation, urination (micturition), defecation (i.e., SLUD), anorexia, coughing, miosis, dyspnea, bradycardia, abdominal pain and distress, and vomiting (emesis). In place of the mnemonic SLUD, the mnemonic DUMBELS is also sometimes used to describe the diarrhea, urination, miosis, bronchospasm (bradycardia), emesis, lacrimation, and salivation.8 NICOTINIC SIGNS. Nicotinic signs include muscle tremors of the head and then of the general body, generalized muscle tetany, stiffness (i.e., sawhorse stance), weakness with paresis, and paralysis caused by the inability of
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membranes to repolarize adequately before the next action potential. Tachycardia and mydriasis are possible because of stimulation of sympathetic ganglia with release of epinephrine and norepinephrine from the postganglionic sympathetic neurons; this is more likely observed with massive oral exposures. CNS SIGNS. CNS signs also occur: anxiety, restlessness, hyperactivity, depression, clonic-tonic seizures, depressed respiration, and coma.
Intermediate syndrome A different syndrome from the classic acute syndrome is sometimes seen in both dogs and cats, usually with exposure to lipophilic OP insecticides. Prolonged dermal exposure from a single dose of insecticide, repetitive exposures to low doses of OP or carbamate insecticides, or a single oral dose of a very lipophilic OP insecticide may produce an intermediate syndrome. The intermediate syndrome may follow an acute crisis within 24 to 72 hours or may be the first syndrome to be noticed after prolonged exposure. This syndrome has been associated with the production of tolerance in cholinergic receptors resulting from prolonged bombardment with ACh.3 Persistent exposure to the ACh agonist is believed to cause down-regulation of cholinergic receptors through an internalization of receptors into cells. Muscarinic receptors are especially prone to downregulation in tolerance situations.9-11 Long-term accumulation of ACh at muscarinic receptors may induce tolerance, causing an intermediate syndrome that lacks the more dramatic muscarinic signs of the acute syndrome. Nicotinic receptors at skeletal muscle sites, which cannot develop tolerance to the ACh bombardment, are primarily involved in the intermediate syndrome. Dermal exposure of cats to chlorpyrifos used around the house has historically been the most common cause of the intermediate syndrome. Fortunately, indoor and outdoor residential uses of chlorpyrifos have been cancelled or phased out by the EPA. Often, clinical signs were not apparent until 3 to 10 days after chlorpyrifos was used. Clinical signs of the intermediate syndrome in dogs or cats include anorexia, diarrhea, generalized weakness, muscle tremors, abnormal posturing, abnormal behavior, cervical ventroflexion, depression, and death.2,12,13 Severely affected animals may have clonic-tonic convulsions. Pupil size in affected animals may range from miotic to mydriatic.2 The chlorine groups on chlorpyrifos make it more lipid soluble than many other OPs. This property may change its toxicokinetics in the body, resulting in a slower stepwise but persistent lowering of AChE levels in the body.
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Other very lipophilic OPs with long half-lives (e.g., fenthion, dimethoate, chlorpyrifos, and phosmet) have been associated with a similar intermediate syndrome in humans.14,15 An intermediate syndrome in dogs was recognized after oral exposure to an insecticide used on roses, probably disulfoton.13 Long-term dietary exposure to carbaryl has been associated with a similar syndrome in swine.16
MINIMUM DATABASE Typically, a history of access to or treatment with an OP or carbamate insecticide within 24 hours before the onset of classic signs of ChE inhibition is enough to begin supportive treatment. Usually, there is not enough time to wait for a ChE assay before administering atropine to an animal showing clinical signs of acute OP or carbamate toxicosis. Acute signs require emergency action. The intermediate syndrome requires a thorough work-up of the patient because of its nondescript clinical signs. A full complete blood count, chemistry panel, and electrolyte values are advisable. A blood ChE assay may lead the clinician to a diagnosis and may also suggest a prognosis and treatment of choice.
CONFIRMATORY TESTS In a live animal, OP or carbamate insecticide toxicosis can be diagnosed by ChE activity of heparinized whole blood. This activity is a combination of true AChE activity of RBCs and pChE activity of serum. The activity of blood ChE differs among species and is reported in different units based on the type of enzyme assay used. Many laboratories use an Ellman assay on heparinized blood; normal whole blood activities for most species using this assay are 1 µmol/mL/min or higher. Blood ChE activities that are less than 50% of normal are suspicious, and activities that are less than 25% of normal are fairly diagnostic. Blood ChE activities correlate fairly well with brain AChE activities, but the correlation is not always perfect. Blood should be kept refrigerated to prevent the loss of enzyme activity. AChE activity can also be depressed when anemia is present; it is recommended to assess a packed cell volume on blood samples for AChE testing. Diagnosis in a live animal is also sometimes based on the animal’s response to treatment (see later section, Suggestive Cholinesterase Inhibitor Toxicosis). Stomach content, vomitus, hair, or suspected baits can also be submitted to the laboratory for an OP or carbamate residue screen.
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In a dead animal, brain ChE can be used to gauge exposure to OP or carbamate insecticides. Usually the brain is cut in half in sagittal sections, and one half is frozen for ChE analysis, and the other half is fixed in formalin. Normal brain ChE values again vary with species, but usually are 1 µmol/g/min or more. The same percentage depressions in ChE activity necessary for diagnosis of blood apply to the brain also. Since OP and carbamate insecticides are metabolized so rapidly in the body, neither body organs (e.g., liver and kidney) nor urine are routinely analyzed for insecticides, but such analysis can be done when stomach content is unavailable. Because chlorpyrifos is more persistent than most OPs, it is sometimes detected in body tissue, such as liver and fat. Measurements of ChE activity in blood or brain from actual cases of carbamate toxicoses are sometimes within the normal range and are not depressed. Just as carbamates bind reversibly with AChE in the body, they may also dissociate from AChE or pChE in a blood tube or brain specimen during transit. Keeping blood refrigerated and brain frozen will help to minimize this dissociation, but ChE inhibition assay results may still be falsely negative with carbamate toxicoses. Some ChE assays also have a long incubation time before enzyme activity is measured, which can also cause false-negative results for carbamate toxicoses. Carbamate screens on stomach content are necessary to absolutely rule out carbamate toxicoses associated with oral ingestions. Some OPs and carbamates do not cross the blood-brain barrier as readily as others, thereby leading to a possible false-negative result on the brain ChE test. Additional biochemical indices that may be altered include hyperamylasemia, hyperlipasemia, hyperglycemia, hypokalemia, acidosis, leukocytosis with or without a left shift, and increased creatine kinase (CK) and aspartate transaminase (AST) levels.7
TREATMENT Known OP or carbamate toxicosis Both OP and carbamate toxicoses that produce severe muscarinic signs should be treated with atropine sulfate. Atropine relieves muscarinic and some CNS-related signs but not nicotinic signs (i.e., muscle fasciculations, paralysis, and weakness). The initial dose of atropine for a known OP or carbamate toxicosis is 0.1 to 0.5 mg/kg, with one fourth of this dose given intravenously (IV) and the rest given intramuscularly (IM) or subcutaneously (SC) (some individuals administer the entire dose slowly IV). The dose is based on the severity of the muscarinic signs. Tachycardia is not a
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contraindication for initial use of atropine in acute OP or carbamate toxicoses. Repeat doses should be lower (0.1 mg/kg) and given as needed based on a combination of signs, including heart rate, pupil size, and degree of salivation. Atropine acts as a noncompetitive antagonist to protect the ACh receptors from excessive bombardment by the accumulating ACh and is a specific physiological antidote for both OP and carbamate toxicoses. In animals poisoned by OP insecticides, AChE can be reactivated with oximes if aging has not occurred. The oxime in greatest use is 2-PAM (Protopam chloride or pralidoxime chloride). Oximes are not necessary in carbamate toxicoses because the inhibition of AChE is very reversible. The dose is 20 mg/kg given IM, SC, or very slowly IV as the insert recommends. The dose may be repeated once or twice at 12-hour intervals if the previous dose appeared to be beneficial (i.e., caused relief of some nicotinic signs or loss of anorexia). Reactivation of ChE may also be monitored by analysis of heparinized blood. The oxime settles into the unoccupied anionic site of AChE, and a bond is formed with the OP moiety in the esteratic site. The combination leaves the reactivated enzyme and is excreted in the urine. Muscarinic, nicotinic, and CNS signs may all be somewhat ameliorated by 2-PAM.6 Other treatments are mostly symptomatic and should be given after the patient has been stabilized. An emetic (e.g., 3% hydrogen peroxide or apomorphine) can be administered to animals with recent oral ingestions if this is not contraindicated (i.e., by the presence of mental depression, convulsions, or loss of gag reflex) and if the animal has not vomited on its own. The emetic should be followed with activated charcoal (1 to 2 g/kg as a 10% to 20% slurry in water) with 0.25 g/kg of sodium sulfate (Glauber’s salts) or magnesium sulfate (Epsom salts) mixed with it. Activated charcoal mixed with sorbitol is also a suitable combination. Enterogastric lavage or just gastric lavage may be appropriate in some cases if a large amount of poison has been ingested and emesis has not occurred or induction of emesis is contraindicated. Anesthesia is induced with a short-acting barbiturate, and an endotracheal tube is placed before gastric lavage is attempted. When the stomach has been lavaged several times with water, activated charcoal and a saline cathartic are left in the stomach. If more than 2 to 3 hours have passed since ingestion, the risks of emesis or anesthesia may outweigh the questionable benefit of minimal recovery of poison from the stomach.17 Activated charcoal with a saline cathartic or sorbitol should be administered without emesis or lavage in these cases of delayed presentation. Animals with dermal exposures are washed with a mild dishwashing detergent (e.g., Dawn) and water. The individuals bathing the animals should wear protective gloves and aprons. The detergent should be rinsed off immediately after lathering-up the animal to prevent further
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absorption of the insecticide. The necessity for fluid therapy to correct possible dehydration, electrolyte imbalances, and acidosis should be evaluated. Convulsions are treated with diazepam or a short-acting barbiturate. Artificial respiration may be necessary in cases of respiratory paralysis. Seizures and severe hypoxia have priority in the treatment protocol. The use of antihistamines in cases of ChE inhibition is controversial. Although antihistamines, such as diphenhydramine, have some effect against muscarinic signs,18 antihistamines are not as specific a physiological antidote as atropine, a muscarinic antagonist. Diphenhydramine purportedly relieves nicotinic signs in some cases of ChE inhibition.19 This alleged antinicotinic action is not commonly recognized in pharmacology texts18 and, in addition, antihistamines are listed as contraindicated for the treatment of ChE inhibition.8,20 Because of the potential adverse CNS effects of antihistamines, diphenhydramine should probably not be administered when ChE inhibition is present.21 Other drugs that are contraindicated include phenothiazine tranquilizers, opiates, local anesthetics, aminoglycoside antibiotics, clindamycin, lincomycin, theophylline, and neuromuscular blockers.7
Unknown cholinesterase inhibitor toxicosis Animals frequently are admitted with classic signs of acute ChE inhibition, but the type of insecticide exposure (OP versus carbamate) is unknown. These animals should be treated with atropine as described previously for known OP or carbamate toxicoses. The decision then becomes whether or not to give 2-PAM. The use of 2-PAM has been beneficial in some carbamate toxicoses, but is definitely contraindicated in carbaryl toxicosis.22,23 Since carbaryl is fairly safe (the rat oral LD50 is approximately 0.5 g/kg), the chance that an unknown ChE inhibitor toxicosis is associated with carbaryl is remote. Therefore, in addition to known OP toxicoses, 2-PAM is recommended for: (1) these unknown ChE inhibitor toxicoses, (2) ChE inhibitor toxicoses associated with dual exposure to both OP and carbamates, and (3) severe carbamate toxicoses that do not respond favorably to atropine alone.8
Suggestive cholinesterase inhibitor toxicosis In animals with suggestive but not classic clinical signs of acute ChE inhibition and no history of insecticide exposure, a test dose of atropine can
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be given IV. The test dose should be low, corresponding to a preanesthetic dose of 0.02 mg/kg. If eyes dilate maximally, heart rate increases dramatically, or salivation stops within 10 to 15 minutes, the original problem was not an OP or carbamate toxicosis. If none of the suggestive clinical signs improve significantly, the disease is very likely an OP or carbamate toxicosis. A dose of 0.1 to 0.2 mg/kg of atropine can then be administered as described previously. Administration of 20 mg/kg of 2-PAM given IM, SC, or very slowly IV is also advisable. If the high dose of atropine (0.1 to 0.5 mg/kg) was administered first and the problem was not an insecticide toxicosis, the animal would very likely experience an atropine toxicosis. Excessive atropine use causes hyperthermia, redness of the skin, decreased salivation, belligerency, intestinal stasis, and possible convulsions (i.e., “hot as a pistol, red as a beet, dry as a bone, and mad as a hatter”). Atropine toxicosis is treated symptomatically with cool water baths, fluids, diazepam, and possibly physostigmine hydrochloride (which should be reserved for heroic circumstances only).
Intermediate syndrome Because the intermediate syndrome lacks the typical muscarinic signs of the acute syndrome, atropine is not indicated. Anorexia and neuromuscular weakness are the most dramatic problems seen in these animals. Supportive care measures depend on the severity and duration of the anorexia. Parenteral nutrition or pharyngostomy tubes may be necessary. Electrolyte imbalances (e.g., hypokalemia) and dehydration if present must be addressed. The animal should be bathed with a mild detergent and water if the exposure was dermal. A ventilator may be required for hypoxemia.13 Typically, AChE values in these animals are severely depressed. Contrary to the insert information provided with 2-PAM, many of these animals respond to 2-PAM many days after the initial OP exposure. Unlike atropine, 2-PAM is capable of working at nicotinic skeletal muscle sites (e.g., diaphragm and cervical muscles) that are primarily affected by the intermediate syndrome. Although 2-PAM has been a miracle worker in some of these animals, anecdotal reports of death in some cats following 2-PAM administration have also surfaced. Administration of 20 mg/kg of 2-PAM IM or SC is indicated in animals that need heroic intervention. The same dose of 2-PAM should be repeated once 12 hours later. These animals probably have blood AChE values of less than 10% of normal. Animals with blood AChE values of more than 25% of normal that are eating reasonably well on their own will probably survive with supportive care only, and their AChE values will regenerate during a few weeks of convalescence. Administration of 2-PAM in some of these less affected
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animals could speed recovery and cut owner expenses, but again it carries a risk of an adverse reaction.
PROGNOSIS Prognosis depends very much on the dose and time of exposure to the insecticide. Because atropine is a specific physiological antidote, it can bring about dramatic and rapid changes in an animal that seems close to death. On the other hand, many of the acute toxicoses progress so rapidly that the animal is dead before it arrives at the clinic. Anecdotal reports of malicious cases of carbamate toxicosis document death within a few minutes of ingestion of a bait. Animals with the intermediate syndrome often respond slowly to supportive care. Owners may not have the financial resources or the patience to go through weeks of supportive care. In the absence of aspiration pneumonia or other rare occurrences of brain anoxia, animals that come through the acute syndrome should experience no chronic problems.
GROSS AND HISTOLOGICAL LESIONS Animals should be examined closely for any signs of excessive parasympathomimetic stimulation, such as excessive salivation on the muzzle or in the trachea and diarrhea in the perineal area. Hair and stomach contents should be examined for unusual odors that smell like petroleum products, sulfur, or garlic.6 Internally, often there are no visible gross lesions. Potential gross lesions include foreign substances in the stomach, diarrhea, an empty bladder, or pulmonary edema. Nonspecific petechial hemorrhages associated with an agonal death may be found on the endocardial and epicardial surfaces. Petechial hemorrhages are also sometimes seen on the subserosal surface of the gastrointestinal tract. Histologically, only pulmonary edema is likely to be seen. However, even pulmonary edema is not a consistent finding. Pancreatitis is also a rare but potential histological finding in dogs but not in cats; it is associated with pChE inhibition of the pancreas.24
DIFFERENTIAL DIAGNOSES Tremorgenic mycotoxicoses Roquefortine and penitrem A are tremorgenic mycotoxins associated with decaying organic matter, moldy walnut hulls, and spoiled dairy products.
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These toxicoses are associated with muscle tremors, salivation, vomiting, and clonic-tonic convulsions. Differential features are the lack of miosis and the general absence of respiratory signs.
Amitraz toxicosis The client should be questioned about the pet’s access to a flea collar and whether or not the collar was one that also was formulated for ticks. Typically, dogs with amitraz toxicosis have muscle weakness, muscle tremors, vomiting, and possible CNS signs. The best differential feature is mydriasis versus the more typical miosis present with OP and carbamate toxicoses.
Pyrethrin and/or pyrethroid toxicosis Pyrethrin and pyrethroid toxicoses often are marked by some salivation, muscle tremors, muscle weakness, depression, and possibly seizures. Usually, pyrethroid toxicoses are associated with known dermal application of a product. Asking the client to bring the preparation to the clinic so that the label can be read will help with the differential.
Cationic surfactant toxicosis Cationic surfactants are components found in some disinfectants, potpourri oils, and dermal pet sprays. Generic names include benzalkonium chloride, benzethonium chloride, and cetylpyridinium. These compounds can cause neuromuscular and ganglionic blockade. Typical clinical signs include vomiting, salivation, muscle tremors, depression, and clonic-tonic convulsions. Cationic surfactants are more likely to cause oral and gastric irritation and ulcers than are OP and carbamate insecticides.
Pancreatitis Pancreatitis lacks the life-threatening respiratory signs characteristic of an acute OP or carbamate toxicosis. Amylase and lipase may be elevated in OP or carbamate toxicoses, but usually not to the same extent as in acute pancreatitis.
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Garbage (endotoxin) intoxication Endotoxins may affect multiple systems in the body and mimic many diseases. Typically, endotoxins would not cause the severe miosis, salivation, or muscle tremors associated with ChE inhibitions. Endotoxins may cause some CNS depression, but would not cause seizures. REFERENCES 1. Smith RA, Tramontin RR, Poonacha KB et al: Carbofuran (Furadan) poisoning in animals. Canine Pract 20(2):8, 1995. 2. Fikes JD: Feline chlorpyrifos toxicosis. In Kirk RW, Bonagura JD, editors: Current veterinary therapy XI: small animal practice, Philadelphia, 1992, WB Saunders. 3. Sultatos LG: Mammalian toxicology of organophosphorus pesticides, J Toxicol Environ Health 43:271, 1994. 4. Fukuto TR: Mechanism of action of organophosphorus and carbamate insecticides, Environ Health Perspect 87:245, 1990. 5. Dorough HW: Metabolism of insecticidal methylcarbamates in animals, J Agric Food Chem 18(6):1015, 1970. 6. Aaron CK, Howland MA: Insecticides: Organophosphates and carbamates. In Goldfrank LR, Flomenbaum NE, Lewin NA et al, editors: Goldfrank’s toxicologic emergencies, ed 5, Norwalk, Conn, 1994, Appleton & Lange. 7. Fikes JD: Organophosphate and carbamate insecticides, Clin North Am Small Anim Pract 20(2):353, 1990. 8. Ellenhorn MJ, Schonwald S, Ordog G et al, editors: Ellenhorn’s medical toxicology: diagnosis and treatment of human poisoning, ed 2, Baltimore, 1997, Williams & Wilkins. 9. Schwab BW, Costa LG, Murphy SD: Muscarinic receptor alterations as a mechanism of anticholinesterase tolerance, Toxic Appl Pharmacol 71:14, 1983. 10. Costa LG, Schwab BW, Murphy SD: Differential alterations of cholinergic muscarinic receptors during chronic and acute tolerance to organophosphorus insecticides, Biochem Pharmacol 31(21):3407, 1982. 11. Costa LG, Schwab BW, Murphy SD: Tolerance to anticholinesterase compounds in mammals, Toxicology 25:79, 1982. 12. Levy JK: Chronic chlorpyrifos toxicosis in a cat, J Am Vet Med Assoc 203(12):1682, 1993. 13. Hopper K, Aldrich J, Haskins SC: The recognition and treatment of the intermediate syndrome of organophosphate poisoning in a dog, J Vet Emerg Crit Care 12(2):99, 2002. 14. De Bleecker J, Lison D, Van Den Abeele K et al: Acute and subacute organophosphate poisoning in the rat, Neurotoxicology 15(2):341, 1994. 15. Guadarrama-Naveda M, Calderon de Cabrera L: Intermediate syndrome secondary to ingestion of chlorpiriphos, Vet Human Toxicol 43(1):34, 2001. 16. Smalley HE, O’Hara PJ, Bridges CH et al: The effects of chronic carbaryl administration on the neuromuscular system of swine, Toxic Appl Pharmacol 14:409, 1969. 17. Hansen SR: Management of organophosphate and carbamate toxicoses. In Bonagura JD, Kirk RW, editors: Current veterinary therapy XII: small animal practice, Philadelphia, 1995, WB Saunders. 18. Babe KS, Serafin WE: Histamine, bradykinin, and their antagonists. In Hardman JG, Limbird LE, Molinoff PB et al, editors: Goodman and Gilman’s the pharmacological basis of therapeutics, ed 9, New York, 1996, McGraw-Hill. 19. Clemmons RM, Meyer DJ, Sundlof SF et al: Correction of organophosphate-induced neuromuscular blockade by diphenhydramine, Am J Vet Res 45(10):2167, 1984. 20. Fernandez G, Gomez MID, Castro JA: Cholinesterase inhibition by phenothiazine and nonphenothiazine antihistaminics: analysis of its postulated role in synergizing organophosphate toxicity, Toxic Appl Pharmacol 31:179, 1975.
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21. Blodgett D, Neer TM, Coates JR: How do I treat? Organophosphate toxicity, Prog Vet Neurol 7(2):56, 1996. 22. Natoff IL, Reiff B: Effect of oximes on the acute toxicity of anticholinesterase carbamates. Toxic Appl Pharmacol 25:569, 1973. 23. Lieske CN, Clark JH, Maxwell DM et al: Studies of the amplification of carbaryl toxicity by various oximes, Toxicol Lett 62:127, 1992. 24. Frick TW, Dalo S, O’Leary JF et al: Effects of the insecticide, diazinon, on pancreas of dog, cat and guinea pig, J Environ Pathol Toxicol Oncol 7(4):1, 1987.
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Dennis J. Blodgett, DVM, PhD
• Organophosphate and carbamate insecticides are common pesticides used in agriculture, around the home, and on and around domestic animals. • Both groups of insecticides inhibit AChE activity. • Clinical signs of acute poisoning are generally divided into three major categories. Muscarinic signs include SLUD, miosis, bronchospasm, bradycardia, and emesis. Nicotinic signs include muscle tremors, ataxia, weakness, and paralysis. CNS signs can range from severe depression to hyperactivity and seizures. • Exposures in dogs and cats to lipophilic organophosphates may lead to an “intermediate syndrome,” characterized mostly by a delayed onset of anorexia, depression, generalized weakness, muscle tremors, ventroflexion of the neck, and abnormal behavior. • Blood or brain AChE activity should be assessed and is an excellent screening tool to determine exposure in clinically affected patients or postmortem. • Treatment options include decontamination, use of atropine sulfate, 2-PAM, diazepam, and respiratory support.
SOURCES Organophosphate (OP) insecticides are marketed for control of insects on plants, animals, soils, and around the house. A few OP products are sold as anthelmintics. Many OPs are designed to remain on the surface of objects to which they are applied; others are meant to be absorbed by plants or animals and to become systemic. Labels on the products have the term phosphate, phos, phoro, or phosphor, somewhere in the long chemical name of the ingredients. Most OPs are very insoluble in water and are 941
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often formulated with oily vehicles or organic solvents. Others are sold as dusts, wettable powders, emulsions, or adsorbed to clay particles. The OPs used to combat fleas are sold as sprays, dips, shampoos, flea collars, or flea bombs. As newer classes of insecticides are marketed for flea control, OPs are involved in fewer cases of poisoning associated with intentional applications. Agricultural uses of OPs are varied, but many are used for control of corn rootworms. Many of these corn rootworm insecticides are very toxic. As such, agricultural OPs have begun to replace strychnine as the malicious pesticide of choice for killing dogs in some areas of the United States.1 Baits are often mixed with food items (e.g., meat, bread, carcasses, and tuna fish) in cases of malicious poisoning. Carbamate insecticides were designed for purposes similar to those described for OPs. Carbamates are similar in structure to physostigmine and neostigmine. The use of carbamates in flea products is also declining with the advent of safer classes of insecticides. Carbamates are still used abundantly for household and agricultural applications. Not all pesticides having the word carbamate in their chemical name are acetylcholinesterase (AChE) inhibitors. Care should be taken to obtain the entire chemical name of a product when working up a possible pesticide exposure. Carbamates, as in the case of OPs, have also gained in popularity as malicious pesticides of choice. Onset times are often shorter than OPs, and the poisoned animal may die within yards of the bait.
TOXIC DOSE The toxicity or lethal dose (LD) of the insecticides is as varied as the types of insects they were formulated to control. Little is known about the lethal doses of OPs and carbamates for dogs and cats. The median lethal dose (LD50) in rats ranges from less than 1 mg/kg to more than 4 g/kg. There is also a large species variation in susceptibility. In general cats, fish, and birds seem to be more susceptible. The indiscriminate tastes of dogs make them likely candidates for insecticide toxicoses, however. Younger animals are usually more susceptible to toxicosis than mature animals. Because OPs and carbamates have similar mechanisms of action, multiple exposures to these types of insecticides are usually additive, especially if the longeracting OP was encountered before the shorter-acting carbamate. A general list of carbamates, listed in order from most to least toxic, includes aldicarb (Temik), carbofuran (Furadan), methomyl, propoxur, and carbaryl (Sevin). A general list of OPs, listed from most to least toxic, includes disulfoton (Di-Syston), terbufos (Counter), phorate (Thimet), parathion,
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chlorpyrifos (Dursban), fenthion (Spotton), diazinon, and malathion. Chlorpyrifos is especially toxic for cats, with an oral minimum LD of 10 to 40 mg/kg.2
TOXICOKINETICS Carbamate and OP insecticides may be absorbed through the skin, respiratory tract, or gastrointestinal tract. Most of the OP insecticides must be metabolized in the body by cytochrome P450 enzymes or flavin monooxygenase enzymes to become active.3 Organophosphate insecticides are formulated with either sulfur or an oxygen atom attached via a double bond to the phosphorus atom. Sulfur products are often denoted by a thio in the name of the insecticide (e.g., phosphorothionate). Sulfur products are inactive until cytochrome P450 enzymes in the liver or other organs (e.g., lungs and brain) replace the sulfur with oxygen. In massive exposures, this time lag for activation is not noticeable and may require only 5 to 10 minutes to activate sufficient amounts of OP to cause a toxicosis. OP products that have oxygen attached to the phosphorus are active as soon as they are absorbed into the body. Carbamates do not require liver activation and are active cholinesterase (ChE) inhibitors as absorbed. Typical metabolic routes for OPs and carbamates are the cytochrome P450 routes, hydrolysis (enzymatic and nonenzymatic), and various conjugation mechanisms.4,5 Distribution occurs throughout the body. The OPs tend to be more lipid soluble than carbamates. Therefore, OPs are more likely to cross the blood-brain barrier. However, many carbamates can and do cause dramatic central nervous system (CNS) signs in animals. Metabolites of OPs and carbamates tend to be primarily eliminated in the urine.5
MECHANISM OF TOXICITY The OP and carbamate insecticides interfere with the metabolism or breakdown of acetylcholine (ACh) at cholinergic sites. ACh is the neurotransmitter found between preganglionic and postganglionic neurons of the autonomic nervous system; at the junction of postganglionic parasympathetic neurons in smooth muscle, cardiac muscle, or exocrine glands; at neuromuscular junctions of the somatic nervous system; and at cholinergic synapses in the CNS. Acetylcholine esterase (i.e., AChE, acetylcholinestase, true ChE) is the enzyme responsible for breaking down ACh at these sites. It is also located on the surface of red blood cells. AChE has an anionic and an esteratic site in which ACh is temporarily bound until
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hydrolysis initially releases choline from the anionic site and then acetate from the “acetylated” esteratic site. AChE is inhibited by both OP and carbamate insecticides at these cholinergic sites, so that ACh keeps depolarizing the postsynaptic membrane and cannot be broken down by synaptic AChE, which is inactive because it is “phosphorylated” or “carbamylated.” Depolarization of postsynaptic membranes begins as a stimulatory process that may progress to paralysis because repolarization of the membranes is inadequate. The OP insecticides were designed to emulate the structure of ACh and occupy both the anionic and esteratic sites of AChE. The OP moiety that fills the anionic site is quickly hydrolyzed away, leaving the phosphorus atom with multiple attached groups bound to the esteratic site. In the case of OP insecticides, binding of the phosphorus atom with its attached groups to the esteratic site of AChE is considered “irreversible” binding since the half-life may be hours or days compared with the microsecond binding of ACh.3,4 The OPs with dimethoxy groups tend to have shorter half-lives than OPs with diethoxy groups. At some point, the organic carbon groups (i.e., methyl or ethyl) attached to the phosphorus may be hydrolyzed away and replaced by hydrogen. This makes the bond between the phosphorus atom and the esteratic site unbreakable. At this point, the enzyme has undergone “aging” and will never be functional again. The body will have to synthesize new AChE at the rate of approximately 1% per day.6 The OPs with dimethoxy groups tend to age faster than OPs with diethoxy groups.3 Carbamate insecticides initially occupy both the anionic and esteratic sites of AchE also. The moiety attached to the carbonyl structure is quickly hydrolyzed away, leaving AChE “carbamylated” at the esteratic site. Carbamylation is considered a “reversible” inhibition of AChE and has a half-life of approximately 30 to 40 minutes.4 No aging of AchE is possible with carbamate insecticides. Although the half-life of AChE inhibition by carbamates is shorter than that of OPs, the time is long enough for clinical signs and sometimes death to occur. Other esterase enzymes in the body act for the most part as a cushion or prevention against clinical signs. Most of these enzymes can be inhibited or can hydrolyze OPs and carbamates without producing any noticeable clinical signs. Arylesterases or type A esterases hydrolyze OPs and carbamates without being inhibited in the process. Type B esterases, consisting of AChE, pseudocholinesterase (pChE), carboxylesterase, and neuropathy target esterase (NTE), are inhibited. Pseudocholinesterase, located in serum, liver, pancreas, and nervous tissue,7 and carboxylesterase (aliesterase), located in serum, liver, muscle, and nervous tissue, can both be inhibited without causing clinical signs. Inhibition of NTE correlates with a loss of myelin and axons in the spinal cord approximately 2 to
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3 weeks after exposure to some OP compounds. The resulting syndrome is called delayed neuropathy or organophosphate-induced delayed neurotoxicity (OPIDN). This effect is monitored in a chicken test model before any insecticide can be registered. Therefore, delayed neuropathy is very rare and is reported only occasionally with exposure to industrial OP chemicals (e.g., tri-o-tolyl phosphate, found in hydraulic fluid). Death from either OPs or carbamates is associated with respiratory problems resulting from massive respiratory tract secretions, bronchiolar constriction, intercostal and diaphragm muscle paralysis, and respiratory paralysis from CNS effects in the medulla.
CLINICAL SIGNS Acute syndrome Clinical signs can be grouped under three broad categories: muscarinic, nicotinic, and CNS signs. Muscarinic signs are usually the first to appear, followed by nicotinic and then CNS effects. Usually the progression of signs is hard to differentiate, however. All of the clinical signs listed here are not necessarily seen in every case. Not all OP and carbamate toxicoses look alike, and the same toxicosis may look much different in different species, at different doses, with different routes of exposure, or at different stages in the toxicosis. Major systems in the body that are affected include the pulmonary and gastrointestinal systems and the CNS. Onset of clinical signs depends on the dose, formulation, and route of exposure. Massive oral doses often produce clinical signs within 10 minutes and can cause death easily within 30 minutes. Dermal exposures or ingestion of delayed-release products may take 12 to 24 hours or sometimes longer to produce clinical signs. MUSCARINIC SIGNS. These signs are sometimes attenuated by sympathetic stimulation via ganglia. They include the following: salivation, lacrimation, urination (micturition), defecation (i.e., SLUD), anorexia, coughing, miosis, dyspnea, bradycardia, abdominal pain and distress, and vomiting (emesis). In place of the mnemonic SLUD, the mnemonic DUMBELS is also sometimes used to describe the diarrhea, urination, miosis, bronchospasm (bradycardia), emesis, lacrimation, and salivation.8 NICOTINIC SIGNS. Nicotinic signs include muscle tremors of the head and then of the general body, generalized muscle tetany, stiffness (i.e., sawhorse stance), weakness with paresis, and paralysis caused by the inability of
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membranes to repolarize adequately before the next action potential. Tachycardia and mydriasis are possible because of stimulation of sympathetic ganglia with release of epinephrine and norepinephrine from the postganglionic sympathetic neurons; this is more likely observed with massive oral exposures. CNS SIGNS. CNS signs also occur: anxiety, restlessness, hyperactivity, depression, clonic-tonic seizures, depressed respiration, and coma.
Intermediate syndrome A different syndrome from the classic acute syndrome is sometimes seen in both dogs and cats, usually with exposure to lipophilic OP insecticides. Prolonged dermal exposure from a single dose of insecticide, repetitive exposures to low doses of OP or carbamate insecticides, or a single oral dose of a very lipophilic OP insecticide may produce an intermediate syndrome. The intermediate syndrome may follow an acute crisis within 24 to 72 hours or may be the first syndrome to be noticed after prolonged exposure. This syndrome has been associated with the production of tolerance in cholinergic receptors resulting from prolonged bombardment with ACh.3 Persistent exposure to the ACh agonist is believed to cause down-regulation of cholinergic receptors through an internalization of receptors into cells. Muscarinic receptors are especially prone to downregulation in tolerance situations.9-11 Long-term accumulation of ACh at muscarinic receptors may induce tolerance, causing an intermediate syndrome that lacks the more dramatic muscarinic signs of the acute syndrome. Nicotinic receptors at skeletal muscle sites, which cannot develop tolerance to the ACh bombardment, are primarily involved in the intermediate syndrome. Dermal exposure of cats to chlorpyrifos used around the house has historically been the most common cause of the intermediate syndrome. Fortunately, indoor and outdoor residential uses of chlorpyrifos have been cancelled or phased out by the EPA. Often, clinical signs were not apparent until 3 to 10 days after chlorpyrifos was used. Clinical signs of the intermediate syndrome in dogs or cats include anorexia, diarrhea, generalized weakness, muscle tremors, abnormal posturing, abnormal behavior, cervical ventroflexion, depression, and death.2,12,13 Severely affected animals may have clonic-tonic convulsions. Pupil size in affected animals may range from miotic to mydriatic.2 The chlorine groups on chlorpyrifos make it more lipid soluble than many other OPs. This property may change its toxicokinetics in the body, resulting in a slower stepwise but persistent lowering of AChE levels in the body.
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Other very lipophilic OPs with long half-lives (e.g., fenthion, dimethoate, chlorpyrifos, and phosmet) have been associated with a similar intermediate syndrome in humans.14,15 An intermediate syndrome in dogs was recognized after oral exposure to an insecticide used on roses, probably disulfoton.13 Long-term dietary exposure to carbaryl has been associated with a similar syndrome in swine.16
MINIMUM DATABASE Typically, a history of access to or treatment with an OP or carbamate insecticide within 24 hours before the onset of classic signs of ChE inhibition is enough to begin supportive treatment. Usually, there is not enough time to wait for a ChE assay before administering atropine to an animal showing clinical signs of acute OP or carbamate toxicosis. Acute signs require emergency action. The intermediate syndrome requires a thorough work-up of the patient because of its nondescript clinical signs. A full complete blood count, chemistry panel, and electrolyte values are advisable. A blood ChE assay may lead the clinician to a diagnosis and may also suggest a prognosis and treatment of choice.
CONFIRMATORY TESTS In a live animal, OP or carbamate insecticide toxicosis can be diagnosed by ChE activity of heparinized whole blood. This activity is a combination of true AChE activity of RBCs and pChE activity of serum. The activity of blood ChE differs among species and is reported in different units based on the type of enzyme assay used. Many laboratories use an Ellman assay on heparinized blood; normal whole blood activities for most species using this assay are 1 µmol/mL/min or higher. Blood ChE activities that are less than 50% of normal are suspicious, and activities that are less than 25% of normal are fairly diagnostic. Blood ChE activities correlate fairly well with brain AChE activities, but the correlation is not always perfect. Blood should be kept refrigerated to prevent the loss of enzyme activity. AChE activity can also be depressed when anemia is present; it is recommended to assess a packed cell volume on blood samples for AChE testing. Diagnosis in a live animal is also sometimes based on the animal’s response to treatment (see later section, Suggestive Cholinesterase Inhibitor Toxicosis). Stomach content, vomitus, hair, or suspected baits can also be submitted to the laboratory for an OP or carbamate residue screen.
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In a dead animal, brain ChE can be used to gauge exposure to OP or carbamate insecticides. Usually the brain is cut in half in sagittal sections, and one half is frozen for ChE analysis, and the other half is fixed in formalin. Normal brain ChE values again vary with species, but usually are 1 µmol/g/min or more. The same percentage depressions in ChE activity necessary for diagnosis of blood apply to the brain also. Since OP and carbamate insecticides are metabolized so rapidly in the body, neither body organs (e.g., liver and kidney) nor urine are routinely analyzed for insecticides, but such analysis can be done when stomach content is unavailable. Because chlorpyrifos is more persistent than most OPs, it is sometimes detected in body tissue, such as liver and fat. Measurements of ChE activity in blood or brain from actual cases of carbamate toxicoses are sometimes within the normal range and are not depressed. Just as carbamates bind reversibly with AChE in the body, they may also dissociate from AChE or pChE in a blood tube or brain specimen during transit. Keeping blood refrigerated and brain frozen will help to minimize this dissociation, but ChE inhibition assay results may still be falsely negative with carbamate toxicoses. Some ChE assays also have a long incubation time before enzyme activity is measured, which can also cause false-negative results for carbamate toxicoses. Carbamate screens on stomach content are necessary to absolutely rule out carbamate toxicoses associated with oral ingestions. Some OPs and carbamates do not cross the blood-brain barrier as readily as others, thereby leading to a possible false-negative result on the brain ChE test. Additional biochemical indices that may be altered include hyperamylasemia, hyperlipasemia, hyperglycemia, hypokalemia, acidosis, leukocytosis with or without a left shift, and increased creatine kinase (CK) and aspartate transaminase (AST) levels.7
TREATMENT Known OP or carbamate toxicosis Both OP and carbamate toxicoses that produce severe muscarinic signs should be treated with atropine sulfate. Atropine relieves muscarinic and some CNS-related signs but not nicotinic signs (i.e., muscle fasciculations, paralysis, and weakness). The initial dose of atropine for a known OP or carbamate toxicosis is 0.1 to 0.5 mg/kg, with one fourth of this dose given intravenously (IV) and the rest given intramuscularly (IM) or subcutaneously (SC) (some individuals administer the entire dose slowly IV). The dose is based on the severity of the muscarinic signs. Tachycardia is not a
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contraindication for initial use of atropine in acute OP or carbamate toxicoses. Repeat doses should be lower (0.1 mg/kg) and given as needed based on a combination of signs, including heart rate, pupil size, and degree of salivation. Atropine acts as a noncompetitive antagonist to protect the ACh receptors from excessive bombardment by the accumulating ACh and is a specific physiological antidote for both OP and carbamate toxicoses. In animals poisoned by OP insecticides, AChE can be reactivated with oximes if aging has not occurred. The oxime in greatest use is 2-PAM (Protopam chloride or pralidoxime chloride). Oximes are not necessary in carbamate toxicoses because the inhibition of AChE is very reversible. The dose is 20 mg/kg given IM, SC, or very slowly IV as the insert recommends. The dose may be repeated once or twice at 12-hour intervals if the previous dose appeared to be beneficial (i.e., caused relief of some nicotinic signs or loss of anorexia). Reactivation of ChE may also be monitored by analysis of heparinized blood. The oxime settles into the unoccupied anionic site of AChE, and a bond is formed with the OP moiety in the esteratic site. The combination leaves the reactivated enzyme and is excreted in the urine. Muscarinic, nicotinic, and CNS signs may all be somewhat ameliorated by 2-PAM.6 Other treatments are mostly symptomatic and should be given after the patient has been stabilized. An emetic (e.g., 3% hydrogen peroxide or apomorphine) can be administered to animals with recent oral ingestions if this is not contraindicated (i.e., by the presence of mental depression, convulsions, or loss of gag reflex) and if the animal has not vomited on its own. The emetic should be followed with activated charcoal (1 to 2 g/kg as a 10% to 20% slurry in water) with 0.25 g/kg of sodium sulfate (Glauber’s salts) or magnesium sulfate (Epsom salts) mixed with it. Activated charcoal mixed with sorbitol is also a suitable combination. Enterogastric lavage or just gastric lavage may be appropriate in some cases if a large amount of poison has been ingested and emesis has not occurred or induction of emesis is contraindicated. Anesthesia is induced with a short-acting barbiturate, and an endotracheal tube is placed before gastric lavage is attempted. When the stomach has been lavaged several times with water, activated charcoal and a saline cathartic are left in the stomach. If more than 2 to 3 hours have passed since ingestion, the risks of emesis or anesthesia may outweigh the questionable benefit of minimal recovery of poison from the stomach.17 Activated charcoal with a saline cathartic or sorbitol should be administered without emesis or lavage in these cases of delayed presentation. Animals with dermal exposures are washed with a mild dishwashing detergent (e.g., Dawn) and water. The individuals bathing the animals should wear protective gloves and aprons. The detergent should be rinsed off immediately after lathering-up the animal to prevent further
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absorption of the insecticide. The necessity for fluid therapy to correct possible dehydration, electrolyte imbalances, and acidosis should be evaluated. Convulsions are treated with diazepam or a short-acting barbiturate. Artificial respiration may be necessary in cases of respiratory paralysis. Seizures and severe hypoxia have priority in the treatment protocol. The use of antihistamines in cases of ChE inhibition is controversial. Although antihistamines, such as diphenhydramine, have some effect against muscarinic signs,18 antihistamines are not as specific a physiological antidote as atropine, a muscarinic antagonist. Diphenhydramine purportedly relieves nicotinic signs in some cases of ChE inhibition.19 This alleged antinicotinic action is not commonly recognized in pharmacology texts18 and, in addition, antihistamines are listed as contraindicated for the treatment of ChE inhibition.8,20 Because of the potential adverse CNS effects of antihistamines, diphenhydramine should probably not be administered when ChE inhibition is present.21 Other drugs that are contraindicated include phenothiazine tranquilizers, opiates, local anesthetics, aminoglycoside antibiotics, clindamycin, lincomycin, theophylline, and neuromuscular blockers.7
Unknown cholinesterase inhibitor toxicosis Animals frequently are admitted with classic signs of acute ChE inhibition, but the type of insecticide exposure (OP versus carbamate) is unknown. These animals should be treated with atropine as described previously for known OP or carbamate toxicoses. The decision then becomes whether or not to give 2-PAM. The use of 2-PAM has been beneficial in some carbamate toxicoses, but is definitely contraindicated in carbaryl toxicosis.22,23 Since carbaryl is fairly safe (the rat oral LD50 is approximately 0.5 g/kg), the chance that an unknown ChE inhibitor toxicosis is associated with carbaryl is remote. Therefore, in addition to known OP toxicoses, 2-PAM is recommended for: (1) these unknown ChE inhibitor toxicoses, (2) ChE inhibitor toxicoses associated with dual exposure to both OP and carbamates, and (3) severe carbamate toxicoses that do not respond favorably to atropine alone.8
Suggestive cholinesterase inhibitor toxicosis In animals with suggestive but not classic clinical signs of acute ChE inhibition and no history of insecticide exposure, a test dose of atropine can
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be given IV. The test dose should be low, corresponding to a preanesthetic dose of 0.02 mg/kg. If eyes dilate maximally, heart rate increases dramatically, or salivation stops within 10 to 15 minutes, the original problem was not an OP or carbamate toxicosis. If none of the suggestive clinical signs improve significantly, the disease is very likely an OP or carbamate toxicosis. A dose of 0.1 to 0.2 mg/kg of atropine can then be administered as described previously. Administration of 20 mg/kg of 2-PAM given IM, SC, or very slowly IV is also advisable. If the high dose of atropine (0.1 to 0.5 mg/kg) was administered first and the problem was not an insecticide toxicosis, the animal would very likely experience an atropine toxicosis. Excessive atropine use causes hyperthermia, redness of the skin, decreased salivation, belligerency, intestinal stasis, and possible convulsions (i.e., “hot as a pistol, red as a beet, dry as a bone, and mad as a hatter”). Atropine toxicosis is treated symptomatically with cool water baths, fluids, diazepam, and possibly physostigmine hydrochloride (which should be reserved for heroic circumstances only).
Intermediate syndrome Because the intermediate syndrome lacks the typical muscarinic signs of the acute syndrome, atropine is not indicated. Anorexia and neuromuscular weakness are the most dramatic problems seen in these animals. Supportive care measures depend on the severity and duration of the anorexia. Parenteral nutrition or pharyngostomy tubes may be necessary. Electrolyte imbalances (e.g., hypokalemia) and dehydration if present must be addressed. The animal should be bathed with a mild detergent and water if the exposure was dermal. A ventilator may be required for hypoxemia.13 Typically, AChE values in these animals are severely depressed. Contrary to the insert information provided with 2-PAM, many of these animals respond to 2-PAM many days after the initial OP exposure. Unlike atropine, 2-PAM is capable of working at nicotinic skeletal muscle sites (e.g., diaphragm and cervical muscles) that are primarily affected by the intermediate syndrome. Although 2-PAM has been a miracle worker in some of these animals, anecdotal reports of death in some cats following 2-PAM administration have also surfaced. Administration of 20 mg/kg of 2-PAM IM or SC is indicated in animals that need heroic intervention. The same dose of 2-PAM should be repeated once 12 hours later. These animals probably have blood AChE values of less than 10% of normal. Animals with blood AChE values of more than 25% of normal that are eating reasonably well on their own will probably survive with supportive care only, and their AChE values will regenerate during a few weeks of convalescence. Administration of 2-PAM in some of these less affected
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animals could speed recovery and cut owner expenses, but again it carries a risk of an adverse reaction.
PROGNOSIS Prognosis depends very much on the dose and time of exposure to the insecticide. Because atropine is a specific physiological antidote, it can bring about dramatic and rapid changes in an animal that seems close to death. On the other hand, many of the acute toxicoses progress so rapidly that the animal is dead before it arrives at the clinic. Anecdotal reports of malicious cases of carbamate toxicosis document death within a few minutes of ingestion of a bait. Animals with the intermediate syndrome often respond slowly to supportive care. Owners may not have the financial resources or the patience to go through weeks of supportive care. In the absence of aspiration pneumonia or other rare occurrences of brain anoxia, animals that come through the acute syndrome should experience no chronic problems.
GROSS AND HISTOLOGICAL LESIONS Animals should be examined closely for any signs of excessive parasympathomimetic stimulation, such as excessive salivation on the muzzle or in the trachea and diarrhea in the perineal area. Hair and stomach contents should be examined for unusual odors that smell like petroleum products, sulfur, or garlic.6 Internally, often there are no visible gross lesions. Potential gross lesions include foreign substances in the stomach, diarrhea, an empty bladder, or pulmonary edema. Nonspecific petechial hemorrhages associated with an agonal death may be found on the endocardial and epicardial surfaces. Petechial hemorrhages are also sometimes seen on the subserosal surface of the gastrointestinal tract. Histologically, only pulmonary edema is likely to be seen. However, even pulmonary edema is not a consistent finding. Pancreatitis is also a rare but potential histological finding in dogs but not in cats; it is associated with pChE inhibition of the pancreas.24
DIFFERENTIAL DIAGNOSES Tremorgenic mycotoxicoses Roquefortine and penitrem A are tremorgenic mycotoxins associated with decaying organic matter, moldy walnut hulls, and spoiled dairy products.
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These toxicoses are associated with muscle tremors, salivation, vomiting, and clonic-tonic convulsions. Differential features are the lack of miosis and the general absence of respiratory signs.
Amitraz toxicosis The client should be questioned about the pet’s access to a flea collar and whether or not the collar was one that also was formulated for ticks. Typically, dogs with amitraz toxicosis have muscle weakness, muscle tremors, vomiting, and possible CNS signs. The best differential feature is mydriasis versus the more typical miosis present with OP and carbamate toxicoses.
Pyrethrin and/or pyrethroid toxicosis Pyrethrin and pyrethroid toxicoses often are marked by some salivation, muscle tremors, muscle weakness, depression, and possibly seizures. Usually, pyrethroid toxicoses are associated with known dermal application of a product. Asking the client to bring the preparation to the clinic so that the label can be read will help with the differential.
Cationic surfactant toxicosis Cationic surfactants are components found in some disinfectants, potpourri oils, and dermal pet sprays. Generic names include benzalkonium chloride, benzethonium chloride, and cetylpyridinium. These compounds can cause neuromuscular and ganglionic blockade. Typical clinical signs include vomiting, salivation, muscle tremors, depression, and clonic-tonic convulsions. Cationic surfactants are more likely to cause oral and gastric irritation and ulcers than are OP and carbamate insecticides.
Pancreatitis Pancreatitis lacks the life-threatening respiratory signs characteristic of an acute OP or carbamate toxicosis. Amylase and lipase may be elevated in OP or carbamate toxicoses, but usually not to the same extent as in acute pancreatitis.
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Garbage (endotoxin) intoxication Endotoxins may affect multiple systems in the body and mimic many diseases. Typically, endotoxins would not cause the severe miosis, salivation, or muscle tremors associated with ChE inhibitions. Endotoxins may cause some CNS depression, but would not cause seizures. REFERENCES 1. Smith RA, Tramontin RR, Poonacha KB et al: Carbofuran (Furadan) poisoning in animals. Canine Pract 20(2):8, 1995. 2. Fikes JD: Feline chlorpyrifos toxicosis. In Kirk RW, Bonagura JD, editors: Current veterinary therapy XI: small animal practice, Philadelphia, 1992, WB Saunders. 3. Sultatos LG: Mammalian toxicology of organophosphorus pesticides, J Toxicol Environ Health 43:271, 1994. 4. Fukuto TR: Mechanism of action of organophosphorus and carbamate insecticides, Environ Health Perspect 87:245, 1990. 5. Dorough HW: Metabolism of insecticidal methylcarbamates in animals, J Agric Food Chem 18(6):1015, 1970. 6. Aaron CK, Howland MA: Insecticides: Organophosphates and carbamates. In Goldfrank LR, Flomenbaum NE, Lewin NA et al, editors: Goldfrank’s toxicologic emergencies, ed 5, Norwalk, Conn, 1994, Appleton & Lange. 7. Fikes JD: Organophosphate and carbamate insecticides, Clin North Am Small Anim Pract 20(2):353, 1990. 8. Ellenhorn MJ, Schonwald S, Ordog G et al, editors: Ellenhorn’s medical toxicology: diagnosis and treatment of human poisoning, ed 2, Baltimore, 1997, Williams & Wilkins. 9. Schwab BW, Costa LG, Murphy SD: Muscarinic receptor alterations as a mechanism of anticholinesterase tolerance, Toxic Appl Pharmacol 71:14, 1983. 10. Costa LG, Schwab BW, Murphy SD: Differential alterations of cholinergic muscarinic receptors during chronic and acute tolerance to organophosphorus insecticides, Biochem Pharmacol 31(21):3407, 1982. 11. Costa LG, Schwab BW, Murphy SD: Tolerance to anticholinesterase compounds in mammals, Toxicology 25:79, 1982. 12. Levy JK: Chronic chlorpyrifos toxicosis in a cat, J Am Vet Med Assoc 203(12):1682, 1993. 13. Hopper K, Aldrich J, Haskins SC: The recognition and treatment of the intermediate syndrome of organophosphate poisoning in a dog, J Vet Emerg Crit Care 12(2):99, 2002. 14. De Bleecker J, Lison D, Van Den Abeele K et al: Acute and subacute organophosphate poisoning in the rat, Neurotoxicology 15(2):341, 1994. 15. Guadarrama-Naveda M, Calderon de Cabrera L: Intermediate syndrome secondary to ingestion of chlorpiriphos, Vet Human Toxicol 43(1):34, 2001. 16. Smalley HE, O’Hara PJ, Bridges CH et al: The effects of chronic carbaryl administration on the neuromuscular system of swine, Toxic Appl Pharmacol 14:409, 1969. 17. Hansen SR: Management of organophosphate and carbamate toxicoses. In Bonagura JD, Kirk RW, editors: Current veterinary therapy XII: small animal practice, Philadelphia, 1995, WB Saunders. 18. Babe KS, Serafin WE: Histamine, bradykinin, and their antagonists. In Hardman JG, Limbird LE, Molinoff PB et al, editors: Goodman and Gilman’s the pharmacological basis of therapeutics, ed 9, New York, 1996, McGraw-Hill. 19. Clemmons RM, Meyer DJ, Sundlof SF et al: Correction of organophosphate-induced neuromuscular blockade by diphenhydramine, Am J Vet Res 45(10):2167, 1984. 20. Fernandez G, Gomez MID, Castro JA: Cholinesterase inhibition by phenothiazine and nonphenothiazine antihistaminics: analysis of its postulated role in synergizing organophosphate toxicity, Toxic Appl Pharmacol 31:179, 1975.
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21. Blodgett D, Neer TM, Coates JR: How do I treat? Organophosphate toxicity, Prog Vet Neurol 7(2):56, 1996. 22. Natoff IL, Reiff B: Effect of oximes on the acute toxicity of anticholinesterase carbamates. Toxic Appl Pharmacol 25:569, 1973. 23. Lieske CN, Clark JH, Maxwell DM et al: Studies of the amplification of carbaryl toxicity by various oximes, Toxicol Lett 62:127, 1992. 24. Frick TW, Dalo S, O’Leary JF et al: Effects of the insecticide, diazinon, on pancreas of dog, cat and guinea pig, J Environ Pathol Toxicol Oncol 7(4):1, 1987.