Insecticides play a most relevant role in the control of insect pests, particularly in developing countries. All of the chemical insecticides in use today are neurotoxicants, and act by poisoning the nervous systems of the target organisms (Table 22–3). The central nervous system of insects is highly developed and not unlike that of mammals. As a class, insecticides have high acute toxicity toward nontarget species compared with other pesticides. Some of them, most notably the organophosphates, are involved in a great number of human poisonings and deaths each year.
The general structure of OP insecticides can be represented by:
where X is the so-called leaving group that is displaced when the OP phosphorylates acetylcholinesterase (AChE), and is the most sensitive to hydrolysis; R1 and R2 are most commonly alkoxy groups (i.e., OCH3 or OC2H5), though other chemical substitutes are also possible; either an oxygen or a sulfur (in this case the compound should be defined as a phosphorothioate) is also attached to the phosphorus with a double bond. Based on chemical differences, OPs can be divided into several subclasses, which include phosphates, phosphorothioates, phosphoramidates, phosphonates, and others. Figure 22–1 shows the chemical structures of some commonly used OPs.
Structures of some organophosphorus insecticides and of the nerve agent sarin. Note that most commonly used compounds are organophosphorothioates (i.e., have a P=S bond), but some, including sarin, have a P=O bond and do not require metabolic activation.
For all compounds that contain a sulfur bound to the phosphorus, a metabolic bioactivation is necessary for their biological activity to be manifest, as only compounds with a P=O moiety are effective inhibitors of AChE. Oxidative desulfuration (leads to the formation of an “oxon,” or oxygen analog of the parent insecticide) and thioether oxidation (formation of a sulfoxide, S=O, followed by the formation of a sulfone, O=S=O) are catalyzed by cytochrome P450s. Catalytic hydrolysis by phosphotriesterases, known as A-exterases (which are not inhibited by OPs), plays an important role in the detoxication of certain OPs. Noncatalytic hydrolysis of OPs also occurs when these compounds phosphorylate serine esterases classified as B-esterases.
Signs and Symptoms of Toxicity and Mechanism of Action
OP insecticides have high acute toxicity, with oral LD50 values in rat often below 50 mg/kg. For several OPs, acute dermal toxicity is also high. Inhibition of AChE by OPs causes accumulation of acetylcholine at cholinergic synapses, with overstimulation of muscarinic and nicotinic cholinergic receptors. As these receptors are localized in most organs of the body, a “cholinergic syndrome” ensues, which includes increased sweating and salivation, profound bronchial secretion, bronchoconstriction, miosis, increased gastrointestinal motility, diarrhea, tremors, muscular twitching, and various central nervous system effects (Table 22–4). Whereas respiratory failure is a hallmark of severe OP poisoning, mild poisoning and/or early stages of an otherwise severe poisoning may display no clear-cut signs and symptoms.
Table 22–4 Signs and symptoms of acute poisoning with anticholinesterase compounds. ||Download (.pdf)
Table 22–4 Signs and symptoms of acute poisoning with anticholinesterase compounds.
Site and Receptor Affected
Exocrine glands (M)
Increased salivation, lacrimation, perspiration
Miosis, blurred vision
Gastrointestinal tract (M)
Abdominal cramps, vomiting, diarrhea
Respiratory tract (M)
Increased bronchial secretion, bronchoconstriction
Urinary frequency, incontinence
Cardiovascular system (M)
Cardiovascular system (N)
Tachycardia, transient hypertension
Skeletal muscles (N)
Muscle fasciculations, twitching, cramps, generalized weakness, flaccid paralysis
Central nervous system (M, N)
Dizziness, lethargy, fatigue, headache, mental confusion, depression of respiratory centers, convulsions, coma
OPs with a P=O moiety phosphorylate a hydroxyl group on serine in the active (esteratic) site of the enzyme, impeding its action on the physiological substrate. Phosphorylated AChE is hydrolyzed by water slowly, and the rate of “spontaneous reactivation” depends on the chemical nature of the R substituents. Reactivation of phosphorylated AChE does not occur once the enzyme-inhibitor complex has “aged,” which occurs when there is loss (by nonenzymatic hydrolysis) of one of the two alkyl (R) groups. When phosphorylated AChE has aged, the enzyme is considered to be irreversibly inhibited, and synthesis of new enzyme is required to restore activity, a process that may take days.
Procedures aimed at decontamination and/or at minimizing absorption depend on the route of exposure. In case of dermal exposure, contaminated clothing should be removed, and the skin washed with alkaline soap. In case of ingestion, procedures to reduce absorption from the gastrointestinal tract do not appear to be very effective. Atropine, a muscarinic receptor antagonist, prevents the action of accumulating acetylcholine on these receptors. Atropine is preferably given intravenously to prevent the signs of excess cholinergic stimulation. The administration of pralidoxime (2-PAM) early after OP exposure can help prevent AChE aging, but its effectiveness is controversial. Diazepam may be used to relieve anxiety in mild cases, and to reduce muscle fasciculations and control convulsions in the more severe cases.
The Intermediate Syndrome
A second distinct manifestation of exposure to OPs is the so-called intermediate syndrome, which is seen in 20 to 50 percent of acute OP poisoning cases. The syndrome develops 1 to several days after the poisoning, during recovery from cholinergic manifestations, or in some cases, when patients have completely recovered from the initial cholinergic crisis. Prominent features include a marked weakness of respiratory, neck, and proximal limb muscles. Mortality due to respiratory paralysis and complications ranges from 15 to 40 percent, and recovery in surviving patients takes up to 15 days. The intermediate syndrome is not an effect of AChE inhibition, and its precise mechanisms are unknown.
Organophosphate-Induced Delayed Polyneuropathy (OPIDP)
A few OPs may cause OPIDP. Signs and symptoms include tingling of the hands and feet, followed by sensory loss, progressive muscle weakness and flaccidity of the distal skeletal muscles of the lower and upper extremities, and ataxia. These may occur 2 to 3 weeks after a single exposure, when signs of both the acute cholinergic and the intermediate syndromes have subsided. OPIDP can be classified as a distal sensorimotor axonopathy.
OPIDP is not related to AChE inhibition. Indeed, one of the compounds involved in several epidemics of this neuropathy is tri-ortho-cresyl phosphate (TOCP), a very poor AChE inhibitor. The target for OPIDP is an esterase, present in nerve tissues as well as other tissues (e.g., lymphocytes), named neuropathy target esterase (NTE). Several OPs, certain carbamates, and sulfonyl fluorides can inhibit NTE. Other compounds that inhibit NTE but cannot undergo the aging reaction are not neuropathic, indicating that inhibition of NTE catalytic activity is not the mechanism of axonal degeneration.
There is still controversy on possible long-term effects of OPs. The possibility exists that low exposure to OPs, at doses that produce no cholinergic signs, may lead to long-term adverse health effects, particularly in the central and peripheral nervous systems. Chronic exposure of animals to OPs, at doses that significantly inhibit AChE but may not be associated with clinical signs, results in the development of tolerance to their cholinergic effects (which is mediated, at least in part, by down-regulation of cholinergic receptors), and has been associated with neurobehavioral abnormalities, particularly at the cognitive level.
Carbamate insecticides are derived from carbamic acid, and most are N-methylcarbamates. Acute oral toxicity ranges from moderate to low toxicity, such as carbaryl, to extremely high toxicity, such as aldicarb. Dermal skin penetration by carbamates is increased by organic solvents and emulsifiers present in most formulations. Carbamates are susceptible to a variety of enzyme-catalyzed biotransformation reactions, and the principal pathways involve oxidation and hydrolysis. The mechanism of toxicity of carbamates is by inhibition of AChE, which is rapidly reversible.
The signs and symptoms of carbamate poisoning include miosis, urination, diarrhea, salivation, muscle fasciculation, and CNS effects (Table 22–4). Acute intoxication by carbamates is generally resolved within a few hours. The treatment of carbamate intoxication relies on the use of atropine. Carbamates can inhibit NTE, but because carbamylated NTE cannot age, they are thought to be unable to initiate OPIDP. Additionally, when given before a neuropathic organophosphate, carbamates offer protection against OPIDP, but when given after, they can promote OPIDP.
Methylcarbamates are not mutagenic, and there is no evidence of carcinogenicity. Embryotoxicity or fetotoxicity is observed only at maternally toxic doses. Limited evidence suggests that carbamates (e.g., aldicarb) may be more acutely toxic to young animals than to adults, possibly because of lower detoxication.
Pyrethrins were first developed as insecticides from extracts of the flower heads of Chrysanthemum cinerariaefolium, whose insecticidal potential was appreciated in ancient China and Persia. Because pyrethrins decompose rapidly on exposure to light, the synthetic pyrethroid analogs were developed. Because of their high insecticidal potency, relatively low mammalian toxicity, lack of environmental persistence, and low tendency to induce insect resistance, pyrethroids now account for more than 25 percent of the global insecticide market. The pyrethroids are used widely as insecticides both in the house and in agriculture, in medicine for the topical treatment of scabies and head lice, and in tropical countries in soaked bed nets to prevent mosquito bites. Pyrethroids alter the normal function of insect nerves by modifying the kinetics of voltage-sensitive sodium channels, which mediate the transient increase in the sodium permeability of the nerve membrane that underlies the nerve action potential.
On absorption, pyrethroids are very rapidly metabolized through two major biotransformation routes: hydrolysis of the ester linkage, which is catalyzed by hepatic and plasma carboxylesterases, and oxidation of the alcohol moiety by cytochrome P450s. These initial reactions are followed by further oxidations, hydrolysis, and conjugation with sulfate or glucuronide.
Signs and Symptoms of Toxicity and Mechanism of Action
Based on toxic signs in rats, pyrethroids have been divided into two types (Table 22–5). The pyrethroids disrupt voltage-gated sodium channels in mammals and insects. Pyrethroids bind to the α subunit of the sodium channel and slow the activation (opening), as well as the rate of inactivation (closing), of the sodium channel, leading to a stable hyperexcitable state. The higher sensitivity of insects to pyrethroid toxicity, compared with mammals, is believed to result from a combination of higher sensitivity of insect sodium channels, lower body temperature (as pyrethroids show a negative temperature coefficient of action), and slower biotransformation. Type II pyrethroids bind to and inhibit GABAA-gated chloride channels at higher concentrations than those sufficient to affect sodium channels (10–7 M versus 10–10 M). This effect is believed to contribute to the seizures that accompany severe type II pyrethroid poisoning.
Table 22–5 Classification of pyrethroid insecticides based on toxic signs in rats. ||Download (.pdf)
Table 22–5 Classification of pyrethroid insecticides based on toxic signs in rats.
Signs and Symptoms
Type I (T syndrome)
Increased sensitivity to external stimuli
Type II (CS syndrome)
Pawing and burrowing
Young animals are more sensitive to the acute toxicity of the pyrethroids deltamethrin and cypermethrin probably because of a lesser capacity for metabolic detoxification.
On occupational exposure, the primary adverse effect resulting from dermal contact with pyrethroids is paresthesia. Symptoms include continuous tingling or pricking or, when more severe, burning. The condition reverses in about 24 h, and topical application of vitamin E has been shown to be an effective treatment. Paresthesia is presumably due to pyrethroid-induced abnormal repetitive activity in skin nerve terminals. Chronic studies with pyrethroids indicate that at high dose levels they cause slight liver enlargement often accompanied by some histopathologic changes. There is little evidence of teratogenicity and mutagenicity. An increased rate of lymphoma incidence in rodents has been reported for deltamethrin, but the effect was not dose-dependent.
The organochlorine insecticides include the chlorinated ethane derivatives, such as DDT and its analogs; the cyclodienes, such as chlordane, aldrin, dieldrin, heptachlor, endrin, and toxaphene; the hexachlorocyclohexanes, such as lindane; and the caged structures mirex and chlordecone. Their acute toxicity is moderate (less than that of organophosphates), but chronic exposure may be associated with adverse health effects particularly in the liver and endocrine disruption of the reproductive system.
DDT is effective against a wide variety of agricultural pests, as well as against insects that transmit some of the world's most serious diseases, such as typhus, malaria, and yellow fever. DDT has a moderate oral acute toxicity and its dermal absorption is very limited. In humans, oral doses of 10 to 20 mg/kg produce illness, but doses as high as 285 mg/kg have been ingested accidentally without fatal results. Toxicity from dermal exposure in humans is also low, as evidenced by the lack of significant adverse health effects when thousands of people were liberally dusted with this compound. On absorption, DDT distributes in all tissues, and the highest concentrations are found in adipose tissue.
Acute exposure to high doses of DDT causes motor unrest, increased frequency of spontaneous movements, abnormal susceptibility to fear, and hypersusceptibility to external stimuli (light, touch, and sound). This is followed by the development of fine tremors, progressing to coarse tremors, and eventually tonic–clonic convulsions. In humans, the earliest symptom of poisoning by DDT is hyperesthesia of the mouth and lower part of the face, followed by paresthesia of the same area and of the tongue. Dizziness, tremor of the extremities, confusion, and vomiting follow; convulsions occur only in severe poisoning. Both in insects and in mammals, DDT interferes with the sodium channels in the axonal membrane by a mechanism similar to that of type I pyrethroids.
An important target for chronic DDT exposure is the liver. DDT and its breakdown product DDE increase liver weight and cause hepatic cell hypertrophy and necrosis, and they are potent inducers of cytochrome P450s, particularly CYP2B and CYP3A. Both DDE and DDD, another breakdown product, are carcinogenic in rodents, causing primarily an increase in hepatic tumors.
Hexachlorocyclohexanes and Cyclodienes
These two families of organochlorine insecticides comprise a large number of compounds that share a similar mechanism of neurotoxic action. Lindane is the γ isomer of benzene hexachloride (BHC; 1,2,3,4,5,6-hexachlorocyclohexane). Cyclodiene compounds include chlordane, dieldrin, aldrin (which is rapidly metabolized to dieldrin), heptachlor, and endrin. Toxaphene is a complex mixture of over 200 chlorinated bornanes and camphenes.
Lindane and cyclodienes have moderate to high acute oral toxicity (Figure 22–2). However, in contrast to DDT, these compounds are readily absorbed through the skin. The primary target for their toxicity is the central nervous system. Unlike DDT, tremor is essentially absent, but convulsions are a prominent aspect of poisoning. Lindane and cyclodienes bind to the picrotoxin-binding site on the chloride channel, thereby blocking its opening and antagonizing the inhibitory action of GABA.
Structure and acute toxicity (oral LD50in rat) of selected organochlorine insecticides of different chemical classes.
Other Old and New Insecticides
The roots of Derris elliptica and those of Lonchocarpus utilis and Lonchocarpus urucu in South America contain at least six rotenoid esters. The most abundant is rotenone, which is used as an agricultural insecticide/acaricide particularly in organic farming. Toxicity of rotenone in target and nontarget species is due to its ability to inhibit, at nanomolar concentrations, the mitochondrial respiratory chain, by blocking electron transport at NADH–ubiquinone reductase, the energy-conserving enzyme complex commonly known as complex I. Poisoning symptoms include initial increased respiratory and cardiac rates, clonic and tonic spasms, and muscular depression, followed by respiratory depression. Rotenone may play a role in the etiology of Parkinson's disease.
Nicotine is an alkaloid extracted from the leaves of tobacco plants (Nicotiana tabacum and Nicotiana rustica), and is used as a free base or as the sulfate salt. Nicotine is a minor insecticide, and the signs and symptoms of poisoning include nausea, vomiting, muscle weakness, respiratory effects, headache, lethargy, and tachycardia. Most cases of poisoning with nicotine occur after exposure to tobacco products, or gum or patches. Workers who cultivate, harvest, or handle tobacco may experience green tobacco sickness, caused by dermal absorption of nicotine.
The avermectins are macrocyclic lactones that are isolated from the fermentation broth of Streptomyces avermitilis. This fungus synthesizes eight individual avermectins that have antiparasitic activity. The semisynthetic derivatives of avermectin B1a, emamectin benzoate, and ivermectin are used as insecticides, and for parasite control in human and veterinary medicine, respectively. Abamectin is used primarily to control mites, whereas emamectin benzoate is effective at controlling lepidopterian species in various crops and emerald ash borer in trees. Ivermectin is used as an antihelmintic and antiparasitic drug in veterinary medicine, and in humans it has proven to be an effective treatment for infection of intestinal threadworms, onchocerciasis (river blindness), and lymphatic filariasis. Signs and symptoms of intoxication include hyperexcitability, tremors, and incoordination, followed by ataxia and coma-like sedation.