Insecticides and Pesticides: Organochlorines, Organophosphates and Carbamates

NTA tests pesticide classification as a flat list of five families with named representatives. Memorise the families, the prototype molecules, and one distinguishing feature per family.

Organochlorines (OC). Chlorinated hydrocarbons: DDT, BHC / lindane (gamma-HCH), endosulfan, dicofol, aldrin, dieldrin. Lipid-soluble, environmentally persistent, and bioaccumulative. DDT was banned for agricultural use in India in 1989 and finally withdrawn from public-health malaria use in 2020. Endosulfan was banned in 2011 by the Supreme Court after the Kasaragod tragedy.

Organophosphates (OP). Phosphate or thiophosphate esters: parathion (and methyl-parathion), malathion, monocrotophos, dimethoate, chlorpyrifos, dichlorvos (DDVP), phorate, quinalphos. OPs are the single largest cause of pesticide-related deaths in India.

Carbamates. Carbamic-acid esters: carbaryl (Sevin), carbofuran (Furadan), propoxur (Baygon), aldicarb. Pharmacologically they behave like short-acting, reversible OPs.

Pyrethroids. Synthetic analogues of natural pyrethrins: deltamethrin, cypermethrin, permethrin, fenvalerate. Mammalian toxicity is low because hepatic esterases hydrolyse them rapidly.

Phosphide rodenticides. Aluminium phosphide (Celphos, Quickphos, Sulfas) and zinc phosphide (Ratol) sold as grain-fumigant tablets. Both release phosphine gas on contact with moisture or gastric acid. AlP is the leading suicidal poison in north Indian farming belts.

The Insecticides Act 1968 and the Central Insecticides Board and Registration Committee (CIBRC) regulate registration, sale and labelling, including the colour-coded toxicity triangle (red, yellow, blue, green) that the SOCO team should record at the scene.

The mechanism is the single most tested sub-bullet in this topic. There is one mechanism per family.

Organochlorines: sodium channel disruption. OCs cross the blood-brain barrier easily and bind voltage-gated sodium channels, keeping them open longer than normal and producing repetitive firing, tremor, ataxia and convulsions. Lindane and dieldrin additionally antagonise GABA-A receptors. Chronic toxicity comes from adipose bioaccumulation; OCs partition into breast milk, which is why DDT residues still appear in Indian biomonitoring surveys.

Organophosphates: irreversible AChE inhibition. OPs phosphorylate the serine hydroxyl at the active site of acetylcholinesterase, forming a stable covalent bond. The enzyme can no longer hydrolyse acetylcholine, which accumulates at every cholinergic synapse (muscarinic, nicotinic, central). The phosphorylated enzyme then undergoes "aging" by losing one alkyl group, after which oxime reactivators cannot remove the phosphoryl group. Aging is why oxime therapy must start early.

Carbamates: reversible AChE inhibition. Carbamates carbamylate the same serine hydroxyl, but the carbamyl-enzyme bond hydrolyses spontaneously over minutes to hours, regenerating active enzyme. The clinical picture mirrors OP poisoning but is shorter and milder. Pralidoxime is not routinely indicated and can worsen carbaryl poisoning, so the antidote is atropine alone.

Pyrethroids: sodium channel disruption (weaker than OC). Prolonged sodium-channel open time produces paresthesia, hypersalivation, tremor and rarely seizures. Type I (no alpha-cyano, T-syndrome) versus Type II (alpha-cyano, CS-syndrome). Mammalian recovery is fast because of rapid ester hydrolysis.

Aluminium phosphide: cytochrome c oxidase block. AlP + H2O (or HCl in stomach) gives phosphine (PH3) gas. Phosphine inhibits cytochrome c oxidase (complex IV), shutting down oxidative phosphorylation. The clinical signature is refractory cardiogenic shock with severe metabolic acidosis. No effective antidote; management is supportive (magnesium sulphate, sodium bicarbonate, vasopressors).

The clinical syndrome of OP and carbamate poisoning splits into three compartments NTA tests as a cluster.

Muscarinic effects are remembered by DUMBELS (Diarrhoea, Urination, Miosis, Bradycardia / bronchorrhoea / bronchospasm, Emesis, Lacrimation, Salivation) or SLUDGE (Salivation, Lacrimation, Urination, Defecation, GI upset, Emesis).

Nicotinic effects at the neuromuscular junction give fasciculations, weakness and paralysis; at autonomic ganglia they give tachycardia, hypertension and mydriasis (which can mask the muscarinic miosis).

Central effects include anxiety, ataxia, seizures, coma and respiratory depression. Death in untreated OP poisoning is usually from bronchorrhoea, bronchospasm and central respiratory failure combined.

Atropine is a competitive antagonist at muscarinic receptors. It does nothing for the nicotinic or central effects, but dries secretions and reverses bradycardia. Dosing is titrated to atropinisation (dry secretions, heart rate above 80, clear chest), starting 2-5 mg IV doubled every 5 minutes.

Pralidoxime (PAM, 2-PAM) is the oxime reactivator. The oxime attacks the phosphorus atom of the phosphorylated enzyme and pulls the phosphoryl group off, regenerating active AChE. PAM must be given before aging. Indian protocols use 1-2 g IV over 15-30 minutes, repeated 6-8 hourly. Obidoxime is the European alternative.

Carbamate poisoning is treated with atropine alone; PAM is unnecessary and can worsen carbaryl toxicity.

Aluminium phosphide poisoning has no antidote. Management is gastric lavage with potassium permanganate, magnesium sulphate, sodium bicarbonate, vasopressors and supportive care. Coconut-oil lavage is on some Indian protocols but evidence is weak. Mortality remains 40-80% in most Indian series.

Cholinesterase activity is the cornerstone biochemical marker of OP and carbamate exposure. NTA tests three facts: the two enzymes, the Ellman principle, and the wavelength.

Plasma butyrylcholinesterase (BChE, pseudo-cholinesterase). Hepatic origin, half-life about 12 days. Falls within hours of OP exposure, recovers in 1-2 weeks. Sensitive but not specific (also depressed in liver disease, malnutrition and certain genetic variants).

RBC acetylcholinesterase (AChE, true cholinesterase). Membrane bound on the erythrocyte, identical to the synaptic enzyme. Recovers only with new RBC turnover (about 60 days), so it is the better marker for long-term inhibition.

Ellman method. Acetylthiocholine (or butyrylthiocholine for BChE) is hydrolysed by cholinesterase to thiocholine plus acetate. The thiocholine reacts with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB, Ellman's reagent) to yield the yellow 5-thio-2-nitrobenzoate (TNB) anion, read at 412 nm. The absorbance change per minute is proportional to enzyme activity. A fall below 50% of reference activity indicates significant exposure, below 20% severe poisoning.

Specimen matrices. Whole blood (EDTA or heparin) for both enzymes. Gastric lavage and stomach contents for the parent insecticide. Viscera (stomach with contents, small intestine, liver, kidney, spleen, blood, urine) collected at autopsy under BNSS Section 176 for parent compound and metabolites; the collection protocol is covered in the book chapter on post-mortem and viscera collection in poisoning.

The toxicology lab workflow for a suspect sample (viscera, stomach contents, vomitus, food, water) follows three steps: extraction, clean-up, detection.

Extraction. Acid digestion with concentrated HCl is the Stas-Otto-style classical method for OC and OP compounds, followed by liquid-liquid partition into n-hexane (OC) or dichloromethane (OP). Carbamates are heat-labile and degrade in acid, so they are extracted at neutral pH into dichloromethane or ethyl acetate. The modern alternative is QuEChERS: acetonitrile shake, MgSO4 + NaCl salt-out, dispersive SPE clean-up with PSA and C18 sorbents. QuEChERS is now standard at most Indian SFSL toxicology divisions because one tube covers every pesticide family.

Detection. Detector choice tracks the heteroatom. Commit this pairing to memory.

NTA likes Indian context questions because they tie a chemistry fact to a public-event memory hook. Three case anchors and one statute show up most.

Kasaragod endosulfan tragedy (Kerala, 1976-2010). Plantation Corporation of Kerala aerially sprayed endosulfan on cashew plantations in Kasaragod district for two decades. Cluster reports of congenital malformations, neurological disability and cancers led to a 2011 Supreme Court order banning endosulfan across India. The textbook Indian example of organochlorine bioaccumulation harm.

Saran mid-day-meal poisoning (Bihar, July 2013). Twenty-three schoolchildren in Gandaman village died after eating a meal cooked in oil contaminated with monocrotophos. The oil had been stored in a used pesticide container. FSSAI tightened school-feeding storage and labelling rules in response. NET takeaway: monocrotophos + accidental ingestion + mass-casualty OP poisoning.

Aluminium phosphide in north Indian farming belts. Celphos, Quickphos and Sulfas tablets, sold as grain fumigants, are by far the leading suicidal poison across Punjab, Haryana, Rajasthan, Uttar Pradesh and parts of Madhya Pradesh. Mortality remains 40-80% even at tertiary centres. The Aluminium Phosphide Tablets (Regulation) Order 2002 and later CIBRC reviews tightened packaging, but enforcement is uneven.

Insecticides Act 1968 and CIBRC. Registration, manufacture, import, sale, transport, storage and use are governed by the Insecticides Act 1968 and the Insecticides Rules 1971. The Central Insecticides Board advises the central government; the Registration Committee registers each molecule. Labels must carry the colour-coded toxicity triangle (red / yellow / blue / green), and an SOCO photograph should capture it for the toxicology report.

In court, defence counsel attacks pesticide casework on three predictable lines: chain-of-custody breaks, specimen instability (AChE degrades on storage; carbamates partially hydrolyse in transit), and screen specificity