Pesticide Poisoning: OP, Carbamate, Pyrethroid, Aluminium Phosphide

Organophosphates remain the single biggest poisoning workload in Indian medical college emergency rooms. The chemistry is straightforward: an OP molecule phosphorylates the serine residue at the active site of acetylcholinesterase, blocking hydrolysis of acetylcholine. Acetylcholine accumulates at muscarinic synapses (DUMBBELS), at nicotinic synapses (fasciculations, weakness, tachycardia that can mask the muscarinic bradycardia) and in the central nervous system (seizures, coma, central apnoea). The phosphorylated enzyme then undergoes a chemical process called aging, in which one alkyl group is lost. After aging, the bond is permanent and no oxime will reactivate the enzyme; the patient depends on resynthesis of new enzyme, which takes weeks. Aging is fast in the chemical warfare agents (soman ages in minutes) and slow in most agricultural OPs (24 to 48 hours for chlorpyrifos, slower for fenitrothion), which sets the practical window for pralidoxime.

The active ingredients seen in Indian casework are a small set. Chlorpyrifos (Class II, the workhorse of Indian agriculture) is sold under dozens of brand names and produces a slow, prolonged clinical course with high pralidoxime requirements. Monocrotophos (Class I, restricted) is fast acting and high mortality and was banned for vegetable crops in 2005, a restriction that predated the Bihar Chhapra mid-day meal disaster of July 2013 in which 23 schoolchildren died from contaminated cooking oil. Dichlorvos (DDVP, sold as Nuvan) is volatile and produces both ingestional and inhalational poisoning. Methyl parathion (Folidol-M) is among the most lethal agents per millilitre swallowed. Malathion (Class III) is the only OP routinely used for urban vector control because of its lower mammalian toxicity. Fenitrothion (Sumithion) and parathion round out the list of recurring autopsy positives.

The clinical picture is the cholinergic toxidrome covered in detail in the antidote topic. A wet, bradycardic, pinpoint-pupil patient with fasciculations of the eyelids and tongue is an OP case until proven otherwise. Two later complications matter for medicolegal documentation and for the inpatient course. Intermediate syndrome appears 24 to 96 hours after acute exposure with proximal muscle weakness, neck flexor weakness and respiratory failure; it is independent of the original cholinergic crisis and needs ventilation. OPIDN, delayed polyneuropathy, appears two to three weeks after exposure to specific OPs (notably TOCP, leptophos and methamidophos) as a distal sensorimotor neuropathy mediated by neuropathy target esterase inhibition. OPIDN is rare with the agricultural OPs currently in use but is documented in the literature and can present years after a known high-level exposure.

Diagnosis is largely clinical but the RBC acetylcholinesterase assay (Ellman DTNB method) gives an objective baseline and tracks recovery. Plasma butyrylcholinesterase is more sensitive but less specific. CFSL Chandigarh and the state forensic science laboratories use gas chromatography with flame photometric detection (GC-FPD) or GC-MS to confirm the parent compound in viscera or gastric aspirate.

Treatment follows the antidote backbone covered in detail elsewhere on the site. Atropine 2 to 4 mg IV bolus, doubled every five minutes until secretions dry, heart rate is above 80 and systolic BP above 80, then infusion at 10 to 20 percent of the loading dose per hour. Pralidoxime 30 mg/kg IV bolus, then 8 to 10 mg/kg per hour, started within 36 hours and continued for 48 to 72 hours. Benzodiazepines for seizures, aggressive pulmonary toilet, ICU admission for any moderate or severe case.

Carbamates carbamylate the same serine residue at the active site of acetylcholinesterase but the resulting bond is spontaneously hydrolysed within hours, so the inhibition is reversible and no aging occurs. The clinical picture is identical to organophosphate poisoning, occasionally a touch milder and almost always shorter. The agents seen in Indian practice are carbofuran (Furadan), propoxur (Baygon, the brown household cockroach gel), aldicarb (Temik, restricted) and carbaryl (Sevin). Propoxur self-poisoning shows up across urban India because it is sold over the counter for household pest control; carbofuran appears in agricultural districts and is implicated in deliberate wildlife poisoning cases that come through the forensic system.

Treatment differs from OP poisoning in one important respect: pralidoxime is not routinely indicated and in some animal models and small clinical series with carbaryl can worsen toxicity by stabilising the carbamylated enzyme. The Indian standard of care is atropine titrated to the same end point as for OPs, supportive care, ICU admission for moderate cases, and pralidoxime only when the agent is uncertain and a coexisting OP cannot be ruled out. The course is shorter; most patients are extubated within 24 to 48 hours and discharged within a week. Intermediate syndrome and OPIDN are not features of pure carbamate poisoning.

Pyrethroids are synthetic analogues of pyrethrin, the natural insecticide from chrysanthemum flowers. They modulate voltage-gated sodium channels in arthropod and (to a much lesser extent) mammalian neurons, prolonging the open state and producing repetitive depolarisation. Pyrethroids divide into Type I (no alpha-cyano group, exemplified by permethrin and allethrin) and Type II (with the alpha-cyano group, exemplified by cypermethrin, deltamethrin and fenvalerate). Type I poisoning gives the T syndrome (Tremor, hyperexcitability, ataxia). Type II poisoning gives the CS syndrome (Choreoathetosis, Salivation, sometimes seizures).

Pyrethroids dominate Indian household and personal pesticide use. Mortein, Hit, Good Knight, Baygon Plus and a long list of mosquito coils, repellents and crawling insect sprays all contain pyrethroids, sometimes mixed with synergists like piperonyl butoxide and (in the case of Baygon Plus) with propoxur as a knock-down booster. Cypermethrin agricultural formulations (Cymbush, Ripcord) are widely sold across the country and appear in deliberate ingestion cases in Maharashtra, Andhra Pradesh and Uttar Pradesh. The picture is usually mild to moderate: oral burning, vomiting, abdominal pain, paraesthesia, tremors and occasionally seizures or short-lived coma. Aspiration of the hydrocarbon vehicle is the commonest serious complication.

There is no specific antidote. Treatment is gastric decontamination if presentation is early (activated charcoal 1 g/kg, avoid lavage because the hydrocarbon solvent raises aspiration risk), benzodiazepines for seizures and tremor, supportive airway management and observation. Mortality in pyrethroid-only ingestion is low (below 5 percent in most Indian case series) but the volume of cases is high enough that the toxidrome is worth knowing on sight. Combination products warrant attention: a Baygon Plus or a cypermethrin-plus-chlorpyrifos formulation can produce a mixed picture, and the treating team should assume an OP component until labelling or gastric aspirate analysis rules it out.

Organochlorines were the first generation of synthetic insecticides: DDT, hexachlorocyclohexane (HCH, lindane is the gamma isomer), endosulfan, aldrin, dieldrin, chlordane and heptachlor. The mechanism is dual: GABA-A receptor antagonism in the central nervous system and slowing of sodium channel inactivation in peripheral neurons. Clinically they produce tremor, hyperexcitability, seizures and in severe cases coma. They are highly lipophilic, persist in adipose tissue for years and accumulate up the food chain, which is the public health reason most of them were banned.

The Indian regulatory record is a series of staged restrictions. DDT was banned for agriculture in 1989 but is still permitted for vector control under the National Vector Borne Disease Control Programme, with the National Malaria Research Institute and state malaria officers as the controlling authorities. HCH was banned for agriculture in 1997. Aldrin, dieldrin, chlordane and heptachlor were withdrawn in the 1990s. Endosulfan was the last major organochlorine to be removed and the most politically charged; the Kerala Plachimada and Padre village cluster of congenital anomalies, neurological disorders and cancers attributed to aerial endosulfan spraying on cashew plantations drove the eventual nationwide ban in 2011, confirmed by the Supreme Court of India.

Acute organochlorine poisoning is rare in current Indian casework but does still appear: occasional accidental ingestion from leftover stocks, occupational exposure in vector control workers using DDT, and historical autopsy material. Treatment is symptomatic, with benzodiazepines for seizures and supportive care; there is no specific antidote. Detection at the forensic toxicology laboratory uses GC-ECD (electron capture detection), which is exquisitely sensitive to the chlorinated structure, with GC-MS confirmation at CFSL Chandigarh and the better-equipped state forensic science laboratories. The longer-term forensic relevance of organochlorines is now mostly in environmental and biomonitoring work rather than acute clinical poisoning.

Aluminium phosphide (AlP) is a grey-brown tablet sold in India as Celphos, Quickphos, Sulfo-Carb, Phostoxin and Fumitoxin in 3-gram unit doses. It is the most widely used grain fumigant in the country; almost every Punjab and Haryana farm household has a tin of tablets in the godown to protect stored wheat from weevils and beetles. The tablet itself is inert until it contacts moisture or gastric acid, at which point it liberates phosphine gas (PH3) along with aluminium hydroxide. One 3-gram tablet releases about 1 gram of PH3, which is enough to kill several adults. The accepted lethal dose is as low as 1.5 grams, half a tablet, swallowed.

The mechanism is metabolic catastrophe. Phosphine inhibits cytochrome c oxidase at complex IV of the mitochondrial electron transport chain, generates reactive oxygen species through redox cycling and produces severe lactic acidosis, profound myocardial depression, refractory cardiogenic shock, acute respiratory distress syndrome and multi-organ failure. The patient typically presents within an hour of ingestion with intractable vomiting, abdominal pain, a characteristic garlic or fishy odour on the breath, agitation, profound hypotension that does not respond to fluids or vasopressors, and progressive metabolic acidosis. Death usually comes within 24 to 48 hours from cardiogenic shock or ARDS, occasionally later from hepatic and renal failure.

The treatment package is supportive and has been refined over four decades of Indian experience. Gastric lavage with sodium bicarbonate solution (raising gastric pH reduces conversion of AlP to PH3) is started immediately, with airway protection. Activated charcoal is given. Coconut oil or olive oil lavage features in several Indian protocols, including the one used at PGIMER Chandigarh, on the rationale that the oil dissolves AlP and reduces evolution of phosphine; the evidence is weak but the intervention is cheap and the harm minimal. Intravenous magnesium sulphate is given for arrhythmia control and as a membrane stabiliser. Trifluoperazine (a phenothiazine with calcium channel effects) has appeared in some Indian protocols, again on weak evidence. The mainstays are intensive cardiovascular support with vasopressors, mechanical ventilation for ARDS, insulin and dextrose for the refractory metabolic acidosis and shock, and renal replacement therapy when needed. Some centres have used intra-aortic balloon counterpulsation and extracorporeal membrane oxygenation in the rare patient who reaches that level of care.

Detection at autopsy has a distinctive workflow. The garlic odour of phosphine is often noticeable when the abdominal cavity is opened. The classical chemical test is the silver nitrate paper test: filter paper soaked in 0.1N silver nitrate is suspended above gently warmed gastric or viscera tissue, and phosphine evolving from the sample reacts to form black silver phosphide (Ag3P), turning the paper black within minutes. Confirmation uses GC-FPD tuned for phosphorus or GC-MS in headspace mode for the trapped phosphine, run routinely at CFSL Chandigarh and the state SFSLs.

The regulatory response has been slow and partial. The 2017 amendment brought AlP under restrictive sale requiring licensed dealers and recorded sales, but enforcement is patchy and the tablets remain easy to obtain. Researchers at PGIMER Chandigarh, AIIMS Bathinda, GMC Patiala and DMC Ludhiana have published the bulk of the Indian clinical literature on AlP and consistently link case volume to seasonal availability at harvest time.

The Indian pesticide regulatory regime sits under the Insecticides Act 1968 and the Insecticides Rules 1971, with the Central Insecticides Board and Registration Committee (CIBRC) as the gatekeeper for new registrations. The Anupam Verma expert committee was constituted in 2013 to review 66 hazardous pesticides and reported in 2015, with the May 2020 draft notification recommending the ban or phase-out of 27 of them: monocrotophos, methomyl, carbofuran, dichlorvos, certain uses of malathion and deltamethrin, thiram, ziram, captan and a long tail of older agents.

The operational map has a few stable features. Class Ia and Ib pesticides are progressively restricted: monocrotophos banned for vegetables, methyl parathion withdrawn except as a 2 percent dust, phorate restricted, parathion withdrawn. Chlorpyrifos and other Class II agents remain the workhorses of Indian agriculture. Organochlorines are largely banned except DDT for vector control. Aluminium phosphide is regulated under the 2017 restrictive sale amendment but remains widely available in practice. Pyrethroids are open-market household chemicals.

The viscera workflow tracks this map because the agents found in samples shift as regulation changes. State forensic science laboratories receive a steady volume of chlorpyrifos and monocrotophos viscera year round, with a seasonal spike in AlP at and after the north Indian harvest (October to December), occasional propoxur and carbofuran cases, regular cypermethrin and deltamethrin samples and a low background of organochlorine positives. CFSL Chandigarh, CFSL Hyderabad and the state SFSLs at Madhuban, Mohali and Kalina handle the largest pesticide caseloads.

The workflow for suspected pesticide poisoning starts at the autopsy table and ends with a confirmatory chromatogram. Five specimens are routinely sent in the viscera box under chain of custody: stomach with contents, 100 to 200 g of liver, one whole kidney, 30 ml of blood preserved with sodium fluoride, and 30 ml of urine. Any suspect container, residue or gastric aspirate goes in a second sealed jar. The preservative is saturated saline (never formalin) for organic poisons. Chain of custody runs under BNSS 2023 Section 105 and BSA Section 63.

1. Scene preservation and exhibit collection
   Photograph the scene. Collect the suspect bottle, sachet, tablet strip or AlP tin and any residue separately. Seal each exhibit in a tamper-evident container with a unique exhibit number and forwarding memo. The investigating officer signs the seizure memo under BNSS Section 105.
2. Autopsy and viscera collection
   Note odour at opening the abdominal cavity (garlic suggests phosphine, kerosene-like suggests pyrethroid hydrocarbon vehicle, characteristic smell of OP formulations). Collect stomach with contents, liver, kidney, blood and urine. Preserve viscera in saturated saline. Seal and dispatch under chain of custody to the state forensic science laboratory.
3. Presumptive testing at the laboratory
   Silver nitrate paper test for phosphide. Reinsch test for arsenic and other metals (negative for pesticides but routinely done as part of screen). Acid hydrolysis and steam distillation of stomach contents for volatile pesticides. RBC AChE assay on a fresh blood sample is run separately at the hospital biochemistry laboratory for clinical follow-up.
4. Screening by chromatography
   GC-FID and GC-ECD screens cover the bulk of organochlorines, organophosphates, carbamates and pyrethroids. Thin layer chromatography is still used as a quick screen at some state laboratories. HPLC with UV or photodiode array detection covers the more polar carbamates that do not run well on GC.
5. Confirmation and quantitation
   GC-MS for organophosphates, organochlorines and pyrethroids. GC-FPD for phosphorus-containing OPs and for AlP detection in headspace mode. LC-MS/MS at CFSL Chandigarh and at the better-equipped state SFSLs for low-level and polar pesticide residues. The final report names the compound and gives concentration in viscera and blood where calibration permits.
6. Reporting and court evidence
   The chemical examiner's report is submitted to the investigating officer and serves as the documentary evidence in court. The chemical examiner is summoned as an expert witness under BSA Section 39 (the rebadged Section 45 of the Indian Evidence Act). The chain of custody from scene to court is the load-bearing piece; any break is the first thing defence counsel will attack.

The clinical and forensic strands meet at the RBC AChE assay. Treating physicians use the serial RBC AChE level to track recovery in OP and carbamate cases, with values rising back from below 30 percent of normal toward normal over days to weeks. Plasma BChE falls and recovers faster, making it a sensitive but less specific screen. Both are run at most medical college biochemistry labs.

The Indian pesticide map is geographically structured. Organophosphate self-poisoning concentrates in the cotton belts of Vidarbha (Maharashtra) and the dryland districts of Telangana and Andhra Pradesh, in chilli-growing parts of Andhra and Karnataka, and in the rice-cotton intermix of West Bengal. The pattern is consistent across two decades of NCRB and ICMR data: agrarian distress, seasonal indebtedness around harvest, an open jar in the field shed, and ingestion of 50 to 200 ml of concentrate during an acute crisis. Recovery depends on adequate atropinisation and on access to pralidoxime within the 36-hour window.

Aluminium phosphide self-poisoning is the northern epidemic, concentrated in Punjab, Haryana, western Uttar Pradesh and northern Rajasthan, with the Malwa belt of Punjab a repeated hotspot. Published series from PGIMER Chandigarh, AIIMS Bathinda, Government Medical College Patiala and DMC Ludhiana put the case fatality rate between 60 and 95 percent, with little change despite four decades of clinical effort, because the mechanism is metabolic and the time window between ingestion and irretrievable shock is short.

The Plachimada and Padre village cluster in Kasaragod district of Kerala is the most thoroughly documented Indian organochlorine episode. Aerial spraying of endosulfan on Plantation Corporation of Kerala cashew estates from 1978 onwards was linked to congenital anomalies, neurological disorders, infertility and cancers in surrounding villages. Kerala banned endosulfan in 2001 and the Supreme Court imposed a nationwide ban in 2011 after litigation and reports from the National Institute of Occupational Health (Ahmedabad) and the Centre for Science and Environment.

The Bihar Chhapra mid-day meal disaster of July 2013 is the standing reference for accidental mass organophosphate poisoning. Twenty-three children aged 4 to 12 at a government primary school in Dharmasati Gandaman village in Saran district died after eating a meal prepared in cooking oil contaminated with monocrotophos. Forensic analysis traced the contamination to a used pesticide container reused for oil storage, and the case led directly to tighter restrictions on monocrotophos for vegetable crops.

Outcomes have improved slowly. Moderate OP poisoning mortality has dropped from around 25 percent in the 1990s to under 10 percent today, driven by better atropinisation, earlier pralidoxime and ICU admission. Carbamate and pyrethroid mortality has stayed low. Aluminium phosphide mortality has barely moved, which is the strongest argument for restricting household sales.