Describe the insecticides used in public health and the control of rodents.

Insecticides and Rodent Control

• The classification of insecticides by chemical group:
• Organochlorines — DDT, BHC (lindane), dieldrin; characterised by long residual action, which was their great advantage, and by persistence in the environment with bioaccumulation in fat and in the food chain, which is why most are now restricted
• DDT remains permitted in India for indoor residual spraying in vector control under the Stockholm Convention exemption, though agricultural use is banned
• Organophosphates — malathion, fenitrothion, temephos, dichlorvos; they inhibit cholinesterase, are less persistent and biodegradable, but are acutely more toxic to humans
• Organophosphate poisoning is a major cause of death in rural India, both accidental and as a means of suicide, presenting with the cholinergic syndrome — salivation, lacrimation, urination, defaecation, gastrointestinal upset, emesis, bradycardia, bronchorrhoea and miosis, treated with atropine and pralidoxime
• The public health response is regulation of the most toxic compounds, and restricting access to them has reduced national suicide rates in comparable countries — which links this topic directly to mental health
• Carbamates — propoxur, carbaryl; also anticholinesterase but reversibly so
• Pyrethroids — permethrin, deltamethrin, cypermethrin; synthetic analogues of natural pyrethrum, with rapid knockdown, an EXCITO-repellent effect and low mammalian toxicity
• They are the insecticide used for long-lasting insecticidal nets, and their low toxicity to humans is precisely why they can be used on a net a child sleeps under
• The modes of application and their appropriate use:
• Residual spraying — applied to indoor wall surfaces where insects rest after feeding; effective only against endophilic vectors, and useless against those that rest outdoors
• Space spraying and fogging — kills adult insects on contact and gives brief relief during an outbreak, with NO residual effect
• It is greatly over-used as a visible political gesture, and achieves almost nothing without simultaneous source reduction
• Larvicides — temephos, and oils and Paris green, applied to breeding sites
• Insecticide-treated and long-lasting nets, which are among the most cost-effective interventions in public health
• Insecticide resistance is the central problem — arising from repeated use of a single compound over generations of insects, which selects resistant survivors
• Its management — rotation of insecticide classes, not merely of products within a class; routine resistance monitoring with susceptibility tests; mosaic and combination spraying; and reducing dependence on chemicals by using environmental and biological methods
• Rodents and their public health importance — they transmit plague (through the rat flea), leptospirosis (urine), salmonellosis, rat-bite fever, scrub typhus, hantavirus and trichinosis; and cause enormous food losses in storage and damage to property and wiring
• The principle of rodent control is that environmental measures come first and poisons last, and the reason should be given:
• Rodent-proofing and depriving rodents of food and harbourage is permanent, whereas killing rodents in a place that still offers food and shelter simply creates vacancies that are refilled from the surrounding population within weeks
• The measures — proper storage of food in rodent-proof containers; solid waste management; sealing entry points and rodent-proof construction; removal of harbourage and clearing of vegetation; trapping; and rodenticides
• The rodenticides — anticoagulants such as warfarin and the second-generation compounds, which act over several days and are preferred because the delay prevents bait shyness and because vitamin K is an antidote if a child or animal is poisoned; and acute poisons such as zinc phosphide, which are more hazardous
• A critical practical point in plague control — the fleas must be killed before the rats
• If rats are killed first, their fleas leave the cooling bodies and seek new hosts, including humans, which precipitates an outbreak — so insecticide dusting must precede rodenticide
• Kill the fleas before the rats, or the fleas leave the cooling bodies and seek human hosts
• Proofing is permanent and poisoning temporary, since a place still offering food and shelter refills within weeks
• Anticoagulants are preferred rodenticides, since the delayed action prevents bait shyness and vitamin K is an antidote
• Organophosphate poisoning is a major rural killer, and regulating the most toxic compounds reduces suicide rates
• Pyrethroids have low mammalian toxicity, which is why they can be used on a net a child sleeps under
• Rotate insecticide classes, not products, since resistance is to the mode of action rather than the brand