Used principally as insecticides, some acaricides. Act by contact, inhalation or
ingestion. Several are used as therapeutic agents, acting as effective nematicides
Animals most affected
All species (notably cattle, dogs).
Compounds in this category
Acephate, azinphos-ethyl, azinphosmethyl*, carbophenothion*, chlorfenvinphos*,
chiormephos, chlorpyrifos~ethyl*, chlorpyrifos~methyl*, ethion demeton-Smethyl*,
demeton-S-methyl sulphone*, dialyfos, diazinon*, dichlorvos*, diethion, dienochlor*,
dimethoate*, dioxathion, disulfoton*, ethoprophos*, fenchlorphos*, fenitrothion*,
fenthion, fonofos*, formothion*, heptenophos*, iodofenphos*, isofenphos, malathion*,
mercarbam, metamidophos, methacrifos*, methidathion, mevinphos*, monocrotophos,
naled*, omethoate*, oxydemeton~methyl*, parathion-ethyl, parathion-methyl, phorate*,
phosalone*, phosmet, phosphamidon, phoxim*, pirimiphos~ethyl*, pirimiphosmethyl*,
propetamphos, prothroate*, sulfotep, temephos, terbuphos, trichlorfon*, vamidothion*.
*Compounds with an asterisk are permitted for use in the UK.
exposure to spray or spray drift due to poorly conducted aerial crop spraying;
ingestion of the agricultural product, treated seeds for sowing, contaminated
fodder or polluted water (the latter occurs when spraying equipment or containers
are washed out into ponds, water courses, etc.).
Deliberate or criminal poisoning
The main targets are dogs and game birds using bait made of meat, eggs or other foods.
Incidents of iatrogenic poisoning may also occur. High ambient temperatures and poor
ventilation increase the risk of poisoning.
The compounds depress cholinesterase production
and consequently lower the levels of blood cholinesterase. Being liposoluble, organophosphorous
compounds diffuse readily into tissues and organs. There is a considerable variation
in toxicity between individual compounds; animals also show marked variation in their
susceptibility to the toxicity of any particular compound (in general, Charolais
cattle are the most sensitive). Age is another factor (juveniles are most affected),
as are the nature of the compound and the particular formulation of the product (encapsulated
preparations are less toxic).
These are due to the accumulation of excess
acetylcholine and overstimulation of the parasympathetic nervous system, leading
to muscarinic, nicotinic and central effects. There is no particular order of appearance
of the clinical signs:
a notable increase in all secretions;
watering eyes, nasal discharge, salivation,
diarrhoea (also affects smooth muscle);
bronchial hypersecretion and bronchospasm
with subsequent dyspnoea and coughing;
incontinence due to relaxation of the
myosis, which may be very severe, tachycardia.
hyperstimulation with local muscle fasciculations,
developing into generalized clonic spasms;
followed by depression, weakness and
paralysis (due to muscular exhaustion).
depression, prostration or coma;
possibility of convulsive attacks or
seizures developing in carnivores, followed by
depression, weakness and paralysis.
Death due to respiratory failure may occur
in several hours to several days (according to the compound involved). Occasionally
there is a very sudden presentation,
with only acute respiratory distress and rapid death due to asphyxia.
Manifestation of toxicity
is frequently delayed in ruminants, birds
and cats (often following
therapeutic use of organophosphates): weakness, ataxia, paraesthesia or paralysis
after several days, or up to several weeks post-exposure.
cardiac and gastric haemorrhages;
bronchial hypersecretion, hypersalivation;
delayed toxicity is often accompanied by damage to or degeneration of the axons
atropine for the muscarinic stage only
(at the onset of poisoning): 0.5-1 mg/kg 25%:75% (iv:sc)
until atropinization has been reached (dry mouth); repeat every 3-5hours during
the first 24 hours;
oximes (which liberate cholinesterase),
such as pralidoxime or pyridyladoxime, at a dose of 20 mg/kg iv in the first
48 hours following poisoning.
Symptomatic and excretory treatment
wash the skin well, in the case of dermal
emetics or adsorbents, in the case of
sedatives, tranquillizers or barbiturates
Level of cholinesterase depression
(Erythrocyte cholinesterase activity or blood enzyme activity)
brain, plasma (carnivores);
brain, whole blood, cardiac blood clot (ruminants).
It is essential to freeze the samples immediately, as the enzyme degrades very rapidly.
Direct analysis for the compound
stomach contents (or ruminal);