Used principally as insecticides, some acaricides. Act by contact, inhalation or ingestion. Several are used as therapeutic agents, acting as effective nematicides and antihelminthics.

[affected.gif] 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.

[etiology.gif] Etiology
Accidental poisoning

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).

[clinical.gif] Clinical features
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:

Muscarinic effects

Nicotinic effects
(Predominantly neuromuscular):

Neurological effects

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.

[lesions.gif] Lesions

[treatm~1.gif] Treatment

Symptomatic and excretory treatment

(do not use phenothiazines).

[labinv~1.gif] Laboratory investigations
Level of cholinesterase depression
(Erythrocyte cholinesterase activity or blood enzyme activity)

It is essential to freeze the samples immediately, as the enzyme degrades very rapidly.

Direct analysis for the compound