ORGANOPHOSPHATES (OPs)

Question 1

SUBSTANCES (for anthelmintic use)

Answer 1

• Dichlorvos (Horses, Pigs, Dogs, and Cats), Cythioate (Dogs and Cats)
• Trichlorfon (Horses and Dogs)
• Naftalofos, Haloxon, Coumaphos, Crufomate (Ruminants)

Question 2

MECHANISM OF ACTION

Answer 2

Cholinesterase inhibition (+ many enzymes) by phosphorylating their esterification sites, this blocks cholinergic nerve transmission in the parasite, which results in spastic paralysis.

Question 3

ANTHELMINTIC SPECTRUM

Answer 3

• Effective against GI-roundworms (and bots).
• Limited efficacy against immature stages.
• Ineffective against migrating larvae tapeworms and flukes.

Question 4

PHARMACOKINETIC FEATURES

Answer 4

• Good absorption after oral administration, readily penetrate through unbroken skin (high lipid solubility).
• Inactivation by the liver.
• Excretion mainly with urine; to a lesser extent with milk.

Question 5

• SIDE-EFFECTS

Answer 5

• Narrow margin of safety. (Host + User safety considerations).
• Signs of overdosing: restlessness, salivation, miosis, emesis, bradycardia, bronchial constriction, diarrhoea, ataxia, tremors. - Strict attention to dosage
• Topical formulations (sprays, collars, washes) can present significant hazards to infant small animals after ingestion, inhalation, or transcutaneous absorption. Basic liquids (soaps) inactivate them.
• Concurrent use of other cholinesterase-inhibiting drugs should be avoided.
• Antidotes: Atropine and 2-PAM-type enzyme-reactivators (e.g. Pralidoxime).
• Contamination of the environment through faecal excretion, or non-careful handling of products and (partly) emptied packaging.

Question 6

APPLICATION

Answer 6

PO limited application (premixes, tablets) External/topical use (see under topic „Ectoparasiticides”)