Trichlorfon is a broad-spectrum organophosphate highly effective against many external livestock and pet parasites (flies, ticks, mites, lice, fleas, mosquitoes, etc.) but is completely ineffective against internal parasites when administered topically (i.e. on the skin).

Nowadays trichlorfon usages is rather modest, both in livestock (cattle, sheep, goats, swine, etc.), horses and pets (dogs a cats). It is also used against crop protection and household pests.

As most organophosphates, triichlorfon products are used mainly for topical administration, either as concentrates for dipping or spraying, or in a few ready-to-use products such as dressings, dusts, and pour-ons for livestock. It is also used in a few low-cost topicals for dogs such as shampoos, soaps, sprays,etc.

Trichlorfon is one of the few organophosphates that is still used as an anthelmintic drench for livestock in a few countries, as well as in oral pastes for horses against horse bot flies (Gasterophilus spp), often mixed with other anthelmintics.

Resistance of several insect and tick species to all organophosphates is already very frequent in many regions, particularly in cattle ticks, houseflies, mosquitoes, fleas, blowfly strike, etc.

The table below indicates some usual dosing recommendations for trichlorfon (also called metrifonate) issued by manufacturers or documented in the scientific literature. They may not be approved in some countries.

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In most finished products, efficacy and safety depend not only on the amount of active ingredient(s) but also on their formulations (i.e. the type and amount of so-called inert ingredients), particularly in injectables and in topical pour-ons and spot-ons. These inert ingredients can significantly affect the pharmacokinetic behavior (e.g. absorption through the skin or from the injection site, distribution within the body, spreading throughout the body surface, etc). Generic products usually contain the same amount of active ingredient(s) as the original product, but often in quite different formulations. In many cases, the curative (therapeutic) efficacy of the different formulations is quite comparable, but the protective (prophylactic) efficacy that determines the length of protection against re-infestations may be rather different.

Dosing recommendations for antiparasitics depend on national regulations. National regulatory authorities determine whether a product is approved for a given indication, i.e. use on a particular host at a specific dose and against a specific parasite. Check the labels of the products available in your country for specific information on approved indications.

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