Trichlorfon is an antiparasitic active ingredient used in veterinary medicine in dogs and livestock mainly against external parasites (lice, mites, fleas, flies, ticks, etc.). It is also used against agricultural and household pests. It belongs to the chemical class of the organophosphates.
Common name: TRICHLORFON = METRIFONATE
EFFICACY AGAINST PARASITES
Type of action: Broad spectrum ectoparasiticide: insecticide, acaricide, tickicide, louisicide, larvicide, as well as endoparasiticide
Main veterinary parasites controlled: flies, lice, fleas, mosquitoes, ticks, mites, fly larvae, etc; several gastrointestinal roundworms (nematodes)
Efficacy against a specific parasite depends on the delivery form and on the dose administered.
Click here for general information on features and characteristics of PARASITICIDES.
Click here to view the article in this site with the most common dosing recommendations for trichlorfon used in domestic animals.
Oral LD50, rat, acute*: 430-630 mg/kg
Dermal LD50, rat, acute*: >2000 mg/kg
* These values refer to the active ingredient. Toxicity has to be determined for each formulation as well. Formulations are usually significantly less toxic than the active ingredients.
MRL (maximum residue limit) set for animal tissues (e.g. beef, mutton pork or chicken)*:
- CODEX: No
- EU: Yes
- USA: No
- AUS: Yes
* This information is an indicator of the acceptance of an active ingredient by the most influential regulatory bodies for use on livestock. MRL's for animal tissues may be set also for agricultural pesticides that are not approved for use on animals but are used on commodities fed to animals. A MRL may be also set in the form of an IMPORT TOLERANCE for active ingredients not approved in a particular country but approved for imported animal commodities.
Withholding periods for meat, milk, eggs, etc. depend on delivery form, dose and national regulations. Check the product label in your country.
Learn more about trichlorfon safety (poisoning, intoxication, overdose, antidote, symptoms, etc.).
General safety information for antiparasitics is available in specific articles in this site (click to visit):
- General safety of antiparasitics for domestic animals
- General safety of antiparasitics for humans
- General safety of antiparasitics for the environment
Never use agricultural or hygiene products with this or any other active ingredient on livestock or pets, even if there are veterinary products with this same active ingredient approved for use on animals. The formulations for agricultural or hygiene use are different and may be toxic for livestock or pets.
It is obvious that veterinary products are not intended for and should never be used on humans!!!
MARKETING & USAGE
Decade of introduction: 1950
Introduced by: BAYER
Some original brands: NEGUVON
Patent: Expired (particular formulations may be still patent-protected)
Use in LIVESTOCK: Yes, low and declining, as all organophosphates
Use in HORSES: Yes, scarce and declining as all organophosphates
Use in DOGS and CATS: Yes, low and declining, as all organophosphates
Main delivery forms:
- Pour-ons and spot-ons
- Premise and environmental treatment
- Shampoos, soaps, powders
Use in human medicine: No
Use in public/domestic hygiene: Yes
Use in agriculture: Yes
Generics available: Yes
- In livestock & horses: Yes, as for all organophosphates: very frequent worldwide in such species as cattle ticks (Boophilus spp), horn flies (Haematobia irritans), sheep lice (Damalinia ovis), poultry mites (Dermanyssus gallinae), houseflies (Musca domestica), mosquitoes
- In pets: Yes, quite frequent worldwide in dog and cat fleas (Ctenocephalides spp)
Trichlorfon is a classical, veteran parasiticide belonging to the organophosphates.
Trichlorfon was the first ectoparasiticide that could be administered as a spot-on to both livestock and pets, but not in single-dose vials or pipettes as most modern spot-ons for pets, but as simple drops.
In livestock it is still used in some countries in cattle, sheep and pig in concentrates for dipping and spraying, in ready-to-use pour-ons and dressings and in drenches against several gastrointestinal roundworms.
Trichlorfon is one of the very few organophosphates still used in some countries against gastrointestinal roundworms of sheep and goats that have become resistant to modern anthelmintics (benzimidazoles, macrocyclic lactones, imidazothiazoles). For this puropose it is administered orally. It has to be done very accurately because the safety margin is extremely low and slight overdosing can easily cause toxic side effects.
In dogs and cats it is still used in shampoos, soaps, baths, sprays, powders and the like, but its usage has strongly declined after the introduction of more modern and safer flea and tick control spot-ons (= pipettes) and tablets.
Nevertheless, there is a clear trend to replace all organophosphates, including trichlorfon, with less toxic compounds.
Efficacy of trichlorfon
As most organophosphates trichlorphon is a broad-spectrum insecticide, acaricide and larvicide. Trichlorfon is particularly effective against various livestock myiases such as maggots from warble flies (Hypoderma spp), human bot flies (e.g. Dermatobia hominis) and screwworm flies.
However, resistance of important veterinary parasites to all organophosphates, including trichlorfon is widespread, especially in cattle ticks (Boophilus spp), horn flies (Haematobia irritans), sheep lice (Damalinia ovis), poultry mites (Dermanyssus gallinae), mosquitoes, dog and cat fleas (Ctenocephalides spp) and houseflies (Musca domestica). As a consequence, products with this active ingredient may not achieve the expected efficacy in many places. The same applies to all other organophosphates. This is also a reason for their progressive replacement with newer active active ingredients with a different mode of action.
Pharmacokinetics of trichlorfon
Percutaneous absorption (i.e. through the skin) of topically administered trichlorfon depends on the animal species, the administered dose, and the extension of the treated body surface. Trichlorfon is easier absorbed through the skin or after inhalation than most other organophosphates.
Animals treated topically can ingest trichlorfon through licking and grooming. However, once absorbed into blood it is very quickly metabolized in the liver to less toxic compounds and very rapidly excreted through urine. Interestingly, one of the main metabolites of trichlorfon is dichlorvos, which itself is unstable and is very quickly hydrolyzed. In cattle, maximum excretion in urine occurs 2 to 4 hours after administration. It is no more detectable in urine 4 days after administration.
Mechanism of action of trichlorfon
As all organophosphates insecticides, trichlorfon acts on the nervous system of the parasites (but also of mammals, birds, fish and many organisms!) as inhibitor of acetylcholinesterase (also known as AchE), an enzyme that hydrolyzes acetylcholine (Ach). Ach is a molecule involved in the transmission of nervous signals from nerves to muscles (so-called neuromuscular junctions) and between neurons in the brain (so-called cholinergic brain synapses).
AchE's role is to terminate the transmission of nervous signals where Ach is the neurotransmitter (there are several other neurotrasmitters). By inhibiting the activity of AchE, carbamates prevent the termination of those nervous signals, i.e. the neurons remain in constant activity and excitation, massively disturbing the normal movements of the parasites. The bottom line for the parasites is that they are paralyzed and die more or less quickly. Organophosphates bind irreversibly to AchE, in contrast with carbamates, another chemical class of parasiticides, which bind reversibly to AchE.
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