Phosmet is an antiparasitic active ingredient used in veterinary medicine in dogs and livestock against external parasites (lice, mites, fleas, flies, ticks, etc.). It is also used against agricultural pests. It belongs to the chemical class of the organophosphates.
Common name: PHOSMET
EFFICACY AGAINST PARASITES
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.
Phosmet is a broad-spectrum organophosphate highly effective against many external livestock (flies, ticks, mites, lice, etc) but is completely ineffective against internal parasites when administered topically (i.e. on the skin).
As most organophosphates, phosmet products are used exclusively for topical administration, mainly as a concentrate for spraying. It is not used in pets. Nowadays usage is rather marginal because it has been replaced by newer, more effective and less toxic compounds.
The table below indicates some usual dosing recommendations for phosmet issued by manufacturers or documented in the scientific literature. They may not be approved in some countries.
|Dosing recommendations for PHOSMET
|Delivery||Parasites||Dose (against phosmet-susceptible parasites)
|Spray||Cattle ticks (R. microplus)||~500 ppm (=mg/L) in the wash|
|Spray||Horn flies||~600 ppm (=mg/L) in the wash|
|Backrubber||Horn flies||~2350 ppm (=mg/L)|
|Spray||Lice||~900 ppm (=mg/L) in the wash|
|Spray||Mites, other ticks||~1175 ppm (=mg/L) in the wash|
|Delivery||Parasites||Dose (against phosmet-susceptible parasites)
|Spray||Lice, mites||~1175 ppm (=mg/L) in the wash|
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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.
Oral LD50, rat, acute*: 113 mg/kg
Dermal LD50, rat, acute*: >4640 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: Yes
- 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.
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: 1960
Introduced by: STAUFFER
Some original brands: PORECT, PORON, STARBAR, PROLATE
Patent: Expired (particular formulations may be still patent-protected)
Use in LIVESTOCK: Yes, scarce and declining, as all organophosphates
Use in HORSES: No
Use in DOGS and CATS: No
Main delivery forms:
Use in human medicine: No
Use in public/domestic hygiene: No
Use in agriculture: Yes
Generics available: Yes, a few
In livestock: 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.
Phosmet is a classical, veteran parasiticide belonging to the organophosphates. It was moderately used in the 1970's to 1990's, in concentrates for dipping and spraying and in ready-to-use pour-ons. But usage has strongly declined since the introduction of systemic acaricides and lousicides (e.g. macrocyclic lactones). Use in pets was vastly abandoned.
Efficacy of phosmet
Phosmet is a broad-spectrum insecticide and acaricide, a "generalist" pesticide with good average efficacy against most external parasites, but not outstanding against a particular one.
Phosmet was one of the very few organophosphates originally available as a pour-on for pigs: most organophosphates are not suited for such a formulation but have to be used for dipping or spraying, basically because they are too toxic as pour-ons. Instead, phosmet shows a rather low dermal toxicity to mammals.
However, resistance of important veterinary parasites to all organophosphates, including chlorpyrifos 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 phoxim
Percutaneous absorption (i.e. through the skin) of topically administered phosmet depends on the animal species, the administered dose, and the extension of the treated body surface. Studies on pigs treated with a phosmet pour-on showed that between 0.5% and 3% of the administered dose was absorbed into the bloodstream. Absorbed phosmet is quickly and vastly metabolized and excreted, almost completely 24 hours after administration, mainly through urine.
Mechanism of action of phosmet
As all organophosphates insecticides, phosmet 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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