Pyrethroids, also called synthetic pyrethroids are analogues of the natural pyrethrins and have a broad spectrum of activity against external parasites (flies, ticks, mites, fleas, lice, mosquitoes, etc.). Natural pyrethrins are extracted from flowers of certain species (e.g. Pyrethrum, Chrysanthemum) and have insecticidal and repellent properties.

Molecular structure of cypermethrin

After the discovery of the chemical structure of natural pyrethrins: in the 1920's, the first synthetic pyrethroid, allethrin, was introduced in the 1950's. It breaks down quickly if exposed to sunlight and is less effective against insects than natural pyrethrins.

In the 1960's and early 1970's second-generation pyrethroids were progressively introduced (e.g. tetramethrin and phenothrin). They were 20 to 50 times more potent than allethrin. A third generation followed (e.g. fenvalerate, permethrin) with a better stability against UV-light.

The last generation (e.g. cypermethrin, deltamethrin, cyfluthrincyhalothrin and flumethrin) was introduced at the end of the 1970's and contributed substantially to the huge success of synthetic pyrethroids in agriculture, animal health and hygiene.

Synthetic pyrethroids are non-systemic insecticides and acaricides with tarsal activity, i.e. they act by contact. Most synthetic pyrethroids currently used on animals were introduced in the 1970's and are still widely used against all kinds of parasites in livestock, horses, cats and dogs, and also in agriculture as well as in public and domestic hygiene.

All pyrethroids are veteran antiparasitics, i.e., they have lost patent protection long ago and are available as generics manufactured by numerous chemical companies (typically in China, India, Israel, Brazil, etc.).

After their introduction, synthetic pyrethroids were highly successful and replaced many products containing organochlorinescarbamates, organophosphates and amidines. The main reason was that they were significantly less toxic for mammals and birds than their predecessors.

Click here for a general introduction to ectoparasiticides and their most important features.

Mode of action and characteristics of pyrethroids

Synthetic pyrethroids have a similar mechanism of action as organochlorines. They act on the membrane of nerve cells blocking the closure of the ion gates of the sodium channel during re-polarization. This strongly disrupts the transmission of nervous impulses. At low concentrations insects suffer from hyperactivity. At high concentrations they are paralyzed and die.

This toxic effect occurs not only on insects, but on many vertebrates as well, since the ion gates of the sodium channel in the cellular membranes of nerve cells work similarly in many living organisms.

However, pyrethroids are substantially safer for mammals than DDT and other organochlorines: they are only poorly absorbed through the skin and what gets into the body is quickly metabolized to non-toxic compounds.

A characteristic of most pyrethroids is their strong knockdown effect on insects, i.e. they paralyze and kill them very quickly. However, at lower dosage, affected insects can recover. Numerous pyrethroids have also a repellent effect, especially on flies and mosquitoes.

Whereas natural pyrethrins and the first synthetic pyrethroids are quickly decomposed by the UV radiation of sunlight, the more modern pyrethroids are UV-resistant, and therefore quite appropriate for topical products for livestock.

As a rule, the residual effect of most topically applied pyrethroids is comparable to the one of organophosphates.

Active ingredients

The synthetic pyrethroids most used against veterinary parasites of livestock, horses and pets are the following:

  • Cyfluthrin: Insecticide. Moderately used in animals, mainly against cattle flies.
  • Cyhalothrin: (Lambda-cyhalothrin): Insecticide, acaricide. Scarcely used in pets.
  • Cypermethrin (= Alphamethrin): Insecticide, acaricide, tickicide, lousicide, scabicide. Probably the most widely used pyrethroid worldwide: there are hundreds if not thousands of brands, mainly for livestock.
  • Cyphenothrin: Insecticide. Scarcely used in pets. 
  • Deltamethrin:  Insecticide, acaricide, tickicide, lousicide, scabicide. Abundant use in livestock, scarce in dogs.
  • Etofenprox:  Insecticide, acaricide, tickicide, lousicide. Scarcely used in dogs.
  • Fenvalerate: Insecticide. Scarcely used in animals, mainly against cattle flies.
  • Flumethrin: Acaricide, tickicide. Abundant use in livestock.
  • Permethrin: Insecticide, tickicide, lousicide. Moderately used in livestock and horses, but massively used in dogs.
  • Phenothrin: Insecticide. Scarcely used in pets.
  • Tetramethrin: Scarcely used in pets.

Delivery forms of pyrethroids

Molecular structure of permethrin

For use on livestock pyrethroids are marketed as concentrates for dipping and spraying. Typical formulations are emulsifiable concentrates (EC) and wettable powders (WP). Since pyrethroids are not soluble in water, these products are designed to produce a stable emulsion of the active ingredient when mixed with water. Such concentrates are typically used against ticks, mites, flies, lice, fleas, etc.

Many such formulations with pyrethroids don't show the stripping effect of organophosphates and amidines, or only to a small extent. This means that the concentration for the initial filling of a dip vat and the concentration for its replenishment are the same, and this makes dip management easier.

Pour-ons (= backliners) with pyrethroids are also very popular for livestock and horses. They are used mainly against flies and lice on cattle, sheep and pigs. Pour-ons often contain a synergist too.

Pyrethroids are also broadly used in insecticide-impregnated ear-tags for the control of flies on cattle. In some countries there are also pyrethroid impregnated plastic strips that are hanged in poultry cages against various mite species.

Many ready-to use topical products (dressings) against fly maggots (screwworms, blowfly strike, etc.) contain pyrethroids too.

Pyrethroids are frequently mixed with other active ingredients, mainly with organophosphates, sometimes with amidines.

For use in dogs and cats pyrethroids are used in collars as well as in thousands of insecticidal shampoos, soaps, sprays, powders and the like.Such products are often mixed with a synergist.

There are also numerous spot-ons with pyrethroids (mainly permethrin) for dogs. They are combined with other active ingredients effective against fleas (e.g. imidacloprid) but not against ticks. 

Pyrethroids and their optical isomers

Due to their chemical structure, all pyrethroids (excepting deltamethrin) can exist in different forms regarding the spatial distribution of their atoms within the molecule. This happens with a lot of molecules, including other parasiticides. However it is especially relevant for pyrethroids, because some optical isomers are substantially less effective and/or toxic than others.

Depending on the particular active ingredient and on the process used for its industrial synthesis, the final product is a mixture of several isomers. For most pyrethroids only one quality, i.e. one standard mixture is produced, and it is not relevant for end users to know the proportions of each isomer in it. However, for other pyrethroids, especially for cypermethrin and permethrin, different mixtures (or grades) are produced and used for the formulation of parasiticides.

Cypermethrin has 8 optical isomers, 4 called cis, and 4 called trans isomers. Although the topic is more complex, the bottom line is that cis isomers are more effective than trans isomers. Usual standard grade cypermethrin is a mixture that contains 40% cis and 60% trans isomers. The higher the content of cis isomers, the more expensive it is to produce. And the quality that is used to formulate a particular product plays a role in its parasiticidal properties.

For the formulation of veterinary parasiticides three main qualities are usually available, depending on the content of cis isomers:

  • Standard cypermethrin: a mixture of 8 different optical isomers, 40% cis, 60%trans
  • High-cis cypermethrin: a mixture of 8 different optical isomers, 80% cis, 20%
  • Alpha-cypermethrin = alphamethrin: a mixture of 2 different optical isomers, both cis, i.e., 100% cis

There are other qualities used mainly in agriculture, e.g. Beta-cypermethrin, Theta-cypermethrin, etc.

Practical consequences?

In a nutshell, to kill a parasite you need less alphamethrin than high-cis cypermethrin or standard cypermethrin.

Theoretically, if in a plunge dip vat you need a concentration of 100 ppm (parts per million) standard cypermethrin to control ticks, 50 ppm of high-cis cypermethrin and 40 ppm of alphamethrin will do the same work. And if you have brand "A" with 10% standard cypermethrin, brand "B" with 5% high-cis cypermethrin, and brand "C" with 4% alphamethrin and you dilute all of them at a rate of 1:1000 (1 liter product in 1000 liters water) you will get the same efficacy. But if you have brand "A" with 10% standard cypermethrin, brand "B" with 10% high-cis cypermethrin, and brand "C" with 10% alphamethrin and you dilute them all 1:1000, efficacy will be higher for brand "C", than for brand "B", than for brand "A". 

The same applies to pour-ons. Pour-on "A" with standard cypermethrin at 2,5%, pour-on "B" with 1.25 high-cis cypermethrin and pour-on "C" with 1% alphamethrin, all applied at a rate of 10 ml per 100 kg will show the same efficacy.

In fact, for most end users and brands, there is no practical difference in using any of those products. A product with standard cypermethrin is usually cheaper, but you need a larger amount to achieve the same efficacy. Or you need a small amount of a product with alphamethrin, but it is more expensive...

Unfortunately it also happens that, e.g. brand "A" with 10% standard cypermethrin and brand "B" with 10% high-cis cypermethrin have the same use recommendations (e.g. 1:1000 dilution), and maybe the same prize. Obviously with brand "A" you are applying only half the amount of the effective cis-isomer, i.e. brand "B" will be less effective.

This story about optical isomers also explains why the use recommendations of apparently similar products with cypermethrin (and permethrin) are sometimes different. Unfortunately also, manufacturers do not always declare which type of cypermethrin they use for their products.

Safety of pyrethroids

Molecular structure of deltamethrin

Synthetic pyrethroids are relatively safe for birds and mammals, mainly due to their poor dermal absorption through the skin and to their fast metabolism. They are also less persistent in the environment. When synthetic pyrethroids became available these were substantial advantages over organochlorinesorganophosphates and amidines, which they largely replaced.

However pyrethroids are irritant for the skin, the eyes, and the respiratory tract of mammals and humans, although correctly used, veterinary products with pyrethroids are unlikely to cause serious problems on operators and pet owners. Nevertheless, both livestock and pets can suffer from such irritations, especially young animals.

The irritant effect of pyrethroid pour-ons on dairy cows is well known for many such products: it may cause significant trouble during milking or handling the cows. 

Pyrethroids are extremely toxic for fish and accidental contamination of waters can have catastrophic consequences for fish populations.

Permethrin and phenothrin are particularly toxic to cats!. The reason is that cats lack glucuronidase, the enzyme that breaks down pyrethroids to less toxic metabolites in the host's body. Other pyrethroids have a similar problem, but their therapeutic dose may be low enough for cats to tolerate them.

Most pyrethroids leave almost no or very few residues in meat and milk and are often approved for use on dairy animals.

Additional specific information (toxicity, intoxication symptoms, adverse drug reactions, antidote, etc.) on the safety for veterinary use of synthetic pyrethroids is available in specific articles in this site:

General information on the safety of veterinary antiparasitics is available in specific articles in this site (click to visit):

Never use livestock, horse or poultry products on dogs and/or cats, unless explicitly approved for dogs and/or cats too. Without reliable use instructions they can be easily overdosed, and pets may not tolerate formulations developed for use on livestock, horses and/or poultry. Some active ingredients may be toxic to particular animals.

Never use agricultural or hygiene products on livestock, horses, poultry or pets, unless explicitly approved for veterinary use, which is very unusual. Even if the specific active ingredient is approved for some veterinary use. The formulations for agricultural and/or hygiene use are mostly different than those for veterinary use and may be toxic to or not be tolerated by animals.

It is obvious that veterinary medicines are not intended for and should never be used on humans!!!

Resistance of parasites to synthetic pyrethroids

Unfortunately, soon after their introduction several important veterinary parasites as well as agricultural and hygiene pests developed resistance to synthetic pyrethroids. Probably it is related to the fact that they have the same molecular mode of action as organochlorines, i.e., they have cross-resistance with the organochlorines massively used during the previous decades.

Nowadays, resistance of several veterinary parasites to pyrethroids is extremely common and strong in many places worldwide. The veterinary parasites most affected by pyrethroid resistance are the following ones:

  • Cattle ticks (Boophilus = Rhipicephalus microplus) in Latin America and Australia and tropical cattle ticks (= blue tick, Boophilus = Rhipicephalus decoloratus) in Southern Africa;
  • Horn flies (Haematobia irritans) worldwide, and Buffalo flies (Haematobia irritans exigua) in Australia
  • Houseflies (Musca domestica) worldwide
  • Cat and dog fleas (Ctenocephalides spp.) worldwide
  • Mosquitoes (various species) worldwide
  • Red poultry mites (Dermanyssus gallinae), worldwide
  • Sheep lice (Damalinia = Bovicola ovis) in Australia and New Zealand

Resistance to pyrethroids is characterized by the high levels it can reach very quickly. The lethal dose of an organophosphate against a parasite resistant to it may be 10 to 100 fold the dose for a susceptible parasite: the so-called resistance factor would be 10 to 100. For pyrethroids, resistance factors can be up to 1000 and higher, i.e., not even an undiluted concentrate would kill the resistant parasites. Whatever pyrethroid product, it would be completely useless against such resistant populations.

Visit also the section in this site about parasite resistance to antiparasitics and more specifically to synthetic pyrethroids.