Permethrin is a synthetic pyrethroid introduced in the late 1970s. Since then, a lot of external parasite species have developed resistance to this chemical class: ticks, flies, fleas, lice, mites, etc.
Resistance to synthetic pyrethroids is found worldwide, is very frequent, develops and spreads very quickly, and can be extremely high (Resistance factors often >1000), which makes many products affected completely useless.
The most critical cases of veterinary parasites resistant to permethrin are:
- One-host Cattle ticks: Rhipicephalus (Boophilus) spp in CATTLE, worldwide in tropical and subtropical regions.
- Horn and Buffalo flies: Haematobia irritans in CATTLE, mainly in the Americas and Australia.
- Sheep body lice: Damalinia (=Bovicola) ovis in SHEEP, only in Australia and New Zealand.
- Red fowl mites: Dermanyssus gallinae in POULTRY, worldwide.
- Houseflies: Musca domestica, in LIVESTOCK OPERATIONS, worldwide.
See below for DETAILS.
There are numerous reports on many other external parasites resistant to synthetic pyrethroids, but so far, such cases remain restricted to limited regions and/or do not represent a global threat for domestic animals yet, and thus are not particularly analyzed in this article. Recommended measures to handle these cases are more or less the same as for the most critical ones: Rotation to chemical classes with different modes of action that remain effective and/or Integrated Pest Management. The following cases can be mentioned:
- Fleas: cat fleas Ctenocephalides felis in DOGS and CATS. Flea resistance to synthetic pyrethroids was common in the 1990s worldwide. However, after the avalanche of new products with many different modes of action in the last decades, flea resistance to insecticides is nowadays not a real issue.
- Multi-host ticks: Cayenne ticks Amblyomma cayennense in CATTLE in Brazil and Mexico LIKELY TO INCREASE in Latin America; Brown dog ticks Rhipicephalus sanguineus in DOGS in several countries; Tropical bont tick Amblyomma variegatum in CATTLE in Ghana; Brown ear ticks Rhipicephalus appendiculatus in Cattle in South Africa; Rhipicephalus bursa in CATTLE in Iran. Hyalomma anatolicum in CATTLE in India.
- Soft ticks: Argas persicus and Ornithodoros lahorensis in Iran.
- Mites: Northern fowl mites: Ornithonyssus sylviarum in POULTRY in the US; Sheep mange mites Psoroptes ovis in SHEEP in the UK.
- Flies: Stable flies Stomoxys calcitrans in all kinds of LIVESTOCK in the US, France, Germany and several other countries: LIKELY TO INCREASE; lesser housefly Fannia canicularis in the US.
- Lice: Goat lice Bovicola spp and horse chewing lice Bovicola (Werneckiella) ocellatus, both in the UK; cattle chewing louse Bovicola bovis in UK and IRL in the 2010s.
- Bed bugs: common bed bugs Cimex lectularius in the US, Denmark and several other countries: LIKELY TO INCREASE; tropical bed bug Cimex hemipterus in Sri Lanka.
- Black flies: Simulium spp in Argentina.
- Mosquitoes: Culex spp, Aedes spp, Anopheles spp, etc. in LIVESTOCK and PETS, worldwide. Resistance of mosquitoes to synthetic pyrethroids is very frequent everywhere and can be rather strong. But their control is usually more a task for health authorities than for producers or pet owners. Mosquitoes are seldom a problem for the livestock industry. For pet owners they are more relevant because mosquitoes are vectors of various diseases (e.g. heartworms), but there is not a lot they can do in terms of chemical protection against mosquitoes.
Synthetic pyrethroids are derivatives of natural pyrethrins. These compounds are produced by some plants (e.g. chrysanths and marigolds) that were known for their insecticidal properties already in the Roman Empire. They are effective against most insects (flies, fleas, mosquitoes, etc.), ticks and mites of veterinary importance. They have no efficacy whatsoever against internal parasites (roundworms, tapeworms, flukes). Synthetic pyrethroids are often used in combination with active ingredients of other chemical classes. They are only available for external use. There are thousands of generic products worldwide.
Permethrin is an all-round synthetic pyrethroid for livestock and pets used worldwide, but with interesting regional differences. Whereas it is the main synthetic pyrethroid used in the USA, it is much less used in Europe and Latin America, where cypermethrin and deltamethrin are often preferred, particularly in livestock.
For livestock and horses permethrin is available mainly as concentrates for dipping or spraying, in pour-ons and in impregnated ear-tags.
For dogs permethrin is available in some spot-ons and collars, as well as in numberless low-cost aerosols, baths, dusts, soaps, sprays, and the like. Most synthetic pyrethroids are not tolerated by cats.
Multiresistance is already common and frequent in some species, particularly in the cattle tick Rhipicephalus (Boophilus) microplus, in the red fowl mite Dermanyssus gallinae, in the housefly Musca domestica, and in mosquitoes.
It is a general rule that compounds that belong to the same chemical class show so-called cross-resistance among them, i.e., if a parasite develops resistance to one compound, it will be more or less resistant to other compounds of the same chemical class.
Other active ingredients of this chemical class used abundantly against veterinary parasites are: cypermethrin, deltamethrin, and flumethrin.
Parasites with resistance to permethrin
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Cattle ticks (one host): Rhipicephalus (Boophilus) spp.
- OCCURRENCE. Resistance of R. microplus and its Australian relative R. australis to synthetic pyrethroids has been reported basically everywhere in tropical and subtropical countries in the Americas, Asia and Australia. Resistance to R. decoloratus in Africa is comparably critical.
- OUTLOOK. Problems will worsen everywhere. Resistance will continue to spread and to strengthen, because synthetic pyrethroids are still being used in mixtures with other chemical classes (e.g. organophosphates, amitraz).
- RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals (see the article on Rhipicephalus (Boophilus) in this site). Where alternative chemicals of other chemical classes are still working against these ticks, rotation is usually a good option, i.e. to stop using synthetic pyrethroids and to use other products with active ingredients of those still effective chemical classes during several years.
- ALTERNATIVE PARASITICIDES for ROTATION. There are several chemical classes of parasiticides that are effective for the control of cattle ticks.
- Amitraz. Amitraz is only available for dipping or spraying. However, resistance of cattle ticks to amitraz is already quite frequent worldwide and spreading, following its increased use as an alternative to synthetic pyrethroids. It controls only ticks as well as lice and mites to some extent. It does not control flies.
- Fipronil. Fipronil is approved for the control of cattle ticks in some countries (e.g. Latin America and India) but in others not (e.g. Australia, USA). It is available only as a pour-on. Usually it has a very long withholding period of at least 12 weeks. However, resistance of cattle ticks to fipronil is spreading and increasing in Latin American countries (e.g. Argentina, Brazil, Mexico, Uruguay) and in India.
- Fluazuron. Fluazuron is available for cattle tick control in most countries where this tick is a problem. It is very specific for ticks and does not control other external parasites. It is available only in pour-ons, often in mixtures. Some cases of resistance of cattle ticks to fluazuron have been already reported in Brazil.
- Macrocyclic lactones (abamectin, doramectin, eprinomectin, ivermectin, moxidectin). Pour-ons and high concentration injectables (>3%) do control cattle ticks and widely used for this purpose in numerous countries. However, resistance of cattle ticks to macrocyclic lactones is spreading and increasing, e.g. in Latin America.
- Organophosphates (chlorpyrifos, coumaphos, diazinon, dichlorvos, etc.). In the past (up to the 1990s) organophosphates were used a lot for tick control worldwide. They were progressively abandoned for safety reasons when less toxic synthetic pyrethroids became available and were almost completely replaced. However, a certain come-back has taken place, e.g. in Latin America, following the exacerbation of cattle tick resistance to synthetic pyrethroids. Availability today is strongly reduced because many products have been banned or discontinued by the manufacturers. Organophosphates are used mainly for dipping or spraying. Most organophosphates are also effective against other external parasites than ticks (lice, mites, flies, etc.). Resistance of cattle ticks to organophosphates was well established before they were replaced and it must be assumed that it remains present in many regions.
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Horn flies Haematobia irritan irritans and Buffalo flies Haematobia irritans exigua in Cattle
- OCCURRENCE. Resistance of these flies to synthetic pyrethroids was reported already in 1982 in the US (horn flies) and Australia (buffalo flies). Nowadays it has spread to most Latin American countries. Resistance can be extremely high (Resistance factor>1000). In Europe and Asia it does not seem to have become a serious problem so far.
- OUTLOOK. Problems will worsen everywhere in the Americas and Australia. It will continue to spread and to strengthen, because synthetic pyrethroids are still being used in mixtures with other chemical classes (e.g. organophosphates, amitraz). Elsewhere the outlook may be optimistic: intensive cattle production, less favorable for horn fly development in the cowpads, is predominant in Europe and Asia.
- RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. Where alternative compounds of other chemical classes are still working against these flies, rotation is usually a good option, i.e. to stop using synthetic pyrethroids and to use other products with active ingredients of those still effective chemical classes during several years. Rotation is also highly recommended to delay resistance development where it has not yet become a problem.
- ALTERNATIVE PARASITICIDES for ROTATION.
- Chlorfenapyr. Ear-tags impregnated with this pyrrole derivative are available as ear-tags in a few countries for the control of horn flies with similar efficacy as other insecticidal ear-tags.
- Fipronil. Fipronil is approved for the control of horn flies in some countries (e.g. Latin America) but in others not (e.g. Australia, USA). It is available only as a pour-on. Usually it has a very long withholding period of at least 12 weeks. So far, horn fly resistance to fipronil has not become a problem in Latin America, although one case of reduced susceptibility to fipronil was reported in Uruguay already in 2013.
- Imidacloprid. This neonicotinoid is available for horn fly control as a pour-on in a few countries (mainly in Latin America).
- Insect Growth Regulators (e.g. diflubenzuron, methoprene). These chemicals do not control adult flies on cattle but stop larval development in the cowpads. Thus they are useful for long-term population control and not for protecting cattle from adult flies. They are available only in a few countries, mainly for oral administration in slow-release devices or in feed-through additives. So far no
- Macrocyclic lactones (abamectin, doramectin, eprinomectin, ivermectin, moxidectin). Pour-ons with these compounds are widely used in numerous countries to control horn flies and are quite effective. A few ear-tags have also been introduced.
- Organophosphates (e.g. diazinon, ethion, chlorpyrifos, coumaphos etc.) mostly in insecticidal ear-tags or pour-ons, or for spraying, often in mixtures with other active ingredients. Resistance of horn flies to organophosphates was already reported in the 1960s. However, nowadays high susceptibility of horn flies to diazinon has been reported in several horn fly populations that were highly resistant to synthetic pyrethroids.
- Tolfenpyrad. Ear-tags impregnated with this pyrazole derivative are available as ear-tags in a few countries for the control of horn flies, with similar efficacy as other insecticidal ear-tags.
- Chlorfenapyr. Ear-tags impregnated with this pyrrole derivative are available as ear-tags in a few countries for the control of horn flies with similar efficacy as other insecticidal ear-tags.
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Sheep body lice: Damalinia (=Bovicola) ovis in Sheep
- OCCURRENCE. Resistance of sheep body lice to synthetic pyrethroids was reported in Australia in the mid 1980s, and in New Zealand in the 1990s. Resistance is now very frequent and strong in Australia, less in New Zealand. Interestingly, no such resistance has been reported in the Americas. In the early 2000s in laboratory tests on lice collected from UK farms showed resistance to deltamethrin. However, to our knowledge a subsequent confirmation of field resistance has not been published so far and it seems not to be an issues nowadays, neither in the UK nor elsewhere in Europe.
- OUTLOOK. Problems will remain in Australia and New Zealand. Elsewhere it is likely to appear because synthetic pyrethroids are still the most used class of chemicals against sheep body lice.
- RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. Where alternative compounds of other chemical classes are still working against these flies, rotation is usually a good option, i.e. to stop using synthetic pyrethroids and to use other products with active ingredients of those still effective chemical classes during several years. Rotation is also highly recommended to delay resistance development where it has not yet become a problem.
- ALTERNATIVE PARASITICIDES for ROTATION.
- Insect Growth Regulators (e.g. diflubenzuron, triflumuron). These chemicals were introduced in Australia and New Zealand in the 1990s after lice resistance to synthetic pyrethroids became a serious problem. However, resistance to rhese compounds developed rather quickly too, particularly in Australia. Currently these chemicals are not available for sheep body lice control in Europe and the Americas.
- Macrocyclic lactones (mainly abamectin, ivermectin, moxidectin). Pour-ons as well as formulations for spraying and jetting against the sheep body louse are available in Australia and New Zealand. Elsewhere they are rather scarce. Injectable or oral formulations of macrocyclic lactones do not control sheep body lice.
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- Neonicotinoids: (imidacloprid, thiacloprid). These compounds are approved for the control of sheep body lice in Australia and New Zealand, mainly in pour-ons, often in mixtures. So far they are not approved for this usage in Europe or in the Americas.
- Organophosphates (e.g. diazinon, ethion, chlorpyrifos, temephos, etc.). These compounds were widely used in the last century, mainly for dipping and spraying sheep. However, they were mostly replaced by less toxic synthetic pyrethroids and other newer lousicides (e.g. diflubenzuron, macrocyclic lactones, spinosad, etc.) in Australia, New Zealand and elsewhere, and availability today is strongly reduced.
- Spinosad. These compound is available for the control of sheep body lice in Australia and New Zealand, mainly in pour-ons and formulations for spraying and jetting. So far they are not approved for this usage in Europe or in the Americas.
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Red fowl mites: Dermanyssus gallinae in poultry
- OCCURRENCE. Resistance of Dermanyssus gallinae to synthetic pyrethroids is a serious problem in layer operations worldwide, including Europe and Latin America. Because rather few alternative chemical classes are approved in many countries, increased illegal use (e.g. of amitraz, fipronil, ivermectin) has been reported in the last years, e.g. in Europe.
- OUTLOOK. Problems will worsen everywhere and will continue to spread and to strengthen, because synthetic pyrethroids continue to be used everywhere against these mites.
- RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. A non-chemical alternative that may be feasible in some layer farms is heating layer hen houses between consecutive laying cycles. Heating at 45°C for 48 hours killed all adults, nymphs and eggs at different locations in such laying houses, including under the floor and in the manure. Where alternative compounds of other chemical classes are still working against these flies, rotation is usually a good option, i.e. to stop using synthetic pyrethroids and to use other products with active ingredients of those still effective chemical classes during several years. Rotation is also highly recommended to delay resistance development where it has not yet become a problem.
- ALTERNATIVE PARASITICIDES for ROTATION.
- Carbamates (e.g. carbaryl, propoxur). Introduced in the 1960s. these compounds show cross-resistance with organophosphates. And many products have also been banned for safety reasons, particularly in the EU. They are mostly available for for spraying and dusting.
- Fluralaner. This compound belongs to the isoxazolines and was the first ot its class to be introduced for use on poultry against Dermanyssus gallinae in the late 2010s. It is administered as an additive for drinking water.
- Organophosphates (e.g. phoxim, diazinon, chlorpyrifos, fenitrothion, etc.). Introduced already in the 1950s-1960s, resistance to these compounds has been also reported in several countries and their availability has been strongly reduced due to legal bans for safety reasons, particularly in the EU. These compounds were mostly used for spraying the facilities. However, resistance of Dermanyssus gallinae to organophosphates is also widespread, and cases of multiresistance have also been reported.
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- Spinosad. This compound has been introduced in the 2010s for Dermanyssus gallinae control in Europe and elsewhere (only for premise treatment), and for Ornithonyssus sylviarum control in the USA (also for directly treating the animals). It is available for spraying.
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Houseflies: Musca domestica, in LIVESTOCK OPERATIONS
- OCCURRENCE. Musca domestica is the world champion in resistance development to insecticides. Resistant populations have been reported virtually everywhere, and multiresistance is often the rule. Houseflies are not veterinary parasites but they are vectors of several diseases and an a source of bacterial contaminations. They are a particular problem in poultry, pig and dairy farms. Synthetic pyrethroids are mostly used for spraying the facilities where houseflies are a problem.
- OUTLOOK. Problems will remain and will continue to spread and to strengthen, because synthetic pyrethroids continue to be used everywhere against houseflies.
- RECOMMENDED MEASURES. The most reasonable measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. A recommended strategy is to alternate the use of adulticides with larvicides with different modes of action, i.e. rotation with chemical classes that still control the flies. Rotation is also highly recommended to delay resistance development where it has not yet become a problem.
- ALTERNATIVE INSECTICIDES or LARVICIDES for ROTATION.
- Carbamates (e.g. methomyl, propoxur). These compounds were introduced in the 1960s and show cross-resistance with organophosphates. And many products have also been banned for safety reasons, particularly in the EU. Were available, they are mostly for off-animal use in the form of baits.
- Insect Growth Regulators (e.g. cyromazine, diflubenzuron, triflumuron). These chemicals were introduced already in the late 1970s as larvicides to kill housefly larvae in manure and organic waste. They are available mainly in feedthrough additives for poultry (mainly cyromazine) and for off-animal use to directly treat manure and other organic waste where houseflies can develop. They are often used in rotation programs with adulticides. Resistance to cyromazine was reported already in the late 1990s. Benzoylphenyl ureas (e.g. diflubenzuron, triflumuron) show cross-resistance with organophosphates, and housefly field populations resistant to IGRs have been also reported.
- Neonicotinoids: (imidacloprid, thiamethoxam). Neonicotinoids were introduced for housefly control in the early 2000s, mainly as baits. Resistant populations were reported already in the late 2000s.
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- Organophosphates (e.g. phoxim, diazinon, chlorpyrifos, fenitrothion, etc.). These compounds were introduced in the 1950s and show cross-resistance with carbamates. Resistance of houseflies to these compounds was reported already in the 1950s and it must be assumed that it still persists worldwide. However, usage of organophosphates for treating manure, organic waste or livestock facilities against houseflies has strongly diminished because these products have been vastly replaced by newer and less toxic compounds with other modes of action (e.g. Insect Growth Regulators, neonicotinoids, etc.).
- Spinosad. This compound was introduced in the 2000s for housefly control, mainly in baits. Resistance in field populations was already reported in the early 2010s.
Where available, follow national or regional recommendations for delaying resistance development or for handling already confirmed cases.
Use of synergists . Quite frequently, products with synthetic pyrethroids contain also synergists (mainly PBO) that are supposed to neutralize resistance. However, PBO works only against the so-called metabolic resistance (enhanced detoxification) caused by mixed function oxidases (= MFO), which is one among several mechanisms by which parasites can become resistant to synthetic pyrethroids, organophosphates and pesticides of other chemical classes. PBO specifically inhibits the activity of MFOs. If metabolic resistance is caused by other enzymes than MFOs, or if resistance is (also) due to other mechanisms such as target site insensitivity, reduced penetration or behavioral modifications, it won't be neutralized by PBO. In the vast majority of cases, producers affected by parasites resistant to synthetic pyrethroids or organophosphates do not know which mechanism makes the parasites resistant, and it is mostly not possible to find it out. Consequently, whether the synergist PBO helps to overcome resistance or not is in fact a lottery.
To evaluate resistance problems it must also be considered that innovation in the field of livestock parasiticides has strongly decreased in the last decades.
- The last "new" chemical class of nematicides for cattle and horses (macrocyclic lactones) was introduced in the 1980s, for sheep and goats in the early 2000s (monepantel, derquantel).
- The last "new" tickicide for cattle (fluazuron belonging to the benzoylphenyl ureas) was introduced in the 1990s.
- The last "new" ectoparasiticides for sheep (dicyclanil, spinosad) were introduced in the 1990s.
- The last "new" flukicide for cattle and sheep (triclabendazole) was introduced in the 1970s.
This means that the likelihood that new chemical classes with new modes of action against resistant parasites become available is quite slim. The reason is that, in the last decades, almost all animal health companies have focused their R&D investments in the much more profitable business of pet parasiticides. As a consequence, regarding resistance management in livestock and horses, almost nothing really new (i.e. with a new mode of action) has been introduced in the last decades: all new products (mostly new formulations or mixtures) have been basically "more of the same".
If you want to learn more about resistance, read one of the following articles in this site:
- Resistance Basics: what is resistance, types of resistance, etc.
- Resistance Development: how does resistance develop and what drives it.
- Resistance Diagnosis: how to find out whether a product failure is due to resistance or not.
- Resistance Prevention and Management: how to prevent, delay or manage resistance.
- Integrated Pest Management (IPM): A global approach to parasite control without relying only on chemicals.
Cick here to get to the section on RESISTANCE in this site.