Synthetic pyrethroids were introduced in the 1970s and 1980s. 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 synthetic pyrethroids are:

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:

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.

For livestock and horses they are available mainly in concentrates for dipping or spraying, in pour-ons and impregnated ear-tags.

For dogs they are 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 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.

The active ingredients of this chemical class most used against veterinary parasites are: cypermethrin, deltamethrin, flumethrin and permethrin.


Parasites with resistance to synthetic pyrethroids

  • 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 alone or 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, eprinomectinivermectin, 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.
  • Horn flies Haematobia irritans 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.
  • 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.
  • 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.
    • OUTLOOKProblems 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.
  • 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.
    • OUTLOOKProblems 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.
      • Organophosphates (e.g. azamethiphos, diazinon, chlorpyrifos, 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.

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:

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