Flumethrin is a synthetic pyrethroid introduced in the late 1970s. Whereas flumethrin was and remains the most effective tickicide and miticide among all synthetic pyrethroids, other compounds are better insecticides. For this reason it was and remains vastly used against ticks of livestock and pets, but has not been used against most insect pests of veterinary importance.
Nowadays, 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 flumethrin are:
- One-host Cattle ticks: Rhipicephalus (Boophilus) spp in CATTLE, worldwide in tropical and subtropical regions.
See below for DETAILS.
There are reports on 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:
- 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.
- Mites: Sheep mange mites Psoroptes ovis in SHEEP in the UK.
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.
Flumethrin is certainly the best tickicide among synthetic pyrethroids and is also excellent against mites. But is is less efective against insects (flies, lice, fleas, etc.) that onter synthetic pyrethroids. (e.g. cypermethrin, deltamethrin, and permethrin). It was and is still used a lot in cattle and sheep in Latin America and other regions, but it was not used in livestock in the USA. There are still dozens of generic products with flumethrin available worldwide.
Multiresistance is already common and frequent in some species, particularly in the cattle tick Rhipicephalus (Boophilus) microplus.
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. However it is known that some cattle tick populations in Australia that were resistant to flumethrin were still susceptible to other synthetic pyrethroids, and vice versa.
Parasites with resistance to flumethrin
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.
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.