The most critical cases of veterinary parasites resistant to amitraz 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 tick species resistant to amitraz, 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 species can be mentioned:
- Multi-host ticks: Cayenne ticks Amblyomma cayennense in CATTLE in Brazil and Mexico. Resistance problems LIKELY TO INCREASE in tropical and subtropical Latin America.
After its introduction in the 1970s, amitraz was used moderately where tick resistance to organophosphates and earlier tickicides was a problem. It was vastly replaced by synthetic pyrethroids in the 1980s-1990s, because synthetic pyrethroids are better tolerated by livestock, have a broader spectrum of activity, and are easier to handle. However, as resistance to synthetic pyrethroids soared at the end of the last century, a come-back of amitraz took progressively place, starting in the 2000s.
Amitraz is effective against all kinds of ticks and many mite species, but not against insects (flies, mosquitoes, fleas, etc.). Amitraz has no efficacy whatsoever against internal parasites (roundworms, tapeworms, flukes).
Nowadays amitraz is only available for external use, mostly for dipping or spraying cattle. There are dozens of generic products available in most countries, often in mixtures with active ingredients of other chemical classes. Amitraz is also used in some spot-ons for dogs to improve tickicidal efficacy, and in some low-cost baths, soaps, sprays and the like. It is not used in horses and cats because these animals do not tolerate amitraz.
Multiresistant populations of the cattle tick Rhipicephalus (Boophilus) microplus simultaneously resistant to amitraz and to actives of other chemical classes (e.g. fipronil, macrocyclic lactones, synthetic pyrethroids, organophosphates) have been reported in several countries (e.g. Brazil, Mexico).
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.
Parasites with resistance to amitraz
Cattle ticks (one host): Rhipicephalus (Boophilus) spp.
- OCCURRENCE. Resistance of R. microplus and its Australian relative R. australis to amitraz has been reported in numerous countries, e.g. Argentina, Australia, Brazil, Colombia, Ecuador, Mexico, India, New Caledonia, Uganda, Uruguay, Venezuela, Zimbabwe, etc. Basically it must be assumed that resistance of one-host ticks to amitraz is already widespread and well established everywhere in tropical and subtropical countries in the Americas, Asia and Australia. Resistance of R. decoloratus in Southern Africa is comparable.
- OUTLOOK. Problems will worsen everywhere. Resistance will continue to spread and to strengthen, because amitraz is still being used, often in mixtures with other chemical classes (e.g. organophosphates, synthetic pyrethroids).
- 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 another 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.
- 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 America.
- 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.
- Synthetic pyrethroids (e.g. cypermethrin, deltamethrin, permethrin, etc.). Synthetic pyrethroids are only available for topical use (dipping, spraying, pour-on). They control numerous external parasites (ticks, mites, lice, flies, etc.). Following their massive use worldwide from the 1980s onwards, resistance of cattle ticks and other external parasites to synthetic pyrethroids is strongly established worldwide and is often extremely high, making them completely useless.
Where available, follow national or regional recommendations for delaying resistance development or for handling already confirmed cases.
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.