Fipronil was introduced in the 1980s for use on dogs and cats against fleas and ticks, and in the 1990s for tick and horn fly control on cattle in Latin America, but not in Australia, the EU or the USA. Nowadays tick resistance to fipronil has been confirmed in several countries.

The most critical cases of veterinary parasites resistant to fipronil are:

See below for DETAILS.

Rumours of resistance of the cat flea (Ctenocephalides felis) to fipronil have been circulating for years, based on increasing product failure reports. However, several studies on this issue have not been able to demonstrate fipronil resistance so far. Instead they conclude that incorrect use is likely to be the main cause of most product failures.

Fipronil is a broad-spectrum phenylpyrazole insecticide and acaricide effective against numerous insects and ticks of veterinary importance. After its introduction in the 1980s, fipronil has been used massively in dogs and cats worldwide, as well as in cattle in Latin America. It is also used massively in agriculture. Fipronil has no efficacy whatsoever against internal parasites (roundworms, tapeworms, flukes).

Fipronil is only available for external use, mostly in spot-ons for dogs and cats, but also in low-cost baths, soaps, sprays and the like, often combined with other active ingredients. For cattle it is used mainly in pour-ons. There are hundreds if not thousands of generic brands available.

Multiresistant populations of the cattle tick Rhipicephalus (Boophilus) microplus simultaneously resistant to fipronil and to actives of other chemical classes (e.g. amitraz, fluazuron, macrocyclic lactones, synthetic pyrethroids, organophosphates) have been reported in several countries (e.g. Brazil, Mexico).

Another active ingredient of the same chemical class as fipronil is pyriprole, moderatly used against fleas and ticks in dogs in some countries, but not in cattle.

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. It has been shown that fipronil shows cross-resistance with lindane, an organochlorine pesticide that was abundantly used up to the 1980s but was vastly abandoned afterwards, replaced by less toxic compounds.

Parasites with resistance to fipronil

  • Cattle ticks (one host): Rhipicephalus (Boophilus) spp.

    • OCCURRENCE. Starting in the 2010s resistance of R. microplus to fipronil has been reported in several Latin American countries (e.g. Argentina, Brazil, Mexico, Uruguay) and in India.
    • OUTLOOK. Problems will worsen everywhere. Resistance will continue to spread and to strengthen, because fipronil is still being used massively against cattle ticks in many countries.
    • 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.
      • 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.
      • Fluazuron. Fluazuron is available for cattle tick control in most countries where these ticks are 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 cattle tick resistance 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.
      • 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.

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:

Cick here to get to the section on RESISTANCE in this site.