Organophosphates were introduced in the 1950s-1960s and were massively used as veterinary parasiticides in the 1960s-1990s. Many external parasite species developed resistance to this chemical class in those years: ticks, flies, fleas, lice, mites, etc.

Despite having been vastly replaced in most countries, resistance of many veterinary parasites to organophosphates persists nowadays almost everywhere and may still increase wherever organophosphates (or carbamates) remain in use.

Multiresistance is 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.

Usually, resistance to organophosphates it is moderately strong (Resistance Factors ~100). Organophosphates show cross-resistance with carbamates.

  • RECOMMENDED MEASURES. The most reasonable measure to handle organophosphate resistance in most cases 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 available, rotation may be a good option, i.e. to stop using organophosphates 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.
  • OUTLOOK. Organophosphate resistance will not disappear, even after decades without using them. And it will increase wherever they remain in use, alone or in mixtures.

The most critical species resistant to organophosphates are:

  • One-host Cattle ticks: Rhipicephalus (Boophilus) spp in CATTLE, worldwide in tropical and subtropical regions. Resistance of Rhipicephalus (Boophilus) microplus to organophosphates was already reported in the 1960s in Australia. In the 1980s resistance of R. microplus and related one-host tick species was common and widespread in most countries where these ticks are a problem. However, organophosphates were almost completely replaced by amitraz and synthetic pyrethroids starting in the 1980s, often before resistance to organophosphates had become too strong. After resistance to synthetic pyrethroids soared in the late 1990s, a certain comeback of organophosphates has taken place, and nowadays they are moderately used again in some countries, often in mixtures with other chemicals, mainly in concentrates for dipping or spraying and in ready-to-use pour-ons.
  • Blowflies (Lucilia cuprina) in SHEEP in AUSTRALIA and NEW ZEALAND. Resistance or tolerance of L. cuprina to organophosphates was reported in Australia, New Zealand and South Africa already in the late 1960s. Starting in the 1980s organophosphates were progressively replaced by newer less toxic active ingredients with different modes of action (cyromazine, dicyclanil, macrocyclic lactones, spinosad, etc.). Replacement was accelerated by progressive withdrawals of organophosphate products by the manufacturers and by increasing regulatory constraints to their use for safety reasons. Nowadays, resistance to OPs is most likely to remain present in L. cuprina populations where it happened before, but it is not considered an issue any more because enough alternatives are available for flystrike control.
  • Red fowl mites: Dermanyssus gallinae in POULTRY, worldwide. Resistance of Dermanyssus gallinae to organophosphates is also widespread worldwide, and cases of multiresistance have also been reported. However, their availability for mite control in poultry has been strongly reduced due to legal bans for safety reasons, particularly in the EU. These compounds were or are still used mainly for spraying the facilities. 
  • Houseflies: Musca domestica, in LIVESTOCK OPERATIONS, worldwide. Resistance of houseflies to these compounds was reported already in the 1950s and 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.).

There are numerous reports on many other external parasites resistant to organophosphates, 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:

Up to the 1990s organophosphates were used massively worldwide in livestock, horses and dogs against many external parasites such as ticks, flies, fleas, lice, mites. They were progressively replaced by less toxic synthetic pyrethroids during the 1990s. And after macrocyclic lactones became easily available, many organophosphate uses in livestock and pets were vastly abandoned, discontinued by the manufacturers or banned by regulatory authorities for safety reasons.

Nevertheless, some organophosphates (e.g. chlorpyrifos, coumaphos, diazinon, dichlorvos, ethion, naphtalophos, temephos, trichlorfon) are still used successfully in several countries against some veterinary parasites, often in mixtures with other compounds. The reasons are that they are still quite effective in some cases, represent a cheap alternative, and safety seems not to be a major issue in the remaining applications. Dozens of generic products are still available.

Up to the 1970s, some organophosphates were also used as anthelmintics for the control of gastrointestinal roundworms in livestock. They were quickly replaced by newer, more effective and less toxic anthelmintics. However, soaring resistance of these worms to newer anthelmintics has led to a certain revival of some less toxic organophosphates in oral drenches for livestock.

Organophosphates were used and partially are still available mainly in dips, sprays and dusts for livestock, farm facilities and households; in insecticide-impregnated ear-tags and collars; in housefly baits; in wound-dressings against fly maggots; in oral pastes or gels for horses, etc.

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 well known for organophosphates, that whereas some compounds completely failed due to resistance, other compounds did work quite well, at least for a certain period of time.

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:

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