Trichlorfon was introduced in the early 1950s. It belongs to the chemical class of the organophosphates, which were massively used 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 trichlorfon are:

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.

Trichlorfon is an all-round ectoparasiticide, highly effective against most external parasites of livestock and dogs. Among the organophosphates it belongs to the moderately toxic ones.

Nowadays trichlorfon is still used significantly in oral pastes or gels for horses against bot flies (Gasterophilus intestinalisin). It is also used in a few dips, sprays, and wound-dressings for cattle and other livestock, as well as in a few low-cost aerosols, baths, dusts, or soaps for dogs, mainly in Latin America. It is very often used in mixtures with other chemicals. It is also used in some Latin American countries as an anthelmintic oral drench for the control of gastrointestinal roundworms in livestock, hoping to overcome multiresistance of these worms to the usual anthelmintics (benzimidazoles, macrocyclic lactones, levamisole, etc.).

Other organophosphates (e.g. chlorpyrifos, coumaphos, diazinon, dichlorvos, ethion, naphtalophos, temephos) 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.

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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