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:

• 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.
• 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:

• Fleas: mainly Ctenocephalides felis in DOGS. Resistance of the cat flea to organophosphates was already reported in the 1980s (e.g. in the USA). Starting in the 1990s, the introduction of new and highly effective active ingredients (fipronil, imidacloprid, lufenuron, etc.) without cross-resistance to organophosphates "solved the problem". Nowadays, resistance of the cat flea to insecticides is not an issue due to the plethora of commercial products with different active ingredients that show almost now cross-resistance among them. Organophosphates (e.g. diazinon) are still being used e.g. in insecticide-impregnated collars. Resistance to organophosphates is likely to remain to some extent in some flea populations, which could reduce the efficacy of such collars.
• Horn and Buffalo flies: Haematobia irritans in CATTLE. Resistance of these flies to organophosphates was already reported in the 1980s (mainly in the USA). However, after organophosphates were vastly replaced by synthetic pyrethroids during the 1980s, strong resistance of horn flies to synthetic pyrethroids developed very quickly. But it was found that some organophosphates were particularly effective against pyrethroid-resistant horn flies, especially diazinon. Nowadays, ear-tags impregnated with organophosphates (e.g. diazinon, coumaphos, chlorpyrifos) are abundantly used in many countries against these flies.
• Other flies: Stable flies Stomoxys calcitrans: resistance cases have been reported in several countries (e.g. the USA, Germany, France). Lesser housefly Fannia canicularis: cases have been reported in the USA.
• Mites: Northern fowl mites: Ornithonyssus sylviarum: cases have been reported in POULTRY in the USA; Sheep mange mites Psoroptes ovis: in SHEEP cases have been reported in the UK.
• Mosquitoes: Culex spp, Aedes spp, Anopheles spp, etc. in LIVESTOCK and PETS, worldwide. Resistance of mosquitoes to organophosphates is very frequent everywhere and can be rather strong. But their control is usually more a task for health authorities than for producers or pet owners. Mosquitoes are seldom a problem for the livestock industry. For pet owners they are more relevant because mosquitoes are vectors of various diseases (e.g. heartworms), but there is not a lot they can do in terms of chemical protection against mosquitoes.
• Multi-host ticks: Cayenne ticks Amblyomma cajennense: resistance cases have been reported in CATTLE in Mexico; Brown dog ticks Rhipicephalus sanguineus in DOGS: resistance cases have been reported in Panama and Brazil; Brown ear ticks Rhipicephalus appendiculatus in CATTLE: cases have been reported in Zambia. Hyalomma dromedarii cases have been reported in India.
• Soft ticks: Otobius megnini: cases have been reported in Sri Lanka; Ornithodoros savigny cases have been reported in Iran.
• Bed bugs: common bed bugs Cimex lectularius: cases have been reported e.g. in Denmark, Israel, Japan, Thailand, and the USA. Tropical bed bugs Cimex hemipterus: cases have been reported e.g. in Thailand and Sri Lanka.

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.

• 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.