Triclabendazole was introduced in the 1970s as a specific flukicide. Since then it has become the most used flukicide for livestock.

Liver fluke resistance to triclabendazole in sheep was already reported in the mid 1990s in Australia. In the following years liver fluke resistance was also reported in goats and cattle in numerous countries. 

Triclabendazole is a narrow-spectrum anthelmintic belonging to the chemical class of the benzimidazoles. It is effective against several fluke species, including liver flukes (Fasciola hepatica), the tropical liver fluke (Fasciola gigantica) and the giant liver fluke (Fascioloides magna). Triclabendazole is effective against adults and all immature stages (from 1 week of age) of liver flukes. Albendazole, the only other flukicidal benzimidazole is only effective against adults. Triclabendazole is ineffective against any gastrointestinal roundworms. 

Triclabendazole is only used in ruminants: flukes are seldom an issue in pets, horses or pigs.

Triclabendazole is available mainly for oral administration: mostly in the form of drenches, often in combination with a nematicide that adds efficacy against gastrointestinal roundworms. A few oral tablets and pour-ons are also available. There are dozens of generic brans available.

Other important benzimidazoles are: albendazole febantel, fenbendazole, flubendazole, mebendazole, oxfendazole, oxibendazole, and thiabendazole. 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 triclabendazole

• Liver flukes (Fasciola hepatica) in catte, sheep and goats

  • OCCURRENCE. Resistance of liver flukes to benzimidazoles (only albendazole and triclabendazole) was first reported in the mid 1990s in sheep in Australia. In the meantime resistant cases have been reported in many countries in sheep (e.g. Argentina, Brazil, Ireland, Netherlands, New Zealand, Peru, Spain, UK, etc.) and in cattle (e.g. Argentina, Australia, Chile, Netherlands, Peru, etc.). It is not as dramatic a problem as resistance of gastrointestinal roundworms, but it is a serious threat, particularly to sheep producers in humid regions where liver flukes are endemic: Fasciola hepatica is more harmful to sheep than to cattle.
    
  • OUTLOOK. Resistance will continue to spread and to strengthen, because little is done in most regions to reduce their use and to encourage non-chemical control and prevention.
  • RECOMMENDED MEASURES. The most recommended measure is to switch to Integrated Pest Management (IPM) and/or to implement whatever preventative measures that reduce the use of any chemicals. Reducing pasture humidity is crucial to prevent infestations with liver flukes because they need snails for their development. Where alternative chemicals of other chemical classes are still working against these worms, rotation is usually a good option, i.e. to stop using benzimidazoles and to use other products of still effective chemical classes during several years.
  • ALTERNATIVE PARASITICIDES for ROTATION.
    • Clorsulon. It is a narrow-spectrum anthelmintic effective against several fluke species, also against liver fluke There are a few reports on liver fluke resistance to clorsulon, but it is certainly not a big issue yet. Clorsulon is almost always used as an injectable in mixtures with  ivermectin.
    • Nitroxinil. It is a narrow-spectrum anthelmintic effective against liver flukes and a few gastrointestinal roundworms (e.g. Bunostomum spp, Haemonchus spp, Oesophagostomum spp. There are a few reports on liver fluke resistance to nitroxinil, but it is certainly not a big issue yet. Nitroxinil is usually administered as an injectable.
    • Salicylanilides (mainly closantel, but also rafoxanide and niclosamide:). Compared with albendazole, they are not effective against numerous gastrointestinal roundworms (narrow spectrum of activity). Salycilanilides are mostly available as oral drenches, often in combination with other anthelmintics. There are also reports on liver fluke populations in sheep resistant to rafoxanide and closantel, but it is certainly not a widespread problem.

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.