Closantel was introduced as an anthelmintic in the 1970s. Since then, a few resistance cases have been reported in several countries in sheep and cattle farms.

So far, the problem is much less widespread and severe than resistance of gastrointestinal roundworms to benzimidazoles, macrocyclic lactones or levamisole.

The most relevant resistant species are:

See below for DETAILS.

Closantel is a narrow-spectrum anthelmintic belonging to the chemical class of the salicylanilides. It is effective against some gastrointestinal roundworms (e.g. Bunostomum, Haemonchus, OesophagostomumOstertagia - Teladorsagia, Strongyloides, Trichostrongylus), liver flukes and some external parasites of cattle and sheep (e.g. sheep nasal bots, cattle grubs). It is not effective against tapeworms.

Closantel was broadly used in ruminants before more effective compounds came to market in the 1980s that vastly replaced it (benzimidazoles, macrocyclic lactones, etc.). In the last decade usage of closantel has increased considerably along with the exacerbation of resistance problems to other chemical classes in many countries. Nowadays usage is still growing.

Closantel is available mostly in the form of oral drenches, injectables and topical pour-ons (only for cattle), often in combination with other compounds. There are dozens of generic products available in numerous countries. It is not used in horses, pigs, dogs or cats.

Other veterinary salicylanilides are: niclosamide, oxyclozanide, and rafoxanide. They are much less used than closantel.

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 closantel

Cattle, sheep and goats:

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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