Abamectin is a macrocyclic lactone that was introduced in the 1980s, shortly after ivermectin, the first macrocyclic lactone that came to market. Since then several veterinary parasites have developed resistance or tolerance to these molecules. The most relevant resistant species are:

See below for DETAILS.

There are reports on other parasites that have developed resistance to macrocyclic lactones, 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:

Multiresistance is spreading. In 2010 one sheep property was reported in Brazil where gastrointestinal roundworms were simultaneously resistant to 7 different chemical classes (levamisole, benzimidazoles, macrocyclic lactones, nitroxinil, disophenol, trichlorfon and closantel). In 2021 a study in cattle farms in Brazil reported multiresistance of gastrointestinal roundworms to 4 chemical classes of anthelmintics in 95% of the twenty farms investigated: macrocyclic lactones, levamisole, benzimidazoles, and closantel.

Abamectin is often considered as the "cheap" ivermectin. It is mostly used in ruminants, particularly in sheep, hardly in horses and pigs, and not at all in pets in most countries. Whereas abamectin is quite popular in Latin America (mainly as an anthelmintic), Australia and New Zealand, it is almost ignored in Europe and the USA. It is mostly available as an injectable, pour-on, or an oral drench. In some contries in Latin America it is also used as an oral paste or gel for horses. There are numerous generic brands available, often in combination with other active ingredients.

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 abamectin

  • Gastrointestinal roundworms in horses

    • Cyathostomins = small strongyles = small red worms a group of about 50 species of gastrointestinal roundworms that affect horses, donkeys and other equids worldwide.
    • Horse roundworm (Parascaris equorum).
    • OCCURRENCE. Tolerance of cyathostomins to macrocyclic lactones, manifested as a low but significant worm egg output after treatment (determined after fecal egg counts) is not yet widespread, but has been already reported in Europe (e.g. in the UK, Germany, Italy), the USA, and Brazil. Resistance of Parascaris equorum to macrocyclic lactones has also been reported in numerous countries. Cross-resistance among all macrocyclic lactones used in horses (mainly abamectin, doramectin, ivermectin, moxidectin) must be assumed.
    • OUTLOOK. Problems are likely to worsen everywhere. 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. Where alternative chemicals of other chemical classes are still working against these worms, rotation is usually a good option, i.e. to stop using macrocyclic lactones and to use other products of still effective chemical classes during several years. However, these worms have already developed resistance to some of the alternative chemical classes as well.
      • Benzimidazoles (e.g. febantel, fenbendazole, oxibendazole, mebendazole, etc.). Resistance of cyathostomins to benzimidazoles is already very frequent and high as well. In a survey from 2009, resistance to benzimidazoles was confirmed in >80% of the investigated yards in UK and Germany. It is known to be also quite frequent in the US, Australia, India and in many other countries, including Latin America.
      • Tetrahydropyrimidines (mainly pyrantel). They are not effective against some important gastrointestinal worm species (narrow spectrum of activity). Cases of resistance of cyathostomins to pyrantel have been reported in Europe and the USA, but so far it is much less frequent and severe than resistance to benzimidazoles. Cases of resistance of Parascaris spp to pyrantel have been reported in Sweden (2018). For the time being resistance of these horse parasites to pyrantel seems not to be an issue in most regions.
      • Piperazine derivatives. Piperazine is a narrow-spectrum anthelmintic used in pets and livestock effective against some gastrointestinal roundworms (particularly against ascarids, e.g. Parascaris equorum).
      • Other anthelmintics such as levamisole and closantel that may still control these worms are not available for horses in most countries.

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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