Pyrantel was introduced as a veterinary anthelmintic in the 1960s. Since then, cases of resistance have been reported in a few gastrointestinal roundworm species of dogs, horses and pigs. A few cases have been reported in ruminants, but mostly as cross-resistance with other chemical classes in multiresistant roundworm populations. 

• In DOGS
  •  Dog hookworms (Ancylostoma caninum).
• In HORSES
  • Cyathostomins = small strongyles = small red worms, a group of about 50 species of gastrointestinal roundworms
  • Horse roundworm (Parascaris equorum).

See below for DETAILS.

There are reports on other parasites that have developed resistance to pyrantel as well, 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 case can be mentioned:

• Pigs: Oesophagostomum dentatum. Resistance of O. dentatum to pyrantel was reported in Denmark as early as 1987. However pyrantel resistance in O. dentatum has not become a major problem elsewhere.

Altogether, resistance to pyrantel in horses and livestock is not a major issue so far. But resistance of dog hookworms to pyrantel is clearly an emerging issue in some countries. Cases of multi-resistance have also been reported.

Pyrantel belongs to the chemical class of the tetrahydropyrimidines (together with morantel and oxantel). They are narrow-spectrum nematicides, effective against some but not all roundworms in the digestive system, but not elsewhere in the body.

Pyrantel is the most used tetrahydropyrimidine. It is mostly used as a salt (citrate, embonate, pamoate, tartrate, etc.). Pyrantel is abundantly used in dogs, cats and horses, very scarcely in livestock. Oxantel is only used in pets. Morantel is only used in livestock and horses, but usage is marginal. Tetrahydropyrimidines are ineffective against tapeworms or flukes.

Pyrantel is available only for oral administration. In livestock it is used mainly in drenches and feed additives. For dogs and cats it is available mainly in tablets or pills. In horses it is mainly used in the form of oral pastes & gels. It is often used in combination with other compounds.

Parasites with resistance to pyrantel

• Dog hookworms (Ancylostoma caninum) in dogs

  • OCCURRENCE. Cases of resistance of A. caninum to pyrantel have been reported in New Zealand (1987), Australia (2007), and in the USA (2019). In the USA populations simultaneously resistant to pyrantel, macrocyclic lactones (milbemycin oxime) and benzimidazoles (fenbendazole) have also been reported.
  • OUTLOOK. Likely to strengthen and to appear elsewhere, because pyrantel is still widely used against these worms.
    
  • RECOMMENDED MEASURES. Compliance, i.e. strictly following the use recommendations are crucial, because incorrect use is the most frequent cause of product failure, and because chronic under-dosing may favor resistance development. Where alternative chemicals of other chemical classe are still working against these worms, rotation is usually a good option, i.e. to stop using pyrantel and to use other products with other effective chemical classes during several years.
  • ALTERNATIVE PARASITICIDES for ROTATION.
    • Emodepside. This active ingredient is available in some countries in the form of oral tablets for dogs combined with praziquantel.
    • Macrocyclic lactones (ivermectin, milbemycin oxime, moxidectin, selamectin). Resistance of A. caninum to macrocyclic lactones has already been reported in the USA (2019) and Australia (2019). Cross-resistance among all macrocyclic lactones must be assumed.
    • Benzimidazoles (e.g. febantel, fenbendazole, oxibendazole, mebendazole, etc.). Cases of resistance of A. caninum to benzimidazoles have already been reported in Brazil (2014), in the USA (2019) and Australia (2019). However, little is known yet about its incidence elesewhere.
    • Other anthelmintics such as levamisole that may control these worms are not available for dogs in most countries.

• 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. 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.
  • OUTLOOK. Uncertain. Since it has taken very long to appear (more than 50 years) spreading and strengthening may be relatively slow.
  • RECOMMENDED MEASURES. Compliance, i.e. strictly following the use recommendations are crucial, because incorrect use is the most frequent cause of product failure, and because chronic under-dosing may favor resistance development. Where alternative chemicals of other chemical classe are still working against these worms, rotation is usually a good option, i.e. to stop using pyrantel and to use other products with other effective chemical classes during several years.
  • ALTERNATIVE PARASITICIDES for ROTATION.
    
    • 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.
    • Macrocyclic lactones (abamectin, doramectin, eprinomectin, ivermectin and moxidectin). Tolerance of cyathostomins to macrocyclic lactones (e.g. ivermectin, moxidectin), 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 with all other macrocyclic lactones used in horses mainly (abamectin, doramectin, moxidectin) must be assumed.
    • 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.

• 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.