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. 

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

  • 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.

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:

Cick here to get to the section on RESISTANCE in this site.