Just as bacteria can become resistant to antimicrobials, so too can worms develop resistance to deworming drugs (anthelmintics). In fact, considerable anthelmintic resistance has already developed with most equine properties already have worm populations that have developed some degree of anthelmintic resistance. Experts predict multi-drug resistant worm populations are likely to develop soon, presenting a major threat to horse health and welfare unless we change our worm management practices. It is therefore, imperative that we collectively act now to prevent further resistance from developing. This is the reasoning behind the new CANTER initiative (Controlling ANTiparasitic resistance in Equines Responsibly), a pan-industry group formed in 2023 to provide collaborative expertise to address the problem of anti-parasitic resistance in equines. https://canterforhorses.org.uk/resistance/ .
To really understand the seriousness of the implications of anthelmintic resistance, first we need to take a look at what worming drugs are available for use in the horse. It might surprise you to know that there are in fact only five anthelmintics in just four drug groups licenced for use in horses in the UK. Table 1 lists which deworming drugs are available, the worms they can be used against, and the level of resistance in the UK. It is important to note that the development of resistance varies from country to country according to the drugs available and common management practices. For example, in the USA, where a daily in feed pyrantel anthelmintic was used for a number of years, widespread cyathostomin resistance to pyrantel has developed, whereas this is still efficacious against cyathostomin adults in the UK.
| Drug (class) and Example Products | Cyathostomins (small red worm) | Parascaris (round worm) | Anoplocephala (tapeworm) | Oxyuris equi (pin worm) |
| Ivermectin (avermectin) e.g. Eraquell®, Noromectin® | Yes (A, L4) (R) | R | No activity | R |
| Moxidectin (avermectin) Equest® | Yes (A, L3, L4) | Same class IVM; don’t use | No activity | Same class IVM; don’t use |
| Pyrantel (pyrimidine) Strongid P® | Yes (A) (R) | Yes | Double dose | Yes |
| Fenbendazole (benzimidazole) Pancur® | R | Yes | No activity | Yes |
| Praziquantel (isoquinoline) Pramox® | No activity | No activity | Yes | No |
The CANTER website contains a tool to help owners assess their horse’s parasite risk profile, which is influenced by several factors that can be remembered using the CANTER acronym: Clinical history, Age Profile, Number of horses, Test results, Environment to give an overall parasite Risk profile. https://canterforhorses.org.uk/horse-owners/. The initiative also intends to issue evidence-based guidance for prescribers of anthelmintics by the end of 2023 to promote consistency of advice to horse owners regarding worming.
Up to now, anthelmintic resistance has very much been focussed on cyathostomins (small red worm) and Parascaris (roundworm) where resistance is well established. However, recent worrying research from the University of Kentucky has raised concerns that the first signs of equine tapeworm resistance may be emerging to praziquantel and pyrantel, the only drugs licensed for tapeworm control in the UK. Tapeworms have a complex lifecycle, part of which is in a forage mite. Adult tapeworms attach near the junction of the small and large intestine and infection is associated with both spasmodic colic, intussusception (where part of the intestine telescopes inside itself) and ileal impaction, where a section of small intestine gets blocked by tapeworms. Whilst spasmodic colic can be recurrent and cause significant pain, the latter two types of colic are far more serious requiring surgery to correct.
At the heart of the CANTER initiative is encouraging the use of diagnostic tests to allow owners to determine if their horse actually requires worming. For example, testing of horses in the UK has shown that 23% are infected with tapeworm parasites so blanket use of tapeworm anthelmintics, rather than a test and treat approach, would mean that over 70% of horses would be treated unnecessarily. Cyathostomins and Parascaris infection can be determined by use of strategic faecal worm egg counts. Tapeworm eggs are shed very intermittently making faecal egg counts not useful, but a saliva testing kit, which detects antibodies to the tapeworm, has been validated for determining tapeworm infection and is widely commercially available.
The frequency with which these tests are required depends on the risk profile of your horse (see above). The lower the risk factors and the better the environmental management, the less frequently diagnostic testing is required. The less anthelmintic drugs used unnecessarily, the lower the risk of anthelmintic resistance increasing.
