ANTHELMINTIC DRUGS HAVE BEEN USED EXTENSIVELY in the horse over the past 40 years.
During this time their widespread use has led to the development of anthelmintic resistance, particularly in cyathostomin populations.
With no new anthelmintic compounds on the horizon, the responsible and strategic use of those compounds currently available is imperative to maintain their effectiveness.
There are three groups of anthelmintic drugs available to treat endoparasites in the horse: benzimidazoles, such as fenbendazole, tetrahydropyrimidines, such as pyrantel, and the macrocyclic lactones, which include moxidectin and ivermectin.
Resistance occurs when parasites develop adaptations that evade the action of that particular drug. When these adaptations confer an evolutionary advantage, the prevalence of the resistant gene will increase. There are many factors that increase the selection pressure for resistant worm, such as treatment frequency, the proportion of the population exposed to the treatment, exposure to sub-lethal doses when drugs are under-dosed and husbandry practices.
Highest prevalence
While horses are exposed to several nematode species, the cyathostomins are by far the most prevalent (Relf et al, 2013). There are over 50 species of cyathostomins that infect the horse.
Third stage larvae (L3) are ingested from the pasture by the horse and these penetrate the wall of the large intestine. Within the wall of the large intestine the larvae develop further and emerge to form adult worms within the intestinal tract.
In some situations the L3 arrest their development or hypobiose, encysted within the wall of the intestine. The precise triggers for this are unknown, although climate, worm burden and immunity have all been suggested.
Re-emergence of encysted L3 results in extensive intestinal inflammation, which can be associated with subclinical disease or severe colitis, known as larval cyathostominosis – a condition with a mortality rate of up to 50% (Love et al, 1999).
Pyrantel and ivermectin are effective against adult cyathostomins and ivermectin is also effective against the non-encysted larval stages. Moxidectin and fenbendazole, however, are the only two anthelmintics with efficacy against the encysted larval stages of cyathostomins.
Benzimidazole resistance in cyathostomin populations is widespread in the UK, with studies demonstrating resistance in 80-100% of the population (Rossano et al, 2010). There is also growing evidence that moxidectin resistance is emerging. A recent publication documented an egg re-appearance period of five weeks following moxidectin administration (Daniels and Proudman, 2016), while a reduced faecal egg count reduction test has been observed in a UK donkey population (Trawford et al, 2005).
The incorporation of faecal egg counts into herd management programmes has led to the development of strategic worming regimes for horses. By identifying and targeting treatment at horses with significant adult worm burdens, the overall use of anthelmintics has been reduced. By allowing a significant portion of the worm population to remain unexposed to the drug, or in refugia, this has reduced the pressure for the development of resistance in these parasites. Unfortunately, the lack of a reliable diagnostic test to identify significant larval cyathostomin burdens currently limits such targeted strategies being applied to the management of cyathostomins in the horse.
Encysted L3 are a significant part of the parasites life-cycle and these would not be detected with a faecal egg count. An IgG(T) ELISA is currently under development (Dowdall et al, 2002) and if successful will be a great asset in detecting pre-patent infections and targeting anthelmintic treatment to those individuals at risk.
At present, blanket treating all animals with a larvicidal product during the winter months remains standard practice (Matthews, 2008).
Given the extensive prevalence of benzimidazole resistance, moxidectin is the only effective treatment option in many areas. This highlights the need to protect the use of moxidectin, as currently no alternative product is available for the treatment of encysted cyathostomins.
Cyathostomins pose a great threat to the equine population, with larval cyathostominosis a life-threatening condition. The current practice of routinely treating all horses with moxidectin during the winter months is less than ideal. We await a reliable diagnostic test to be able to tackle this further.
Throughout the rest of the year, strategic worming should be implemented, avoiding the use of moxidectin where alternative drug classes are available, to limit the selection pressure on nematode populations.
References
Daniels, S. P. and Proudman, C. J. (2016) Shortened egg reappearance after ivermectin or moxidectin in horses in the UK. Vet J 218: 36-39.
Dowdall, S. M., Matthews, J. B., Mair, T., Murphy, D., Love. S. and Proudman, C. J. (2002) Antigen-specific IgG(T) responses in natural and experimental cyathostominae infection in horses. Vet Parasitol 106: 225- 242.
Love, S., Murphy, D. and Mellor, D. (1999) Pathogenicity of cyathostome infection. Vet Parasitol 85: 113-121.
Matthews, J. B. (2008) An update on cyathostomins: anthelmintic resistance and worm control. Eq Vet Ed 20: 552-560.
Relf, V. E., Morgan, E. R., Hodgkinson, J. E. and Matthews, J. B. (2013). Helminth excretion with regard to age, gender and management practices on UK Thoroughbred stud farms. J Parasitol 44: 507-514.
Rossano, M. G., Smith, A. R. and Lyons, E. T. (2010) Shortened egg reappearance periods in naturally infected horses treated with moxidectin and failure of lavicidal dose of fenbendazole to reduce faecal egg counts. Vet Parasitol 173: 349-352.
Sangster, N. C. (2001) Managing parasiticide resistance. Vet Parasitol 98: 89-109.
Trawford, A. F., Burden, F. and Hodkinson, J. E. (2005) Suspected moxidectin resistance in cyathostomes in two donkey herds at the donkey sanctuary, UK. In: Proceedings of the 20th International Conference of the WAAVP. P196.
