Anthelmintic resistance in equine helminth parasites – a growing issue for horse owners and veterinarians in New Zealand?

Abstract

There is growing concern that given the high frequency with which anthelmintics are being administered to many horses, anthelmintic resistance amongst equine helminth populations will be an increasing problem, rendering many of the currently available products unusable with little prospect of new products becoming available, at least in the near future. Worldwide, much reliance has been placed on the macrocyclic lactone (ML) group of anthelmintics, but resistance has been reported to these products as well as to the two other major anthelmintic classes used in this species, the benzimidazoles (BZ) and the tetrahydropyrimidines (e.g. pyrantel). In New Zealand, resistance has been reported to the ML and BZ groups, but not yet to pyrantel. As an alternative to interval-based anthelmintic regimens, the highly overdispersed nature of parasite populations in horses can be utilised to decide whether treatment is required, based on whether or not animals exceed a predetermined level of shedding of parasite eggs. If well managed, such a targeted and selective approach can be utilised to eliminate the majority of egg output whilst still providing a refuge for susceptible parasites to persist. Such a system would require that an adequate standard of monitoring be in place and cognisance needs to be taken of parasites or their lifecycle stages that cannot be diagnosed by routine methods. At the same time, using anthelmintics with high levels of efficacy, avoiding practices such as under-dosing, as well as utilising non-chemical means of parasite control when possible, e.g. regular removal of faeces from pasture, should all be considered. Combinations of anthelmintics, specifically of anthelmintics that target the same or a similar spectrum of parasite species, should play an important role in parasite control in horses. As well as providing arguably the highest levels of efficacy, combinations may also slow the rate at which anthelmintic resistance develops.

Cross‐sectional survey of parasite control practices on Thoroughbred and Standardbred training yards in New Zealand

“…Half of both Thoroughbred and Standardbred trainers were using an interval regimen for spellers and racehorses. The reliance on the interval strategy is consistent with previous studies of Thoroughbreds and Standardbred stud farms in New Zealand . Similarly, the median number of treatments horses received annually was comparable with the number of treatments on stud farms, with racehorses treated as frequently as yearlings at up to 15 times annually .…”

Questionnaire study on parasite control practices on Thoroughbred and Standardbred breeding farms in New Zealand

“…It has been accepted that resistance of the cyathostomes to the benzimidazole anthelmintics is widespread in New Zealand (Bishop et al, 2013;Scott et al, 2015). Furthermore, a recent study on three Thoroughbred stud farms in New Zealand indicated that resistance of Parascaris equorum to ivermectin has developed (Bishop et al, 2013).…”

Especies de Ciatostomas Resistentes al Albendazol en Equinos, Cuba

“…Actualmente la resistencia a los BZs resulta la más prevalente en los ciatostomas (Traversa et al, 2012), reportándose en al menos 14 países (Peregrine et al, 2014). Por su parte, la periodicidad de los tratamientos antihelmínticos en el Rancho Turiguanó imprime una alta presión de selección sobre la población parasitaria (Scott et al, 2015).…”

“…Esta práctica y el empleo inadecuado de drogas antiparasitarias condujo a la aparición de nematodos resistentes en ganaderías equinas en todo el mundo (Kaplan y Nielsen, 2010; Scott et al, 2015). La gran mayoría de los helmintos gastrointestinales de los equinos están representados por los miembros de la familia Strongylidae, con al menos 63 especies (Lichtenfels et al, 2008), de las cuales se reportan 24 en Cuba (Pérez-Vigueras, 1936; Varus y Valle, 1966).…”