Parasite Control Strategies: A Review of the Evidence

Strategies for parasite control in horses are undergoing major changes these years. Traditional approaches are abandoned and more surveillance-based strategies are being implemented instead. These changes are largely driven by scientific evidence documenting the widespread occurrence of anthelmintic resistance in equine parasites across the world. The pharmaceutical industry has not introduced new anthelmintic drug classes with new modes of action for treatment of equine parasites since ivermectin in the early 1980s, and it remains unknown when such new formulations will reach the equine market. The following section outlines the basic premises of traditional deworming programmes applied in horse populations and presents the evidence behind recommended surveillance-based programmes.

Interval-dose programme

The interval-dose programme was introduced by Drudge and Lyons in 1966. At that time, the first modern paste-based anthelmintics had become available making it feasible to perform treatments at regular intervals. Drudge and Lyons identified Strongylus vulgaris to be the major target of their programme, but devised their recommendations to also provide control over small strongyle parasites. In essence, their recommendation was to apply anthelmintic treatments to the entire herd every other month yearround (Drudge and Lyons 1966). Questionnaire surveys performed over the past decades have illustrated that the interval-dose approach is widely used (Relf et al. 2012), and this high treatment intensity is believed to be the main reason for the current levels of anthelmintic resistance observed in equine parasites. The advent of new anthelmintic drug classes led parasitologists to recommend rotating between these to counteract development of resistance. Despite the intuitive logic behind this approach, available evidence from the sheep industry suggests that drug rotation does not reduce or delay the development of anthelmintic resistance (Barnes et al. 1995). Instead, work performed in the New Zealand sheep industry has suggested a benefit from combining 2 nematodicidal anthelmintics in the same formulation. If no resistance has developed to any of the 2 drugs, resistance development can be delayed significantly by combining drug classes (Leathwick 2012). As a result, combination anthelmintic products are now promoted and marketed for ruminants in New Zealand and Australia. The long-term consequences of this approach have yet to be evaluated, and it remains unknown if combination dewormers will be applied for equine parasite control. One possible issue of concern is the recent association of anthelmintic resistance with P-glycoprotein (PgP) transporters, which are independent of drug class and could potentially mediate multidrug resistance (Beech et al. 2011).[...]