Ivermectin performance against equine strongylids: Efficacy, egg reappearance periods, and fecal egg counting method comparison

Equine strongylids are ubiquitous and can cause severe health issues. Anthelmintic resistance is widely common in cyathostomin parasites, and recent studies have documented increasing incidence of resistance to the macrocyclic lactone drug class. Several European countries have implemented prescription-only restrictions of anthelmintic usage by law to reduce anthelmintic treatment intensity and decrease the selection pressure for drug resistance. However, long term outcomes of this approach have not been thoroughly evaluated. The aim was to determine ivermectin efficacy in strongylids by means of faecal egg count reduction test (FECRT) and egg reappearance period (ERP) in Danish horses, compare two fecal egg count techniques, and determine the prevalence of Strongylus vulgaris in the study population. Herds and horses were selected by four collaborating veterinary practices. A total of 299 horses from 30 herds were enrolled in the study. Fecal egg counts (FEC) were determined using a concentration McMaster technique as well as an artificial intelligence-based automated fecal egg counting system. All horses with FEC > 0 eggs per gram of feces (EPG) were treated with ivermectin. Ivermectin efficacy and ERP were determined following current guidelines. Coproculture and PCR were employed for detection of S. vulgaris. FECRTs based on McMaster egg counts suggested inconclusive ivermectin efficacy in two equine operations and full efficacy in all remaining populations, whereas the automated system suggested ivermectin resistance in 6 operations and inconclusive results in another 8. Ivermectin ERP was determined to be at least 8 weeks with both methods in all cases. The prevalence of S. vulgaris was 2.7 % and 5.7 % with coproculture and PCR, respectively, and all samples were negative with both methods at 8 and 24 weeks post treatment. In general, ivermectin efficacy was high, although some results suggest that reduced efficacy could be occurring, which warrants further monitoring going forward. The ERP estimates all exceeded 8 weeks, which suggests that no reduction has occurred. The two egg counting techniques were in general agreement, but the automated system detected more positives at low egg count levels, which led to lower efficacy estimates in some populations. Strongylus vulgaris remains enzootic in Danish horses.