Selective anthelmintic therapy of horses in the Federal states of Bavaria (Germany) and Salzburg (Austria): an investigation into strongyle egg shedding consistency.

For 9 consecutive months (March-November 2008), faecal samples were collected monthly from 129 horses residing within 40 km of Salzburg, Austria. Samples were analysed quantitatively using a modified McMaster egg counting technique. Whenever a faecal egg count (FEC) result exceeded 250 eggs per gram (EPG), the horse was treated with pyrantel, ivermectin or moxidectin. In 52 of 129 horses (40.3%), no strongyle eggs were ever detected over the course of 9 months. In 39 horses (30.2%), strongyle eggs were detected in at least 1 sample, but the egg count never exceeded 250 EPG. The remaining 38 (29.5%) horses were treated at least once in response to a FEC that exceeded 250 EPG. As a result of this selective anthelmintic scheme, the total number of anthelmintic treatments was reduced to 54% of the number of treatments administered to the same horses in the previous year. Both the maximum and mean FEC dropped significantly after initiation of the study. A statistically significant, negative correlation was demonstrated between the maximum and mean FEC of a horse and its age. Pasture hygiene appeared to reduce FECs, but the effect was not statistically significant. The magnitude of the initial FEC was significantly correlated with the maximum FECs in the subsequent 8 months (p<0.01). The same relationship was observed for the maximum FEC of the first 2 samples. Furthermore, horses which required several anthelmintic treatments had a higher initial FEC and a greater maximum FEC in the first 2 samples than horses which received only one or no treatment. These results suggest that selective anthelmintic treatment accomplished a reduced pasture contamination with strongyle eggs, while simultaneously decreasing the number of anthelmintic treatments. Sustained implementation of a selective treatment strategy has the potential to reduce selection pressure for anthelmintic resistance. These results reported herein will assist equine practitioners in designing and monitoring sustainable anthelmintic treatment programs.