Equine strongylid egg re-appearance period after ivermectin or moxidectin treatment in Italy

INTRODUCTION: Drug resistance in cyathostomins is a potential threat for horse health, welfare and production. While resistance to benzimidazoles (BZs) is worldwide recognized and to tetrahydropirimidines (THPs) is increasing, it is up until recently that the first evidence of reduced efficacy of macrocyclic lactones (MLs) has been reported. Recent studies indicated a reduction in egg reappearance period (ERP) after treatment with ivermectin (IVM) in different countries, while moxidectin (MOX) is still highly effective in Europe (von Samson-Himmelstjerna et al., 2007, Vet. Parasitol. 144,74-80; Lyons et al., 2011, Parasitol. Res. 108,1315-1319; Traversa et al., 2009, Parasit. Vectors 2:S2; Traversa et al., 2012, Vet. Parasitol., 188, 294-300). This study investigated the ERP after treatment with IVM or MOX in horses infected with cyathostomins in Italy. MATERIALS AND METHODS: Ten farms have been selected in Italy on the basis of a history either of <100% efficacy of MLs or of frequent use of IVM or MOX. In each farm ten horses have been selected on the basis of a minimum pre-treatment faecal egg count (FEC) of 50 (range 50 -2425) per gram of faeces (EPG). Horses were randomly allocated to one of two treatment groups based on pre-treatment FEC and treated either with IVM or MOX on Day 0. Individual faecal samples were taken from all animals on Day 14, 28, 42 and 56 for IVM and MOX, and 70 and 84 for MOX. All samples were examined using a modified McMaster technique with a sensitivity of 25 EPG and the efficacy for each treatment was calculated based on the arithmetic mean (AM) FEC before and at each time point after treatment with the following formula: %reduction= 100x (AMpre-treatment - AMpost-treatment)/AMpre-treatment. RESULTS: Efficacy at Day 14 was high for both IVM and MOX in all sites. The ERP was as expected for MOX except on one farm, in which decreased ERP was found for both MLs. Decreased ERP and efficacy was found in another farm on Day 56 for IVM, while decreased ERP and efficacy was detected in tail (i.e. on Day 84) for MOX on a third farm. CONCLUSIONS: The reduced cyathostomin ERP was found for IVM and/or MOX in 3 farms of this study, consistently with the results of a similar study carried out in Belgium and the Netherlands which demonstrated reduced ERP for IVM and MOX (Geurden et al. 2014, Vet. Parasitol., submitted). The duration of the ERP varies with anthelmintic class and worm populations, thus effective molecules must be evaluated separately in each herd to characterize local properties. Given that a reduction in the ERP is possibly indicative of early anthelmintic resistance, appropriate use of anthelmintics in worm control strategies, including reduced treatment frequency or selective treatments, along with maintenance of refugia, should be implemented to delay the occurrence of drug resistance in horse cyathostomins.