Original Contribution INVESTIGATION ON THE BENZIMIDAZOLE RESISTANCE IN GASTROINTESTINAL STRONGYLIDS OF SHEEP IN SOUTHEASTERN BULGARIA

Benzimidazoles are the most commonly used anthelmintic drugs for the chemical control of sheep strongylidoses in Bulgaria. The intensive and not always proper application could be able to cause a reduction of their efficacy. To establish benzimidazole resistances among gastrointestinal strongylids, two in vitro assays were performed – Egg Hatch Assay (EHA) and Larval Development Assay (LDA). The effective concentrations required to inhibit 50 % of egg hatching (egg dead ED50) or larval development (larva dead LD50) were: for EHA from 0.0114 to 0.2023 μg/ml and for LDA from 0.0017 to 0.5817 μM thiabendazole. All the data were analysed. Based on the reference we assumed that ED50 values showing a presence of resistance are over 0.1 μg/ml and LD50 over 0.12 μM thiabendazole. According to the results discovered here, in two of 13 sheep flocks examined, a benzimidazole resistance was detected with values of ED50 between 0.1132 and 0.2023 μg/ml and for LD50 between 0.2940 and 0.5817 μM thiabendazole.