Development of a novel equine combination anthelmintic and a study of anthelmintic susceptibility using a larval development assay : a thesis presented in partial fulfilment of the requirements for the degree of Master of Philosophy in Veterinary Science at Massey University

Abstract

The occurrence of anthelmintic resistance to benzimidazoles and avermectin in cyathostome nematodes in New Zealand horses was investigated using a larval development assay (LDA) to conduct a small survey. For the benzimidazoles the analogue, thiabendazole was used and for avermectin the analogues, ivermectin and ivermectin aglycone were used in the LDA. The normal range of LD50 values was estimated by assaying eggs from Kaimanawa feral horses (n = 22) for each analogue used. From these the mean LD50 values + 2xstandard deviation was taken as the upper limit of normal. The survey involved domestic horses (n = 47) from several locations around New Zealand. For ivermectin and ivermectin aglycone 12% had LD50 values higher than normal with resistance factors up to 5.3 and 6.8 respectively. This represents horses from three separate farms. For the benzimidazoles 43% of domestic horses had higher than normal LD50 values with resistance factors up to 4.8. These results suggest that some nematodes in these domestic horses had an increased tolerance to avermectins and as well as to benzimidazoles. Two studies were conducted to assess the efficacy of a combination of abamectin (0.2mg/kg), oxibendazole (10mg/kg) and bithionol (5mg/kg or 7mg.kg). In the first study bithionol was included at 5mg/kg and in the second study it was included at 7mg/kg. These studies showed the efficacy of the combination was >98% against adult luminal stages of S. vulgaris, S. edentatus, cyathostomes, migratory stages of S. edentatus and third instar stages of Gasterophilus intestinalis. Efficacy against mucosal stages of cyathostomes (about 64%) and arterial stages of S. vulgaris (71%) was poor. Inclusion of bithionol at 7mg/kg achieved an efficacy of 100% against A. perfoliata whereas at 5mg/kg the efficacy was only 84.6%. In these studies the following species were identified: two species of Strongylus (S. vulgaris and S. edentatus); three species of Triodontophorus (T. serratus, T. minor and T. tenuicollis); and twelve species of cyathostomes (Cyathostomum coronatum, Cyathostomum labiatum, Cyathostomum catinatum, Cylicocyclus nassatus, Cylicocyclus leptostomus, Cylicocyclus radiatus, Cylicocyclus insigne, Cylicostephanus poculatus, Cylicostephanus minutus, Cylicostephanus calicatus, Cylicostephanus longibursatus and Cylicostephanus goldi).