THE IDENTITY OF NEMATODE GENERA INVOLVED IN CASES OF OVINE ANTHELMINTIC RESISTANCE IN THE SOUTHERN NORTH-ISLAND OF NEW-ZEALAND

Larval cultures and faecal egg count reductions were used to determine the identities and frequencies of occurrence of nematode genera invol ved in cases of ovine anthelmintic resistance submitted to the Batchel ar Animal Health Laboratory between 1992 and 1994. Based on an examina tion of 102 cases, it was found that, while there were some difference s relating to the method of assessment, the genera most frequently inv olved were Trichostrongylus, Ostertagia and Nematodirus. Also implicat ed, although less commonly, were infections of Oesophagostomum/Chabert ia, Haemonchus and Cooperia. The majority of cases involved resistance in two or more genera, with resistance in a single nematode genus occ urring in 45% of cases. Most such monogeneric resistances comprised in fections of Trichostrongylus (52%), Ostertagia (17%) or Nematodirus (1 1%). Anthelmintic resistance involving Haemonchus alone was found in o nly three cases. These results suggest that the potential role of narr ow-spectrum drenches in the management and control of anthelmintic-res istant worms in New Zealand sheep is likely to be strictly limited.