Anthelmintic resistance. An overview of the situation in the Nordic countries

Introduction Gastrointestinal nematodes in grazing animals cause major production losses and represent an animal welfare problem worldwide. For decades use of anthelmintics has been central in the control programs of these parasites. This intensive use of anthelmintic drugs has resulted in problems with resistance to the anthelmintic drugs available today. Resistance to all classes of broadspectrum anthelmintics available benzimidazoles (BZ), imidothiazoles-tetrahydropyrines and macrocyclic lactones has been reported [1].The time from introducing a new class of anthelmintic drugs until resistance has been detected seems to be less than 10 years [1]. As time has passed problems of multiresistance to more than one class has occurred as well. Multiresistant Haemonchus contortus has become a major threat to the whole small ruminant industry in part of South Africa and in the South-East of USA [2,3]. At present, resistant nematode populations are detected in all our naturally grazing species; sheep, goats, cattle and horses [1] . In pigs, resistance to pyrantel, levamisole and benzimidazoles in Oesophagostumum spp have been detected [4,5].