Refugia--overlooked as perhaps the most potent factor concerning the development of anthelmintic resistance.

VAN WYK J.A. 2001. Refugia-overlooked as perhaps the most potent factor concerning the devel­ op ment of anthelmintic resistance. Onderstepoort Journal of Veterinary Research, 68:55-67 Anthel mintic resistance involving particularly the gastro intestinal nematodes of small rum inants is escalating globall y, to the extent that in certain countries, such as South Africa, it has already reached alarming proportions, and is affecti ng practicall y all the anthelm intics. In th is paper it is argued that the high leve ls of resistance in nematodes of veterinary importance in­ dicate that the drugs have been used incorrectly. It is suggested that the phenomenon of refugia plays a much more important ro le in the selection of anthelmintic res istance than other phenomena that are more frequently investigated and recommended for counteracting it, such as reduced drenching frequency and avoiding under-dosing. While refugia is commonly mentioned in passing in most pa­ pers on anthelmintic res istance, it is, almost without exception , not incorporated in the final controll management proposals. On the strength of the conclusions arrived at in the present paper strategies such as the drench-and­ move system in which all the animals in a flock are drenched before they are moved to pastures containing few or no worms in refugia, and the system of strategic drenching on safe pastures should be condemned and never recommended. If such strategies are indeed unavoidable, the farmer should be warned that the farming system would probably not be sustainable even in the short term, in view of the generally high levels of resistance already present in most of the important sheep-producing regions. Farmers should be educated to consider refugia above all else when designing worm man­ agement programmes.