Anthelmintic resistance.

In populations of livestock in which anthelmintics have become the only means of control, individual worms in some species of helminths evade the effects of specific anthelmintics. If these resistant individuals are selected (by removing the susceptible individuals in the population) then the resistant worm population on individual farms becomes dominant in both numbers and effect. This selection of anthelmintic resistant populations is most often reported in equids and small ruminants (the hosts in which anthelmintics are used most frequently). There is little evidence of anthelmintic resistance by Ostertagia in cattle but an understanding of the problem in other grazing species may aid in delaying its onset. Benzimidazole/phenothiazine resistance by small strongyles in horses in widespread. Resistance by various geographic strains of Haemonchus, Teladorsagia and Trichostrongylus against all of the commonly used classes of anthelmintics in small ruminants has been verified. The failure of levamisole to remove adult Ostertagia ostertagi from cattle has been reported in several geographical areas. Resistance is an individual farm problem and each premise has to be evaluated as to which anthelmintics can be used. Drug rotation, especially rapid rotation (less than 1 year) selects for multiple drug resistance. Epidemiologic treatment selects for resistance, but if properly used, requires considerably fewer treatments to prevent disease and the development of resistant populations will be slowed. A combination of strategic and tactical treatments along with evaluation of anthelmintics will be required to enable producers to raise livestock in maximum production units.