Activation of host defence mechanisms. The drug may have no direct effect on the parasite, but exert its effect through the host by activating defence mechanisms. Levamisole is an immunostimulant. There are also several drugs which damage the surface membranes of cestodes and digeneans, this exposes new antigens on the surface of the parasite and increases their susceptibility to immune attack. The widespread use of anti-parasite compounds has lead to the emergence of drug resistant strains of parasite and it is becoming an increasingly worrying problem.
Drug treatment exerts a tremendous selection pressure on the parasite population.
Resistance in an individual parasite or in a population may be due to the presence or absence of a single gene or it may be polygenetic, involving a combination of different genes. The selection pressure exerted on the parasite population by drug treatment results in an increase in the frequency of the resistant alleles in the treated population. The survivors of drug treatment contribute their genes to the next generation, and if selection is maintained the frequency of resistant alleles can increase rapidly. In the absence of drug pressure, resistant individuals are often less fit than the wild type. One way of combating the spread of drug resistance is to relax drug pressure for while.