There are a number of factors that may be involved in the development of resistance; two important ones are:

1) The level of exposure to a specific wormer:

– Frequency of use – the more often a wormer is used, i.e. the shorter the time between dosing, the more often a parasite population is exposed to the drug and the more frequently selection pressure will be exerted, and so enhances the onset of resistance (Dipietro et al, 1997).

– Underdosing – parasites are exposed to the wormer, but not at a sufficient dosage to kill them and so will pass on their immunity to subsequent generations.

2) The effectiveness of the wormer:

– If a wormer is only moderately effective i.e. has no larval activity (Reinemeyer C, 2003), then those parasites that survive treatment may be likely to produce offspring with inherited resistance.