I would like to add something to the answers offered by by Dan Holdworth
y Stephen Keyes about this subject. I liked Dan's answer although he
seems more concern with plant parasites than with animnal parasites. I differ
somewhat with Stephen, however.
I think that the answer is simple: pesticides kill the parasites
that are susceptible to that dose but do not affect those that are resistant.
If the resistance is genetically codified (as is often the case), these latter
parasites reproduce and generate a population that is more resistant than the
original population. As it was said before, just a matter of selection.
I had never heard that the animal (the host, I assume)
learns to detoxify the drug more effectively. This does not seem
likely to me. I would like to know about some solid evidence that this
actually happens in other than geological times.
I have never heard of that gene for multiple resistance either (resistance
to different drugs with different mechanisms of action). On the contrary, the
many mechanisms of resistance revealed so far suggest that drug resistance is
due to many and different genes. I would like to know solid evidence about
this gene too.
What I wanted to add is that the population resistance to the
drug accumulates in the field (in the case of anthelmintics, for example) so
the hosts that acquire further infections are infected with drug-resistant
parasites and this creates a permanent, self-replicating problem. There are
ways, however,how farmers may minimize the survival of the drug-resistant
parasites in the field and delay the spread of the resistance. The methods are
a little technical to explain here but you can ask any veterinarian that has
been a student of mine and he/she will explain it to you :-)
Omar O. Barriga, DVM, PhD