It could be resistant. I have read in veterinary journals that resistance can come and many farmers were going back to natural remedies, I might be able to find that article if you need the reference.
The key to killing is like finding the right key to open the lock. So you have to consider the following:
1/ the right substance
2/ the right concentration to use
3/ the duration of treatment
I have studied resistance for quite some time, keep in mind that these studies have been on bacteria and protozoa resistance, so I am not sure how much applies to non-protozoa parasites. Some of the experts agree that the use of natural substances along with prescriptons, enhances the prescription ability. Many naturals can stop resistance.
You have seen these in the stool I presume, was this following a specific protocol? Are the protocols just prescriptions?
f there is resistance then there
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