You mean you believe that a widespread administration of ivermectin would lead to development of ivermectin resistance in parasites?

That’s certainly already the case. Ivermectin has lost effectiveness against particularly insidious parasites such as Haemonchus Contortus (barber pole worm). We’ve resorted to other treatments such as Moxidectin, Levamisole, etc, all of which are more expensive and harder to dose safely and effectively.

I can totally understand how Ivermectin got its following— it’s an amazing general purpose medicine around the farm. I generally use it responsibly by verifying the presence of parasites before administering (which can be literally any animal on the farm with almost any parasite) … but then there are times such as when my daughter had a single pet chicken (among many healthy ones) that wasn’t looking so hot … Whereas previously I’d just dispatch the animal and be done with it, now I give a dose of Ivermectin and isolate for a few days before calling in the grim reaper. So far the success rate is something like 90% where now my 9 year old daughter administers it herself. While I acknowledge this isn’t the most responsible, I can imagine others have had similar experiences which explain some of the popularity of Ivermectin with rural folk.

What's the downside of giving Ivermectin to livestock that don't have parasites?

Ivermectin stops working.

Through what mechanism?

Natural selection.

How will natural selection act on parasites if there are no parasites in the animal?

The drug may end up in the manure, and the parasites may encounter it there at a marginally-lethal dose. When the parasite progresses further in its life cycle, its offspring may be resistant when they infect the next livestock animal.

Most parasites have a life-cycle that includes time spent outside the preferred host animal, including in zoonotic species that may not have symptomatic infections. They may acquire resistance in any stage, in any place they encountered the drug.

It obviously cannot. But the issue isn't giving it to animals without any parasites, it is in giving it to animals with parasites but that do not need the anti-parasitic in order to recover. Once you have determined that the animal is sick enough to need treatment and that the sickness is most likely caused by the parasites, then you can make the decision that the risk of furthering resistance is outweighed by the need to treat the animal. If you have determined that there is no chance the animal has parasites, then there's no risk of furthering resistance but also no need to give the treatment.

I think this largely right, but resistance generally does not just evolve in the animals "that do not need it to recover", as all parasites/bacteria are more likely die in that case, but in more extensive cases where some of the parasites/bacteria survive and there are more reproductive events that can introduce a resistant genetic variation. This is why some consider that people not completing the full course of antibiotics- only taking them until they feel better, may be a greater contributor.

In nature, both the antiparasitic and the parasite will have a tendency to end up in places that you don't necessarily want them to be for the purposes of motivated hypothetical reasoning.

aka the chicken will urinate out some amount of ivermectin, unprocessed by the chicken, and then some amount will make it elsewhere that will allow for natural selection to take place (eggs or wherever). Once is infinitesimal, but give every animal some ivermectin for funsies, and you've now bred ivermectin resistance.

There's an issue with collies and collie-derived dog breeds; used to be warnings (even mentioned in an episode of "House"). But. Most of the lines running around today are the ones that weren't sensitive and didn't die from ivermectin. So it's no longer such an issue.

Widespread administration does lead to the development of resistant parasites. That's not an hypothetical, that's what you see on any farm if you prolong the treatment for too long.

It's always recommended that you cycle through different anti-parasite treatments.

Why would I take human medical advice from you? Are you saying there are no human uses beyond an anti parasitic?

I guess in theory you could use the topical cream form as a moisturiser. Perhaps even use the tube as a doorstop in a pinch.