Basically it talks about people going nuts trying to buy Ivermectin. Now the poor animals who need this for parasites might not have access due to stupid humans. Plus, the dosage sold in feed stores in for an animal that weighs about 1000 lbs, No wonder people are getting sick from this drug. Idiocracy is here.
They should use whatever dose is suitable for a foal, not a full-grown horse.
Normal Weights of Horse Foals at Birth | Animals.mom.com - a newborn foal typically weighs about 10% of its mother's weight. Which puts foals' weights in the adult-human range.
They shouldn't, because it's the wrong medicine for the disease.
IF they had a parasitic condition for which Ivermectin was appropriate, then they should use a preparation of that medication formulated and manufactured for humans, and at the dose prescribed by a qualified medical professional.
In the absence of a zombie apocalypse or similar collapse of civilisation, there are no situations where it is appropriate for humans to take veterinary medicines. Even in a zombie apocalypse, you should raid a pharmacy as your first option, before looting a veterinarian's office.