>>only a small portion of the antibiotics ZTS (and other animal-health companies) sells are ones that are relevant to human health
It is a mistake to assume that using antibiotics that are not used by humans will solve the problem. Many of the antibiotic resistance traits - efflux pumps and the like - are not specific to one antibiotic at all and can be spread to other bacteria.
Few countries outside the U.S. have a comprehensive plan to fight resistance, monitoring is infrequent and sales of medicines without a prescription are rampant, according to a 2015 World Health Organization survey. So while regulators and consumers seek to reduce use in the U.S., developing nations serve as a willing market.
Maybe the hit to such companies as ZTS from the FDA’s new regulations (effective Jan 2017) will be as small as the companies say:
“We don’t think that this will represent a negative impact to our business,” said [LLY’s animal-health president, Jeff Simmons].