But RVETS does more than vaccinate animals or fix their teeth. The group has also changed the way that people treat the horses, mules and donkeys they rely on to fetch water, plow fields, ride competitively or go to school.

At the clinic, Brenda Arias and Martín Cuevas Jr., both veterinary students, gently approached two mares and a colt. Syringes in hand, the students prepared to squirt a pale-yellow liquid — the anti-parasitic drug ivermectin — into the animals’ mouths. Some rural horses, unfamiliar with people other than their owners, “won’t even let themselves be touched,” Ms. Arias said.

What to do, then? “Seduce them,” Mr. Cuevas said. “Talk to them nicely, pet them” — an unfamiliar tactic to an earlier generation.