Gemara Moed Katan 10b (translation credits to Artscroll Schottenstein Talmud):
Rav ruled it permissible to curry [cur•ry 1 1. To groom (a horse) with a currycomb] horses with a metal comb (Footnote #9: … since it is deemed a festival need that the horse one rides on Ch”H look presentable.) … Rava ruled it permissible to let blood of an animal on Ch”H (Footnote #12: Since this forestalls the loss of the animal. Alternatively, it is permitted in order to alleviate the animal’s suffering.) Abaye said to Rav: A beraisa was taught that supports you on this matter: We may let the blood of an animal, and we do not withhold medical treatment from an animal on Ch”H. (Footnote #13: It is permitted to perform one of the thirty-nine prohibited labors to cure an animal on Ch’H.)
Hilchos Chol Hamoed Zichron Shlomo, by Rabbis Zucker and Francis, p. 62:
Even skilled melacha is permitted on Ch”H to treat sick pets or livestock in order to avoid a ‘davar ha-aveid’ (necessity to avoid loss or damage).