He thought it was an allergic reaction, because other horses that had eaten leaves did not appear to be affected. Research by Dr. Art Craigmill, University of California toxicologist, has indicated that avocado leaves from the Guatemalan race, such as Reed and Nabal, are toxic to animals such as goats and horses. Pure Mexican varieties such as Bacon, Mexicola, Topa Topa and Zutano do not appear to be toxic. Fuerte, a Mexican variety with some Guatemalan, has been reported to be toxic to rabbits. Although fruit has not been studied, we do not believe that the flesh of fruit is toxic to animals, only the leaves.
Avocado seeds are also toxic and chopped seeds are sometimes used as rat and mouse bait.
Dr. Miller indicated that he had seen pigs die after eating wind-dropped avocados and seeds. According to Craigmill, the worst symptom shown by the ingestion of avocado leaves in horses and goats is a full cardiac arrest within 48 hours. Essentially, if the lactating animal is allowed to eat Guatemalan avocado leaves, the udder will be severely injured and the animal may not be able to provide milk for future offspring. It is important that owners of livestock be diligent in keeping their animals away from Guatemalan avocado trees at all times. Make sure that trees leaning over into pens are pruned back, or removed entirely.