There are three main ‘races’ of avocado: the Guatemalan; the Mexican; and the West Indian avocado. These are all variants of Persea americana.
In Australia the most common cultivars of Persea americana are the Haas and the Shepard, with lesser known varieties including the Reed, Fuerte, Pinkerton and Gem. The Haas avocado is from the Guatemalan race.
The toxic principle are the acetogenins, persin and avocadene 1-acetogen. Persin is the toxin that causes disease in animals. It is likely the amount of acetogenins varies between cultivars. The toxin can be found in leaves and fruit (seed and pulp).
Avocado poisoning has been reported in a number of species, including horses, dogs, sheep and goats, rabbits, birds and ostriches.
Signs of poisoning
Mastitis (necrohemorrhagic) has been reported in Australian horses. Other signs include mild colic and oedematous swelling of the head and pharynx, potentially spreading down the neck. The swelling of the head of horses involves the lips, tongue and eyelids. Dyspnea is common with this presentation, and is secondary to airway compromise and likely not cardiogenic pulmonary oedema. This was a common feature in 13 horses introduced to an abandoned avocado farm in SE Queensland.
Horses can show classical signs are cardiac failure – tachycardia, dyspnea, cold extremities, congested mucous membranes with a prolonged capillary refill time. This may be related to the cultivar involved and the amount consumed. This does not appear to be a feature of Haas avocado toxicity.
Pathological changes
Widespread fibrosis and hemorrhages of the heart muscle (myocardium), endocardium, and epicardium are present in horses with cardiac disease. Less severe changes are present in the kidneys. The pathological changes responsible for the head oedema presentation is not known.
Treatment and outcome
Treatment is supportive, including non-steroidal anti-inflammatory drugs, anti-histamines and corticosteroids. The outcome is dependent on cultivar type and the amount ingested. Poisoning with Hass avocado leaves and fruit rarely results in death.
Reading
McKenzie and Brown 1991 Avocado (Persea americana) poisoning of horses