Dear Beverlee,
The flesh of ripe apples doesn’t pose a problem for pets. However, apple stems, leaves and seeds contain substances known as cyanogenic glycosides, which can cause vomiting, diarrhea, a lack of oxygen in the blood, decreased heart rate, respiratory congestion, seizures, coma, and if large amounts are ingested, can lead to respiratory failure and death.
Small animals who consume a few pieces of the plant don’t typically show problems beyond gastrointestinal upset. Grazing animals—such as horses or livestock—are more prone to severe effects. Nonetheless, it’s still a good idea to discourage your pets from nibbling on the stems, leaves and seeds of apple trees.