According to the ASPCA, oleander is toxic to cats, dogs and horses. The plant, also called rose-bay, contains a digoxin-like substance that causes difficult breathing, disorientation, vomiting, diarrhea, muscle tremors and death from heart failure.
The entire oleander plant is poisonous, including stems, leaves and flowers. House pets commonly receive exposure by eating debris from pruning and gardening, according to the ASPCA. Horses receive exposure by nibbling on oleander bushes.
All animals are at risk of oleander poisoning, according to the Merck Veterinary Manual. The plant, commonly used as a decorative shrub in southern gardens, is toxic all year round. Symptoms occur immediately after ingestion. There is no antidote, but immediate supportive treatment may save the pet's life.