Yes, hydrangeas (Hydrangea macrophylla and other species within the Hydrangea genus) are considered toxic to both dogs and cats. The primary toxic component in these popular ornamental shrubs is a compound called amygdalin, which is a cyanogenic glycoside. This compound is not directly poisonous in its stored form. However, when plant material is chewed and consumed by a pet, the amygdalin is metabolized and broken down, ultimately releasing cyanide into the animal's system. It is important to note that the concentration of amygdalin is highest in the leaves and flower buds, making these the most dangerous parts of the plant if ingested.
From a botanical perspective, the presence of cyanogenic glycosides like amygdalin is a sophisticated chemical defense strategy evolved by the hydrangea plant. This system acts as a powerful deterrent against herbivores, such as insects, deer, and other browsing animals. When a predator bites into the leaf or bud, cellular damage occurs, bringing the stored amygdalin into contact with specific enzymes. This enzymatic reaction triggers the rapid release of hydrogen cyanide gas (HCN), which is highly toxic. This mechanism effectively protects the plant's foliage and reproductive structures (flowers and buds) from being consumed, thereby increasing its chances of survival and successful reproduction.
While the term "cyanide" is alarming, the overall toxicity level of hydrangeas is generally classified as mild to moderate for dogs and cats. The severity of poisoning is highly dependent on the amount of plant material ingested relative to the size of the animal. A large dog would need to consume a considerably larger quantity of leaves or buds to suffer severe effects compared to a small cat or a puppy. Furthermore, different cultivars and individual plants may have varying concentrations of amygdalin, influenced by factors such as soil conditions, climate, and the plant's overall health. Nevertheless, any ingestion should be taken seriously due to the potential for uncomfortable and dangerous symptoms.
The release of cyanide interferes with cellular respiration, preventing cells from using oxygen. This leads to a range of clinical signs that typically manifest within a few hours of ingestion. Pet owners should watch for symptoms including gastrointestinal distress such as vomiting and diarrhea, which are often the first and most common signs. Other symptoms can include lethargy, depression, and abdominal pain. In more severe cases of significant ingestion, symptoms may progress to include increased heart rate, hyperventilation, and in extreme circumstances, seizures or coma due to oxygen deprivation at the cellular level.
