Hydrangeas (genus Hydrangea) are popular ornamental shrubs prized for their large, showy flower heads. From a botanical perspective, it is important to understand that many plants, including hydrangeas, have evolved chemical defense systems to deter herbivores and insects. These chemical compounds, while natural, can be problematic for non-specialist animals that ingest them. In the case of hydrangeas, the primary toxic compound is a cyanogenic glycoside. This is not a single substance but a class of compounds that have the potential to release hydrogen cyanide (HCN) when the plant tissue is chewed, crushed, or digested.
The specific cyanogenic glycoside found in hydrangeas is hydrangin. It is present throughout the plant but is typically most concentrated in the leaves and flower buds. For the toxin to become active, the plant's cellular structure must be broken down. When a pet chews on a hydrangea leaf, the hydrangin comes into contact with enzymes also present within the plant, triggering a reaction that releases cyanide. It is crucial to note that the concentration of these glycosides can vary significantly between different hydrangea species and even cultivars, and can be influenced by environmental factors like soil conditions and fertilization.
All parts of the hydrangea plant contain cyanogenic glycosides and are therefore considered toxic. However, the highest concentrations are generally found in the leaves and the budding flowers. The risk to a pet is directly related to the amount of plant material ingested. A small nibble may only cause mild stomach upset, while consuming a larger quantity of leaves or buds could lead to more severe poisoning. The size of the pet is also a major factor; a large dog may experience only minor effects from an amount that would be very serious for a small dog or a cat.
The release of cyanide interferes with cellular respiration, preventing cells from using oxygen. This leads to a range of gastrointestinal and systemic symptoms. The most common signs of hydrangea poisoning in dogs and cats include vomiting and diarrhea, which may sometimes contain blood. Due to the nausea and abdominal pain, pets will often exhibit signs of depression and lethargy. As the toxin affects the system, more serious symptoms can develop, such as increased heart rate, labored breathing, and a drop in body temperature. In severe cases, poisoning can lead to seizures, coma, and can be fatal, though such extreme outcomes are rare.
