All parts of the rhododendron are toxic and have the potential of causing serious damage to cats because of their small body size. It is the presence of toxic resins called “grayanotoxins” that make this plant so dangerous. These grayanotoxins are composed of water soluble diterpenoid compounds that bind themselves to channels on cell membranes inside the cat. This leads to the depolarization of the cells, causing serious issues throughout the body and inhibiting normal organ and central nervous system function. In addition to a negative gastrointestinal response, pneumonia, hemorrhage, and kidney or liver failure may develop from rhododendron ingestion.

The rhododendron is a common shrub that grows world wide. Its various subspecies come in both deciduous and evergreen forms. The thick foliage of the rhododendron is shiny on the top of the leaves and matte on the undersides. While the foliage is ornamental, the plant is most loved for its clusters of large, brightly colored, bell-shaped flowers. These blossoms come in white, yellow, orange, red, pink or purple. Rhododendrons generally do not exceed 5 feet in height. They are grown in many gardens throughout the United States. 

Most symptoms of rhododendron poisoning will manifest within 6 hours of plant consumption. Severe symptoms may require immediate medical attention and should be treated as an emergency. All signs to watch for are listed as follows:

The leaves of most Rhododendrons stay green throughout the winter, which may lead to outdoor cats being more prone to sample them as there may not be any other foliage around. The stem, leaves, and flowers of the plant are all highly toxic. Eating as few as three leaves may be a large enough dosage to kill a cat. 

If you have witnessed you cat eating a rhododendron, take it into a nearby veterinary clinic or animal hospital immediately. If you saw your cat eating a plant or see evidence of plant material in its mouth but are unsure what type of plant it was, and negative symptoms have now begun to develop, bring a sample of the plant with you to be identified by the veterinarian. If there is no sign of plant consumption but serious effects are being exhibited, a diagnosis may be more difficult. Providing your cat's medical history can help the vet to differentiate pre-existing health issues from a poisoning. You may be asked if you allow your cat outdoors, and which types of plants are grown in your garden.

The veterinarian will then perform a complete physical examination of the cat. A stethoscope may be used to listen to the cat's breathing and heart rate. Abnormalities in both may be noted. The cat's temperature will be taken and its blood pressure will be measured. A sample of the cat's blood will be drawn so that measurements of the cells and minerals in the bloodstream can be taken. This will reveal the cat's overall health condition. Urinalysis may also be needed to assess if damage has occurred to the liver or kidneys.

There is no exact treatment course for Rhododendron poisoning, so treatment will depend on symptoms and their severity. Rhododendron consumption can be a medical emergency in some instances. Hospitalization along with a close monitoring of the cat will likely be needed.

This is when the vet induces the cat to vomit, often using hydrogen peroxide, to remove any undigested plant material from the stomach. 

Administering activated charcoal multiple times on the day of ingestion can help absorb many plant toxins in the digestive tract. The toxins may then safely pass through the cat without causing further damage.

General care to support vital organ function may be needed in severe poisoning cases. This may include intravenous fluid administration and/or respiratory support.

Medication 

Various medications may be used to stabilize the heart including atropine or isoproterenol to quicken the heart rate and prevent heart block.

The illness caused by rhododendron poisoning may last anywhere from 24 hours to several days. After the first two days, the cat will either start to recover or take a turn for the worst, become comatose and die. Recovery is possible, but it depends much on the health of the cat, the amount of plant material that has been consumed, and how fast treatment was received. Some cats do make a recovery without treatment, however, this is rare and the chance of survival vastly increases with medical attention. 

If you have rhododendrons in your garden, there are natural deterrents you can use to keep cats out including vinegar or cayenne pepper along the edge of the garden bed.  Some cat owners may choose to remove the plant from their garden to reduce the risk of poisoning. The most effective way to prevent your cat from contacting rhododendrons is by keeping it indoors.