Winterberry, also referred to by its scientific name Ilex opaca or the common names English holly, European holly, Oregon holly, inkberry or American holly, is toxic to cats if ingested. Winterberry is a shrub found throughout North America with green foliage and bright red berries. Winterberry toxicity is generally mild. Ingestion in cats causes symptoms such as vomiting, diarrhea and depression, which rarely results in a severe condition. Recent production of winterberry oil that is used as in fragrance or for holistic treatments presents and additional hazard for cats if exposed. If you suspect winterberry poisoning in your cat you should contact your veterinarian, due to a cat’s small size and susceptibility to poisoning, symptoms may need to be addressed.

Symptoms resulting from the ingestion of winterberry plants in cats are mostly gastrointestinal and rarely severe:

If ingestion of winterberry oil occurs, symptoms may become more severe due to higher concentrations of plant compounds.

Although cats are not plant eaters by nature, cats may ingest plants to help with passing digested hair or as a result of playing with leaves or berries that make attractive playthings. Winterberry leaves and berries are mildly toxic and contain saponins and alkaloid compounds with toxic action similar to that of caffeine. 

Although winterberry is not usually known for producing severe toxic reactions, cats can be particularly susceptible to plant poisoning due to their small size and lack of enzymes produced by their liver for breaking down plant compounds. The availability of winterberry oil used for holistic purposes and fragrance can present an additional hazard in your home to your cat. Oils contain much higher concentrations of plant compounds including toxins that, if your cat is exposed, will result in a much larger dosage of toxin being introduced to your cat. In the rare case that a large amount of the plant is ingested or poisoning with essential oil containing high saponin concentrations occurs, medical attention may become more urgent.

If you suspect winterberry poisoning, take a sample of the plant you suspect your cat ingested with you to the veterinarian to confirm diagnosis. Your veterinarian will perform a physical examination of your cat and note severity of symptoms and any compromised vital signs that need addressing. Blood and urine tests, as well as radiography tests, may be taken to rule out other causes of gastrointestinal distress. Mild gastrointestinal symptoms and exposure to winterberry plants that may have been ingested is an indication of winterberry poisoning. 

If the ingestion of winterberry is recent your veterinarian may induce vomiting with 3% hydrogen peroxide solution or other medication. Your veterinarian may administer activated charcoal which binds with plant compounds in the digestive tract, preventing them from absorbing into your cat's system, and instead passing quickly through with your cat's stool. Kapectolin may be administered to treat stomach upset and diarrhea. Sucralfate may also be administered as it reacts with stomach acid and forms a paste that prevents stomach contents from irritating the stomach lining. If dehydration occurs, your veterinarian will treat with intravenous fluids. Additional supportive treatment will be provided as required. 

Toxic effects of winterberry tend to be mild and limited to gastrointestinal upset. Your cat should be allowed to recover in a quiet warm place and given mild, bland food and water as they regain their appetite. Ensure your cat does not have the opportunity to further ingest winterberry by removing or restricting access to the shrub outside, removing plants used for decorative purposes in your home, and ensuring exposure to winterberry oil does not occur. Monitor your cat for signs of illness or secondary complications to poisoning that may require follow up from your veterinarian.