Mistletoe is one of those plants which is poisonous to cats and indeed dogs. I think it is less poisonous to dogs than cats but nonetheless it is toxic and it can affect the cat’s cardiovascular system when ingested. The cat may vomit, drool and have diarrhoea. They may have difficulty in breathing and if you suspect that your cat has ingested mistletoe you might perform first aid by inducing vomiting unless your cat is unconscious or having convulsions at which point you will need to obtain immediate veterinary assistance.