American mistletoe contains multiple toxins including phoratoxins, polysaccharides, and lectins. All parts of the plant are considered toxic to most mammals. The phoratoxins especially are capable of causing negative effects in cats ranging from a standard gastrointestinal response to severe cardiac and central nervous system abnormalities. Fatalities from American Mistletoe poisoning do occur, although a large amount of plant material needs to be eaten for this to take place. Humans have long used various species of mistletoe during the time of the winter solstice dating back to the Celtic Druids who believed the plant had magical powers. 

Mistletoe has long been associated with rituals surrounding the Christmas holidays. American mistletoe is a specific subspecies that is native to the southern and eastern States. It is an evergreen shrub that leaches off of other trees, drawing its water and nutrients from them. American mistletoe actually grows on the host tree that it feeds off of, somewhat like a parasite. The plant has thick, green leaves and very distinct white berries. Scientifically it is classified as a Phoradendron serotinum of the Loranthaceae family of plants

Mild to moderate symptoms may develop depending on how much of the plant has been consumed. As some reactions can be life-threatening, call your vet or local poison control center for advice on the best course of action as soon as you suspect a poisoning has occurred. All signs to watch for include:

As many people use American mistletoe to decorate their home during the Christmas season, indoor cats may be exposed to the plant at this time. The plant is usually hung up, but it is not uncommon for leaves or berries to fall as the plant dries out. American mistletoe can also be found outdoors in warm climates throughout the United States. Outdoor cats may come into contact with the plant, particularly if they are avid tree climbers, as American mistletoe grows in pre-existing trees.

If you suspect your cat has eaten American mistletoe, or have witnessed it doing so, take it into the nearest veterinary clinic or animal hospital for immediate medical attention. If the cat's condition is bad, treatment may begin before a diagnosis has been made. Providing your cat's full medical history can help the vet to determine if the episode is from an existing health problem or if a poisoning has occurred. The time of year may point to possible mistletoe consumption, although ingestion of other toxic holiday plants will have to be differentiated from. 

The veterinarian will then perform a complete physical examination of the cat to note all symptoms that have developed. The vet will listen to the cat's heart rate and breathing, and may notice irregularities. The cat's temperature and blood pressure will also be taken. An electrocardiogram may be needed if the cat's pulse is weak or abnormal. A sample of the cat's blood will be drawn so that routine tests can be run. These generally include a complete blood count and a biochemical profile, which give a more complete picture of the cat's overall health. The cat's urine may also be collected for analysis to see what is being poured out of the body.

The sooner treatment can be started on the cat, the better the outlook becomes. There is no specific course of action when dealing with a cat who has consumed American mistletoe, so treatment usually is based on addressing symptoms as they arise and stabilizing the cat's vital functions.

This can be given to the cat to absorb the many toxins that may have been released into the stomach and digestive system from the plant material. Once absorbed, the toxins are trapped and can safely pass through the cat.

In extreme cases, supportive care such as oxygen supplementation may be needed to assist a cat whose breathing is hindered. Intravenous fluids may also be given to help prevent the cat from becoming seriously dehydrated.

Various medications may be administered to the cat to help ease gastrointestinal distress or to increase blood pressure. 

The period of illness after American mistletoe has been eaten can last for up to three days. The overall prognosis greatly depends on how much of the plant was eaten and how fast treatment was given to the affected cat. Death is unlikely if treatment has been received in a timely manner. 

To prevent your cat from being poisoned by American mistletoe, be sure to keep it far out of reach. There are many faux alternatives that pose no danger to pets that can be used for decoration instead of real American mistletoe clippings. If you live in an area where American mistletoe grows naturally, keeping your cat indoors will greatly reduce the possibility of exposure to this and other toxic plants.