Plant toxicity

Buttercups are on many lists of poisonous plants because they can contain an irritant that can cause dermatitis in humans that handle buttercups and salivation, oral ulceration and gastrointestinal irritation in animals that eat them. Rabbits can eat small, young leaves that are growing in pastureland without ill effects. The mature leaves, tall plants and flowers are unpalatable, so they do not eat them. There are no reports of buttercup toxicity in rabbits.

Plant toxicity is a cause of anxiety to rabbit owners and there are many sources of information that will add to their concerns. It would easy if plants could be classified as ‘poisonous’ or ‘non-poisonous. Unfortunately, this is not the case, which is why there are long lists of plants that ‘could be poisonous’ that are unnecessarily alarming to rabbit owners. These lists are compiled from toxicity in any species and there are big differences in susceptibility to plant poison.

Unfortunately, long lists of poisonous plants are so disturbing for owners that they frighten them into buying commercial foods instead, which are more likely to cause long term health problems, such as obesity and dental disease, than fresh plants. Some lists even include vegetables, which is illogical as we feed ourselves and our children on them. Chemicals that are sprayed on to vegetables are more likely to be a problem than the vegetables themselves

In reality, rabbits are very difficult to poison with plants. History has proven this to be the case. Anxiety about poisonous plants can prevent owners picking plants for their pet or allowing the rabbit to have access to a garden. This is a shame because a diet of fresh plants and is enjoyable for the rabbit and has many health benefits. Wild plants also cost nothing, which may be important for owners with many rabbits to feed. 

What is meant by 'poisonous'?

The word poisonous is emotive. It can make owners believe that if their rabbit has even a small mouthful of a plant on the poisonous plant lists, it will die. This is not the case.   Some lists of poisonous plants use the term ‘poisonous’ interchangeably with 'toxic', ‘hazardous’ or 'dangerous'. This difference may seem trivial but it allows plants, such as nettles, buttercups or clematis that cause a skin reaction in humans to be included in lists of 'poisonous' plants.  Any plant with thorns is also considered to be hazardous, which is not the same as poisonous.

What makes a plant poisonous

These leaves of the foxglove (Digitalis purpurea) have been nibbled by a young wild rabbit. It is not unusual for rabbits to taste plants and never try them again.Plants are poisonous if they contain a toxic compound (e.g alkaloids or glycosides)  that could harm an animal that ingests that plant. These compounds may irritate the digestive tract causing nausea, vomitting and diarrhoea in some species. Other toxic principles affect the nervous system causing convulsions and other neurological signs. The amount of the toxic compound is variable in the parts of the plant. Sometimes only parts of the plant contain the toxic priciple. For example, the leaves of the yew tree (Taxus baccata) are extremely poisoning and can cause sudden death in livestock that eat them but the fleshy part of the berries are non toxic. Birds may eat them. Even some humans eat them but this is not recommended. Another example is rhubarb where the stems are not poisonous but the leaves might be. The poisonous content of a plant may change from early growth to maturity. Some plants, are more toxic in advanced stages of growth, whereas others are more poisonous when they are young. In some cases, drying may increase or decrease the potential of a plant to be poisonous.  

The amount of a toxic compound that an animal ingests affects the ability of that compound to be harmful so if a rabbit nibbles the edge of a leaf it will only consume a small dose of a potentially poisonous plant. At the other end of the scale, if a rabbit only eats one type of plant or vegetable and little else, it could receive a high enough dose of a toxic compound. A theoretical example is cabbage, which contains a goitregenic compound that inhibits iodine absorption.      

Rabbit's resistance to toxic compounds in plants 

Rabbits are resistant to some of the toxic compounds in poisonous plants. For example, rabbits are resistant to pyrrolizidine alkaloids that are the toxic principle of several plants including common ragwort (Senecio jacobea) (http://pubs.aic.ca/doi/abs/10.4141/cjas84-224) and some rabbits are resistant to the toxic effects of deadly nightshade because have high levels of plasma atropinesterase that breaks down atropine.  http://www.ncbi.nlm.nih.gov/pubmed/16736160. Atropine is the toxic principle of deadly nightshade.

Apple pips 

Many websites say that apple seeds are toxic to rabbits because they contain cyanide.  Although, it is true that apple seeds contain a small amount of cyanide, the toxin is protected by the hard seed coating so it passes straight through the digestive system of most animals. Although there is a possibility that a rabbit might chew the seeds so the contents would be released into the gut, the cyanide is present in such small quantities that it can be detoxified by the body. The rabbit would have to eat a large amount of apple pips to suffer any ill effect. There are no reported cases of a rabbit dying from eating apple pips. In the autumn, wild rabbits feast on windfalls, including the pips, with no ill effects.

Deliberate attempts to poison rabbits

Domesticated pet rabbits are descended from wild rabbits that originated in Spain and Portugal and were introduced to many parts of the world (including the UK) to provide meat, fur and sport for voyagers and settlers. Some of the introduced rabbits either escaped or were released into the wild where they bred successfully to become a pest species that destroyed the natural habitat and changed the landscape and ecology dramatically.  Australia is a good example of a location where this has taken place. In some states, drastic control measures have been deployed to control rabbits, including building a fence 2000 miles long and dropping poison in baited food, such as apples or carrots, from helicopters and aeroplanes. The cost has been enormous and illustrates how difficult it is to kill rabbits. Growing a palatable, poisonous plant that rabbits would eat would be a much easier and cheaper solution. Such a plant doesn’t exist. For example, buttercups and apples pips are on many lists of poisonous plants for rabbits. These would definitely have been used if they killed rabbits easily. They don't. 

Rabbit resistant plants

Any gardener who has wild rabbits eating their herbaceous border or entering their vegetable plot knows that rabbits will eat a wide variety of plants. As a result, there are many lists of rabbit resistant plants that gardeners can grow because rabbits will not eat them. An example is on the Royal Horticultural Society website  https://www.rhs.org.uk/Advice/Profile?PID=209. If this list is cross referenced with lists of poisonous plants, it can be seen that rabbits do not eat the poisonous varieties.  The list of poisonous plants is the same as the rabbit resistant plants.

When rabbits have no choice..

Although it is difficult to poison rabbits with plants, it is not impossible. There are experiments where laboratory rabbits have been forced to eat a poisonous plant or plant extract, either because they have no choice or because it has been ground up and incorporated into the pellets that they are fed on. The results of these experiments have shown that rabbits can be ill or die after ingestion of plant poisons. This is not a natural situation because there is usually a choice of plants for a rabbit to eat but it could be mimicked in a house where there are ornamental houseplants and no other vegetation for a rabbit to eat. As a general rule, houseplants and rabbits are a bad combination. Not only is there a risk of poisoning the rabbit but there is the likelihood of damage or destruction of the houseplant.

Picking wild plants 

While reports of rabbit deaths following ingestion of poisoning are disturbing, they are rare and should not put owners off allowing their rabbits to graze in the garden or to pick plants for them. The VPIS reported no incidents of toxicity associated with wild plants although, in the literature, there is a case of rabbits dying after being given hemlock in mistake for carrot tops (Short B, Edwards C. 1989 Accidental Conium maculatum poisoning in the rabbit. Vet Hum Toxicol 31:54–57). Hemlock is a rare plant and usually found in damp marshy areas. My advice to owners that have access to weeds, herbs and plants in their gardens, on verges or next to footpaths is to choose 5-6 varieties that they recognise and know their rabbit will enjoy. They can pick these in the knowledge that they are safe.