MYXOMATOSIS

Myxomatosis is caused by the myxoma virus, a type of pox virus that only affects rabbits. It was first discovered in 1896 in Uruguay and was imported to Australia in 1951 to control its large rabbit populations - initially having the desired devastating effect. The disease was illegally introduced to France in 1952 and it appeared in Britain the following year. It quickly spread to both wild and domestic rabbit populations and within a few years had spread throughout Europe. Myxomatosis has been a threat to wild and domestic rabbits ever since.

Who is at risk?

All rabbits, whether wild or domestic are at risk of myxomatosis.

How is it spread?

Myxomatosis is typically spread by blood sucking insects and in particular the rabbit flea, Spilopsyllus cuniculi. This flea is frequently found on wild rabbits and transmission in the absence of bites is unusual. All breeds of domestic rabbit can be affected, with little to suggest that one breed is more susceptible than another, and whatever the lifestyle of your rabbit there is a potential risk of this disease.

Signs and symptoms

The incubation period varies depending on the strain and its virulence and is typically at least five days. Accompanying the classic bulging eyes that most of us associate with myxomatosis, are localised swellings around the head, face, ears, lips, anus and genitalia. Severe swellings can lead to blindness and distortion around the face within a day or so of the onset of symptoms, leading to difficulty with feeding and drinking. Bacterial respiratory infection often complicates the disease resulting in a fatal pneumonia.

Progress of the disease may be slower in well cared for pet rabbits and recovery is sometimes possible with intensive care. However, myxomatosis can be a very protracted and profoundly unpleasant disease and euthanasia is generally recommended. Recovery in the wild occasionally occurs but for animals with severe signs death usually occurs about 12 days after initial infection.

Management of myxomatosis

There is no specific treatment for the virus and any treatment offered is merely supportive. Treatment is occasionally contemplated but would not usually be recommended for rabbits with the full-blown disease since affected individuals suffer dreadfully, have a low chance of survival and they remain a source of infection for other rabbits. The occasional individuals with milder disease may, however, recover with appropriate care.
Control of myxomatosis

To help prevent your rabbit from contracting myxomatosis, it is important to put various controls in place, for which there are two main methods: control of parasites and vaccination.

Flea control
Always keep a regular check on pets for any signs of fleas and consider the regular use of an insecticidal treatment from your vet. There is also evidence to suggest that mosquitoes and other biting flies may transmit myxomatosis in the UK, so nets and insect repellent can be used to combat this threat in warmer weather. Your vet will be able to advise you further on these measures, since not all products are suitable or safe for rabbits.

Vaccination
There is one licensed rabbit vaccine to immunise against myxomatosis available in the UK. It is a live vaccine containing the Shope fibroma virus, which is closely related to myxomatosis but does not cause disease in the European domestic rabbit. The only clinical sign that may occasionally be seen is a slight temporary lump, or nodule, at the vaccination site. The technique of using a different virus in order to protect an individual against a serious disease is an old and well-proven technique, first used by Edward Jenner when he used the live cowpox virus to vaccinate people against smallpox.

Vaccination schedule
It is recommended that a single dose of vaccine is given to all rabbits over six weeks of age who are healthy and not pregnant, ideally in early spring so that rabbits have the best protection during the period of the year when they are most at risk – the peak season for disease is late August-October – however, it must be remembered that myxomatosis can affect animals at any time of the year. Following vaccination, rabbits should not be exposed to infection for at least 14 days, allowing adequate time for immunity to develop. Boosters are recommended once or twice a year, depending on the likely risk of exposure to myxomatosis. Higher risk may be seen in multi-rabbit households, where other health problems are present, where there may be close association with wild rabbits or indeed if there is a succession of warmer winters resulting in increased prevalence of disease.

Strains of myxomatosis and vaccine development

Although myxomatosis is typically fatal in rabbits without immunity, there are many different strains and some are more virulent than others. As evolution has progressed and the virus adapts accordingly, the modern myxoma virus may not kill rabbits quite as readily or rapidly as older strains. Use of the Shope fibroma virus strain is well proven in terms of safety and achieves good levels of efficacy. Inactivated (killed) myxomatosis vaccines are generally ineffective. Other approaches that have been taken with mixed results have included using a weakened strain of the myxoma virus as a vaccine.

Can rabbits contract myxomatosis after vaccination?

Vaccination can never guarantee 100% protection against any disease. However, when used as recommended, vaccination offers the best chance of immunity against this dreadful disease. Rabbits exposed to myxoma virus who have a vaccinal immunity, typically do not develop the disease. Vaccination may, however, be ineffective if given to rabbits already incubating the disease, or for those suffering overwhelming challenge of the disease, or those who are immunologically incompetent – which could be due to a number of factors, including underlying health problems, poor nutrition, genetic factors, stress and drug therapy. For maximum protection, care is needed in administration of the vaccine to ensure that a small volume is introduced within the skin (intradermally) as well as beneath (subcutaneously) as this stimulates a more powerful immune response.

In addition, control of fleas, good basic husbandry and steps to reduce stress should be undertaken to reduce the risk of myxomatosis and complement the protection afforded by vaccination.