Myxomatosis in rabbits
Esther van Praag, Ph.D.
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Sanarelli first recognized the myxomatosis disease in 1896, in Uruguay, where it causes sporadic lethal infections in the American cottontail species (Sylvilagus sp.). The virus has spread over the entire American continent, and has become endemic in some regions (Chili, in O. cuniculi; Western USA, in Sylvilagus bachmani).
It was soon discovered that the European rabbit, (Oryctolagus cuniculi) was very sensitive to this virus, causing severe skin abscesses, infections, and ultimately death. In the 1950se, the myxomatosis virus was introduced and spread among wild rabbits in Australia, in order to reduce its population. This operation decimated almost all the wild rabbit population, except a few individuals that seemed resistant to this virus. The surviving rabbits started to reproduce offspring and colonized the country anew. In Europe, the virus spread rapidly and has now become endemic in some regions, which are populated by the European rabbit.
The lagomorph’s groups largely affected by the myxoma virus are the European rabbit (Oryctolagus cuniculi), the European hare (Lepus europaeus), the Brush cottontail (S. bachmani) and the eastern cottontail (S. floridanus).
Myxomatosis is caused by a virus belonging to the family of the Poxviridae, and is a type species of the genus Leporipoxvirus. The later comprises close related viruses that affect he American cottontail rabbit (Sylvilagus sp.) and the “hare fibroma virus”, among others. All these viruses lead to the development of tumors of the skin connective tissues (fibroma). Various strains exist. Some are very virulent (e.g. Standard laboratory, Lausanne, California), others manifest their presence chronically. Genetic studies show a relationship between the myxoma and the Shope fibroma virus
Blood sucking insects (fleas, mosquitoes, lice and mite) are efficient mechanical vectors of this disease. It was observed that the virus is present in the mouth’s parts of the rabbit flea Spilopsyllus cuniculi, where it can survive over 100 days, independently of the environmental conditions. It is furthermore speculated that the disease may spread from one rabbit to another during skin and fur contact.
The development of myxomatosis follows the typical pattern of a poxvirus infection. Once inoculated in the skin, the virus starts to reproduce in the skin and local lymph nodes. The virus is then spread through the body. The viruses are then spread throughout the body (viremia) and into the skin.
The first evident signs of the disease appear 3 days after the infection: swelling (edema) of the eyelids, followed by the lips, genital organs and purulent conjunctivitis. At later stages of the disease, the rabbit becomes blind. The disease is usually fatal between day 8 to 15 after the infection with the virus.
In the chronic form of the disease, the most prominent signs are the formation of skin tumors, called myxoma, on the ears, nose and limbs. These tumors will resorb by themselves after some time.
A side effect of the chronic form of myxomatosis is the development of secondary bacterial infection. Pneumonia caused by Pasteurella sp. or Staphylococcus aureus is often observed. It is accompanied by respiratory distress (dyspnea).
Although the disease depends on the strain of myxoma virus, it is usually severe and almost always fatal.
The clinical symptoms are sufficient for diagnosis. One must keep in mind though, that early stages of the spirochetosis disease (caused by the parasite Treponema sp., affecting the perianal parts of the rabbit) look similar to those of myxomatosis. Indeed, tumors of those diseases show close similarities, so spirochetosis and myxomatosis must be carefully differentiated from each other.
Myxomatosis should furthermore be differentiated from an upper respiratory infection, like e.g. Pasteurellosis. In the later, no swelling is observed in the perianal region, on the contrary to myxomatosis.
In the case myxomatosis is chronic, it is recommended to do a biopsy and check it for the presence of viruses.
If a rabbit is affected by the aggressive form of myxomatosis, its chances of survival are close to zero. It is then recommended to humanely put the affected animal to sleep.
If treatment is chosen, intensive care over a longer period of time is needed. It is important to keep the sick rabbit in a warm environment (21-22°C). Eyes and ears must be regularly cleaned. As much fluids and food should be given to the rabbit as possible, even if the rabbit is drinking good amounts of water by itself. Skin tumors can be removed surgically.
Unfortunately, secondary complications often appear. The most common one are respiratory disease and pneumonia, due to secondary infection by Pasteurella sp. or Staphylococcus sp.
Rabbit that suffer a chronic form of myxomatosis recover by themselves. Antibiotics can be given to avoid respiratory complications.
In regions where myxomatosis is endemic and present among the wild rabbit or cottontail population, prevention of the disease in pet rabbits is is possible by regular vaccination. This is not available in all countries.
Depending on the vaccine used and the age or breed of rabbits, vaccinated rabbits may develop a mild to serious form of the disease. In rare cases, the rabbit must be put to sleep.
For detailed information on myxomatosis in rabbits,
by E. van Praag, A. Maurer and T. Saarony,
408 pages, 2010.
A special thanks to Denise Baart, for sharing the pictures of her rabbit Bucks.
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