Myxomatosis
Myxomatosis is a disease caused by Myxoma virus, a poxvirus in the genus Leporipoxvirus. The natural hosts are tapeti (Sylvilagus brasiliensis) in South and Central America, and brush rabbits (Sylvilagus bachmani) in North America. The myxoma virus causes only a mild disease in these species, but causes a severe and usually fatal disease in European rabbits (Oryctolagus cuniculus).
This article is about the disease in rabbits. For the Radiohead song, see Hail to the Thief.Myxomatosis is an example of what occurs when a virus jumps from a species adapted to the virus to a naive host, and has been extensively studied for this reason. The virus was intentionally introduced in Australia, France, and Chile in the 1950s to control wild European rabbit populations.
Transmission[edit]
Myxomatosis is transmitted primarily by insects. Disease transmission commonly occurs via mosquito or flea bites, but can also occur via the bites of flies and lice, as well as arachnid mites. The myxoma virus does not replicate in these arthropod hosts, but is physically carried by biting arthropods from one rabbit to another. Seasonality is driven by the availability of arthropod vectors and the proximity of infected wild rabbits.[3]
The myxoma virus can also be transmitted by direct contact. Infected rabbits shed the virus in ocular and nasal secretions and from areas of eroded skin. The virus may also be present in semen and genital secretions. Poxviruses are fairly stable in the environment and can be spread by contaminated objects such as water bottles, feeders, caging, or people's hands.[3] They are resistant to drying but are sensitive to some disinfectants.[4]
Pathophysiology[edit]
A laboratory study in which European rabbits received intradermal injections of a South American strain of the myxoma virus demonstrated the following progression of disease. Initially the virus multiplied in the skin at the site of inoculation. Approximately two days following inoculation the virus was found in nearby lymph nodes, and at three days it was found in the bloodstream and abdominal organs. At approximately four days the virus was isolated from non-inoculated skin as well as from the testes. Slight thickening of the eyelids and the presence of virus in conjunctival fluid was detectable on day five. Testicular engorgement was noticed on day six.[5]
Diagnosis[edit]
Diagnosis of myxomatosis in European rabbits is often made on the basis of the characteristic clinical appearance. If a rabbit dies without exhibiting the classic signs of myxomatosis, or if further confirmation is desired, a number of laboratory tests are available. Historically these have included histopathology, electron microscopy, and virus isolation. Histopathologic examination of affected skin typically shows undifferentiated mesenchymal cells within a matrix of mucin, inflammatory cells, and edema. Intracytoplasmic inclusions may be seen in the epidermis and in conjunctival epithelium.[6]
Negative-stain electron microscopic examination can also be used for diagnosis due to the large size and distinctive structure of poxviruses. This method allows rapid visualization of poxviruses, but does not allow specific verification of virus species or variants.[7] Virus isolation remains the "gold standard" against which other methods of virus detection are compared. Theoretically at least, a single viable virus present in a specimen can be grown in cultured cells, thus expanding it to produce enough material to permit further detailed characterization.[8]
The more recent development of molecular methods such as polymerase chain reaction (PCR) and real-time polymerase chain reaction assays has created faster and more accurate methods of myxoma virus identification.[7] Real-time PCR simplifies the diagnosis of myxomatosis by allowing nasal, ocular, or genital swabs to be quickly tested. It can also be used on paraffin-embedded tissue samples to confirm the presence of Myxoma virus and identify the viral strain.[9]
Treatment[edit]
At present, no specific treatment exists for myxomatosis. If the decision is made to attempt treatment, careful monitoring is necessary to avoid prolonging suffering. Previously vaccinated rabbits or those infected with an attenuated strain may recover given supportive care with fluids, food, and broad spectrum antivirals. Cessation of food and water intake, ongoing severe weight loss, or rectal temperatures below 37 °C (98.6 °F) are reasons to consider euthanasia.[3]
Prevention[edit]
Vaccination[edit]
Vaccines against myxomatosis are available in some countries. All are modified live vaccines based either on attenuated myxoma virus strains or on the closely related Shope fibroma virus, which provides cross-immunity. It is recommended that all rabbits in areas of the world where myxomatosis is endemic be routinely vaccinated, even if kept indoors, because of the ability of the virus to be carried inside by vectors or fomites. In group situations where rabbits are not routinely vaccinated, vaccination in the face of an outbreak is beneficial in limiting morbidity and mortality.[1] The vaccine does not provide 100% protection,[3] so it is still important to prevent contact with wild rabbits and insect vectors. Myxomatosis vaccines must be boostered regularly to remain effective, and annual vaccinations are usually recommended.[1]
In Europe and the United Kingdom a bivalent vectored vaccine called Nobivac Myxo-RHD[10] is available that protects against both myxomatosis and rabbit haemorrhagic disease. This vaccine is licensed for immunization of rabbits 5 weeks of age or older, with onset of immunity taking approximately 3 weeks. Protection against myxomatosis and rabbit hemorrhagic disease has a duration of immunity for 12 months, and annual vaccination is recommended to ensure continued protection. The vaccine has been shown to reduce mortality and clinical signs of myxomatosis.[11]
Vaccination against myxomatosis is currently prohibited in Australia due to concerns that the vaccine virus could spread to wild rabbits and increase their immunity to myxomatosis. As feral rabbits in Australia already cause a great deal of environmental damage, this concern is taken seriously by the government.[12] Many pet rabbits in Australia continue to die from myxomatosis due to their lack of immunity.[13] There is at least one campaign to allow the vaccine for domestic pets.[14] The Australian Veterinary Association supports the introduction of a safe and effective myxomatosis vaccine for pet rabbits,[15] and the RSPCA of Australia has repeatedly called for a review of available myxoma virus vaccines and a scientific assessment of their likely impacts in the Australian setting.[16]
Although myxomatosis is endemic in parts of Mexico and the United States, there is no commercially available vaccine in either of these countries. Information on recently reported cases in the United States is available from the House Rabbit Society.[17] In the United States the importation of vaccines is overseen by the Animal and Plant Health Inspection Service, part of the Department of Agriculture.[18]
Other preventive measures[edit]
In locations where myxomatosis is endemic but no vaccine is available, preventing exposure to the myxoma virus is of vital importance. Even vaccinated rabbits need protection, as the vaccines are not 100% effective. The risk of a pet contracting myxomatosis can be reduced by preventing contact with wild rabbits, keeping rabbits indoors (preferred) or behind screens to prevent mosquito exposure, and using rabbit-safe medications to treat and prevent fleas, lice, and mites. Any new rabbit that may have been exposed should be quarantined, and rabbits suspected of having myxomatosis should be immediately isolated until the diagnosis is ruled out. If the disease is confirmed all contaminated cages, dishes, or other objects should be disinfected with 10% bleach, 10% sodium hydroxide, or 1%–1.4% formalin.[19]
In culture[edit]
Myxomatosis is referred to as "the white blindness" by the rabbit characters of the novel Watership Down (1972) by Richard Adams, and in the story a rabbit chief had driven out all rabbits who seemed to be afflicted. In one of the novel's folk tales about the rabbit hero El-ahrairah, the transmission of the disease is explained to him by the lord of the rabbit underworld, the Black Rabbit of Inle ("it is carried by the fleas in rabbits' ears; they pass from the ears of a sick rabbit to those of his companions").[34]
The sixth album of British band Radiohead, Hail to the Thief, contains a song "Myxomatosis". The disease is used as analogy to the journalist attention the band has. Thom Yorke has said:
"I remember my parents pointing out all these dead rabbits on the road when I was a kid. I didn’t know that much about the virus, or even how to spell it, but I loved the word. I loved the way it sounded."[35]