Myxomatosis is caused by the myxoma virus, a kind of pox virus. There are different strains of the virus which vary in their virulence (basically the ability to cause disease). Domestic rabbits are at high risk!
In the United States, Myxomatosis is prevelant in Santa Cruz and Santa Clara counties, There have been 3 cases in 2021:
July 20 2021 - San Jose
early August - Santa Cruz mountains
mid-August - San Jose
Cases in prior years are listed here: https://rabbit.org/myxo/
How is Myxomatosis Spread?:
The virus is spread by biting insects (e.g. fleas, mosquitoes, mites, lice, and flies) as well as by direct contact (between rabbits), indirect contact (via items that such as food dishes or clothes that carry the virus from rabbit to rabbit), and by aerosols.
Signs of Myxomatosis:
Myxomatosis can take several courses. Rabbits may suddenly become very ill with conjunctivitis (red, runny eyes), a high fever, loss of appetite and lethargy, and may die within 48 hours. Sometimes the illness lasts longer, and the mucous membranes and other tissues become swollen, including the eyes, nose, mouth, ears (which become droopy) and the genital and anal areas. The entire face may become very swollen, and thick pus may be discharged from the nose and the rabbit may have difficulty breathing. Most rabbits die within 14 days.
In more chronic cases (and depending on the strain of virus and immunity of the rabbit) lumps and nodules (myxomas) may develop over the body. Rabbits with this form may survive, and become immune to myxomatosis virus. This seems to be a less likely course of disease in domestic rabbits, however, with most suffering from the acute forms with eventual death.
Here is some more techincal information, derived from an article in "The Biology & Medicine of Rabbits & Rodents":
Myxomatosis is a viral disease of domestic rabbits. Wild rabbits (Sylvilagus) act as the natural or reservoir host. The virus causes local skin tumors in wild lagomorphs.
Etiology: The disease is caused by several strains of poxviruses on the more virulent end of the myxoma-fibroma spectrum of viruses. The highly virulent variant occurring in western California and Oregon is, appropriately, the California strain; other strains exist in South America, Australia, and Europe.
Transmission: The highly virulent virus is transmitted from the wild reservoir primarily by arthropod vectors. The dimensions of the transmission vary with the populations of mosquitoes, mite, and fleas feeding on rabbits. An increase in myxomatosis is seen from August to November. As these populations vary with seasons and years, so does the incidence of myxomatosis. Arthropods act as mechanical vector, as might birds, plants and fomites.
Clinical signs: The first signs following infection with California strain produce a "sleepy eyed" rabbit, mild lethargy, red eyes and swollen lids, fever, and a watery ocular discharge. If the rabbit survives the acute stage, the reddening and swelling extend to the lips, face, ears, and ano-genital areas. Death follows in a large percentage of rabbits. Those surviving develop cutaneous hemorrhages. Affected rabbits are anorectic and dehydrated, but many do survive and the lesions regress over 1 to 3 months. In chronic cases, the disease is often complicated by Pasteurellosis, which causes the deaths.
Lesions in the European and South American disease forms include the development of skin tumors, which may eventually rupture and ooze.
Necropsy: Gross necropsy signs following infection with the California strain are subcutaneous edema and widespread visceral hemorrhage. Microscopically, there is extensive epithelial proliferation with ballooning of cells in the stratum granulosum and hyperkeratinization. Large eosinophilic inclusion bodies are present in the stratum germinativum. There is lympocytic depletion in the spleen and necrosis of lymphatic tissue in several organs. European rabbits develop mucinous skin tumors and endothelial proliferation in capillaries and small venues.
Diagnosis: Diagnosis is based on clinical signs, necropsy findings and the characteristic histopathologic appearance of the lesions.
Treatment: Limited options
Prevention: Vector control through spraying and screening, avoidance of wild rabbits, quarantine of new arrivals, and vaccination with attenuated vaccines prepared in the face of an outbreak are methods of preventing myxomatosis. The vaccine used to combat myxomatosis in Europe is not approved for use in the United States.
Public Health Significance: The Myxomatosis virus does not affect humans.
Reprint from "The Biology & Medicine of Rabbits & Rodents" by John E., Harkness & Joseph E. Wagner.
Contact your Veterinarian immediately if these symptoms occur.