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Sandbox in which Rabbit Vet will be playing... Myxomatosis in North America (Californian Myxomatosis)


Myxoma virus
Myxoma virus (transmission electron microscope)
Virus classification Edit this classification
(unranked): Virus
Realm: Varidnaviria
Kingdom: Bamfordvirae
Phylum: Nucleocytoviricota
Class: Pokkesviricetes
Order: Chitovirales
Family: Poxviridae
Genus: Leporipoxvirus
Species:
Myxoma virus

Myxomatosis is a disease caused by the myxoma virus, a poxvirus in the genus Leporipoxvirus. The natural host in North America is the brush rabbit (Sylvilagus bachmani). The myxoma virus causes only a mild disease in the brush rabbit, but causes a severe and usually fatal disease in European rabbits (Oryctolagus cuniculus), the species of rabbit commonly kept as pets or used as a food source.


Cause

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The myxoma virus is in the genus Leporipoxvirus (family Poxviridae; subfamily Chordopoxvirinae). Like other poxviruses, myxoma viruses are large DNA viruses with linear double-stranded DNA. Virus replication occurs in the cytoplasm of the cell. In North America the natural host of the virus is the brush rabbit (Sylvilagus bachmani). The myxoma virus causes only a mild disease in brush rabbits, with signs limited to the formation of short-lived skin nodules.Cite error: The <ref> tag has too many names (see the help page). The distribution of Myxomatosis in North America is limited to the brush rabbit’s native habitat, which extends from the Columbia River in Oregon to the north, the Sierra Nevada and Cascade mountains to the East, and the tip of the Baja California peninsula to the south.[1] The brush rabbit is the sole carrier of myxoma virus in North American because other native lagomorphs, including cottontail rabbits and hares, are incapable of transmitting the disease.[2][3]

 
Pet rabbit with myxomatosis caused by the MSW strain of myxoma virus (Santa Cruz, CA); note the facial swelling and subcutaneous edema

Transmission

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Myxomatosis is transmitted primarily by insects. The myxoma virus does not replicate in these arthropod hosts, but is physically carried by biting arthropods from one rabbit to another. In North America mosquitoes are the primary mode of viral transmission, and multiple species of mosquito can carry the virus.[3][4][5][6] Transmission via the bites of fleas, flies, lice, and mites have also been documented.[7][8] Outbreaks have a seasonal nature and are most likely to occur between August and November, but cases occur in other months as well.[9] Seasonality is driven by the availability of arthropod vectors and the proximity of infected wild rabbits.[10]

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.[10]

Pathophysiology

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Clinical presentation

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Pet rabbit with myxomatosis caused by the MSW strain of myxoma virus (Santa Cruz, CA); note the swollen eyelids and genitals

The clinical signs of myxomatosis depend on the strain of virus, the route of inoculation, and the immune status of the host. Cases of myxomatosis in northern California, southern California, Oregon, and Mexico are caused by different strains of myxoma virus, and clinical signs vary.

A 2024 study found that myxomatosis caused by the MSW strain (California/San Francisco 1950) of the myxoma virus occurs regularly in the greater San Jose and Santa Cruz regions of California.[9] Common physical examination findings included swollen eyelids, swollen genitals, fever, and lethargy. Swollen lips and ears and subcutaneous edema also occurred in some rabbits. Domesticated rabbits that spent time outdoors were at greater risk of acquiring the disease, and most of the cases occurred between August and October. It was postulated that that the number of cases brought to veterinarians underestimates the number of cases occurring, as sudden death was sometimes the first sign noted.[9]

A 1930 report of myxomatosis in southern California described rabbits as developing swelling of the nose, lips, ears, and genitals as well as purulent discharge from the nose and eyes. Rabbits that survived for over a week developed cutaneous nodules on the face.[11]

A 1977 report of a myxomatosis outbreak in Lane County and other parts of western Oregon described a single rabbit that developed copious mucopurulent ocular discharge and a chronic form of myxomatosis from which it eventually recovered.[12]

A 2003 report from Oregon state reported necrotizing hemorrhagic appendicitis in nearly 50% of cases.[13]

A 2000 report from the Baja Peninsula in Mexico in 1993 described a nodular form of the disease characterized by respiratory symptoms.[14]

Diagnosis

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Because the clinical signs of myxomatosis are distinctive and nearly pathognomonic, initial diagnosis is largely based on physical exam findings.[15][16][17] 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.

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.[18] 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.[19]

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.[18] 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.[20]

Treatment

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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.[10]

Prevention

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Vaccination

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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.[21] The vaccine does not provide 100% protection,[10] 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.[21]

In Europe and the United Kingdom a bivalent vectored vaccine called Nobivac Myxo-RHD[22] 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.[23]

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.[24] In the United States the importation of vaccines is overseen by the Animal and Plant Health Inspection Service, part of the Department of Agriculture.[25]

Other preventive measures

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In locations where myxomatosis is endemic but no vaccine is available, preventing exposure to the myxoma virus is of vital importance. 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. Using rabbit-safe medications to treat and prevent fleas, lice, and mites is also warranted. New rabbits that could have been exposed to the virus should be quarantined for 14 days before being introduced to current pets.

Any rabbit suspected of having myxomatosis should be immediately isolated, and all rabbits moved indoors. To prevent the virus from spreading further any cages and cage furnishings that have come into contact with infected rabbits should be disinfected, and owners should disinfect their hands between rabbits. Poxviruses are stable in the environment and can be spread by fomites but are highly sensitive to chemical disinfection; bleach, ammonia, and alcohol can all be used to deactivate myxoma virus.[26] They are resistant to drying but are sensitive to some disinfectants.[27]

Reporting

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References

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  1. ^ Regnery, David C.; Marshall, Ian D. (1971). "Studies in the epidemiology of myxomatosis in California". American Journal of Epidemiology. 94 (5): 508–513. doi:10.1093/oxfordjournals.aje.a121348.
  2. ^ Silvers, L.; Barnard, D.; Knowlton, F.; Inglis, B.; Labudovic, A.; Holland, M.K.; Janssens, P.A.; van Leeuwen, B.H.; Kerr, P.J. (March 2010). "Host-specificity of myxoma virus: Pathogenesis of South American and North American strains of myxoma virus in two North American lagomorph species". Veterinary Microbiology. 141 (3–4): 289–300. doi:10.1016/j.vetmic.2009.09.031.
  3. ^ a b Grodhaus, G; Regnery, DC; Marshall, ID (1963). "Studies in the epidemiology of myxomatosis in California. II. The experimental transmission of myxomatosis in brush rabbits (Sylvilagus bachmani) by several species of mosquitoes". Am J Hyg. 77: 205–212.
  4. ^ Marshall, ID; Regnery, DC; Grodhaus, G (1963). "Studies in the epidemiology of myxomatosis in California I. Observations on two outbreaks of myxomatosis in coastal California and the recovery of myxoma virus from a brush rabbit (Sylvilagus bachmani)". American Journal of Epidemiology. 77 (2): 195–204. doi:10.1093/oxfordjournals.aje.a120310.
  5. ^ Regnery, DC; Marshall, ID (1971). "Studies in the epidemiology of myxomatosis in California. IV. The susceptibility of six leporid species to Californian myxoma virus and the relative infectivity of their tumors for mosquitoes". American Journal of Epidemiology. 94 (5): 508–513. doi:10.1093/oxfordjournals.aje.a121348.
  6. ^ Regnery, DC; Miller, JH (1972). "A myxoma virus epizootic in a brush rabbit population". Journal of Wildlife Diseases. 8 (4): 327–331. doi:10.7589/0090-3558-8.4.327.
  7. ^ Bertagnoli, S; Marchandeau, S (2015). "Myxomatosis". Revue Scientifique et Technique de l'OIE. 34 (2): 539–556. doi:10.20506/rst.34.2.2378.
  8. ^ Ross, J; Tittensor, AM; Fox, AP; Sanders, MF (1989). "Myxomatosis in farmland rabbit populations in England and Wales". Epidemiology and Infection. 103 (2): 333–357. doi:10.1017/s0950268800030703.
  9. ^ a b c Stern, HS; Biswell, E; Kiupel, M; Ossiboff, RJ; Brust, K (2024). "Epidemiologic, clinicopathologic, and diagnostic findings in pet rabbits with myxomatosis caused by the California MSW strain of myxoma virus: 11 cases (2022–2023)". Journal of the American Veterinary Medical Association. 262 (9): 1–11. doi:10.2460/javma.24.02.0139.
  10. ^ a b c d Kerr, P (2013). "Viral Infections of Rabbits". Veterinary Clinics of North America: Exotic Animal Practice. 16 (2): 437–468. doi:10.1016/j.cvex.2013.02.002. PMC 7110462. PMID 23642871.
  11. ^ Kessel, JF; Prouty, CC; Meyer, JW (1931). "Occurrence of infectious myxomatosis in southern California". Exp Biol Med (Maywood). 28 (4): 413-414.
  12. ^ Patton, NM; Holmes, HT (1977). "Myxomatosis in domestic rabbits in Oregon". J Am Vet Med Assoc. 171 (6): 560–562.
  13. ^ Löhr, CV; Kiupel, M; Bildfell, RJ (2005). "Outbreak of myxomatosis in domestic rabbits in Oregon: pathology and detection of viral DNA and protein". Veterinary Pathology. 42 (5): 680–729. doi:10.1177/030098580504200501.
  14. ^ Licón Luna, RM (2000). "First report of myxomatosis in Mexico". J Wildl Dis. 36 (3): 580-583.
  15. ^ Kerr P (2013). "Myxomatosis". In Mayer, J; Donnelly, TM (eds.). Clinical Veterinary Advisor: Birds and Exotic Pets. Elsevier Saunders. p. 398–401.
  16. ^ Smith MV (2023). "Urogenital diseases". In Smith, MV (ed.). Textbook of Rabbit Medicine (3rd ed.). Elsevier. p. 314–331.
  17. ^ Girolamo ND; Selleri P (2021). "Disorders of the urinary and reproductive systems". In Quesenberry, KE; Orcutt, CJ; Mans, C; Carpenter, JW (eds.). Ferrets, Rabbits, and Rodents (4th ed.). Elsevier. p. 201–219.
  18. ^ a b MacLachlan, J (2017). Fenner's Veterinary Virology, 5th Edition. Elsevier. p. 158. ISBN 978-0-12-800946-8.
  19. ^ MacLachlan, J (2017). Fenner's Veterinary Virology, 5th Edition. Elsevier. p. 112. ISBN 978-0-12-800946-8.
  20. ^ Albini, S; Sigrist, B; Güttinger, R; et al. (2011). "Development and validation of a real-time polymerase chain reaction assay" (PDF). Journal of Veterinary Diagnostic Investigation. 24 (1): 135–137. doi:10.1177/1040638711425946. PMID 22362943. S2CID 32171325.
  21. ^ a b Cite error: The named reference Meredith 2013 was invoked but never defined (see the help page).
  22. ^ "Nobivac Myxo RHD". MSD Animal Health. Retrieved 20 July 2019.
  23. ^ "Nobivac Myxo RHD Data Sheet". European Medicine Agency. Archived from the original on 20 July 2019. Retrieved 20 July 2019.
  24. ^ "Myxomatosis in the US". House Rabbit Society. Retrieved 23 August 2019.
  25. ^ "Veterinary Biologics". United States Department of Agriculture, Animal and Plant Health Inspection Service. Retrieved 23 July 2019.
  26. ^ Delhon, Gustavo (2022). "Poxviridae". Veterinary Microbiology: 522–532. doi:10.1002/9781119650836.ch53.
  27. ^ "Disinfection". The Center for Food Security and Public Health. Retrieved 21 July 2019.

Further reading

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