Myxomatosis – An effective method of rabbit control or an issue of animal welfare?
By Nicola Foreman
This article is an essay written towards the degree of Bachelor of Animal and Veterinary Bioscience at The University of Sydney.
Introduction
Myxomatosis has had a long history within Australia; it has been used since its first introduction in the 1950’s as a lethal biological control agent of the wild population of European rabbits with a kill rate of close to 100%. Today it is still affecting wild populations with a large percentage of deaths still being attributed to myxomatosis [1] [2]. The manner in which the virus affects the host after infection has been an ongoing issue of animal welfare; the symptoms are prolonged for a minimum of 8 days, but can last up to two weeks. The virus is transferred throughout the tissues of the body from the site of infection by the lymphatic system, symptoms include large lesions found on the ears, muzzle, genital regions, mucosal orifices, eyelids, which eventually close over due to swelling, and infection of the respiratory tract; all of which result in death [3].
Discussion
Recent scientific studies have examined just how the myxoma virus affects the immunological system of the rabbit following infection, and the results seen raise the welfare issue of whether the successes of the virus as a method of control is worth the pain, suffering, and prolonged death to the rabbit. In a study conducted by [2] the effects of the virus to the host immune system were assessed; their results revealed a succession of events that led to death. The rabbits in their research developed an inflammation of the inoculation site at day two. At day four, conjunctivitis was noted, at day five secondary lesions were seen on the eyelids and ears as well as a swelling of the genital region. Day six and seven saw a discharge from the eyes, on day seven the primary lesion at the site of inoculation darkened in colour, became necrotic on day eight and by day ten was oozing a serous fluid. On day eight the eyes of the rabbits were nearly closed due to substantial swelling of the eyelids, respiratory distress was noted, and by day eleven displayed acute dyspnoea, open mouthed breathing, and secondary lesions were hard, protuberant, and spread over the body of the rabbit. Following this the rabbits were euthanised due to the distress of the rabbits, severity of the symptoms, and the terminal nature of the disease. Also in the same experiment by [2] they assessed the lymphocyte activity and distribution following infection, they observed a decrease in T cells in the lymph node closest to the site of infection on day four, and from day six similar changes in the collateral lymph nodes, spleen and blood. They discovered that the lymphocyte activity noted occurred only in the presence of the myxoma virus.
[3] discovered similar symptoms in their research. From day four the primary lesion was raised, by day six lesions were forming on the ears and eyelids, by day seven the lesion was large and protuberant, eyelids were red and swelling; lesions were also forming in each ear. By day eight the primary lesion measured up to 3cm, the head of the rabbit was enlarged, the muzzle and nose contained lesions, and the ears were greatly swollen, heavy and purple in colour. Days 9-11 showed how the severity of the symptoms affected the rabbit with a decline in food and water consumption, increased lesions on nose, difficulties in breathing, and heavy droopy ears with multiple lesions. Their research also assessed the viral gene M130R and its role in the replication of the myxoma virus. [3] discovered that M130R was essential for a full myxomatosis response. Without this protein the immune system was able to initiate a response that limits the spread and severity of the reaction to infection.
A third study conducted by [4] went further in to the investigation of repeating proteins. Their study explored both M148R and M149R and presented the first characterisation of these proteins. Their research concluded that through a constructed deletion of one or both proteins a delay in the onset of the symptoms of myxomatosis was seen, as well as an increase in survival time of an affected rabbit, and a remarkable reduction in mortality rate. However [4] have concluded that further research is needed to determine any interaction between M148R and M149R.
All of these research studies have provided evidence on the way in which the myxoma virus interacts with the host. They have shown through experimentation that the death of the European rabbit through infection of myxomatosis is a process that can last up to two weeks, it is a virus that spreads through the tissues of the body by the lymphatic system and that the effects of this virus produce symptoms that have a severe impact on the host and that death is nearly always inevitable.
The welfare implications of this virus were considered by [5] in 1986 prior to the release of myxomatosis in New Zealand. Some of the argument against the release included the length of time the disease took to claim the life of the host, and that eight days of pain and suffering was too long when other methods were available for consideration. They reasoned that with rabbits having a nervous system similar to our own the pain and suffering felt by them would be in a similar way to how a human would experience it.
Conclusion
It has to be considered that the knowledge of the myxoma virus that has been obtained by the research teams referenced in this report has furthered the welfare of the European rabbit in that there is a reference of the severity of the symptoms, the length of time before the rabbit is finally claimed by the virus, and the nature of proteins affecting the immunological response of the host. These results must be considered for the welfare of the European rabbit prior to any release of the virus.
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References
[1] Saunders G, Cooke B, McKoll K, Shine R, Peacock T, 2010, ‘Modern Approaches for the Biological Control of Vertebrate Pests: An Australian Perspective’ Biological Control, vol. 52, pp. 288-295.
[2] Jeklova E, Leva L, Matiasovic J, Kovarcik K, Kudlackova H, Nevorankova Z, Psikal I, Faldyna M, 2008, ‘Characterisation of Immunosuppression in Rabbits After Infection With Myxoma Virus’, Veterinary Microbiology, vol. 129, pp. 117-130.
[3] Barrett J.W, Werden S.J, Wang F, McKillop W.M, Jimenez J, Villeneuve D, McFadden G, Dekaban A, 2009, ‘Myxoma Virus M130R is a Novel Virulence Factor Required for Lethal Myxomatosis in Rabbits’, Virus Research, vol. 144, pp. 258-265.
[4] Blanie S, Mortier J, Delverdier M, Bertagnoli S, Camus-Bouclainville C, 2009, ‘M148R and M149R are Two Virulence Factors for Myxoma Virus Pathogenesis in the European Rabbit’ Vet. Res, vol. 40, no. 11, pp. 1-14.
[5] Gumbrell R.C, 1986, ‘The Association, Rabbit Control and Myxomatosis’, N.Z. vet. J, vol. 34, pp. 21.
For further information, read:
Berman D, Kerr P.J, Stagg R, van Leeuwen B.H, Gonzalez T, 2006, Should the 40 Year Old Practice of Releasing Virulent Myxoma Virus to Control Rabbits (Oryctolagus cuniculus) be Continued, Wildlife Research, vol. 33, pp. 549-556.
Ferreira C, Ramirez E, Castro F, Ferreras P, Alvez P.C, Redpath S, Villafuerte R, 2009, ‘Field Experimental Campaigns Against Myxomatosis and their Effectiveness in the Wild’, Vaccine, vol. 27, pp. 6998-7002.
Fouchet D, Guitton J, Marchandeau S, Pontier, D, 2008, ‘Impact of Myxomatosis in Relation to Local Persistence in Wild Rabbit Populations: The Role of Waning Immunity and the Reproductive Period’, Journal of Theoretical Biology, vol. 250, pp. 593-605.
Scanlan J. C, Berman D.M, Grant W.E, 2006, ‘Population Dynamics of the European Rabbit (Oryctolagus cuniculus) in North Eastern Australia: Simulated Responses to Control’, Ecological Modelling, vol. 196, pp. 221-236.
Stanford M.M, Werden S.J, McFadden G, 2007, ‘Myxoma Virus in the European Rabbit: Interactions Between the Virus and its Susceptible Host’, Vet. Res, vol. 38, pp. 299-318.
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