4. Discussion
The presence of external signs of myxomatosis in three adult and three juvenile farm European rabbits in an outbreak registered in Aljustrel (Alentejo, South Portugal) in August 2020, the clinical outcome (83% mortality), along with the histopathological and molecular data, confirmed that five out of six infected rabbits died from this disease and that the virus involved was the recombinant myxoma virus , thus demonstrating the susceptibility of rabbits to this new virus.
Lesions described in the three necropsied rabbits, in particular skin lesions, were fully coincident with the ones of classical myxomatosis also found in hares(Carvalho et al., 2020) , such as ballooning degeneration of keratinocytes and myxoid change of the dermal connective tissue.
Given that ha-MYXV, originally considered hare specific, is also being detected in rabbits, we rather refer to ha-MYXV (also known as MYXV-Tol) as recombinant myxoma virus (rec-MYXV), since it affects both leporid species.
The mortality found in this small backyard rabbitry was ≈83%, apparently higher than what was estimated in Iberian hare’ outbreaks (55.4%) (García-Bocanegra et al., 2020), despite the small number of animals involved may biased this evaluation.
Interestingly, the oldest of the rabbits (a two years old male) developed very mild symptoms and survived the infection developing high antibody titter (RI10 = 18.4). No serum was available from the five dead rabbits due to blood coagulation. The owner reported no previous vaccination, but being the oldest animal in the group, we cannot exclude that it may have contacted with classic MYXV strains in the past, acquiring some level of cross-immunity, allowing its survival.
Despite the biosecurity measures adopted in the rabbit industry, virus introduction from the outside is always at risk. The rabbit industry is one of the largest expanding meat industries in the world, having had a positive evolution of 85% between 1998 and 2017. In fact, in 2017, around 1,483,000 tons of rabbit meat were produced in the world (Trocino et al., 2019), being the China, Democratic People’s Republic of Korea and Spain the bigger producers. In China, the research funding for rabbit science reaches values of 2,443 million of Euros and in France and Germany, the research in this area is funded with around 600-700 millions of Euros a year (Trocino et al., 2019).
When myxomatosis emerged in the Europe around 1950’s, and until the development of vaccines, millions of domestic rabbits died with incalculable economic losses. Just in Great Britain, tens of millions of rabbits died (Bartrip, 2008). Spill over between the wild and domestic animals has been appointed as source of infection for outbreaks in the industry (C. L. Carvalho et al., 2017). Given the impact that viral infectious diseases may have in this sector, it is critical to monitor the sanitary status of industry and of the wild populations.
To the best of our knowledge this is the first documented description of myxomatosis in domestic rabbits by recombinant MYXV (referred as ha-MYXV or MYXV-Tol) infection. It is crucial to verify if commercial vaccines fully/effectively protect rabbits from this recombinant virus.