Summary
Myxomatosis is an emergent disease in Iberian hare, having been considered a rabbit disease for decades. Genome sequencing of the strains obtained from affected Iberian hares showed to be distinct from the classical strains that circulated in rabbits since the virus introduction in Europe, in 1952. The main genomic difference concerns the presence of an additional 2.8Kb region disrupting the M009L gene and adding a set of genes with homology to the MYXV genes M060R, M061, M064 and M065R originated in poxviruses. After the emergence of this recombinant virus (MYXV-Tol, ha-MYXV ), in the summer of 2019, recombinnat MYXV was not detected in rabbit surveys suggesting apparent species segregation with the MYXV classic strains persistently circulating in rabbits.
Recently, a group of six unvaccinated European rabbits (Oryctolagus cuniculus cuniculus ) from a backyard rabbitry in the South Portugal, developed signs of myxomatosis (anorexia, dyspnoea, oedema of eyelids, head, ears, external genitals and anus, and skin myxomas in the base of the ears), five of them dying within 24-48 hours of symptoms onset. Molecular analysis revealed that only the recombinant myxoma virus was present.
This is the first documented report of a recombinant myxomatosis virus in farm rabbits associated with high mortality, which aggravates the concern for the future of the Iberian hare and wild rabbits and the safety of the rabbit industry against which the existing vaccines may not be fully protective.
Keywords: European rabbit, Oryctolagus cuniculus cuniculus , rabbit farming, Recombinant myxoma virus, ha-MYXV, MYXV-Tol, Myxomatosis, Myxoma virus
Introduction
Myxoma virus (MYXV) is a Leporipoxvirus from the Poxviridae family that causes an infectious, often fatal, systemic disease in the European rabbit called myxomatosis. The virus is a dsDNA that replicates in cytoplasm (Murphy et al., 1995). After the virus introducing in France, in 1952, MYXV spread throughout all Europe, with mortalities rates as high as 90% (Fenner and Ratcliffe, 1965). Over the years, the virulence of the strains decreased significantly due to increase of genetic resistance by the host, development of immunity and natural selection of less virulent strains (Fenner and Fantini, 1999; Kerr, 2012). Nowadays, the disease persists and is considered one of the main reasons behind the fragile conservation status (danger of extinction) of the European rabbit (Duarte et al., 2018; Villafuerte and Delibes-Mateos, 2019; C. L. Carvalho et al., 2020).
The development and production of heterologous and homologous vaccines against myxomatosis for rabbits, proved to confer seroconversion in almost 100% of rabbits (Hipra, 2003), and, along with the application of biosafety measures, allow to control the disease in the industrial rabbit farming and small rabbitries, contrarily to wild life.
During almost seven decades, myxomatosis has been rarely reported in European brown hares (Collins, 1955; Wibbelt and Frolich, 2005), never in Iberian hare, being considered a rabbit disease. However, in late 2018, a natural recombinant MYXV (ha-MYXV) emerged in Iberian hare, leading to the first epidemic outbreaks in this species, throughout Portugal and Spain (Bocanegra et al., 2019; C. L. Carvalho et al., 2020). The change in host tropism was attributed to a genomic modification comprising a 2.8kb insertion containing genes with homology to the M060R, M061R, M064 and M065R genes of MYXV, which are located around position 60Kb in the genome. The insert disrupted the M009L gene (Dalton et al., 2019; Pinto et al., 2019) located around position 12Kb in the MYXV genome, which became divided in two smaller ORFs.
To date, no cases of mortality in European rabbit due to this recombinant virus have been documented, although cases of myxomatosis in wild and domestic rabbits continue to be monitored and genotyped in Portugal, under the scope of a national surveillance program (+Coelho Project, dispatch 4575, 31 may 2017, MAFDR).
Here we report a case of high mortality (≈83%) in a small backyard rabbitry. Genotyping confirmed the unique presence of a recombinant virus, similar to the ha-MYXV identified in Iberian hares. None of the rabbits was vaccinated impeding any inference on the degree of protection conferred by commercial vaccines against the recombinant MYXV. However, the confirmation that recombinat MYXV induces severe disease in rabbits, raises new concerns for the wild and domestic leporid species, emphasizing the need of continuous monitoring and genomic characterization of viruses circulating in these species in the Iberia.