Partial sequencing of recent Portuguese myxoma virus field isolates exhibits a high degree of genetic stability.

To study genetic changes underlying myxoma virus evolution in its new host, the European rabbit (Oryctolagus cuniculus), we sequenced selected genomic regions of nine recent virulent field strains and a live attenuated vaccine strain ("MAV", Germany). DNA was extracted from cell culture passaged myxoma virus. A total of 4863 bp (approximately 3% of the genome) of 10 regions spanning 12 genes of the myxoma viruses was sequenced and compared to the original virulent strain "Lausanne" and its attenuated field derivative strain "6918". The field strains displayed a maximum of three (strains C43, C95) and a minimum of one (strains CD01, CD05) nucleotide substitutions. These were distributed through all analysed coding regions, except gene M022L (major envelope protein), where all strains were identical to "Lausanne" and "6918". Two new single nucleotide insertions were observed in some of the field strains: within the intergenic region M014L/M015L and within gene M009L, where it leads to a frameshift. These insertions were located after homopolymeric regions. The vaccine strain displayed 37 nucleotide substitutions, predominantly (95%) located in genes M022L and M036L. Interestingly, regions M009L and M014L/M015L of the vaccine were not amplified successfully, suggesting major genomic changes that could account for its attenuated phenotype. Our results support a high degree of genetic stability of myxoma virus over the past five decades. None of the analysed genome regions by its own seems sufficient for the genetic characterisation of field strains.