Evolutionary history and molecular epidemiology of rabbit haemorrhagic disease virus in the Iberian Peninsula and Western Europe.

BACKGROUND: Rabbit haemorrhagic disease virus (RHDV) is a highly virulent calicivirus, first described in domestic rabbits in China in 1984. RHDV appears to be a mutant form of a benign virus that existed in Europe long before the first outbreak. In the Iberian Peninsula, the first epidemic in 1988 severely reduced the populations of autochthonous European wild rabbit. To examine the evolutionary history of RHDV in the Iberian Peninsula, we collected virus samples from wild rabbits and sequenced a fragment of the capsid protein gene VP60. These data together with available sequences from other Western European countries, were analyzed following Bayesian Markov chain Monte Carlo methods to infer their phylogenetic relationships, evolutionary rates and demographic history. RESULTS: Evolutionary relationships of RHDV revealed three main lineages with significant phylogeographic structure. All lineages seem to have emerged at a common period of time, between ~1875 and ~1976. The Iberian Peninsula showed evidences of genetic isolation, probably due to geographic barriers to gene flow, and was also the region with the youngest MRCA.Overall, demographic analyses showed an initial increase and stabilization of the relative genetic diversity of RHDV, and a subsequent reduction in genetic diversity after the first epidemic breakout in 1984, which is compatible with a decline in effective population size. CONCLUSIONS: Results were consistent with the hypothesis that the current Iberian RHDV arose from a single infection between 1869 and 1955 (95% HPD), and rendered a temporal pattern of appearance and extinction of lineages. We propose that the rising positive selection pressure observed throughout the history of RHDV is likely mediated by the host immune system as a consequence of the genetic changes that rendered the virus virulent. Consequently, this relationship is suggested to condition RHDV demographic history.

Molecular characterisation and recent evolution of myxoma virus in Spain.

Myxoma virus and the European rabbit are one of the best-studied examples of coevolution of pathogen virulence and host resistance. Since the introduction of the virus in Spain in 1953, a decrease in its virulence has been observed; however, most strains are still considered highly virulent. To determine whether this attenuation is due to molecular differences, and to characterise the field strains in Spain and the genetic changes that have occurred since the introduction of the virus, we analysed 7,741 bp in 97 virus samples from 12 localities. We found an extremely low genetic variability and an absence of a geographic structure. We defined 35 haplotypes, none of which were identical to the original Lausanne strain. Three genetic groups were determined and were found to occur at different frequencies in different locations. Overall, virus evolution deviated from neutrality and did not conform to a strict molecular clock, probably due to the existence of a strong selective pressure that acts differently across the viral genome.