Study of the virulence of five strains of amyxomatous myxoma virus in crossbred New Zealand White/Californian conventional rabbits, with evidence of long-term testicular infection in recovered animals

The virulence of five amyxomatous myxoma virus (MV) strains, the clinical a nd pathogenetic effects of which had been studied previously in specific :p athogen-free (SPF) rabbits, was determined by inoculation of five groups-of 10 crossbred New Zealand White/Californian conventional rabbits, A much mo re acute myxomatosis syndrome was produced in conventional rabbits than tha t reproduced previously in SPF animals. However, the main clinical signs we re of the respiratory type. The MV strains MYX 254/95 and 801 appeared very virulent, killing all the inoculated animals. The strains MYX 217/95, MYX 555/94 and Saint Benoist were somewhat attenuated, killing only seven, six and six rabbits, respectively. Extensive lung lesions due to supervening ba cterial infections were observed in 36 of the 39 rabbits that died. Lethali ty was found to be a better estimate of virulence than mean survival time. By 98 days after viral inoculation, all the surviving animals had completel y recovered. At that time, they were immunosuppressed by treatment with adr enocorticotrophic hormone (ACTH) for 10 days to determine whether they stil l harboured the virus. After the ACTH treatment, eight of the 1 1 surviving rabbits showed clinical signs that resembled amyxomatous myxomatosis. All the virological examinations performed on naso-conjunctival exudate, on mon onuclear cells, on eyelids and on ovaries remained negative but infectious virus was isolated from the testes of three of six surviving male rabbits. (C) 2000 Harcourt Publishers Ltd.