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A new virus of minimal pathogenicity associated with friend virus. I. Isolation by end‐point dilution

In an attempt to purify a strain of Friend virus (FV) by passage in mice at end‐point dilution, we injected groups of BALB/c mice with ten‐fold dilutions of the virus. After 3 weeks, plasma was collected from each mouse and the spleen weighed. Mice given virus diluted 10−7 and untreated controls had spleens weighing between 100 and 120 mg. Of the six mice given virus diluted 10−1, two had spleens weighing over 200 mg; plasma samples from both of these and from two mice of the same group, which had normal sized spleens, were tested in mice. After 3 weeks, only the recipients of plasma from mice with splenomegaly had enlarged spleens (200–350 mg). The virus obtained by end‐point dilution is referred to in this paper as the Rowson‐Parr virus (RPV). It has been passaged by plasma seven times at 3‐weekly intervals and regularly produces splenomegaly of a minor degree which does not progress. The virus is present in the plasma of infected animals for at least 122 days and the spleen does not return to normal but the infection is not lethal. Mice infected with RPV develop gross splenomegaly following a subsequent injection of FV but the disease progresses more slowly than in mice infected with FV alone. Neutralizing FV antibodies could not be demonstrated in the plasma of RPV infected mice. On filtration through gradocol membranes RPV appeared to be the same size as FV. It was chloroform sensitive, resistant to terramycin and did not affect the plasma lactate dehydrogenase level of infected mice.