Ecotropic leukemia viruses in congenic C57BL mice: natural dissemination by milk-borne infection.

The incidence of infectious murine leukemia virus (MuLV) in congenic-resistant C57BL mice was studied. No constant relationship between the incidence of MuLV and the H-2 complex was apparent. Two lines, B10 and B10.A, were examined in detail because the incidence of MuLV in B10 was low and the virus appeared relatively late in life, whereas B10.A animals had a relatively high incidence of infection by MuLV early in life. Further studies of B10.A mice revealed an almost universal concordance between the virologic status of the mother and her offspring. This was particularly evident when (B10 times B10.A)F1 animals were compared with (B10.A times B10)F1 mice: Although genetically identical, the incidence of MuLV in the latter was high, whereas in the former it was low. Transmission of MuLV by milk was proved by foster-nursing experiments; when the infants of MuLV-positive B10.A mothers were suckled on MuLV-negative B10.A mothers, they were free of MuLV. Milk-borne infection may account for the natural dissemination of MuLV among some inbred lines of mice.