Influenza virus infection of a newborn rats: virulence of recombinant strains prepared from a cold-adapted, attenuated parent.

Infant rats were infected with one of a series of influenza A viruses. The growth of viruses in the turbinates or lungs, and the ability of virus infection to potentiate a subsequent bacterial infection by Haemophilus influenzae (HIb), were measured. The three virus strains known to be virulent for man grew to relatively high titres of 10(5.2)--10(6.8) EBID50/ml in the turbinates of infant rats at 48 hours post-infection, and virus infection enhanced subsequent systemic infection following intranasal inoculation of rats with HIb. In contrast, influenza virus A/Ann Arobr/6/60--P17 and the three recombinant viruses prepared from this strain, all of which are attenuated for man, replicated to significantly lower titres of 10(2.6)--10(4.1) EBID50/ml in infant rats turbinates, and failed to promote systemic infection by HIb to the samest that the behaviour of influenza viruses in infant rats may be an indication for virus virulence for man, and thus provide a test which could facilitate the development of live, attenuated virus vaccines.