Influenza virus infection in newborn rats: A possible marker of attenuation for man

Abstract

The growth of parent influenza viruses A/England/939/69 and A/PR/8/34, and clones 6, 7, and 64C, derived by recombination, was studied in newborn rats. Using an inoculum of 10^4.0 EID₅₀, influenza virus A/England/939/69 produced the highest titres of virus in rat turbinates at 48 hours after inoculation; clones 6 and 7 and A/PR/8/34 grew to lower titres; and clone 64C grew to the lowest titre. These differences were less apparent when 10^2.0 EID₅₀ of virus was used as an inoculum, and rats were not infected by smaller inoculum of any of the virus strains. Infection with 10^4.0 EID₅₀ of all viruses produced lung infection; at 48 hours after infection, the highest titres were recovered from rats infected with A/PR/8/34 and A/England/939/69 virus. Prior infection with A/England/939/69 or A/PR/8/34 increased the incidence of bacteraemia and meningitis following intranasal inoculation of Haemophilus influenzae type b; infection with clone 64C did not enhance bacterial meningitis, while infection with clone 6 gave an intermediate result. Volunteer studies with these viruses have shown that influenza virus A/England/939/69 was virulent, clones 6 and 7 were attenuated, clone 64C was overattenuated, and A/PR/8/34 virus was noninfective for man. The relative titres of virus recovered from turbinates taken 48 hours after infection with 10^4.0 EID₅₀ of virus and the ability of virus infection to enhance bacterial infection correlated with the property of virus attenuation for man for four of the five strains tested; however, no correlation was seen for A/PR/8/34 virus, which is a result also found in other laboratory tests designed to measure virulence for man.