Comparison of Overt and Inapparent Influenzal Infection in Ferret

ABSTRACT

In relation rto the size of viral inoculum, influenzal infection in ferrets caused by the inoculation of a mouse-adapted subline of PR 8 strain of type A influenza virus was studied. The results are summarized as follows; 1) When ferrets were inoculated with a highly diluted virus (10-7), a small proportion of them experienced inapparent infections and the rest of them escaped the infection. 2) With the increased size of viral inocula, there was a good correlation between the size of infecting does and the frequency of overt infections in ferrets. 3) Nasal tissues were the main locus of viral multiplication in ferrets at 72 hours after viral inoculation. Viral multiplication in nasal tissues was demonstrated only in a small proportion of ferrets which were inocu1ated with a 10-7 dilution of virus; however, when the size of vira1 inoculum was increased above this level, all ferrets had viral growth in their nasal tissues. 4) The involvement of pulmonary tissues, viral growth in those tissues and the development of gross lung lesions were significantly rare. There was no dear-cut relationship between the size of infecting doses and the frequency of such plumonary involvements in ferrets.