Tissues of the upper respiratory tract of ferrets are more susceptible to influenza virus infection than those of the lower respiratory tract. An organ culture technique has been standardized sufficiently to examine the basis for these differences.

In experiments with an Asian and two Hong Kong influenza viruses maintenance of organ cultures for 24 h before inoculation consistently and markedly increased the susceptibility of lung, the least susceptible tissue. The susceptibility of the most susceptible tissue, nasal turbinates, was increased significantly (but to a lesser extent than lung) in some experiments but not others. Tracheal organ cultures showed two patterns of susceptibility; about half were poorly susceptible but showed a marked increase with maintenance. The remainder were moderately susceptible and showed no increase with maintenance.

The increased susceptibility of lung with maintenance was not due to fibroblast outgrowth, to loss of inhibitory material easily removed by washing, to killing of commensal bacteria by the bactericidal medium nor to changes in mucus distribution. Although central necrosis occurred in the maintained tissue pieces it was not correlated with increased susceptibility and virus replicated only in peripheral cells which remained healthy throughout culturing. The increase in susceptibility was demonstrated after maintenance for 6 h and increased to a maximum by 24 h.

The reason for the low susceptibility of the lung may be established by comparing organ cultures of fresh lung and nasal turbinates and by analysing the basis for the increased susceptibility of the lung with maintenance.


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