|hybrid Tx/91 virus carrying the 1918 HA and NA genes, and the hybrid Tx/91virus carrying only the 1918 NA gene were able to replicate without trypsin added. This suggests that the 1918 NA is able to facilitate HA cleavage, therefore offering a possible explanation for its high pathogenicity.
In order to directly compare the pathogenicty of the 1918 influenza virus with the contemporary Tx/91 H1N1 virus, mice were infected and had their weight and lung virus titers monitored. Just four days after infection, mice with 1918 influenza had 39,000 times the number of virus particles in their lungs than mice infected with the Tx/91 control strain. In addition, the control mice lost some weight in the first few days of infection, but all survived and had regained the weight by the end of the fourteen day study. Mice infected with the1918 strain, however, very rapidly lost weight and had all died within five days. Baslers team also looked at the lung tissue and other organs of infected mice to determine the type and extent of disease. Just as described in autopsy reports from victims of the Spanish flu pandemic, there was severe disease in the lungs, but not in the spleen or other organs.