Parallel to the survivorship test, and concurrent with it, the activity of the reticulo-endothelial system (RES) was measured at frequent intervals in identically infected BSVS and BRVR mice and in their non-infected controls. For this measurement the carbon clearance method was used. No pre-infection difference of activity of the RES could be discerned in susceptible BSVS mice vs. resistant BRVR. Following infection, increases in the RES activity were detected in about the same magnitude in both mouse stocks. However, a difference was found in the time of onset. The susceptible animals showed an early and short increase in the activity of the RES, followed by a drop to control levels at the time of death. The resistant group exhibited a considerably delayed, but significant increase in RES activity, which returned to control levels approximately 4 weeks after infection.

The absolute white blood count did not undergo significant change in either of the two infected groups, but the susceptible animals showed a relative increase of their neutrophils at the expense of their lymphocytes.

Extensive anatomical changes were observed in both mouse strains, mainly confined to liver and spleen. These consisted of stasis, swelling of Kupffer cells, necrotic foci, histiocytic-monocytic nodules, widespread thrombosis of branches of the portal and splenic veins, and extensive areas of necrosis. These changes appeared earlier in susceptible than in resistant animals.

The implications of these findings are discussed.