Relationship of Size of Inoculum to Time to Death in Mice Infected with Toxoplasma gondii

Abstract

The relationship between time to death and size of inoculum for Toxoplasma gondii is well described by a linear regression on the logarithm of the number of free organisms injected intra-peritoneally into white mice. For organisms suspended in serum the equation, T = 12.05 - 1.27 X, best defines the relationship. From this equation it was determined that a 31-fold increase in the size of the inoculum was necessary to decrease the time to death by one day. The LD50 for the organisms suspended in serum was approximately one, indicating that a single Toxoplasma organism may initiate a fatal infection. From this it follows that the median infective dose is also one, and this was confirmed by the fact that mice surviving after the inoculation of small calculated numbers of parasites could be shown to have infections by subinoculation in only one of 22 instances. It is believed that the techniques of measurement developed may be of aid in the study of the comparative virulence of different strains of Toxoplasma.