The 226 positive and 172 negative smears accruing during the daily (in some instances every second day) examination of the peripheral blood of fifty canaries mosquito-infected with Plasmodium cathemerium (Hartman) have been studied with the object of arranging them according to the time required to count a given number of asexual organisms. It was observed that the rate of progression and diminution in these infections occurred in such manner that a system of plus counting could be devised as follows.
Negative—No organisms in 3 minutes
1+—Less than twenty organisms in 3 minutes
2+—Twenty organisms in 2 to 3 minutes
3+—Twenty organisms in 1 to 2 minutes
4+—Twenty organisms in 30 to 60 seconds
5+—Twenty organisms in 30 seconds or less
I believe this to be a simple, quick and reliable method of determining the degree of infection upon a comparative basis for all ordinary practical purposes.