Improved quantification of Plasmodium exoerythrocytic forms in rodents.

The result of a Plasmodium sporozoite challenge is currently evaluated either by detecting the emergence or not of parasites in the blood, or by estimating the "prepatent period", which is the time between sporozoite inoculation and the appearance of parasites in the blood. This type of measurement is relatively rough and has given way to another method of measuring sporozoite infectivity, which is to enumerate the exoerythrocytic forms (EEF) by microscopic examination of liver sections. Up until now, two different methods have been proposed to calculate and estimate the number of Plasmodium EEF forms in the livers of infected rodents, both of which are unfortunately biased to some extent. Here, we propose a different method of calculation, which more faithfully reflects the EEF number in the liver. This method is based on the calculated mean number of consecutive liver sections in which a schizont appears, and is host related.