Studies on parasitic crisis in malaria: I. Signs of impending crisis in Plasmodium berghei infections of the white rat.

White rats infected with Plasmodiam berghei were observed throughout the course of infection for indicators of impending crisis (penultimate stage). Two ratios, percentage young compact forms:percentage trophozoites and percentage reticulocytes:percentage parasitaemia, were reliable markers of the penultimate period and impending crisis. When the percentage young compact form:percentage trophozoite ratio exceeded 1.0 and was coincident with an increase of uninfected reticulocytes in the circulation (as given by a percentage reticulocyte:percentage parasitaemia ratio greater than 1.0) the parasitaemic decline of crisis consistently occurred 24-48 hours later.