Plasmodium yoelii nigeriensis: biological mechanisms of resistance to chloroquine.

The sensitivity to chloroquine according to the degree of synchronicity of Plasmodium yoelii nigeriensis, which is considered to be the most resistant of the rodent malaria strains, was studied. The infection was synchronised by means of a Percoll-glucose gradient which separates rings and young trophozoites from other stages. The mid-term trophozoite, when it predominated in the blood at the time of treatment, was shown to be as sensitive to chloroquine as Plasmodium vinckei petteri. According to previous results indicating that part of the population of merozoites is latent and penetrates around midnight, the inoculations were timed in order to obtain a lower or higher degree of synchronisation. The infection appeared to be better synchronised if rings and young trophozoites, were inoculated at 06:00 hrs rather than at 15:00 hrs and consequently the efficacy of chloroquine was higher in the former than in the latter.