Survival of rodent malaria merozoites in the lymphatic network: potential role in chronicity of the infection

Experiments performed during the last few years, lead us to hypothesise the existence of latent asexual forms of murine Plasmodium. In the present report we examined the organs of infected animals and describe novel structures, which we call merophores, containing merozoites which have resisted lysis seen with other asexual stage parasites. We propose that these merozoites represent a latent form of the parasite. Merophores were also found in the lymphatic circulation, and were demonstrated by subinoculation to have retained their viability. Depending on the parasite species two types of merophores were observed. For P. yoelii nigeriensis merophore sacks, with the latent merozoites found inside vesicles, were usually observed. Merophore leucocytes, where latent merozoites dispersed in the cytoplasm of macrophages or neutrophils, were solely seen with P. vinckei petteri. Both structures were seen in P. chabaudi chabaudi infections. Merophores were found in lymph nodes of rodents after the asexual parasitaemia had apparently subsided. They were formed soon after schizogony, principally in the spleen, either by pitting or by macrophage phagocytosis. Merophore numbers appeared to be proportional to the number of maturing schizonts. We propose that merophore formation and their circulation in the lymphatics play an important role in the pattern of recrudescences and chronicity of rodent malaria infections. It is further suggested that the lymphatic network, a privileged pathway for many parasites, might play a similar role in human malaria infections.