Why do Plasmodium malariae infections sometimes occur in spite of previous antimalarial medication?

Quartan malaria due to Plasmodium malariae is commonly regarded as being preventable by current antimalarials. A case of P. malariae infection occurred in spite of previous treatment of Plasmodium falciparum malaria 4 months earlier with a full therapy course of intravenous quinine hydrochloride and oral doxycycline followed by artemether + lumefantrine. Since the patient was not anymore exposed to agents of malaria in the meantime, a new infection by P. malariae after therapy is unlikely. The present observation is difficult to explain by the current view on the origin of latent P. malariae infections and recurrences which are thought to arise from intra-erythrocytic development stages susceptible to common antimalarials. The most likely explanation of our observation is a delayed pre-erythrocytic development. The latency between infection by P. malariae and the quartan malaria fever attack might have been extended further by an initial subclinical circulation of a low number of intra-erythrocytic asexual parasites in the blood stream preceeding the clinical quartan malaria breakthrough.