Humans Affected by Same Malaria Parasite as Monkeys

The quartan malaria parasite Plasmodium malariae is the widest spread and best adapted human malaria parasite while the simian P. brasilianum causes quartan fever in New World monkeys and resembles P. malariae morphologically.

Malaria parasites of the Plasmodium brasilianum species have been discovered in the Yanomami, an indigenous people on the border between Venezuela and Brazil, and this malaria species had only been found in monkeys in the past.

Tropical medicine experts at the University of Tübingen (Germany) and their colleagues took thick and thin blood smears from individuals who presented with a history of fever and/or headache and/or malaise. Blood samples were collected by finger prick and stored on filter papers. 633 samples were used for the retrospective screening to investigate the molecular genetics of P. malariae parasites. Parasite DNA from field samples was extracted from dried blood spots on filter paper using a commercial extraction kit, the QIAamp DNA Blood Mini Kit, (Qiagen; Venlo, Netherlands).

Screening for Plasmodium spp. infection was carried out by conventional nested-polymerase chain reaction (PCR) assay with genus and species specific primers based on is sensitive and typically employed in the differential diagnosis of species, the 145 bp product of this primer set is too short for extensive sequence analysis. Therefore, the team designed new sequencing primers targeting an amplicon spanning the entire variable region 5 (V5), one of the eight highly variable regions in the 18S gene which has considerable sequence variations among Plasmodium species.

The determination of malaria by conventional nested-PCR detected the presence of P. malariae DNA in 75 of 633 samples collected from different individuals in Yanomami villages, constituting an 11.8% carrier rate. Out of the 75 samples PCR-positive for P. malariae, the 18S gene from 33 samples (27 mono-infections and six mixed infections) was successfully amplified and the resulting 763 bp product was analyzed by sequencing. Based on 18S rRNA gene sequencing, the scientists identified 12 patients harboring malaria parasites which were 100% identical with P. brasilianum isolated from the red howler monkey, Alouatta seniculus. Translated amino acid sequences of the circumsporozoite protein (CSP) gene showed identical immunodominant repeat units between quartan malaria parasites isolated from both humans and monkeys.

The authors concluded that as quartan parasites are the only global human malaria parasites successfully infecting multiple mammalian hosts, they could evade control measures that do not account for the animal reservoir. These hidden and possibly emerging parasites could, therefore, represent an important area for future investigations. Recently P. knowlesi, another monkey parasite has found to naturally infect humans. The study was published on September 25, 2015, in the specialist journal EbioMedicine.

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