Orangutans and Plasmodium vivax or P. cynomolgi | CDC EID

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Volume 15, Number 10–October 2009
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Orangutans Not Infected with Plasmodium vivax or P. cynomolgi, Indonesia
Balbir Singh and Paul Cliff Simon Divis
Author affiliation: Universiti Malaysia Sarawak, Kuching, Sarawak, Malaysia

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Abstract
After orangutans in Indonesia were reported as infected with Plasmodium cynomolgi and P. vivax, we conducted phylogenetic analyses of small subunit ribosomal RNA gene sequences of Plasmodium spp. We found that these orangutans are not hosts of P. cynomolgi and P. vivax. Analysis of >1 genes is needed to identify Plasmodium spp. infecting orangutans.

Parasites belonging to the genus Plasmodium cause malaria and are usually host specific. For example, humans are natural hosts for P. falciparum, P. vivax, P. malaria, and P. ovale, and orangutans are naturally infected with P. pitheci and P. silvaticum (1,2). However, simian malaria parasites can infect humans (1); for example, P. knowlesi, normally associated with infections in long-tailed and pig-tailed macaques, has recently been found to have caused malaria in humans in several countries in Southeast Asia (3–8). This finding raises the possibility that other zoonotic malaria parasites may emerge in humans.

Malaria parasites have distinct small subunit ribosomal RNA (SSU rRNA) genes that are developmentally regulated (9). Each Plasmodium species has at least 2 types of SSU rRNA genes, and the stage-specific expression of these genes varies among species. In general, the A-type genes are transcribed or expressed mainly during the asexual stages, and the S-type genes are transcribed mainly during the sporozoite stage. P. vivax also has O-type genes, which are expressed during ookinete and oocyst development. Phylogenetic analysis of the P. vivax and P. cynomolgi SSU rRNA genes has indicated that the genes appear to have evolved as a result of 2 gene duplication events (10). A more recent study, involving SSU rRNA sequence data from a much larger number of Plasmodium spp., demonstrated that gene duplication events giving rise to the A-type and S-type sequences took place independently at least 3 times during the evolution of Plasmodium spp. (11).

Reid et al. (12) analyzed the DNA sequences of SSU rRNA genes of Plasmodium spp. from blood of orangutans in Kalimantan, Indonesia. Using phylogenetic analysis, they concluded that, in addition to P. pitheci and P. silvaticum, the orangutans were infected with the human malaria parasite P. vivax and the macaque malaria parasite P. cynomolgi. Their report implies that human and macaque malaria parasites could be transmitted to orangutans and that orangutans could act as reservoir hosts for at least 1 of the human malaria parasites.

When taxonomic inferences of species within a genus are made from phylogenetic trees, trees must be reconstructed by using orthologous genes and must include as many species sequences as possible. However, Reid et al. used sequence data of only the S-type SSU rRNA genes for P. vivax, P. cynomolgi, and P. knowlesi and data of only the A-type genes for P. inui and P. fragile. Furthermore, they analyzed sequence data from only 4 simian malaria parasites. Nishimoto et al. recently included data from 10 simian malaria parasites (11). We therefore reanalyzed the SSU rRNA sequence data of malaria parasites of orangutans together with the A-type, S-type, and O-type SSUrRNA gene sequence data for various Plasmodium spp.

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