Fragmented Population Structure of Plasmodium falciparum in a Region of Declining Endemicity

Background. The population genetic structure of Plasmodium falciparum differs between endemic regions, but the characteristics of a population recently fragmented by effective malaria control have been unknown. Methods. Genotypic analysis of 10 microsatellite loci widely separated in the parasite genome was conducted on 288 P. falciparum isolates from 8 foci in Malaysian Borneo, a region in which malaria incidence has been progressively reduced. Results. Within all P. falciparum foci, moderate levels of allelic diversity were found, but levels of multilocus linkage disequilibrium were extremely variable. The population with the highest proportion of mixed-clone infections also had the highest allelic diversity and nonsignificant linkage disequilibrium. In contrast, several populations showed evidence of clonal expansion, and one offshore island population had exceptionally high levels of linkage disequilibrium. Genetic differentiation between many populations was very high and strongly associated with the geographical distance between them. Conclusions. High levels of differentiation and contrasting population structure among P. falciparum populations in Malaysian Borneo indicate that they are genetically independent. This supports the feasibility of individually eradicating the remaining P. falciparum foci

Identification of malaria parasites and molecular characterisation of Plasmodium Knowlesi from fatal human infections

Until recently, only four species of Plasmodium parasites were thought to be the aetiological agents for human malaria. However, recent nested polymerase chain reaction (PCR) based studies have shown that P. knowlesi, a parasite that naturally infects macaques is responsible for a significant number of human malaria cases in Southeast Asia. Since P. knowlesi blood forms are identical to P. malariae and the early trophozoites are identical to P. falciparum, most of these cases were diagnosed by microscopy as P. malariae. Plasmodium knowlesi infections are potentially fatal and four fatal cases occurring in Sarawak between 2004 and 2005 were found to have been caused by P. knowlesi using nested PCR assays. According to the Sarawak State Health Department, between 1999 and 2005, there were 35 fatal malaria cases reported in Sarawak with 29 of them due to P jalciparum infections, three to P. vivax, three to P. malariae and one due to mixed P. falciparum-P. vivax infection. The aims of the study were therefore to identify the species of Plasmodium from the 35 fatal malaria cases reported in Sarawak from 1999 to 2005 by morphological and molecular methods. The purpose was to determine whether the microscopy results were accurate and whether there have been any other fatalities due to P. knowlesi prior to 2005 among these 35 fatal cases. Archived blood films which consisted of thin and thick blood smears' from the 35 fatal malaria cases were obtained from the Sarawak State Health Department

A large focus of naturally acquired Plasmodium knowlesi infections in human beings.

BACKGROUND: About a fifth of malaria cases in 1999 for the Kapit division of Malaysian Borneo had routinely been identified by microscopy as Plasmodium malariae, although these infections appeared atypical and a nested PCR assay failed to identify P malariae DNA. We aimed to investigate whether such infections could be attributable to a variant form of P malariae or a newly emergent Plasmodium species. METHODS: We took blood samples from 208 people with malaria in the Kapit division between March, 2000, and November, 2002. The small subunit ribosomal RNA and the circumsporozoite protein genes were sequenced for eight isolates that had been microscopically identified as P malariae. All blood samples were characterised with a genus-specific and species-specific nested PCR assay together with newly designed P knowlesi-specific primers. FINDINGS: All DNA sequences were phylogenetically indistinguishable from those of P knowlesi, a malaria parasite of long-tailed macaque monkeys, but were significantly different from other malaria parasite species. By PCR assay, 120 (58%) of 208 people with malaria tested positive for P knowlesi, whereas none was positive for P malariae. P knowlesi parasites in human erythrocytes were difficult to distinguish from P malariae by microscopy. Most of the P knowlesi infections were in adults and we did not note any clustering of cases within communities. P knowlesi infections were successfully treated with chloroquine and primaquine. INTERPRETATION: Naturally acquired P knowlesi infections, misdiagnosed by microscopy mainly as P malariae, accounted for over half of all malaria cases in our study. Morphological similarities between P knowlesi and P malariae necessitate the use of molecular methods for correct identification. Further work is needed to determine whether human P knowlesi infections in the Kapit division are acquired from macaque monkeys or whether a host switch to human beings has occurred