The malaria vaccine Combination B comprises recombinant Plasmodium falciparum ring-infected erythrocyte surface antigen and 2 merozoite surface proteins (MSP1 and MSP2) formulated in oil-based adjuvant. A phase 1-2b double-blind, randomized, placebo-controlled trial in 120 children (5-9 years old) in Papua New Guinea demonstrated a 62% (95% confidence limits: 13%, 84%) reduction in parasite density in children not pretreated with sulfadoxine-pyrimethamine. Vaccinees had a lower prevalence of parasites carrying the MSP2-3D7 allelic form (corresponding to that in the vaccine) and a higher incidence of morbid episodes associated with FC27-type parasites. These results demonstrate functional activity of Combination B against P. falciparum in individuals with previous malaria exposure. The specific effects on parasites with particular msp2 genotypes suggest that the MSP2 component, at least in part, accounted for the activity. The vaccine-induced selection pressure exerted on the parasites and its consequences for morbidity strongly argue for developing vaccines comprising conserved antigens and/or multiple components covering all important allelic types.

Abstract. Plasmodium falciparum (Pf), P. vivax (Pv), P. malariae (Pm), and P. ovale (Po) infections are endemic in coastal areas of Papua New Guinea. Here 2,162 individuals living near Dreikikir, East Sepik Province, have been analyzed for complexity of malaria infection by blood smear and polymerase chain reaction (PCR) diagnoses. According to blood smear, the overall prevalence of Plasmodium infection was 0.320. Most individuals (0.283) were infected with a single species only. The prevalence of mixed species infections was low (0.037). Further analysis of a 173-sample subset by nested PCR of small subunit ribosomal DNA resulted in an overall 3.0-fold increase in prevalence of infection, with a 17.5-fold increase in the frequency of mixed species infections. Among mixed species infections detected by PCR, the frequency of double species was 0.364, and that of triple species was 0.237. Nine individuals (0.052) were infected with all 4 species. To determine if infection status (uninfected, single, and multiple infections) deviates from an independent random distribution (null hypothesis), observed versus expected frequencies of all combinations of Plasmodium species infections, or assemblages (Pf-, Pv-, Pm-, Po-, to Pfϩ, Pvϩ, Pmϩ, Poϩ), were compared using a multiple-kind lottery model. All 4 species were randomly distributed whether diagnosed by blood smear or PCR in the overall population and when divided into age group categories. These findings suggest that mixed species malaria infections are common, and that Plasmodium species appear to establish infection independent of one another.

No abstract

Plasmodium falciparum isolates from 24 Papua New Guinean patients with symptomatic malaria were tested for susceptibility to pyrimethamine and cycloguanil. Thirteen isolates were sensitive to both agents and the remainder exhibited varying degrees of resistance. No isolates were found to be resistant to one agent yet sensitive to the other and a positive correlation suggesting cross-resistance was found. Parasite DNA extracted from the patients' stained blood slides was amplified and sequenced to examine point mutations in the dihydrofolate reductase (DHFR) and dihydropteroate synthetase genes (DHPS) associated with antifolate resistance. All resistant isolates possessed mutations in the DHFR gene at codon 108, the majority changing from Ser to Asn, but one isolate from Ser to Thr, a change not previously reported in field isolates. A second mutation of the DHFR gene at Cys-59 to Arg was present in isolates with higher level resistance, but not exclusively so. Sequencing the DHPS gene, as a predictor of sulfadoxine resistance, revealed only one example that was different from DHPS alleles of sensitive isolates.

Abstract. Malaria is holoendemic in the lowlands of Papua New Guinea (PNG), and interactions among Plasmodium species may influence prevalence of mixed infections. Previously, field samples from a cross-sectional survey in Dreikikir, East Sepik Province, analyzed by blood smear and polymerase chain reaction (PCR), showed that mixed infections were common and randomly distributed in this malaria endemic region. To evaluate further whether Plasmodium species distribution is random, blood smear-and PCR/sequence-specific oligonucleotide probe hybridizationbased analyses of cross-sectional survey samples were conducted in 2 additional malaria holoendemic regions of northern PNG. Despite ecologic, species prevalence, and transmission season differences in these new surveys, all 4 Plasmodium species were found to be randomly distributed in each area; random distribution patterns also were observed when study populations were divided into age groups. These findings provide consistent evidence that Plasmodium species infections occur independently of one another in PNG malaria holoendemic sites. This independent occurrence suggests that age-dependent, acquired malaria immunity has limited influence on the distribution pattern of Plasmodium species infections in endemic human populations; infection by 1 human malaria parasite species does not reduce susceptibility to infection by others; and malaria vaccines would exhibit limited protection against blood-stage infection by heterologous Plasmodium species.