Two cases of long-lasting, sub-microscopic Plasmodium malariae infections in adults from coastal Tanzania

Abstract

Background: Malaria is endemic in Tanzania with majority of clinical cases caused by Plasmodium falciparum. Additionally, Plasmodium malariae and Plasmodium ovale spp. are also present and clinical manifestations caused by these infections are not well described. Clinical episodes caused by P. malariae infections are often characterized by a relatively mild illness with a low number of parasites, which can persist for long periods. In this report, two cases of P. malariae infections that were identified during a clinical trial evaluating the P. falciparum malaria vaccine candidate, PfSPZ Vaccine are described. The two participants were followed up and monitored for clinical and laboratory parameters to assess vaccine safety providing the opportunity to study clinical manifestations of P. malariae over 4 months.

Case presentation: Two young, healthy Tanzanian men infected with low density asexual blood stage P. malariae diagnosed by quantitative polymerase chain reaction (qPCR) are described. Retrospective analysis of collected and stored blood samples revealed that the two volunteers had constant asexual blood stage parasitaemia for more than 4 months. During the 132 days of infection, the volunteers' vital signs, body temperature and serum biochemistry all remained within normal ranges. Haematological abnormalities, which were transiently outside normal ranges, were regarded as not clinically significant. During this time period, four consecutive evaluations of blood samples by thick blood smear microscopy conducted by an experienced microscopist were all negative, indicating the presence of low-density sub-microscopic infections.

Conclusions: The two cases of P. malariae infections presented here confirm the ability of this Plasmodium species to persist at low density in the human host for extended time periods without causing clinical symptoms. The presented data also demonstrate that clinical study sites in malaria endemic regions need to have a strong malaria diagnostic infrastructure, including the ability of capturing sub-microscopic parasitaemia and differentiation of Plasmodium species. Trial registration ClinicalTrials.gov: NCT02613520, https://ichgcp.net/clinical-trials-registry/NCT02613520 , Registered: November 24th 2015, Enrolment of the first participant to the trial: December 15th 2015, Trial was registered before the first participant was enrolled.

Keywords: Asymptomatic malaria; Plasmodium malariae; Quantitative polymerase chain reaction (qPCR).

Conflict of interest statement

LWP Church, TL Richie and SL Hoffman are salaried, full time employees of Sanaria Inc, the developer and sponsor of Sanaria® PfSPZ Vaccine. These individuals were not responsible for the collection, recording or entry of the clinical and parasitological data presented here. The other authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in the subject matter or materials discussed in the manuscript.

Figures

Fig. 1

Elevated eosinophil counts for volunteer 2 over a time period of more than 400 days. Eosinophil counts of volunteer 2 covering all visits, from study enrolment to completion, are shown. The dashed line represents the upper limit of the normal range (0.78 × 103 cells/µL)

Fig. 2

qPCR data for the Plasmodium spp. screening and Plasmodium species identification assays. The upper panel shows the amplification curves for the Plasmodium spp. target of the screening assay. The lower panel shows the amplification for the four Plasmodium species-specific targets. All samples were run in triplicates and DNA from P. falciparum, P. malariae, P. ovale wallikeri, P. ovale curtisi and P. vivax were included as positive controls during the qPCR