Gametocyte carriage in Plasmodium falciparum-infected travellers

Lactic Acid Supplementation Increases Quantity and Quality of Gametocytes in Plasmodium falciparum Culture

Malaria infection by Plasmodium falciparum continues to afflict millions of people worldwide, with transmission being dependent upon mosquito ingestion of the parasite gametocyte stage. These sexually committed stages develop from the asexual stages, yet the factors behind this transition are not completely understood. Here, we found that lactic acid increases gametocyte quantity and quality in P. falciparum culture. Low-passage-number NF54 parasites exposed to 8.2 mM lactic acid for various times were monitored using blood film gametocyte counts and RNA analysis throughout 2 weeks of gametocyte development in vitro for a total of 5 biological cohorts. We found that daily continuous medium exchange and 8.2 mM lactic acid supplementation increased gametocytemia approximately 2- to 6-fold relative to controls after 5 days. In membrane feeding mosquito infection experiments, we found that gametocytes continuously exposed to 8.2 mM lactic acid supplementations were more infectious to Anopheles stephensi mosquitoes, essentially doubling prevalence of infected midguts and oocyst density. Supplementation on days 9 to 16 did not increase the quantity of gametocytes but did increase quality, as measured by oocyst density, by 2.4-fold. Lactic acid did not impact asexual growth, as measured by blood film counts and luciferase quantification, as well as radioactive hypoxanthine incorporation assays. These data indicate a novel role for lactic acid in sexual development of the parasite.

Associations between Season and Gametocyte Dynamics in Chronic Plasmodium falciparum Infections

INTRODUCTION

In a markedly seasonal malaria setting, the transition from the transmission-free dry season to the transmission season depends on the resurgence of the mosquito population following the start of annual rains. The sudden onset of malaria outbreaks at the start of the transmission season suggests that parasites persist during the dry season and respond to either the reappearance of vectors, or correlated events, by increasing the production of transmission stages. Here, we investigate whether Plasmodium falciparum gametocyte density and the correlation between gametocyte density and parasite density show seasonal variation in chronic (largely asymptomatic) carriers in eastern Sudan.

MATERIALS AND METHODS

We recruited and treated 123 malaria patients in the transmission season 2001. We then followed them monthly during four distinct consecutive epidemiological seasons: transmission season 1, transmission-free season, pre-clinical period, and transmission season 2. In samples collected from 25 participants who fulfilled the selection criteria of the current analysis, we used quantitative PCR (qPCR) and RT-qPCR to quantify parasite and gametocyte densities, respectively.

RESULTS AND DISCUSSION

We observed a significant increase in gametocyte density and a significantly steeper positive correlation between gametocyte density and total parasite density during the pre-clinical period compared to the preceding transmission-free season. However, there was no corresponding increase in the density or prevalence of total parasites or gametocyte prevalence. The increase in gametocyte production during the pre-clinical period supports the hypothesis that P. falciparum may respond to environmental cues, such as mosquito biting, to modulate its transmission strategy. Thus, seasonal changes may be important to ignite transmission in unstable-malaria settings.

Targeting Human Transmission Biology for Malaria Elimination

Malaria remains one of the leading causes of death worldwide, despite decades of public health efforts. The recent commitment by many endemic countries to eliminate malaria marks a shift away from programs aimed at controlling disease burden towards one that emphasizes reducing transmission of the most virulent human malaria parasite, Plasmodium falciparum. Gametocytes, the only developmental stage of malaria parasites able to infect mosquitoes, have remained understudied, as they occur in low numbers, do not cause disease, and are difficult to detect in vivo by conventional methods. Here, we review the transmission biology of P. falciparum gametocytes, featuring important recent discoveries of genes affecting parasite commitment to gametocyte formation, microvesicles enabling parasites to communicate with each other, and the anatomical site where immature gametocytes develop. We propose potential parasite targets for future intervention and highlight remaining knowledge gaps.

A review of the effects of artemether-lumefantrine on gametocyte carriage and disease transmission

While significant advances have been made in the prevention and treatment of malaria in recent years, these successes continue to fall short of the World Health Organization (WHO) goals for malaria control and elimination. For elimination strategies to be effective, limited disease transmission, achieved through rapid reduction in the infectious parasite reservoir and decreased gametocyte carriage, will be critical. Artemisinin-based combination therapy (ACT) forms the cornerstone of WHO-recommended treatment for uncomplicated Plasmodium falciparum malaria, and in combination with other effective interventions will undoubtedly play a vital role in elimination programmes. The gametocytocidal properties of artemisinins are a bonus attribute; there is epidemiological evidence of reductions in malaria incidence and transmission in African regions since the introduction of these agents. Many studies and analyses have specifically investigated the effects of the ACT, artemether-lumefantrine (AL) on gametocyte carriage. In this systematic review of 62 articles published between 1998 and January 2014, the effects of AL on gametocyte carriage and malaria transmission are compared with other artemisinin-based anti-malarials and non-ACT. The impact of AL treatment of asymptomatic carriers on population gametocyte carriage, and the potential future role of AL in malaria elimination initiatives are also considered. Despite the inherent difficulties in comparing data from a range of different studies that also utilized different diagnostic approaches to assess baseline gametocyte counts, the gametocytocidal effect of AL was proportionately consistent across the studies reviewed, suggesting that AL will continue to play a vital role in the treatment of malaria and contribute to clearing the path towards malaria elimination. However, the specific place of AL is the subject of much ongoing research and will undoubtedly be dependent on different demographic and geographical scenarios. Utilizing ACT, such as AL, within malaria elimination strategies is also associated with a number of other challenges, such as balancing potential increased use of ACT (e g, treatment of asymptomatic carriers and home-based treatment) with rational use and avoidance of drug resistance development.

Managing malaria in the intensive care unit

The number of people travelling to malaria-endemic countries continues to increase, and malaria remains the commonest cause of serious imported infection in non-endemic areas. Severe malaria, mostly caused by Plasmodium falciparum, often requires intensive care unit (ICU) admission and can be complicated by cerebral malaria, respiratory distress, acute kidney injury, bleeding complications, and co-infection. The mortality from imported malaria remains significant. This article reviews the manifestations, complications and principles of management of severe malaria as relevant to critical care clinicians, incorporating recent studies of anti-malarial and adjunctive treatment. Effective management of severe malaria includes prompt diagnosis and early institution of effective anti-malarial therapy, recognition of complications, and appropriate supportive management in an ICU. All cases should be discussed with a specialist unit and transfer of the patient considered.

Emergence of resistance to atovaquone-proguanil in malaria parasites: insights from computational modeling and clinical case reports

The usefulness of atovaquone-proguanil (AP) as an antimalarial treatment is compromised by the emergence of atovaquone resistance during therapy. However, the origin of the parasite mitochondrial DNA (mtDNA) mutation conferring atovaquone resistance remains elusive. Here, we report a patient-based stochastic model that tracks the intrahost emergence of mutations in the multicopy mtDNA during the first erythrocytic parasite cycles leading to the malaria febrile episode. The effect of mtDNA copy number, mutation rate, mutation cost, and total parasite load on the mutant parasite load per patient was evaluated. Computer simulations showed that almost any infected patient carried, after four to seven erythrocytic cycles, de novo mutant parasites at low frequency, with varied frequencies of parasites carrying varied numbers of mutant mtDNA copies. A large interpatient variability in the size of this mutant reservoir was found; this variability was due to the different parameters tested but also to the relaxed replication and partitioning of mtDNA copies during mitosis. We also report seven clinical cases in which AP-resistant infections were treated by AP. These provided evidence that parasiticidal drug concentrations against AP-resistant parasites were transiently obtained within days after treatment initiation. Altogether, these results suggest that each patient carries new mtDNA mutant parasites that emerge before treatment but are killed by high starting drug concentrations. However, because the size of this mutant reservoir is highly variable from patient to patient, we propose that some patients fail to eliminate all of the mutant parasites, repeatedly producing de novo AP treatment failures.

Travellers as sentinels: Assaying the worldwide distribution of polymorphisms associated with artemisinin combination therapy resistance in Plasmodium falciparum using malaria cases imported into Scotland

There is growing evidence that Plasmodium falciparum parasites in southeastern Asia have developed resistance to artemisinin combination therapy. The resistance phenotype has recently been shown to be associated with four single nucleotide polymorphisms in the parasite's genome. We assessed the prevalence of two of these single nucleotide polymorphisms in P. falciparum parasites imported into Scotland between 2009 and 2012, and in additional field samples from six countries in southeastern Asia. We analysed 28 samples from 11 African countries, and 25 samples from nine countries in Asia/southeastern Asia/Oceania. Single nucleotide polymorphisms associated with artemisinin combination therapy resistance were not observed outside Thailand and Cambodia.