Abstract
BackgroundThe sexual stages (gametocytes) of Plasmodium falciparum do not directly contribute to the pathology of malaria but are essential for transmission of the parasite from the human host to the mosquito. Mature gametocytes circulate in infected human blood for several days and their circulation time has been modelled mathematically from data of previous in vivo studies. This is the first time that longevity of gametocytes is studied experimentally in vitro.MethodsThe in vitro longevity of P. falciparum gametocytes of 1 clinical isolate and 2 laboratory strains was assessed by three different methods: microscopy, flow cytometry and reverse transcription quantitative real-time PCR (RT-qPCR). Additionally, the rate of gametocytogenesis of the used P. falciparum strains was compared.ResultsThe maximum in vitro lifespan of P. falciparum gametocytes reached almost 2 months (49 days by flow cytometry, 46 days by microscopy, and at least 52 days by RT-qPCR) from the starting day of gametocyte culture to death of last parasite in the tested strains with an average 50% survival rate of 6.5, 2.6 and 3.5 days, respectively. Peak gametocytaemia was observed on average 19 days after initiation of gametocyte culture followed by a steady decline due to natural decay of the parasites. The rate of gametocytogenesis was highest in the NF54 strain.ConclusionsPlasmodium falciparum mature gametocytes can survive up to 16–32 days (at least 14 days for mature male gametocytes) in vitro in absence of the influence of host factors. This confirms experimentally a previous modelling estimate that used molecular tools for gametocyte detection in treated patients. The survival time might reflect the time the parasite can be transmitted to the mosquito after clearance of asexual parasites. These results underline the importance of efficient transmission blocking agents in the fight against malaria.
Highlights
The sexual stages of Plasmodium falciparum do not directly contribute to the pathology of malaria but are essential for transmission of the parasite from the human host to the mosquito
A summary of the longevity data evaluated by the different methods is given in Table 1, showing that analysis by reverse transcription quantitative real-time PCR (RTqPCR) was the most sensitive method, revealing the Discussion Density, sex ratio and life span of stage-V gametocytes of P. falciparum are the most important factors determining the probability of transmission to the mosquito
It was observed that all gametocytes matured at the day of peak gametocytaemia, and gametocyte peak was used as the day of gametocyte maturity to calculate longevity of mature gametocytes
Summary
The sexual stages (gametocytes) of Plasmodium falciparum do not directly contribute to the pathology of malaria but are essential for transmission of the parasite from the human host to the mosquito. Mature gametocytes circulate in infected human blood for several days and their circulation time has been modelled mathematically from data of previous in vivo studies. Mathematical modelling in vivo after clearance of asexual stages by anti-malarial treatment in patients and subsequent detection by microscopy or molecular methods was so far the main method by which longevity of gametocytes was estimated. In these modelling approaches, assumptions were made considering gametocyte conversion probability, sequestration periods and decay of gametocytes [2, 10, 13]
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