Microbe prevents Malarial Transmission, Scientists have now discovered a microbe that can prevent mosquitoes from being infected with malarial parasites; a revolutionary stride in fight against one of the world’s deadly disease.
About half of the world’s population resides in areas that are at risk of malarial transmission in 91 countries and territories. Malaria has turned into a severe public health issue worldwide, a leading cause of death and disease in many developing countries with young children and pregnant women at high risk.
Report from the World Health Organisation (WHO) in 2018, estimates that there are 228 million cases of malaria worldwide and around 4,00,000 deaths reported every year.
Malaria is an acute febrile illness with an incubation period of 7 days or longer. It is caused by the protozoan parasite Plasmodium. Malaria in humans is caused by four different species of Plasmodium: P. falciparum, P. malariae, P. ovale and P. vivax. However, the most severe form of Malaria is caused by P. falciparum.
Though billions of dollars are spent on developing strategies to eradicate the diseases with huge advances have been made over the last two decades, the progress has de-escalated in recent years- with coronavirus hampering the further gains.
A team of researchers from Kenya and the United Kingdom conducted a study on malarial mosquitoes in their natural environment that lead to the discovery of malaria-transmission blocking microbe known as Microsporidia MB. The study was published in Nature Communications on 4th May 2020 and presents with the hope of controlling malaria by the natural method using microbe-based tools.
Few known examples of Microsporidians that interfere with infection and development of Plasmodium are Nosema stegomyiae, Vavraia culicis. However, only partial reduction of transmission was observed in these previous Plasmodium experiments.
The current study led by Dr Jeremy Herren detected the presence of a microsporidian designated as Microsporidia MB, non-pathogenic single-celled spore-forming fungi, found to occur in high intensity and with low to moderate prevalence (approx. 5%) in a geographically dispersed population of Anopheles arabiensis in Kenya. Not a single Microsporidia-infected mosquito was found to harbour the malarial parasites.
Phylogenetic analysis of the 18S gene sequence of Microsporidia MB revealed 97% homology with Crispospora chironomi. Both the microbes exhibit polysporoblastic and diplosporoblastic sporogenies and are found in the larval mosquito gut epithelium, which further explains their close resemblance.
The researchers examined the effect of this microbe on Plasmodium infection using direct membrane feeding assays on both MB positive and negative mosquitoes. The Microsporidia MB positive mosquitoes were identified by sequencing of 18S amplicons and rpoB gene fragment.
Transmission of Plasmodium: Sporozoites of Plasmodium parasite travel to the salivary glands from the hemocoel, traversing into the epithelium and mixing with the saliva of Anopheles resulting into infectious mosquitoes, which is usually after 8-14 days after the blood-meal.
To understand at which stage the development stage of Plasmodium was impaired, the researchers quantified the presence of Plasmodium in the head and thorax compartment after 10 days of DMFA.
The absence of Plasmodium in the head and thorax compartment of Microsporidia MB infected mosquitoes was detected using the qPCR-based assay of cytochrome b gene of Plasmodium. This absence indicates that this microbe prevents Anopheles salivary gland from being colonised by the Sporozoites and hence prevents the transmission.
Dr Jeremy Herren of International Centre of Insect Physiology and Ecology, Kenya who led the research told the BBC: ‘The data we have so far suggest it is 100% blockage, it’s a very severe blockage of malaria.’
The study also reported that Microsporidia MB is transmitted in female Anopheles with high efficiency and the microbe does not have any effect on the Anopheles fitness nor result in any sterilising effect on females, which was previously reported in other Microsporidians.
This microbe has been reported to increase the expression of several genes involved in digestion and immunity. Therefore, immune priming in Anopheles maybe confers to the impairment of Plasmodium.
Since the microbe can be transmitted both vertically and horizontally, researchers have put forth two main strategies to increase the number of Microsporidia-infected mosquitoes for significant combat against Malaria.
- Microsporidia is spore-forming, hence spores can be released and mixed with sugars to infect the mosquitoes
- Infecting male mosquitoes under laboratory conditions and releasing them in the wild to infect females when they mate.
Thus, these findings can play a significant role in understanding the regional transmission of Malaria, epidemiology as well as in risk mapping particularly in the development of a microbe-based tool for blocking the transmission of Plasmodium. Though the concept of using microbes to combat mosquito-borne diseases is not uncommon. Wolbachia a transmission-blocking symbiont used in controlling Dengue.
Further studies are needed to understand the precise mechanistic and dynamics behind Microsporidia MB to control malaria. The result of this study does provide with key information in the development of further prospects for malaria control.
Jeremy K. Herren et al. A microsporidian impairs Plasmodium falciparum transmission in Anopheles arabiensis mosquitoes, Nature Communications (2020). DOI: 10.1038/s41467-020-16121-y
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