Malaria is one of the many diseases transmitted by mosquitoes. It represents a significant public health problem, with 229 million cases and 409,000 deaths recorded worldwide in 2019 [1]. Traditional methods of controlling the spread of malaria include the use of insecticides and mosquito nets, however, researchers have been considering the utility of a far more unorthodox weapon: bacteria.
This brings us to the paper of today’s post.
Wolbachia Infections Are Virulent and Inhibit the Human Malaria Parasite Plasmodium Falciparum in Anopheles Gambiae [article link]
In this 2011 paper, Hughes et al. (and others) continue an ongoing investigation of Wolbachia bacteria’s malaria-thwarting ability. Prior to this paper, several things had been established about this curious genus of bacteria.
Firstly, Wolbachia (which can be found in this host-symbiote relationship in the wild) provide mosquitoes with resistance against various pathogens, ranging from viruses to parasites. More officially, this phenomena is called “pathogen interference,” or PI for short.
Secondly, researchers found that even greater pathogen interference could be created in mosquitoes by artificially infecting them with a different strain of Wolbachia than the one they’re usually infected with in the wild [2].
Finally, Wolbachia was shown to provide PI against Plasmodium berghei, a rodent malaria parasite, but it was yet to be determined whether it could do the same for human malaria parasites. This is what Hugh et al. set out to do.
In a pretty straightforward experiment, they injected Anopheles gambiae (one of the moquitoes that carry human malaria) with two different strains of Wolbachia in separate treatments. Seven days later, they fed these mosquitoes a blood meal contaminated with Plasmodium falciparum, one of the human malarial parasites. The mosquitoes were then dissected and examined for the extent of malarial presence. Here’s the data from that experiment.
Plot details (or you can read the caption if you want)
The x axis is the treatments given. Moss55 is more or less the liquid the Wolbachia was grown in. It serves as an experimental control, ruling out the possibility that it’s doing anything by itself. This is important, as this liquid is pretty much inevitably injected along with the two strains (wMelpop and wAlbB) in their separate treatments. In case you’re wondering what the difference between wMelpop and wAlbB, wMelpop is basically a more aggressive strain of Wolbachia than wAlbB.
The y axis is the number of P. falciparum oocysts (basically latent malaria) present.
Finally, the difference between the left and right plots is slower and faster-reproducing forms of P. falciparum were used for the experiments that yielded the data for the left and right plots, respectively.
Takeaway
The most important takeaway here though is that there is a significant difference (P < 0.05) between the Moss55 control and the Wolbachia treatments. Wolbachia works here, in other words.
Great, that means we can eradicate malaria now?
Not exactly. There was one catch mentioned early on in the paper: The Wolbachia used here, for unknown reasons, seems unable to infect the ovaries of the Anopheles gambiae mosquitoes when injected as in this experiment (see below). This means Wolbachia is unable to be transmitted vertically, or across generations. So it seems to me a stable, malaria-blocking Wolbachia presence in Anopheles mosquitoes is difficult to create, or at least for the species of A. gambiae.
There is some hope though. A more recent paper in 2018 discussed the detection of Wolbachia in both A. gambiae and A. coluzzii in the wild, across a span of 5 years(!) [2]. However, the authors seem to suggest that the challenge remains to both supplant uninfected mosquitoes in sufficient numbers, and replace them with stably infected ones.
Thanks for reading, stay safe!
- BJW
1. Malaria. [cited 16 Jun 2021]. Available: https://www.who.int/news-room/fact-sheets/detail/malaria
2. Gomes FM, Barillas-Mury C. Infection of anopheline mosquitoes with Wolbachia: Implications for malaria control. PLoS Pathog. 2018;14: e1007333.
3. Hughes GL, Koga R, Xue P, Fukatsu T, Rasgon JL. Wolbachia infections are virulent and inhibit the human malaria parasite Plasmodium falciparum in Anopheles gambiae. PLoS Pathog. 2011;7: e1002043.
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