Hello everyone, please buckle up. This week we are mostly focusing on the evolution of malaria parasites and the modes and routes of transmission. Be reminded that in this blog “modes” will mean the vectors and “routes” the pathways parasites are transmission through. Now that that is out of the way, let’s jump right into it...
In case you missed my first blog, there are many vectors of malaria. Genetic variation in malaria has been well documented indicating that different Anopheles species transmit different Plasmodium parasites but not all are equally efficient vectors. In recent years, it was discovered that although An. quadriannulutus and An. gambiae are phylogenetically very close, the former is not a vector of malaria but harbors the parasites. It has been demonstrated that An. quadriannulutus is highly resistant to infections by P. falciparum. The implication is that if the mosquito cannot become infective, the parasites fail to migrate to the salivary glands where they can be deposited into a human host, therefore transmission is halted. Additionally, this mosquito kills the parasites at subsequent stages of their development. It seems like the parasite has lost the ability for horizontal transmission in this mosquito or could it be that the parasites will eventually co-evolve with the mosquito, adding another vector to the transmission list? Well, that's a discovery for another day.
The one discussed above is just one-way malaria parasites can be transmitted - through vectors, and although that is the primary mode, they can also be transmitted via other modes. Because the parasites that cause malaria affect red blood cells, people can also catch malaria through blood transfusions, from mother to unborn child, and by sharing needles used to inject drugs. This may not be the preferred mode because more often than not, we people die when we are infected with the parasites. If the density of the host is reduced, the rate at which the host and pathogen come in contact is reduced and transmission may cease. There are theories that mosquito parasites have evolved with humans and apes. However, the transmission was aided by the bite of a mosquito. The parasites can alter the biting behavior of a mosquito. They become increasingly aggressive at probing and take large amounts of blood. That is why the parasites need mosquitoes that are not negatively affected to keep the transmission going at a faster rate.
As someone studying to reduce the abundance of mosquitoes, this week’s topic was thought-provoking. Suppose we exclude mosquitoes from the transmission of malaria. Would human-human transmission intensify considering the belief that with the reduction of host density, we can increase transmission through other modes?
Sources
https://www.malariasite.com/history-parasites/#:~:text=Recent%20molecular%20studies%20have%20found,the%20bites%20of%20vector%20mosquitoes.&text=Malaria%20seems%20to%20have%20been%20known%20in%20China%20for%20almost%205%2C000%20years.
Smallegange, R. C., van Gemert, G. J., van de Vegte-Bolmer, M., Gezan, S., Takken, W., Sauerwein, R. W., et al. (2013). Malaria infected mosquitoes express enhanced attraction to human odor. PLoS One 8:e63602. DOI: 10.1371/journal.pone.0063602.
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