Mosquitoes beat humans as the world’s deadliest animals.1 This is not because they directly kill people, but instead they are really good at being vectors, which are organisms that act as vehicles for pathogens to transfer between hosts. Mosquitoes are notorious for transmitting viruses like yellow fever and dengue, bacteria that cause Lyme disease and typhus, protozoans that cause malaria, and metazoans that cause lymphatic filariasis, to name a few. These pathogens are often acquired by the mosquito via horizontal transmission through an intermediate host. The ability of the mosquito to acquire and transmit pathogens to susceptible hosts, or its ‘vector competence,’ is influenced by numerous internal factors like its preference for host, time taken for the pathogen to develop within the host, and the microbiome, as well as external factors like temperature and predator density.2
Malaria is one of the most devastating diseases transmitted by mosquitoes. This disease is endemic in over 100 countries with a tropical climate, infecting about 229 million people as of 2020 and causing about 409,000 deaths annually, mostly in children.3 Symptoms of the disease may include fever, chills, vomiting, exhaustion, anemia, and organ failure. The Plasmodium parasites are the causative agents of malaria in humans. Plasmodium falls under the Apicomplexa phylum which consist of parasitic, unicellular eukaryotes that use an apical complex to invade a host cell during part of their lifecycle. For Plasmodium, its lifecycle involves two hosts – mosquitoes and humans. In the mosquito, the parasite undergoes sexual reproduction; the macro and micro gametocytes ingested during a blood meal encounter each other in the mosquito’s gut to form oocytes, which progress to form several thousand copies of sporozoites. When a mosquito encounters a human host, it delivers the sporozoites along with its saliva. The sporozoites migrate to the liver where they are amplified and progress to form merozoites that can infect red blood cells. The merozoites replicate to produce about 30 more copies each, before lysing the red blood cells, which causes anemia, and going on to replenish gametocytes that get ingested by mosquitoes to propagate the cycle.2
Considering that we would be saving many lives each year by eradicating mosquitoes, why don’t we just kill them off? The truth is that not all mosquitoes are bad; there are about 3,500 species of which only the blood-sucking females of 100 species are vectors for parasites. Most mosquitoes feed off plant-based sources and serve as food for birds, fish, and other animals in the ecosystem.4 Hence, killing them off would most likely come with negative environmental consequences. Alternative approaches have turned to making mosquitoes resistant to the parasites they transmit.4 For example, the Australian-based World Mosquito Program breeds mosquitoes that carry Wolbachia bacteria to prevent the spread of viruses like dengue, Zika, yellow fever and chikungunya.5 Wolbachia is proposed to function by activating host immunity in mosquitoes or by competing for cellular resources with the RNA viruses they transmit.6 Other groups are looking to genetically modify mosquitoes to prevent parasitic transmission.7
References
1. Gates, B. The deadliest animal in the world. https://www.gatesnotes.com/health/most-lethal-animal-mosquito-week.
2. Hillyer, J., IGP 8002 - Adventure guide to the microbial world: Journey into a mosquito. 2022.
3. World malaria report 2020; Global Malaria Programme, 30 November, 2020.
4. Bates, C., Would it be wrong to eradicate mosquitoes? 28 January, 2016.
5. World Mosquito Program. https://www.worldmosquitoprogram.org/en/work/about-us.
6. Pimentel, A. C.; Cesar, C. S.; Martins, M.; Cogni, R., The Antiviral Effects of the Symbiont Bacteria Wolbachia in Insects. Frontiers in Immunology 2021, 11.
7. Pike, A.; Dong, Y.; Dizaji, N. B.; Gacita, A.; Mongodin, E. F.; Dimopoulos, G., Changes in the microbiota cause genetically modified Anopheles to spread in a population. Science 2017, 357 (6358), 1396-1399.
