Killing around 619,000 people in 2021, malaria is one of the most significant global health challenges faced today (WHO, 2022). The coevolution of parasites, mosquitoes and humans drives an ongoing arms race that complicates malaria control in many ways. Parasites have been shown to evolve drug resistance, mosquitoes are able to adapt to avoid insecticides, and human genetic traits, such as sickle cells, offer partial protection (Mackinnon and Marsh, 2010; Russell et al., 2011; Band et al., 2022). Hence, understanding of these three factors is crucial for malaria control efforts in order to lower the astonishing death rate.
Parasites play a key role in malaria transmission. Although Plasmodium falciparum is not the exclusive cause of malaria, it is the most dangerous protozoan parasite responsible for malaria in humans. One of the main reasons for this is that it reproduces both asexually in red blood cells and sexually in mosquitos, allowing it to thrive in the host and then transmit through vectors (Mackinnon and Marsh, 2010). Additionally, it employs antigenic variation through the expression of the PfEMP1 protein on the surface of infected red blood cells, which helps to evade the host's immune system. This strategy allows the parasite to repeatedly change its surface antigens, preventing the immune system from creating an effective, long-lasting response against it (Niang et al., 2017). This gives P. falciparum the ability to create chronic infections - complicating efforts to develop lasting immunity and making malaria harder to control globally. Over time, P. falciparum has also developed resistance to key antimalarial treatments, such as chloroquine and artemisinin, due to mutations in PfCRT and K13 genes (Mackinnon and Marsh, 2010). These mutations enable the parasite to survive despite drug exposure, making it harder to achieve effective treatment outcomes and ultimately making the control of the disease more difficult.
Changes in mosquito behaviour through evolution have had a negative effect on malaria transmission. Russell et al. (2011) found that increased use of insecticide-treated nets in Tanzania led to a shift in mosquito feeding behaviour, with Anopheles mosquitoes feeding more outdoors, rather than indoors. This adaptation allowed mosquitoes to avoid the insecticidal effects of the nets, reducing their effectiveness. Not only this, but a study conducted by Moiroux et al. (2012) similarly found that after the widespread distribution of insecticide-treated nets, the mosquito population exhibited a shift toward increased outdoor biting, particularly during the early evening and late night. This behavioural adaptation reduced the success of the nets in controlling malaria transmission, as mosquitoes were able to evade exposure to the insecticide (Moiroux et al., 2012). These changes in mosquito behaviour highlight the need for adaptive vector control strategies that consider the evolution of mosquito populations.
On the other hand, human genetics could play a vital role in malaria protection through sickle cells. Band et al (2022) investigated how sickle cell haemoglobin provides protection against malaria and found that this protective effect depends on both the genotype of the individual and the specific Plasmodium falciparum strain. People with the sickle cell trait, especially those who are heterozygous, tend to have an advantage in areas with a high risk of malaria, due to the parasite's reduced ability to survive in sickled red blood cells. However, the degree of protection varies, as some P. falciparum genotypes can evade the effects of sickle haemoglobin, making the coevolutionary dynamics between human genetics and parasite genotypes a critical factor in malaria resistance, influencing both disease outcomes and control strategies.
In addition to this, humans are constantly hunting for more effective malaria treatments and technologies, providing another facet of human rebuttal to the evolution of parasites and mosquitos (WHO, 2022). However, human economic factors can play a negative role in malaria control efforts. In areas where poverty is widespread, access to healthcare, prevention tools, such as insecticide-treated nets and effective treatments can be limited, resulting in higher malaria transmission rates. Studies in Nigeria emphasized that, in both rural and urban settings, socio-economic status influences the use of anti-malarial drugs and adherence to treatment protocols, often complicating efforts to tackle malaria effectively (Worrall et al., 2003).
Overall, the interrelation of the coevolution of Plasmodium falciparum, Anopheles mosquitos and humans has made controlling the spread and treatment of malaria increasingly difficult. While it’s worth noting other factors, such as socio-economic status, the constant evolution of parasites and vectors, providing them with increased effectiveness, proves to be a pressing issue in the fight to lower the high death rate malaria possesses. However, human genetic adaptations and the advancement of malaria treatment are a beacon of hope for the future of malaria control.
Bibliography:
World Health Organization (WHO) (2022). ‘Despite continued impact of COVID-19, malaria cases and deaths remained stable in 2021’, WHO. Available at: https://www.who.int/news/item/08-12-2022 despite-continued-impact-of-covid-19-malaria-cases-and-deaths-remained-stable-in-2021 (Accessed: 16 November 2024)
Niang, M. et al. (2017) ‘Temporal analysis of IgG antibody responses to Plasmodium falciparum antigens in relation to changing malaria epidemiology in a West African setting’, Malaria Journal, 16(1).
Mackinnon, M. J. and Marsh, K. (2010) ‘The selection landscape of malaria parasites’, Science, 328(5980), pp. 866–871.
Russell, T. L. et al. (2011) ‘Increased proportions of outdoor feeding among residual malaria vector populations following increased use of insecticide-treated nets in rural Tanzania’, Malaria Journal, 10(1).
Moiroux, N. et al. (2012) ‘Changes in Anopheles funestus biting behavior following universal coverage of long-lasting insecticidal nets in Benin’, The Journal of Infectious Diseases, 206(10), pp. 1622–1629.
Band, G. et al. (2022) ‘Malaria protection due to sickle hemoglobin depends on parasite genotype’, Nature, 602(7896), pp. 106–111.
Worrall, E., Basu, S. & Hanson, K., 2003. 'The relationship between socio-economic status and malaria: A review of the literature.' SES and Malaria. Available at: https://www.researchgate.net/publication/267679401
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