A study of more than 2,200 schoolchildren in Cameroon suggests that sickle cell trait is associated with differences in malaria parasite genotypes even when infections occur without symptoms, highlighting the complex evolutionary relationship between human genetic variation and Plasmodium falciparum.
Sickle cell trait may influence not only susceptibility to symptomatic malaria but also the genetic composition of the parasites that infect an individual, according to a study published in Nature Microbiology. The findings provide new insights into the biological relationship between host genetics and parasite diversity and suggest that malaria protection involves mechanisms extending beyond the human response to infection.
Researchers investigated 2,246 healthy schoolchildren in the Mfou region of Cameroon, an area with high malaria transmission. Using high-throughput DNA sequencing, they analyzed human hemoglobin genotypes and genetic markers in Plasmodium falciparum, the parasite responsible for the most severe form of human malaria. The analysis included 1,701 asymptomatic P. falciparum infections, enabling the researchers to examine parasite diversity in individuals who were infected but had no apparent clinical symptoms.
Sickle cell trait occurs when an individual inherits one copy of the hemoglobin S variant (HbS) and one normal hemoglobin A allele. This genotype, designated HbAS, differs from sickle cell disease, which generally results from inheriting two abnormal hemoglobin alleles. Although sickle cell trait does not usually cause the clinical manifestations associated with sickle cell disease, it is known to provide substantial protection against severe malaria.
This protective association is an important example of natural selection driven by infectious disease. In malaria-endemic regions, the survival advantage associated with carrying one HbS allele has contributed to the persistence of the variant in human populations. However, the mechanisms underlying this protection are complex and may involve both reduced susceptibility to disease and an increased ability to tolerate infection without developing severe symptoms.
Previous research had identified P. falciparum parasites carrying particular sickle-associated alleles, termed Pfsa+, among individuals with HbAS who developed symptomatic malaria. These observations raised questions about whether such parasite variants could partially overcome the protection associated with sickle cell trait. Their significance in asymptomatic infections, however, had remained uncertain.
The new study found that individuals with HbAS and those with the HbAA genotype had similar rates of asymptomatic malaria infection. However, the genetic composition of the infecting parasites differed according to the host’s hemoglobin status. Individuals carrying HbS were more likely to harbor parasites with Pfsa+ alleles at two genetic loci, Pfsa1 and Pfsa3. These associations indicate that the host hemoglobin genotype is linked to the distribution of particular parasite variants, even in the absence of clinical symptoms.
The findings suggest that sickle cell hemoglobin status may influence parasite populations before the onset of symptomatic disease. Rather than preventing malaria infection altogether, the protective genetic variant may alter the relative success of different parasite genotypes in establishing or maintaining infection.
The researchers also reported lower parasite multiplicity and parasitemia associated with HbS status. These observations provide additional evidence that the relationship between sickle cell trait and malaria extends beyond whether an individual becomes infected. A central finding is the distinction between malaria infection and symptomatic disease. The presence of parasites does not necessarily result in clinical illness, and individuals may carry P. falciparum without developing recognisable symptoms. By examining asymptomatic infections, the researchers could investigate host–parasite relationships that might be overlooked in studies restricted to patients presenting with malaria symptoms.
The enrichment of Pfsa+ alleles among HbS carriers suggests that parasite genetic characteristics may be associated with the host environment in which infection occurs. However, the study does not demonstrate that these alleles directly cause reduced disease severity or explain the full protective effect of sickle cell trait.
The findings instead support a more complex model in which host resistance, tolerance of infection and parasite genetic variation may interact to influence clinical outcomes. Resistance refers to mechanisms that limit parasite infection or burden, whereas tolerance describes the ability to limit disease-related harm despite infection. Both processes may contribute to the protection associated with HbS. The findings highlight the importance of studying both human and parasite genetics to understand malaria outcomes. They also challenge the assumption that parasite variants associated with symptomatic infections necessarily have the same biological significance in asymptomatic carriers.
Further research is needed to determine how Pfsa-associated variants affect parasite growth, host–parasite interactions and the development of clinical disease. Investigations across different geographical populations and transmission settings will also be important to establish whether the observed associations are consistent beyond the study population in Cameroon.
A better understanding of these relationships could eventually inform parasite genomic surveillance and help researchers identify genetic changes relevant to malaria epidemiology. However, the current findings do not establish an immediate clinical application or a new preventive or therapeutic intervention. The study adds a new dimension to the understanding of malaria protection associated with sickle cell trait. It suggests that the evolutionary interaction between humans and P. falciparum may shape not only human genetic variation but also the distribution of parasite genotypes. Clarifying these interactions could improve understanding of why some infected individuals remain asymptomatic while others develop clinically significant disease.
Reference
- Hopson HD, Herbert-Mainero A, Bouopda-Tuedom AG, Nanssong-Vomo CT, Fotso BT, Kiam BC, et al. Sickle cell haemoglobin status shapes malaria parasite genotype in asymptomatic infections. Nat Microbiol. 2026 Oct 8. doi:10.1038/s41564-026-02502-4.