Hypoglycemia (< 70 mg/dL) has been detected in approximately 16% of patients and is considered a clinical and laboratory features of severe malaria (34). Careful glucose monitoring should be targeted to these complications, especially in those patients with G6PD deficiency. However, none of the G6PD alleles segregated with low level glucose among the severe malaria patients.
In this study, we found in uncomplicated malaria patients a three times higher frequency of genotype FY*01/FY*02 (45.3%) when compared to the FY*01/FY*01 (15.4%) and FY*02/FY*02 (14.8%). In severe malaria patients, the frequency of genotype FY*01/FY*02 (39.2%) was approximately two times higher with regards to the genotypes FY*01/FY*02N.01 (20.6%) and FY*02/FY*02 (17.5%). These findings corroborate to the study reported by Cavasini et al. (2007), that correlated the high frequency of the FY*01 and FY*02 alleles among P. vivax malaria patients. The FY*01/FY*02 and FY*01/FY*01 genotypes were associated with a high frequency of P. vivax infection suggesting that these individuals have a higher risk of developing disease (35). The FY*01/FY*01 and FY*01/FY*02 genotypes are associated with increased frequency of P. vivax infection, while FY*01/ FY*02W.01 and FY*02/ FY*02W.01 were associated with low parasitic density levels (17).
The presence of a single case with null Duffy variants in this study supports a possible advantageous selection, driven by defense mechanisms against P. vivax. In endemic areas, the Duffy negative blood group is reported as a protective factor against P. vivax malaria infection (36). In a study conducted in São Paulo, the phenotypic frequencies found in blood donors for Duffy blood system antigens were 19.8% for the phenotype Fy(a + b-) in Caucasians and 14.0% in African-Brazilian, Fy(a + b+) in 41.4% of Caucasians and 1.6% of African-Brazilian, Fy(a-b+) in 37.8% of Caucasians and 17.5% of African-Brazilian and Fy(a-b-) in 1.1% of Caucasians and 66.9% of African-Brazilian (37). Our results indicate that the Manaus-Amazonas region has individuals who express three Duffy phenotypes Fy(a + b+), Fy(a + b-) and Fy(a-b+) more frequently with expression of FY*01 and FY*02 antigens.
For the process of parasite invasion into red blood cells, Duffy protein is functionally important. In regions of high malaria transmission rates, as in the inhabitants of the Amazon, Duffy protein is naturally immunogenic. Woolley et al. demonstrated (in vitro) that the expression level of the Fy6 epitopes was significantly lower in reticulocytes and red blood cells from individual carriers of the FYB/FYB genotypes compared with individuals of the FYA/FYA or FYA/FYB genotypes. Another similar study showed that the presence of the FY*02N.01 allele resulted in a 50% reduction of that protein on invasion by P. vivax (38, 39).
In a study with P. vivax malaria patients, the FY*01 and FY*02 alleles were found to have low, medium, and high parasitic density. However, in the presence of the GATA variant, genotypes with alleles FY*02N.01 and FY*02W.01 were found only in patients with low parasitic density and low symptomatology (17).
A study performed in the state of Pará with a population of African descent demonstrated the presence of the c.202G > A variant was 0.060, the Duffy blood group included 24.3% Duffy negative and 41.3% individuals heterozygous for FY*02w. The frequency of the FY*02w allele was 41.0%. These findings support the monitoring of individuals with G6PD deficiency for use of primaquine during the routine care of Afro-descendant communities of the Trombetas, Erepecuru, and Cumná rivers, to assess risks of hemolytic crisis in recurrent cases of malaria in the region (40).
Several studies have revealed that the phenotypic and genotypic variation complexity of the Duffy system and the variants of G6PD (A-) can have a significant impact on the distribution of human populations in areas where malaria is endemic. The Duffy system and G6PD are polymorphic systems that offer great challenges to researchers not only due to their academic importance, but also due to their potential applications in the treatment of vivax malaria (41). Whenever natural selection occurs within a population in an area of endemicity for malaria, natural adaptations may result from genetic variation that provides a partial defense mechanism against P. vivax infections (38).
In this study, we were careful to include hemoglobinopathies status like sickle cell anemia (HbSS) or Sickle cell diseases (HbSC, HbSD, HbSE) of the patients before antimalarial treatment intervention.
One of the major confounders of our analysis and limitations in this study is the inherent disadvantage of G6PD deficiency that goes against the consensus that G6PD potentially protect against malaria. Despite most G6PD-deficient persons are asymptomatic, a hemolytic anemia, main clinical sign, can occur 1–3 days after exposure, eating fava beans, triggered by infections or by certain drugs, such as those used to treat malaria (42, 43). In many cases, this acute hemolytic anemia is usually self-limiting and resolve on their own (44). In addition, we acknowledge that an important limitation of our analysis is the small sample size and we understand that additional studies of larger samples will be required to confirm our results.