The prevalence of P. vivax and P. falciparum and Duffy-blood group genotype distribution was studied in the population of Anajás, State of Pará, an area of malaria transmission in the Brazilian Amazonian, in order to analyse the presence of P. vivax infection in Duffy-negative individuals.
Until recently, the Duffy-negative phenotype was seen as giving complete protection against infection by P. vivax, since this parasite requires the presence of Duffy antigen receptor for chemokines on the RBC surface to be able to invade cells and cause disease. However, the universality of this specificity has been questioned by recent reports that P. vivax infects and causes disease in Duffy-negative people in some parts of the world [16–21].
In this study, 6.9% (2/29) of the Duffy-negative subjects were diagnosed as P. vivax-infected, a finding that confirms previous reports in patients from Rondonia, western Brazilian Amazon [17, 18], as well as in Kenya, East Africa , in Madagascar , Equatorial Guinea and Angola, African West Coast  and Mauritania, north-western Africa . Moreover, the data obtained in the population of Anajás are in accordance with those found in children from Madagascar , where in individual study sites with sufficient numbers of PCR-positive P. vivax infections to enable comparisons, prevalence ratios were not significantly different between Duffy-positive and -negative children. That is, the risk of malaria infection due to P. vivax was not different between the Duffy-negative and Duffy-positive groups.
It is of note that other genotypes besides Duffy-negative have been shown capable of influencing the susceptibility to P. vivax infection [18, 25], but this was not observed in Anajás, since the genotype frequencies did not differ significantly between controls and P. vivax-infected patients both in the whole sample and in each site investigated. One possible explanation for the results obtained in Anajás would be that the intensity of transmission of P. vivax in this region is such that it provides a constant source of parasites that infect Duffy-positives, providing ample opportunities for infection of hepatocytes Duffy-negative and selecting strains of P. vivax with a new capacity to invade erythrocytes, possibly through a Duffy-independent mechanism. In this scenario, less prominent protective effects against infection by P. vivax conferred by other genotypes seem to have been abolished or are less evident.
This study was unable to assess possible effects of Duffy genotypes on the risk of developing clinical malaria, since the patients were not followed after medical attention. Anyway, it is important to note that only one of the two Duffy-negative patients diagnosed as P. vivax-infected through mtDNA-qPCR method was confirmed by microscopic examination of blood smear, and had parasitemia classified as low (12 parasites per field under oil immersion microscope). The other patient was only detected by qPCR, but also presented a profile consistent with low parasitemia. In addition, with respect to clinical manifestations, both were asymptomatic at the time of medical consultation.
The observed frequencies of Duffy-negative genotype in P. vivax patients and controls in Anajás population (2.1% and 4.6%, respectively), which are not significantly different, are smaller than those found in Afro-Brazilian communities in eastern Amazonia (Pará and Amapá) , in which the genotype frequencies ranged from 0.323 to 0.588, but are quite similar to those found in most Amazonian populations already studied (frequencies ranging from 0 to 12%) [17, 18, 27, 28]. Moreover, the frequencies of Duffy genotypes found in Anajás are the expected for a population with a genetic background resulting from the admixture between Europeans, mainly Portuguese, Africans and Amerindians in very close proportions . Thus, the observed distribution of Duffy genotypes in the population of Anajás with a high frequency of Duffy-positive associated with a high prevalence of malaria, predominantly P. vivax, appears to fulfil the conditions considered by Ménard et al as necessary to clear the barrier of Duffy negativity, providing conditions for the parasites have sufficient exposure to Duffy-negative red cells, allowing more opportunities for de novo selection or optimization of an otherwise cryptic invasion pathway that nevertheless seems less efficient than the Duffy-dependent pathway.