Vivax malaria in Duffy-negative patients shows invariably low asexual parasitaemia: implication towards malaria control in Ethiopia

Background The increase in detections of Plasmodium vivax infection in Duffy-negative individuals in Africa has challenged the dogma establishing the unique P. vivax Duffy Binding Protein-Duffy antigen receptor for chemokines (PvDBP-DARC) pathway used by P. vivax merozoites to invade reticulocytes. Information on the impact of Duffy antigen polymorphisms on the epidemiology of P. vivax malaria remains elusive. The objective of this study was to determine the distribution of asexual parasitaemia of P. vivax according to the Duffy antigen polymorphisms in Ethiopia. Methods DNA was extracted from dried blood spots (DBS) collected from prospectively recruited 138 P. vivax-infected patients from health centres. The identification and estimation of P. vivax asexual parasitaemia were performed by microscopic examination and quantitative real-time polymerase chain reaction (PCR). Duffy genotyping was conducted by DNA sequencing in a total of 138 P.vivax infected samples. Results The proportion of Duffy-negatives (FY*B-ES/FY*B-ES) in P. vivax infected patients was 2.9% (4/138). Duffy genotype FY*B/FY*B-ES (48.6%) was the most common, followed by FY*A/FY*B-ES genotype (25.4%). In one patient, the FY*02 W.01/FY*02 N.01 genotype conferring a weak expression of the Fy(b) antigen was observed. All P.vivax infected Duffy-negative patients showed low asexual parasitaemia (<= 110 parasites/mu L). The median P. vivax parasitaemia in Duffy-negative patients (53 parasites/mu L) was significantly lower than those found in homozygous and heterozygous individuals (P < 0.0001). Conclusion Plasmodium vivax in Duffy-negative patients shows invariably low asexual parasitaemia. This finding suggests that the pathway used by P. vivax to invade Duffy-negative reticulocytes is much less efficient than that used in Duffy-positives. Moreover, the low asexual parasitaemia observed in Duffy-negative individuals could constitute an 'undetected silent reservoir', thus likely delaying the elimination of vivax malaria in Ethiopia.