Reduced Plasmodium vivax erythrocyte infection in PNG Duffy-negative heterozygotes

Abstract

Background: Erythrocyte Duffy blood group negativity reaches fixation in African populations where Plasmodium vivax (Pv) is uncommon. While it is known that Duffy-negative individuals are highly resistant to Pv erythrocyte infection, little is known regarding Pv susceptibility among heterozygous carriers of a Duffy-negative allele (+/-). Our limited knowledge of the selective advantages or disadvantages associated with this genotype constrains our understanding of the effect that interventions against Pv may have on the health of people living in malaria-endemic regions.

Methods and findings: We conducted cross-sectional malaria prevalence surveys in Papua New Guinea (PNG), where we have previously identified a new Duffy-negative allele among individuals living in a region endemic for all four human malaria parasite species. We evaluated infection status by conventional blood smear light microscopy and semi-quantitative PCR-based strategies. Analysis of a longitudinal cohort constructed from our surveys showed that Duffy heterozygous (+/-) individuals were protected from Pv erythrocyte infection compared to those homozygous for wild-type alleles (+/+) (log-rank tests: LM, p = 0.049; PCR, p = 0.065). Evaluation of Pv parasitemia, determined by semi-quantitative PCR-based methods, was significantly lower in Duffy +/- vs. +/+ individuals (Mann-Whitney U: p = 0.023). Overall, we observed no association between susceptibility to P. falciparum erythrocyte infection and Duffy genotype.

Conclusions: Our findings provide the first evidence that Duffy-negative heterozygosity reduces erythrocyte susceptibility to Pv infection. As this reduction was not associated with greater susceptibility to Pf malaria, our in vivo observations provide evidence that Pv-targeted control measures can be developed safely.

Figure 1. Susceptibility to blood-stage infection during monthly prevalence surveys (1998–99); unmatched longitudinal analyses.

Diagnosis of Pv and Pf erythrocyte infection was performed by light microscopy (LM) . Light gray line represents A+/A− (n = 11) and dark gray line represents A+/A+ (n = 453). A, Cumulative Pv erythrocyte infection in Duffy A+/A− vs A+/A+ individuals in unmatched analyses (p = 0.026; log-rank test). B, Cumulative Pf erythrocyte infection in Duffy A+/A− vs A+/A+ individuals in unmatched analyses (p = 0.875; log-rank test).

Figure 2. Susceptibility to blood-stage infection during monthly prevalence surveys (1998–99); matched longitudinal analyses.

Evaluation of cumulative Pv and Pf erythrocyte infection was performed among Duffy A+/A− vs A+/A+ individuals matched by age, sex, and village of residence. A, Diagnosis of Pv erythrocyte infection was performed by light microscopy (LM) (p = 0.049; log-rank test) . B, Diagnosis of Pv erythrocyte infection was performed by LDR-FMA (p = 0.065; log-rank test) . C, Diagnosis of Pf erythrocyte infection was performed by LM (p = 0.675; log-rank test). D, Diagnosis of Pf erythrocyte infection was performed by LDR-FMA (p = 0.453; log-rank test). In A through D, light gray lines represent A+/A− (n = 11) and dark gray lines represent A+/A+ (n = 33).

Figure 3. Semi-quantitative LDR-FMA Diagnosis of Pv and Pf infections split by Duffy Genotype and Age Group.

A, Comparison of Pv fluorescent signal intensity (log₁₀-transformed) of individuals who had a Pv-positive blood sample split by Duffy genotype (matched on age, sex, village of residence, and survey period). For children less than 15 years of age, A+/A+ (n = 105) mean fluorescent signal intensity = 2.96, A+/A− (n = 18) mean fluorescent signal intensity = 2.37 (Mann-Whitney P = 0.023). For individuals over 15 years, A+/A+ (n = 66) mean fluorescent signal intensity = 2.22, A+/A− (n = 15) mean fluorescent signal intensity = 2.16 (Mann-Whitney P = 0.86). B, Comparison of Pf fluorescent signal intensity (log₁₀-transformed) of individuals who had a Pf-positive blood sample split by Duffy genotype (matched on age, sex, village of residence, and survey period). For children less than 15 years of age, A+/A+ (n = 146) mean fluorescent signal intensity = 3.08, A+/A− (n = 17) mean fluorescent signal intensity = 3.22 (Mann-Whitney P = 0.742). For individuals over 15 years, A+/A+ (n = 89) mean fluorescent signal intensity = 2.51, A+/A− (n = 22) mean fluorescent signal intensity = 2.69 (Mann-Whitney P = 0.529).