High heterogeneity of malaria transmission and a large sub-patent and diverse reservoir of infection in Wusab As Safil district, Republic of Yemen

Abstract

Background: Yemen remains the country with the highest malaria transmission within the Arabian Peninsula and a source of imported cases to neighbouring countries.

Methods: This study collected samples from individuals resident in a valley in Western Yemen as a baseline to examine infection prevalence for a future trial. As well as rapid diagnostic test (RDT) and microscopy, a filter paper blood spot was collected for molecular and serological analyses.

Results: Samples were collected from 2261 individuals from 12 clusters across a study area of approximately 100 km(2). Plasmodium falciparum infection prevalence was 12.4, 11.1 and 19.6% by RDT, microscopy and polymerase chain reaction (PCR), respectively. RDT and microscopy did not detect 45% of infections present, suggesting many infections were low-density. Infection prevalence and seroprevalence were highly heterogeneous between clusters, with evidence of higher exposure in clusters close to the wadi. The mean multiplicity of infection (MOI) was 2.3 and high heterozygosity and allelic richness were detected.

Conclusions: This highly diverse parasite population suggests a high degree of transmissibility and coupled with the substantial proportion of low-density infections, may pose challenges for malaria control and elimination efforts.

Keywords: Heterogeneity; Heterozygosity; Malaria transmission; Molecular; Multiplicity of infection; PCR.

Fig. 1

Location of study district, South West Yemen

Fig. 2

Proportion of patent (detectable by a RDT and b microscopy) and sub-patent (detected by PCR) P. falciparum infections by cluster

Fig. 3

Prevalence for a RDT b Microscopy c PCR and d seroprevalence by cluster

Fig. 4

Proportion of patent (detectable by a RDT and b microscopy) and sub-patent (detected by PCR) P. falciparum infections by distance to the wadi

Fig. 5

Seroprevalence curves to either P. falciparum antigen for a people living within 500 m of the wadi and b further than 500 m from the wadi. Black circles represent actual data, plotted at median percentiles for age and black line represents the results from a reversible catalytic model. In the clusters close to the wadi, there was evidence for a change in transmission 9 years prior to the survey—this dataset has been fitted allowing for two lambdas

Fig. 6

Frequency of a 3D7 and b Fc27 allelles in P. falciparum samples, Yemen