Most rapid diagnostic tests for Plasmodium falciparum malaria target the Histidine-Rich Proteins 2 and 3 (HRP2 and HRP3). Deletions of the hrp2 and hrp3 genes result in false-negative tests and are a threat for malaria control. A novel assay for molecular surveillance of hrp2/hrp3 deletions was developed based on droplet digital PCR (ddPCR). The assay quantifies hrp2, hrp3, and a control gene with very high accuracy. The theoretical limit of detection was 0.33 parasites/µl. The deletion was reliably detected in mixed infections with wild-type and hrp2-deleted parasites at a density of >100 parasites/reaction. For a side-by-side comparison with the conventional nested PCR (nPCR) assay, 248 samples were screened in triplicate by ddPCR and nPCR. No deletions were observed by ddPCR, while by nPCR hrp2 deletion was observed in 8% of samples. The ddPCR assay was applied to screen 830 samples from Kenya, Zanzibar/Tanzania, Ghana, Ethiopia, Brazil, and Ecuador. Pronounced differences in the prevalence of deletions were observed among sites, with more hrp3 than hrp2 deletions. In conclusion, the novel ddPCR assay minimizes the risk of false-negative results (i.e., hrp2 deletion observed when the sample is wild type), increases sensitivity, and greatly reduces the number of reactions that need to be run.

Background Transmission stemming from asymptomatic infections is increasingly being recognized as a threat to malaria elimination. In many regions, malaria transmission is seasonal. It is not well understood whether Plasmodium falciparum modulates its investment in transmission to coincide with seasonal vector abundance. Methods We sampled 1116 asymptomatic individuals in the wet season, when vectors are abundant, and 1743 in the dry season, in two sites in western Kenya, representing different transmission intensities (Chulaimbo, moderate transmission, and Homa Bay, low transmission). Blood samples were screened for P. falciparum by qPCR, and gametocytes by pfs25 RT-qPCR. Results Parasite prevalence by qPCR was 27.1% (Chulaimbo, dry), 48.2% (Chulaimbo, wet), 9.4% (Homabay, dry), and 7.8% (Homabay, wet). Mean parasite densities did not differ between seasons (P = 0.562). pfs25 transcripts were detected in 119/456 (26.1%) of infections. In the wet season, fewer infections harbored detectable gametocytes (22.3% vs. 33.8%, P = 0.009), but densities were 3-fold higher (wet: 3.46 transcripts/uL, dry: 1.05 transcripts/uL, P < 0.001). In the dry season, 4.0% of infections carried gametocytes at moderate-to-high densities likely infective (> 1 gametocyte per 2 uL blood), compared to 7.9% in the wet season. Children aged 5–15 years harbored 76.7% of infections with gametocytes at moderate-to-high densities. Conclusions Parasites increase their investment in transmission in the wet season, reflected by higher gametocyte densities. Despite increased gametocyte densities, parasite density remained similar across seasons and were often below the limit of detection of microscopy or rapid diagnostic test, thus a large proportion of infective infections would escape population screening in the wet season. Seasonal changes of gametocytemia in asymptomatic infections need to be considered when designing malaria control measures.

Background: The most commonly used Plasmodium falciparum rapid diagnostic tests target the Histidine-Rich Proteins 2 and 3 (HRP2, HRP3). An increasing number of countries report parasites that carry hpr2 and/or hrp3 gene deletions, resulting in false negative test results. Molecular surveillance of hrp2 and hrp3 deletions is crucial but adequate protocols have been lacking. Methods and Findings: We have developed novel assays for deletion typing based on droplet digital PCR (ddPCR), targeting hrp2 exon1, hrp2 exon 2, and hrp3. In the ddPCR assay, hrp2 or hrp3 and a control gene were quantified with very high accuracy in a single tube. The theoretical limit of detection of the ddPCR assay was 0.33 parasites/uL, and thus well suited for typing of low-density asymptomatic infections. The deletion was reliably detected in mixed infections with wild-type and hrp2-deleted parasites when the proportion of parasites carrying the deletion was >40%. For a side-by-side comparison with the conventional nested PCR (nPCR) assay, 248 samples from asymptomatic individuals from western Kenya were screened in triplicate by ddPCR and nPCR. No deletions were observed by ddPCR, while by nPCR no band for hrp2 was observed in 8% of samples. The ddPCR assay was applied to screen 777 samples from six countries in Africa and South America. No deletions were observed in Kenya (n=241) and Zanzibar/Tanzania (n=91), and very few in Ghana (3/170 hrp3 deletions). In southwestern Ethiopia, 1/47 (2.1%) samples carried hrp2 deletion, and 35/47 (74.5%) hrp3 deletions. In Brazil, 87/187 (46.5%) samples carried hrp2 deletions, and 116/187 (62%) hrp3 deletions. In Ecuador, no hrp2 deletions were observed, but 22/41 (53.7%) samples carried hrp3 deletions. Conclusions: Compared to nPCR, the ddPCR assay minimizes the risk of false-negative results (i.e. hrp2 deletion observed when the sample is wild type), increase sensitivity, and greatly reduces the number of reactions that need to be run. Pronounced differences in the prevalence of deletion were observed among sites, with more hrp3 than hrp2 deletions.