Temporal and Microspatial Heterogeneity in Transmission Dynamics of Coendemic Plasmodium vivax and Plasmodium falciparum in Two Rural Cohort Populations in the Peruvian Amazon

Evidence-Based Malaria Control and Elimination in the Amazon: Input from the International Center of Excellence in Malaria Research Network in Peru and Brazil

Malaria remains endemic in 17 countries in the Americas, where 723,000 cases were reported in 2019. The majority (> 90%) of the regional malaria burden is found within the Amazon Basin, which includes nine countries and territories in South America. Locally generated evidence is critical to provide information to public health decision makers upon which the design of efficient and regionally directed malaria control and elimination programs can be built. Plasmodium vivax is the predominant malaria parasite in the Amazon Basin. This parasite species appears to be more resilient to malaria control strategies worldwide. Asymptomatic Plasmodium infections constitute a potentially infectious reservoir that is typically missed by routine microscopy-based surveillance and often remains untreated. The primary Amazonian malaria vector, Nyssorhynchus (formerly Anopheles) darlingi, has changed its behavior to feed and rest predominantly outdoors, reducing the efficiency of core vector control measures such as indoor residual spraying and distribution of long-lasting insecticide-treated bed nets. We review public health implications of recent field-based research carried out by the Amazonia International Center of Excellence in Malaria Research in Peru and Brazil. We discuss the relative role of traditional and novel tools and strategies for better malaria control and elimination across the Amazon, including improved diagnostic methods, new anti-relapse medicines, and biological larvicides, and emphasize the need to integrate research and public health policymaking.

Malaria Resilience in South America: Epidemiology, Vector Biology, and Immunology Insights from the Amazonian International Center of Excellence in Malaria Research Network in Peru and Brazil

The 1990s saw the rapid reemergence of malaria in Amazonia, where it remains an important public health priority in South America. The Amazonian International Center of Excellence in Malaria Research (ICEMR) was designed to take a multidisciplinary approach toward identifying novel malaria control and elimination strategies. Based on geographically and epidemiologically distinct sites in the Northeastern Peruvian and Western Brazilian Amazon regions, synergistic projects integrate malaria epidemiology, vector biology, and immunology. The Amazonian ICEMR's overarching goal is to understand how human behavior and other sociodemographic features of human reservoirs of transmission-predominantly asymptomatically parasitemic people-interact with the major Amazonian malaria vector, Nyssorhynchus (formerly Anopheles) darlingi, and with human immune responses to maintain malaria resilience and continued endemicity in a hypoendemic setting. Here, we will review Amazonian ICEMR's achievements on the synergies among malaria epidemiology, Plasmodium-vector interactions, and immune response, and how those provide a roadmap for further research, and, most importantly, point toward how to achieve malaria control and elimination in the Americas.

Assessment of IgG3 as a serological exposure marker for Plasmodium vivax in areas with moderate-high malaria transmission intensity

A more sensitive surveillance tool is needed to identify Plasmodium vivax infections for treatment and to accelerate malaria elimination efforts. To address this challenge, our laboratory has developed an eight-antigen panel that detects total IgG as serological markers of P. vivax exposure within the prior 9 months. The value of these markers has been established for use in areas with low transmission. In moderate-high transmission areas, there is evidence that total IgG is more long-lived than in areas with low transmission, resulting in poorer performance of these markers in these settings. Antibodies that are shorter-lived may be better markers of recent infection for use in moderate-high transmission areas. Using a multiplex assay, the antibody temporal kinetics of total IgG, IgG1, IgG3, and IgM against 29 P. vivax antigens were measured over 36 weeks following asymptomatic P. vivax infection in Papua New Guinean children (n = 31), from an area with moderate-high transmission intensity. IgG3 declined faster to background than total IgG, IgG1, and IgM. Based on these kinetics, IgG3 performance was then assessed for classifying recent exposure in a cohort of Peruvian individuals (n = 590; age 3-85 years) from an area of moderate transmission intensity. Using antibody responses against individual antigens, the highest performance of IgG3 in classifying recent P. vivax infections in the prior 9 months was to one of the Pv-fam-a proteins assessed (PVX_125728) (AUC = 0.764). Surprisingly, total IgG was overall a better marker of recent P. vivax infection, with the highest individual classification performance to RBP2b_1986-2653 (PVX_094255) (AUC = 0.838). To understand the acquisition of IgG3 in this Peruvian cohort, relevant epidemiological factors were explored using a regression model. IgG3 levels were positively associated with increasing age, living in an area with (relatively) higher transmission intensity, and having three or more PCR-detected blood-stage P. vivax infections within the prior 13 months. Overall, we found that IgG3 did not have high accuracy for detecting recent exposure to P. vivax in the Peruvian cohort, with our data suggesting that this is due to the high levels of prior exposure required to acquire high IgG3 antibody levels.

Relative contribution of low-density and asymptomatic infections to Plasmodium vivax transmission in the Amazon: pooled analysis of individual participant data from population-based cross-sectional surveys

Background

Low-density and asymptomatic Plasmodium vivax infections remain largely undetected and untreated and may contribute significantly to malaria transmission in the Amazon.

Methods

We analysed individual participant data from population-based surveys that measured P vivax prevalence by microscopy and polymerase chain reaction (PCR) between 2002 and 2015 and modelled the relationship between parasite density and infectiousness to vectors using membrane feeding assay data. We estimated the proportion of sub-patent (i.e., missed by microscopy) and asymptomatic P vivax infections and examined how parasite density relates to clinical manifestations and mosquito infection in Amazonian settings.

Findings

We pooled 24,986 observations from six sites in Brazil and Peru. P vivax was detected in 6·8% and 2·1% of them by PCR and microscopy, respectively. 58·5% to 92·6% of P vivax infections were asymptomatic and 61·2% to 96·3% were sub-patent across study sites. P vivax density thresholds associated with clinical symptoms were one order of magnitude higher in children than in adults. We estimate that sub-patent parasite carriers are minimally infectious and contribute 12·7% to 24·9% of the community-wide P vivax transmission, while asymptomatic carriers are the source of 28·2% to 79·2% of mosquito infections.

Interpretation

Asymptomatic P vivax carriers constitute a vast infectious reservoir that, if targeted by malaria elimination strategies, could substantially reduce malaria transmission in the Amazon. Infected children may remain asymptomatic despite high parasite densities that elicit clinical manifestations in adults.

Funding

US National Institutes of Health, Fundação de Amparo à Pesquisa do Estado de São Paulo, and Belgium Development Cooperation.

Multicopy targets for Plasmodium vivax and Plasmodium falciparum detection by colorimetric LAMP

BACKGROUND

Loop-mediated isothermal amplification (LAMP) for malaria diagnosis at the point of care (POC) depends on the detection capacity of synthesized nucleic acids and the specificity of the amplification target. To improve malaria diagnosis, new colorimetric LAMP tests were developed using multicopy targets for Plasmodium vivax and Plasmodium falciparum detection.

METHODS

The cytochrome oxidase I (COX1) mitochondrial gene and the non-coding sequence Pvr47 for P. vivax, and the sub-telomeric sequence of erythrocyte membrane protein 1 (EMP1) and the non-coding sequence Pfr364 for P. falciparum were targeted to design new LAMP primers. The limit of detection (LOD) of each colorimetric LAMP was established and assessed with DNA extracted by mini spin column kit and the Boil & Spin method from 28 microscopy infections, 101 malaria submicroscopic infections detected by real-time PCR only, and 183 negatives infections by both microscopy and PCR.

RESULTS

The LODs for the colorimetric LAMPs were estimated between 2.4 to 3.7 parasites/µL of whole blood. For P. vivax detection, the colorimetric LAMP using the COX1 target showed a better performance than the Pvr47 target, whereas the Pfr364 target was the most specific for P. falciparum detection. All microscopic infections of P. vivax were detected by PvCOX1-LAMP using the mini spin column kit DNA extraction method and 81% (17/21) were detected using Boil & Spin sample preparation. Moreover, all microscopic infections of P. falciparum were detected by Pfr364-LAMP using both sample preparation methods. In total, PvCOX1-LAMP and Pfr364-LAMP detected 80.2% (81 samples) of the submicroscopic infections using the DNA extraction method by mini spin column kit, while 36.6% (37 samples) were detected using the Boil & Spin sample preparation method.

CONCLUSION

The colorimetric LAMPs with multicopy targets using the COX1 target for P. vivax and the Pfr364 for P. falciparum have a high potential to improve POC malaria diagnosis detecting a greater number of submicroscopic Plasmodium infections.

New Records of Anopheles benarrochi B (Diptera: Culicidae) in Malaria Hotspots in the Amazon Regions of Ecuador and Peru

The increase in malaria transmission in the Amazon region motivated vector control units of the Ministry of Health of Ecuador and Peru to investigate Anopheles (Diptera: Culicidae) species present in transmission hotspots. Mosquitoes were collected using prokopack aspirators and CDC light traps (Ecuador) and human landing catch in Peru. In Ecuador, 84 Anopheles were captured from Pastaza, Morona Santiago, and Orellana provinces and identified morphologically [An. (An.) apicimacula Dyar and Knab, An. (Nys.) near benarrochi, An. (Nys.) near oswaldoi, An. (Nys.) near strodei, An. (An.) nimbus (Theobald, 1902), and An. (Nyssorhynchus) sp.]. In Peru, 1,150 Anopheles were collected in Andoas District. A subsample of 166 specimens was stored under silica and identified as An. near oswaldoi, An. darlingi, and An. (An.) mattogrossensis Lutz and Neiva. COI barcode region sequences were obtained for 137 adults (107 from Peru, 30 from Ecuador) identified by ITS2 PCR-RFLP as An. benarrochi Gabaldon, Cova Garcia, and Lopez and retained in the final analysis. Haplotypes from the present study plus An. benarrochi B GenBank sequences grouped separately from Brazilian An. benarrochi GenBank sequences by 44 mutation steps, indicating that the present study specimens were An. benarrochi B. Our findings confirm the presence of An. benarrochi B in Ecuador and reported here for the first time from the Amazonian provinces of Orellana and Morona Santiago. Furthermore, we confirm that the species collected in Andoas District in the Datem del Maranon Province, Peru, is An. benarrochi B, and we observed that it is highly anthropophilic. Overall, the known distribution of An. benarrochi B has been extended and includes southern Colombia, much of Peru and eastern Ecuador.

Heterogeneity in response to serological exposure markers of recent Plasmodium vivax infections in contrasting epidemiological contexts

BACKGROUND

Antibody responses as serological markers of Plasmodium vivax infection have been shown to correlate with exposure, but little is known about the other factors that affect antibody responses in naturally infected people from endemic settings. To address this question, we studied IgG responses to novel serological exposure markers (SEMs) of P. vivax in three settings with different transmission intensity.

METHODOLOGY

We validated a panel of 34 SEMs in a Peruvian cohort with up to three years' longitudinal follow-up using a multiplex platform and compared results to data from cohorts in Thailand and Brazil. Linear regression models were used to characterize the association between antibody responses and age, the number of detected blood-stage infections during follow-up, and time since previous infection. Receiver Operating Characteristic (ROC) analysis was used to test the performance of SEMs to identify P. vivax infections in the previous 9 months.

PRINCIPAL FINDINGS

Antibody titers were associated with age, the number of blood-stage infections, and time since previous P. vivax infection in all three study sites. The association between antibody titers and time since previous P. vivax infection was stronger in the low transmission settings of Thailand and Brazil compared to the higher transmission setting in Peru. Of the SEMs tested, antibody responses to RBP2b had the highest performance for classifying recent exposure in all sites, with area under the ROC curve (AUC) = 0.83 in Thailand, AUC = 0.79 in Brazil, and AUC = 0.68 in Peru.

CONCLUSIONS

In low transmission settings, P. vivax SEMs can accurately identify individuals with recent blood-stage infections. In higher transmission settings, the accuracy of this approach diminishes substantially. We recommend using P. vivax SEMs in low transmission settings pursuing malaria elimination, but they are likely to be less effective in high transmission settings focused on malaria control.