Infectivity of symptomatic and asymptomatic Plasmodium vivax infections to a Southeast Asian vector, Anopheles dirus

https://doi.org/10.1016/j.ijpara.2016.10.006Get rights and content

Highlights

  • The mosquito infection rate of Plasmodium vivax is correlated with parasitemia.

  • The threshold of Anopheles dirus infection by P. vivax is ∼10–100 parasites/μl.

  • Plasmodium vivax asymptomatic reservoir likely contributes significantly to malaria transmission.

Abstract

Plasmodium vivax is now the predominant species causing malarial infection and disease in most non-African areas, but little is known about its transmission efficiency from human to mosquitoes. Because the majority of Plasmodium infections in endemic areas are low density and asymptomatic, it is important to evaluate how well these infections transmit. Using membrane feeding apparatus, Anopheles dirus were fed with blood samples from 94 individuals who had natural P. vivax infections with parasitemias spanning four orders of magnitude. We found that the mosquito infection rate was positively correlated with blood parasitemia and that infection began to rise when parasitemia was >10 parasites/μl. Below this threshold, mosquito infection is rare and associated with very few oocysts. These findings provide useful information for assessing the human reservoir of transmission and for establishing diagnostic sensitivity required to identify individuals who are most infective to mosquitoes.

Introduction

Over the past decade, malaria incidence has steadily declined in various parts of the worlds. In many places where Plasmodium falciparum and Plasmodium vivax coexist, including South America, Southeast Asia and the Western Pacific region, the latter has now become the predominant species (Oliveira-Ferreira et al., 2010, Rodriguez et al., 2011, Imwong et al., 2015, Waltmann et al., 2015). The resilience of P. vivax relative to P. falciparum against malaria controls can be attributed, at least partially, to the parasite’s ability to remain dormant as hypnozoites in the host’s liver (Krotoski et al., 1982, White et al., 2014, Robinson et al., 2015) and its greater transmission efficiency (Boyd, 1937, Pethleart et al., 2004). Plasmodium vivax thus poses a great challenge for malaria eradication. Due to the lack of an in vitro culture system that produces infectious gametocytes, information about P. vivax transmission efficiency is limited, and has mostly relied on direct or membrane feeding experiments using blood from human malaria infections (Sattabongkot et al., 1991, Sattabongkot et al., 2003, Gamage-Mendis et al., 1993, Zollner et al., 2006, Rios-Velasquez et al., 2013, Vallejo et al., 2016).

Several studies have reported that the majority of P. vivax infections in the endemic areas of Asia, South America, and Oceania are asymptomatic (Harris et al., 2010, Baum et al., 2015, Imwong et al., 2015, Waltmann et al., 2015, Vasquez-Jimenez et al., 2016) and submicroscopic (Cheng et al., 2015), even in areas where malaria transmission intensity has declined. Previous studies have also shown that blood from both P. vivax-infected patients and asymptomatic carriers can infect Anopheles dirus, a Southeast Asian vector (Sattabongkot et al., 1991, Sattabongkot et al., 2003, Coleman et al., 2004, Pethleart et al., 2004), but the relationship between P. vivax parasitemia and the mosquito infection rate is often described as weak if not absent (Graves et al., 1988, Gamage-Mendis et al., 1991, Bharti et al., 2006, Coleman et al., 2004). At present, the relative contributions to transmission of asymptomatic and symptomatic P. vivax-infected populations remain unclear. Such information is important for improving the current disease control and elimination programs. If asymptomatic carriers are contributing substantially to transmission, then malaria interventions will need to also target these carriers to be effective.

To determine how well P. vivax transmits from humans to mosquitoes and to assess the contribution of asymptomatic carriers to transmission, we performed membrane feeding experiments on An. dirus using blood samples from both P. vivax malaria patients and asymptomatic carriers. These samples covered a broad range of parasitemias, from submicroscopic to 10,000 parasites/μl.

Section snippets

Study sites

The study was conducted in Tha Song Yang District of Tak Province and Sai Yok District of Kanchanaburi Province in western Thailand between 2014 and 2015. Both areas were mountainous and populations were composed mainly of Thai and Karen ethnicities. The main occupation of the study participants was farming. Malaria in the study areas was seasonal, with the major peak season lasting from May to August, and a secondary peak in November to December. The prevalence of P. vivax and P. falciparum in

Results

We performed membrane feeding experiments on 222 blood samples (Table 1). The demographic information of the blood donors as well as the raw membrane feeding data are available in Supplementary Table S1. In total, 94 of the samples were P. vivax-positive by light microscopy, P. vivax-specific LAMP (Pv-LAMP), or qRT-PCR. Twenty-one samples were Plasmodium-positive by genus-specific LAMP or qRT-PCR, but the species were indeterminate due to low parasitemias. The last 107 samples were

Discussion

We used membrane feeding assays to examine mosquito infectivity of blood samples from acute P. vivax patients, asymptomatic carriers, and individuals living in the endemic areas who appeared to be free of blood-stage parasites. Although the relationship between P. vivax parasitemias and infection rates has often been described as weak or absent (Graves et al., 1988, Gamage-Mendis et al., 1991, Coleman et al., 2004, Bharti et al., 2006), we could detect significant positive correlation between

Acknowledgements

This study was supported by the TransEPI consortium funded by the Bill & Melinda Gates Foundation, USA (www.gatesfoundation.org) and National Institutes of Health, USA, International Centers of Excellence in Malaria Research grant (U19 AI089672, www.niaid.nih.gov). We greatly appreciate the contribution to microscopic examinations of blood samples by Mrs. Nongnuj Maneechai. IM is supported by a National Health and Medical Research Council of Australia Senior Research Fellowship. WN is supported

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