Singapore\u2019s Anopheles sinensis Form A is susceptible to Plasmodium vivax isolates from the western Thailand\u2013Myanmar border

Sook-Cheng Pang,Sai-Gek Lam-Phua,Chiara Andolina,Bruce Russell,Peter R Christensen,Daiqin Li,Laurent Rénia,Cindy S Chu,Francois Nosten,Benoit Malleret,Muhammad Aliff Bin Abdul Razak,Chee-Seng Chong,Lee-Ching Ng

doi:10.1186/s12936-017-2114-3

Abstract

BackgroundSingapore has been certified malaria-free by the World Health Organization since November 1982. However, sporadic autochthonous malaria outbreaks do occur. In one of the most recent outbreaks of vivax malaria, an entomological investigation identified Anopheles sinensis as the most probable vector. As metaphase karyotype studies divided An. sinensis into two forms, A and B, with different vector competence: the investigation of vector competence of An. sinensis found in Singapore was thus pursued using Plasmodium vivax field isolates from the Thailand–Myanmar border.MethodsAdults and larvae An. sinensis were collected from Singapore from 14 different locations, using various trapping and collection methods between September 2013 and January 2016. Molecular identification of An. sinensis species were conducted by amplifying the ITS2 and CO1 region using PCR. Experimental infections of An. sinensis using blood from seven patients infected with P. vivax from the Thailand–Myanmar border were conducted with Anopheles cracens (An. dirus B) as control.ResultsPhylogenetic analysis showed that An. sinensis (F22, F2 and collected from outbreak areas) found in Singapore was entirely Form A, and closely related to An. sinensis Form A from Thailand. Artificial infection of these Singapore strain An. sinensis Form A resulted in the development of oocysts in four experiments, with the number of sporozoites produced by one An. sinensis ranging from 4301 to 14,538.ConclusionsInfection experiments showed that An. sinensis Form A from Singapore was susceptible to Thai–Myanmar P. vivax strain, suggesting a potential role as a malaria vector in Singapore.

Highlights

Singapore has been certified malaria-free by the World Health Organization since November 1982
Infection experiments showed that An. sinensis Form A from Singapore was susceptible to Thai–Myan‐ mar P. vivax strain, suggesting a potential role as a malaria vector in Singapore
Anopheles sinensis larvae were collected by the Environmental Health Officer (EHO) of National Environment Agency (NEA) during the routine malaria surveillance and were submitted to Environmental Health Institute (EHI) for identification

Summary

Introduction

Singapore has been certified malaria-free by the World Health Organization since November 1982. In one of the most recent outbreaks of vivax malaria, an entomological investigation identified Anopheles sinensis as the most probable vector. The last outbreaks occurred in the middle of 2009, when three clusters with a total of 29 vivax malaria patients, with no recent travel history, were identified by the Ministry of Health. Relapse cases in vivax malaria amongst foreign workers from malaria endemic countries are common and defining if the cluster is due to local transmission is challenging. Molecular epidemiology was performed using the msp3a and msp genes of the parasite. It confirmed only two independent local transmissions in Mandai-Sungei Kadut and in Sembawang [8]. An. sinensis has been implicated as the malaria vector in some parts of Asia, including Korea, China, Japan and Vietnam, it has never been reported as a vector in Singapore [10,11,12,13,14,15,16,17,18,19,20,21,22,23]

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