Cross-sectional study of asymptomatic malaria and seroepidemiological surveillance of seven districts in Gia Lai province, Vietnam

Abstract

Background: Malaria elimination by 2030 is an aim of many countries in the Greater Mekong Sub-region, including Vietnam. However, to achieve this goal and accelerate towards malaria elimination, countries need to determine the extent and prevalence of asymptomatic malaria as a potential reservoir for malaria transmission and the intensity of malaria transmission. The purpose of this study was to determine the prevalence of asymptomatic malaria and seropositivity rate in several districts of Gia Lai province in the Central Highlands of Vietnam.

Methods: A cross-sectional survey of asymptomatic malaria and serological testing was conducted in 3283 people living at 14 communes across seven districts in Gia Lai province in December 2016 to January 2017. Finger prick capillary blood samples were tested for malaria using rapid diagnostic testing and polymerase chain reaction (PCR), as well as detecting antibodies against 3 Plasmodium falciparum and 4 Plasmodium vivax antigens by indirect enzyme-linked immunosorbent assay (ELISA). Age-seroprevalence curves were fitted using reverse catalytic models with maximum likelihood.

Results: The study population was predominantly male (65.9%, 2165/3283), adults (88.7%, 2911/3283) and of a minority ethnicity (72.2%, 2371/3283), with most participants being farmers and outdoor government workers (90.2%, 2960/3283). Using a small volume of blood (≈ 10 µL) the PCR assay revealed that 1.74% (57/3283) of the participants had asymptomatic malaria (P. falciparum 1.07%, P. vivax 0.40%, Plasmodium malariae 0.15% and mixed infections 0.12%). In contrast, the annual malaria prevalence rates for clinical malaria in the communities where the participants lived were 0.12% (108/90,395) in 2016 and 0.22% (201/93,184) in 2017. Seropositivity for at least one P. falciparum or one P. vivax antigen was 38.5% (1257/3262) and 31.1% (1022/3282), respectively. Age-dependent trends in the proportion of seropositive individuals in five of the districts discriminated the three districts with sustained low malaria prevalence from the two districts with higher transmission.

Conclusions: Asymptomatic Plasmodium carriers were found to be substantially more prevalent than clinical cases in seven districts of Gia Lai province, and a third of the population had serological evidence of previous malaria exposure. The findings add knowledge on the extent of asymptomatic malaria and transmission for developing malaria elimination strategies for Vietnam.

Keywords: Asymptomatic malaria; Drug resistance; Gia Lai province; PCR; Plasmodium falciparum; Plasmodium vivax; Serology; Vietnam.

Fig. 1

Map of Gia Lai province, Vietnam. The study districts highlighted are in blue: Ia Grai (1), Duc Co (2), Chu Prong (3), Chu Se (4), Chu Puh (5), Kon Chro (6) and Krong Pa (7)

Fig. 2

Seroprevalence of participants in seven districts in Gia Lai province against P. falciparum and P. vivax antigens. Error bars indicate 95% CI

Fig. 3

Age-seroprevalence curves for P. falciparum exposure across five districts in Gia Lai province, Vietnam. Exposure is defined as seropositivity to at least one P. falciparum antigen. Reversible catalytic conversion models allowing one seroconversion rate (λ) were fit to the data (dashed line indicates 95% CI)

Fig. 4

Age-seroprevalence curves for P. vivax exposure across five districts in Gia Lai province, Vietnam. Exposure is defined as seropositivity to at least one P. vivax antigen. Reversible catalytic conversion models allowing one seroconversion rate (λ) were fit to the data (dashed line indicates 95% CI)