Seroprevalence of Zika Virus in Amphawa District, Thailand, after the 2016 Pandemic

Abstract

In 2016, Zika virus (ZIKV) infection was declared a public health emergency of international concern because of the neurological consequences in babies born to infected people. Because of the mild and nonspecific symptoms, serological tests are essential in epidemiological studies. However, cross-reactive antibodies between other Flaviviridae members may complicate the interpretation of results of these tests. This study investigated the seroprevalence of ZIKV infection in Samut Songkhram in central Thailand which was affected by the Zika outbreak of 2016. Three hundred and fifty volunteers aged 5-50 years in Amphawa District, Samut Songkhram, were enrolled between April 2017 and April 2018. ZIKV nonstructural protein 1 (NS1) immunoglobulin G enzyme-linked immunosorbent assay (ELISA) was used to screen serum samples collected on the first day of enrollment and after 6 and 12 months. The seroprevalence and seroconversion of ZIKV were assessed. Cases of ZIKV seroconversion were verified as evidence of ZIKV infection by NS1 blockade-of-binding ELISA and plaque reduction neutralization test (PRNT50). ZIKV seroprevalence in Amphawa was 15.1-17.8% with no significant change over the year. The total seroconversion rate throughout the year was 7/100 person-years. The ratio of asymptomatic to symptomatic infections was 4.5:1. The cases in our study confirmed the occurrence of occult ZIKV infections in the community. These undetected infections might promote the spread of ZIKV in vulnerable groups of the community.

Keywords: ELISA; PRNT; Thailand; Zika virus; epidemiology; flavivirus.

Figure 1

(a) The location of Samut Songkhram Province, Thailand. (b) The map of Amphawa District. The blue stars mark three subdistricts from which the participants were recruited.

Figure 2

Schematic diagram of the study design and procedures. Zika virus (ZIKV) and dengue virus nonstructural protein 1 antibodies were examined at enrollment (Day 0) and after 6 and 12 months. Urine ZIKV RT-PCR was performed for volunteers who visited a hospital with acute febrile illness.

Figure 3

The change in the serostatus of the participants at each visit. The orange color represents the proportion of seroconverted participants between Day 0 and Month 6. The green color represents subjects with seroconversion between Month 6 and Month 12 (N/A: participants lost to follow-up).