Zika Antibody Signals in Pregnant Thai Women Blur the Line Between Zika and Dengue

Pregnant women in Bangkok showed Zika-reactive antibody signals at a surprisingly high rate during 2021 and 2022, yet none of them had detectable viral genetic material in their blood, and their immune profiles pointed as strongly toward dengue as toward Zika. The findings, from a prospective cohort study conducted at Rajavithi Hospital and published in PLOS Neglected Tropical Diseases, illustrate a persistent dilemma in flavivirus medicine: in regions where dengue circulates constantly, the antibodies that flag possible Zika infection may instead reflect the echo of prior dengue exposure, making definitive diagnosis during pregnancy extraordinarily difficult.

The research team, led by Salin Sirinam of Rajavithi Hospital and colleagues across Thai and international institutions, enrolled 199 pregnant women attending routine antenatal clinic visits and followed them across their pregnancies between 2021 and 2022. The investigators used a rapid immunochromatography assay, the DPP ZCD IgM/IgG System manufactured by Chembio, which simultaneously detects immunoglobulin M and immunoglobulin G antibodies against Zika virus, chikungunya virus, and dengue virus. Because Zika infection during pregnancy has been linked to severe congenital neurological abnormalities, including microcephaly, identifying infections in real time is a public health priority, particularly in Southeast Asia where the virus circulates at low levels amid dense dengue transmission.

The study defined a Zika virus serologic response in two ways. A participant was classified as having a response if she tested reactive for Zika-specific immunoglobulin M at enrollment, indicating a relatively recent antibody response, or if she seroconverted from non-reactive to reactive for either Zika immunoglobulin M or immunoglobulin G between scheduled study visits. Seroconversion of this kind suggests new exposure to a flavivirus or a boosting of existing flavivirus immunity, but it does not by itself identify which virus triggered the response. This ambiguity sits at the heart of the study’s conclusions.

Over the course of follow-up, the rapid test revealed incident Zika-reactive serologic responses at a rate of 15.4 per 100 person-years, a figure that would be alarming if interpreted at face value in a population of pregnant women. Notably, every participant with a reactive serologic response was completely asymptomatic. Zika infection is known to be mild or silent in the large majority of cases, so the absence of symptoms does not rule out true infection. However, the molecular testing told a different story.

All samples tested by real-time reverse transcription polymerase chain reaction, the diagnostic gold standard for active Zika infection, came back negative. RT-qPCR detects viral RNA in blood and is most sensitive during the first week of illness, when viremia peaks. In an asymptomatic population, the window for catching viremia is narrow, and the researchers acknowledge that negative PCR results in this context cannot definitively exclude infection that occurred between visits. Still, the combination of negative PCR and reactive antibodies left the team without molecular confirmation of a single Zika infection in the cohort.

To probe the specificity of the antibody responses further, the investigators performed plaque reduction neutralization testing, a more discriminating serologic method that measures whether a participant’s antibodies can actually neutralize a given virus in vitro. Among participants who showed Zika immunoglobulin G seroconversion, the neutralization assays revealed antibodies against Zika virus and against all four dengue virus serotypes. This broad neutralizing profile is characteristic of people with extensive flavivirus exposure history and does not single out Zika as the most recent infecting agent. In dengue-endemic Thailand, where most adults carry neutralizing antibodies against multiple dengue serotypes from lifetime exposure, such patterns are expected even in the absence of new Zika infection.

The epidemiological analysis added another layer of interpretation. A prior history of dengue infection was associated with significantly increased odds of a Zika-reactive serologic response, with an odds ratio of 3.53 and a P value of 0.046. This association is consistent with the well-documented phenomenon of antibody cross-reactivity among flaviviruses. Immunoglobulin M raised against dengue can bind Zika antigens on a rapid test strip, and prior dengue immunity can be boosted by subsequent flavivirus exposure of any kind, producing reactive signals that mimic new Zika infection. The odds ratio suggests that dengue immune history, rather than undetected Zika transmission, may account for a substantial share of the reactive results.

The authors carefully enumerate the possible explanations for the serologic responses they observed. In a highly dengue-immune setting, a reactive Zika rapid test result could reflect true Zika virus exposure, dengue virus reinfection, flavivirus immune boosting, or serologic cross-reactivity between related viruses. Because all four possibilities produce overlapping antibody patterns on the assays used, the responses cannot be attributed specifically to Zika virus. This limitation is not unique to Thailand. Systematic reviews of Zika diagnostics in Latin America, led in part by co-authors Thomas Jaenisch and May Chu, have reached similar conclusions about the interpretive challenges of serologic testing in flavivirus-endemic populations.

The practical implications for clinical care and surveillance are significant. Rapid point-of-care serologic tests offer real advantages in resource-limited settings: they require minimal equipment, deliver results within minutes, and can be deployed in antenatal clinics where PCR capacity is scarce. The DPP ZCD assay, which screens for three viruses simultaneously, represents the kind of multiplex tool that surveillance programs need in co-endemic regions. The study’s findings suggest that such tests may support screening and surveillance by identifying populations with recent flavivirus antibody responses, but they cannot establish Zika infection or Zika-specific exposure without careful interpretation and additional testing.

For pregnant women and their physicians, the message is one of calibrated vigilance rather than reassurance or alarm. The incidence of 15.4 reactive serologic responses per 100 person-years in an asymptomatic cohort underscores how frequently flavivirus antibody signals arise during pregnancy in dengue-endemic settings, and how little those signals reveal on their own. Confirming Zika infection during pregnancy would require molecular testing timed to catch viremia, neutralization testing to characterize antibody specificity, and longitudinal follow-up to distinguish new infection from immune boosting. Until diagnostic tools capable of flavivirus-specific serologic discrimination become widely available, the study suggests that reactive rapid tests in dengue-endemic regions should trigger careful additional evaluation rather than immediate attribution to Zika virus.

Subject of Research: A prospective cohort study of 199 pregnant women in Bangkok found frequent Zika-reactive serologic responses detected by a rapid test, but negative RT-qPCR results and broad dengue cross-neutralization prevented attribution to Zika virus specifically.

Article Title: Zika virus serologic responses during pregnancy in a dengue-endemic setting: A prospective cohort study in Thailand

Article References: Zika virus serologic responses during pregnancy in a dengue-endemic setting: A prospective cohort study in Thailand. (n.d.). https://doi.org/10.1371/journal.pntd.0014805

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014805

Keywords: Zika virus, dengue, pregnancy, serology, rapid diagnostic test, IgM, IgG, cross-reactivity, Thailand, flavivirus, RT-qPCR, neutralization test