Evidence of Ongoing Transmission of Zika Virus in Mérida, Mexico

Abstract

Since the Zika virus (ZIKV) pandemic in 2015-2017, there has been a near absence of reported cases in the Americas outside of Brazil. However, the conditions for Aedes-borne transmission persist in Latin America, and the threat of ZIKV transmission is increasing as population immunity wanes. Mexico has reported only 70 cases of laboratory-confirmed ZIKV infection since 2020, with no cases recorded in the Yucatán peninsula. Here, we provide evidence of active ZIKV transmission, despite the absence of official case reports, in the city of Mérida, Mexico, the capital of the state of Yucatán. Capitalizing on an existing cohort, we detected cases in participants with symptoms consistent with flavivirus infection from 2021 to 2022. Serum samples from suspected cases were tested for ZIKV RNA by polymerase chain reaction or ZIKV-reactive IgM by ELISA. To provide more specific evidence of exposure, focus reduction neutralization tests were performed on ELISA-positive samples. Overall, we observed 25 suspected ZIKV infections for an estimated incidence of 2.8 symptomatic cases per 1,000 persons per year. Our findings emphasize the continuing threat of ZIKV transmission in the setting of decreased surveillance and reporting.

Figure 1.

Incidence of ZIKV, DENV, and CHIKV cases in Mexico and the state of Yucatán reported to the Mexican Ministry of Health from January 2017 to June 2023. CHIKV = chikungunya virus; DENV = dengue virus; ZIKV = ZIKA virus.

Figure 2.

Heatmaps of the fold increase in FRNT₅₀ titers from the most recent baseline sample and either the convalescent sample (active surveillance) or the 2022 annual serological surveillance sample (passive surveillance) for each participant for ZIKV and the indicated DENV serotype (full dataset in Supplemental Figures 1–4, Supplemental Tables 1 and 2). Participants have been divided based on category of likely exposure: ZIKV, ZIKV/DENV cross-reactive, or DENV. DENV = dengue virus; FRNT = focus reduction neutralization test; PID = participant identification; ZIKV = Zika virus.

Figure 3.

(A) Map of Mérida indicating the hot spot area for dengue and other Aedes-borne viruses (light gray) and clusters of the TIRS trial cohort (dark gray), together with likely ZIKV cases (red) and ZIKV⁺/DENV⁺ cross-reactive cases (black). (B) Results from K-function analysis showing the observed value of the function (solid dark line) against the 95% CI for complete spatial randomness (dotted line). A pattern is random when the observed values are within the 95% CI band. DENV = dengue virus; TIRS = Targeted Indoor Residual Spraying; ZIKV = Zika virus.