Health authorities in Bangladesh have officially confirmed a local outbreak of the Zika virus, with ten laboratory-confirmed cases identified in and around the capital city of Dhaka between September and December 2024, providing conclusive evidence of domestic transmission and raising significant public health concerns in a region already grappling with other mosquito-borne illnesses. The Zika virus, primarily transmitted by Aedes species mosquitoes, is known for causing an illness that is often mild or entirely asymptomatic, with common symptoms like fever, rash, and joint pain that can be easily confused with other viral infections endemic to the area, such as dengue and chikungunya. Despite its typically mild presentation in most individuals, the virus poses a grave threat to pregnant women, as an infection during gestation can lead to Congenital Zika Syndrome, a devastating condition that causes severe birth defects, including microcephaly and other profound brain abnormalities in newborns. This outbreak is not the first time Zika has been detected in the country; the virus was retrospectively identified in an archived sample from 2014, and a small cluster of five active cases was found in Dhaka in 2023. However, the 2024 outbreak’s wider geographic spread and the confirmation of the virus in local mosquito populations suggest that Zika is transitioning from a sporadic, imported threat to a more established, endemic pathogen.
A Diagnostic Puzzle Unveiled
The complex investigation into the 2024 outbreak was catalyzed on September 4, when the Institute of Epidemiology, Disease Control and Research (IEDCR) received a critical notification from a private hospital in Dhaka. A 29-year-old non-pregnant woman had presented with a constellation of symptoms—including fever, joint pain, and a rash—that were highly suggestive of dengue fever, a common and well-known arboviral illness in Bangladesh. Standard procedure led to a rapid diagnostic test for the dengue nonstructural protein-1 (NS1) antigen, but the result came back negative. This unexpected outcome became a crucial turning point, prompting astute clinicians to consider other possibilities. Suspecting another arbovirus like chikungunya, they ordered a more sophisticated and comprehensive multiplex reverse transcription–polymerase chain reaction (RT-PCR) assay, a powerful diagnostic tool designed to simultaneously detect the genetic material of dengue, chikungunya, and Zika viruses from a single sample. The test yielded a definitive answer: the patient was positive exclusively for Zika virus RNA. To formally verify this finding, IEDCR’s own state-of-the-art virology laboratory obtained serum and urine samples from the patient and independently confirmed the diagnosis via RT-PCR, solidifying the presence of an active infection and officially triggering a national public health emergency response.
Upon this definitive laboratory confirmation, IEDCR moved swiftly, formally declaring a Zika virus disease outbreak in Dhaka on September 9, 2024. This rapid declaration was consistent with established public health policy in Bangladesh, where the identification of even a single laboratory-confirmed case with suspected local transmission is sufficient to activate a full-scale outbreak response. An intensive investigation was immediately launched into the index patient’s background. A detailed travel history revealed no international trips in the preceding weeks, effectively ruling out the possibility that the infection was acquired abroad. Furthermore, investigators meticulously examined and excluded less common transmission routes, such as sexual contact, blood transfusion, or organ transplantation, thereby cementing the conclusion that the virus had been transmitted locally. In response, IEDCR initiated a robust, multi-faceted public health campaign. The agency issued urgent alerts to all other hospitals in Dhaka, disseminated critical information to the public through a series of media briefings to foster awareness and encourage preventative measures, and formally notified the Obstetrical and Gynecological Society of Bangladesh to ensure a heightened index of suspicion among clinicians responsible for managing the care of pregnant patients, the population most vulnerable to the virus’s devastating effects.
Tracing the Spread Through Integrated Surveillance
The implementation of this heightened alert system proved remarkably effective in uncovering the broader scope of the outbreak. Shortly after the public health notifications were circulated, three other hospitals in the capital began to identify and report additional cases. These new identifications followed a similar diagnostic pattern to the index case: patients presented with arbovirus-like symptoms, initially tested negative for dengue, and were subsequently confirmed to have Zika virus following more comprehensive testing. This process led to the discovery of four more individuals with the disease. In one particularly insightful instance, the public health response team was able to establish the only direct epidemiological link found during the entire outbreak. After an outpatient was diagnosed with Zika, investigators proactively tested the patient’s symptomatic household member, who was also confirmed to have the virus. This discovery of a household cluster, though limited to this single instance, provided strong evidence of the virus’s efficient transmission in close-contact settings. The identification of these additional cases in September and October underscored the success of the awareness campaign, as clinicians, now equipped with new information, were better prepared to look beyond the more common diagnoses of dengue and chikungunya.
The investigation was further complicated, yet ultimately aided, by the fact that the Zika outbreak occurred concurrently with a separate and significant chikungunya outbreak in Dhaka. In response to the chikungunya emergency, IEDCR had already established a dedicated sample collection booth to test referred patients presenting with suspected arboviral illnesses. This pre-existing public health infrastructure became an invaluable tool for Zika surveillance. Between October and December 2024, a total of 394 patients, the vast majority of whom were initially suspected of having chikungunya, were tested using the comprehensive multiplex RT-PCR assay. This parallel surveillance effort, initially designed for another purpose, incidentally led to the identification of four additional Zika virus disease cases. This accidental discovery was a powerful demonstration of the inherent challenges in distinguishing these diseases based on clinical symptoms alone, as the overlap is substantial. More importantly, it highlighted the immense strategic value of an integrated surveillance approach, where patients with generalized arboviral symptoms are systematically tested for a panel of potential pathogens rather than being screened for just one, thereby preventing misdiagnosis and revealing the true co-circulation of multiple viruses within the community.
Characterizing the Outbreak and Its Vector
A detailed analysis of the ten confirmed cases provided a clear demographic, clinical, and geographic profile of the outbreak. The median age of the patients was 37 years, with a range spanning from 23 to 52 years. The data revealed a notable gender disparity, with females accounting for seven of the ten confirmed cases. Critically, and to the great relief of public health officials, none of the female patients were pregnant at the time of their diagnosis, averting the most severe potential consequence of the virus. All ten individuals experienced a relatively mild course of illness, with symptoms resolving without the need for hospitalization or intensive medical intervention; supportive care was sufficient for a full recovery. The most consistently reported symptoms, present in all ten patients, were fever, arthralgia (joint pain), and myalgia (muscle pain). Other frequently observed clinical signs included a generalized rash (seen in nine patients), headaches (seven patients), and conjunctivitis (six patients). The investigation into each patient’s history consistently ruled out any international travel in the two weeks preceding symptom onset, providing unequivocal evidence that Zika virus transmission was occurring locally. Moreover, the geographic distribution of the cases—with nine patients residing in various locations across Dhaka and one in the adjacent Gazipur district—indicated a widespread presence of the virus, contrasting sharply with the highly localized cluster identified in 2023 and suggesting that the virus and its mosquito vector were broadly established across the metropolitan region.
To supplement the compelling epidemiological evidence, IEDCR conducted a targeted entomological investigation aimed at finding direct biological proof of the Zika virus within the local mosquito population. Public health teams meticulously selected seven sites in Dhaka, each located within a one-kilometer radius of a confirmed patient’s home, to serve as collection points. At these sites, mosquito larvae were carefully collected from various bodies of stagnant water, such as ponds and lakes, which serve as primary breeding grounds for Aedes mosquitoes. The collected larvae were then transported to a secure laboratory, where they were reared to adulthood under controlled conditions. Once the mosquitoes matured, they were pooled together by collection site and tested for the presence of Zika virus RNA using the same highly sensitive multiplex RT-PCR assay employed for human diagnostics. This methodical investigation yielded a landmark result: one of the seven pooled mosquito samples tested positive for Zika virus RNA. This finding was of immense significance, as it provided the first direct biological evidence of the virus’s circulation within the local Aedes mosquito population, definitively confirming that the vector for transmission was present, active, and infectious in Dhaka. This entomological confirmation completed the chain of evidence, solidifying the understanding of the local transmission cycle.
A Strategic Shift in Public Health Defense
The thorough investigation into the 2024 Zika outbreak provided several critical insights that prompted a strategic enhancement of Bangladesh’s national public health framework. The combined evidence—from cases with no travel history, the wide geographic distribution, and the detection of Zika RNA in local mosquitoes—unequivocally confirmed that established local transmission of the virus was occurring in and around Dhaka. The recurrence of cases in 2024, following the initial cluster in 2023, strongly suggested that the virus was transitioning from a sporadic threat to an endemic pathogen. The investigation also highlighted the profound challenge of underdetection, concluding that the ten identified cases likely represented only a small fraction of the true scale of the outbreak. This underreporting was attributed to the absence of routine Zika surveillance, the limited availability of specific diagnostic tests outside of specialized labs, the high proportion of mild or asymptomatic infections that go unnoticed, and the significant clinical overlap with dengue and chikungunya, which frequently leads to misdiagnosis. Consequently, a central conclusion of the report was the urgent need to dismantle disease-specific surveillance silos and adopt an integrated arboviral surveillance system.
In response to these findings, IEDCR coordinated a multi-faceted and forward-looking public health strategy. The agency formally incorporated Zika virus screening into the standard testing protocol for all patients referred for suspected dengue or chikungunya, a measure that had already proven its worth during the outbreak investigation. This integrated screening for Zika, dengue, and chikungunya was continued into the 2025 arboviral season to ensure early detection of future outbreaks. Furthermore, a new surveillance program for acute febrile illness was initiated at six sites across Bangladesh where Zika is now considered a priority disease, expanding monitoring beyond the capital. An arboviral serosurvey was also launched in Dhaka to better understand the true prevalence and population-level immunity to these viruses. The response also emphasized leveraging ongoing vector control programs organized by city authorities, which are primarily aimed at controlling dengue, and raising specific awareness among obstetricians and gynecologists to refer any pregnant patients with compatible clinical signs for comprehensive arboviral testing at IEDCR. This strategic shift represented a move toward a more holistic and resilient public health defense against the complex and overlapping threat of mosquito-borne diseases.
