In a recent study published in eBioMedicine, researchers conducted entomological, wastewater, and case surveillance of the Zika virus (ZIKV) in Singapore.
Study: Case report: Zika surveillance complemented with wastewater and mosquito testing. Image Credit: Tacio Philip Sansonovski/Shutterstock.com
Background
ZIKV is a mosquito-borne virus from the Flaviviridae family. ZIKV cases and outbreaks have increased over the last two decades in the Americas, the Pacific, Asia, and Africa.
The World Health Organization (WHO) declared a public health emergency of international concern in 2016 when over a million ZIKV cases were reported in Brazil and the Americas.
Moreover, infections were associated with severe neurologic complications and congenital Zika syndrome.
Infected individuals are often asymptomatic; if present, symptoms include fever, rash, joint and muscle pain, and conjunctivitis. Singapore recorded a ZIKV outbreak in 2016-17, and sporadic infections have been since reported.
The low population immunity and presence of competent vectors in Singapore underscore the risk of ZIKV outbreaks, warranting comprehensive surveillance and vector control measures.
While case surveillance is limited to those with symptoms, wastewater and entomological surveillance can provide non-intrusive, comprehensive surveillance.
About the study
In the present study, researchers conducted entomological, case, and wastewater surveillance of ZIKV. A definitive ZIKV case was defined based on the detection of viral RNA in whole blood, serum, urine, or semen.
Hospitals and primary healthcare clinics had access to test services, and all cases were required to be notified to the Health Ministry, Singapore.
Epidemiological data included the weekly number of ZIKV infections and areas with infection/case clusters.
Mosquito surveillance was undertaken at sites with reported Zika cases, including the area of concern (AOC), its vicinity, and the neighborhood of sporadic cases. Mosquito populations were monitored using the National Gravitrap Surveillance System.
Additional Biogents (BG)-Sentinel traps or Gravitraps were set up at the AOC. Mosquito specimens were collected and processed.
Adult female Aedes mosquitos were identified at the species level. RNA was extracted from samples and subject to real-time reverse-transcription polymerase chain reaction (RT-qPCR).
Besides, wastewater autosamplers were deployed at the AOC and its neighborhood. Wastewater samples were concentrated through ultrafiltration; RNA was extracted, and RT-qPCR was performed.
ZIKV envelope (E) gene was sequenced. A phylogenetic tree was constructed based on its sequence. Spearman’s correlation between ZIKV wastewater signals and new weekly Zika cases was analyzed.
Findings
A residential area in northeast Singapore recorded two Zika cases in May 2023. Infections increased to 15, with five sporadic cases in other zones by mid-June 2023.
The researchers screened 590 Aedes mosquito pools for ZIKV. These samples comprised 43 pools from the National Gravitrap Surveillance System at the AOC, 128 from its vicinity, and 419 from areas with sporadic cases.
Ninety mosquitos were captured using additionally set-up BG-Sentinel traps or Gravitraps in the AOC and its vicinity. Virus-positive mosquitos were detected in samples from the AOC only.
Three pools with eight mosquitos and three individually screened mosquitos were positive for viral RNA. Wastewater signals were positive for viral RNA between May 15 and 25, 2023, in the AOC and surrounding locations.
One area (A) had a higher frequency of equivocal (20.2%) and positive (6%) samples than other areas (B and C).
Notably, the period of wastewater detection coincided with the peak number of cases reported in the AOC and a peak in the detection of positive mosquitos. ZIKV RNA signals in wastewater positively correlated with weekly new infections.
Retrospective and prospective wastewater samples from other areas were not positive for ZIKV.
Phylogenetic analyses revealed that ZIKV sequences were genetically distinct from strains identified in 2016 in Singapore and belonged to the Asian genotype. Further, the sequences were closely related to those detected in 2012 and 2016 in the Philippines.
Conclusions
In sum, this surveillance analysis showed that the peak ZIKV outbreak in Singapore occurred in the epidemiological week of May 14, 2023, and declined by mid-June 2023.
The absence of viral RNA in samples from regions not vicinal to AOC and those with sporadic cases highlights the limited transmission and low incidence of ZIKV infections in the country.
Of note, all mosquito samples could not be identified at the species level. Moreover, the team could not determine whether ZIKV in wastewater was from new infections or those in recovery.
Overall, the study demonstrated the emerging applicability of wastewater surveillance for ZIKV, suggesting that regular surveillance of wastewater could provide early alerts and facilitate timely measures.
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