Detection of flaviviruses and alphaviruses in mosquitoes in Central Macedonia, Greece, 2018

The 2022 West Nile Virus Season in Greece; A Quite Intense Season

Since 2010, the West Nile virus (WNV) has been established in Greece. We describe the epidemiology of diagnosed human WNV infections in Greece with a focus on the 2022 season. During the transmission period, clinicians were sending samples from suspected cases for testing. Active laboratory-based surveillance was performed with immediate notification of diagnosed cases. We collected clinical information and interviewed patients on a timely basis to identify their place of exposure. Besides serological and molecular diagnostic methods, next-generation sequencing was also performed. In 2022, 286 cases of WNV infection were diagnosed, including 278 symptomatic cases and 184 (64%) cases with neuroinvasive disease (WNND); 33 patients died. This was the third most intense season concerning the number of WNND cases, following 2018 and 2010. Most (96%) cases were recorded in two regions, in northern and central Greece. The virus strain was a variant of previous years, clustering into the Central European subclade of WNV lineage 2. The 2022 WNV season was quite intense in Greece. The prompt diagnosis and investigation of cases are considered pivotal for the timely response, while the availability of whole genome sequences enables studies on the molecular epidemiology of the disease.

West Nile Virus in Culex Mosquitoes in Central Macedonia, Greece, 2022

In 2022, Greece was the second most seriously affected European country in terms of the West Nile virus (WNV), after Italy. Specifically, Central Macedonia was the region with the most reported human cases (81.5%). In the present study, 30,816 female Culex pipiens sensu lato mosquitoes were collected from May to September 2022 in the seven regional units of Central Macedonia; they were then grouped into 690 pools and tested for WNV, while next-generation sequencing was applied to the samples, which showed a cycle threshold of Ct < 30 in a real-time RT-PCR test. WNV was detected in 5.9% of pools, with significant differences in the detection rate among regional units and months. It is of interest that in the Thessaloniki regional unit, where most of the human cases were observed, the virus circulation started earlier, peaked earlier, and lasted longer than in the other regional units. All sequences clustered into the Central European subclade of WNV lineage 2, and the virus strain differed from the initial Greek strain of 2010 by 0.52% and 0.27% at the nucleotide and amino acid levels, respectively. Signature substitutions were present, such as S73P and T157A in the prM and E structural proteins, respectively. The screening of mosquitoes provides useful information for virus circulation in a region with a potential for early warning, while the availability of whole-genome sequences is essential for further studies, including virus evolution.

Detection and molecular characterization of West Nile virus in Culex pipiens mosquitoes in Central Macedonia, Greece, 2019-2021

Since 2010 when West Nile virus (WNV) emerged in Greece, it causes seasonal outbreaks of human infections almost every year. During May-October of 2019-2021 a total number of 51,504 Culex pipiens mosquitoes were trapped in all seven regional units of Central Macedonia in northern Greece. They were grouped into 1099 pools and tested for WNV. The virus was detected in 5% of the mosquito pools (1.5%, 3.6% and 9.6% pools in 2019, 2020, and 2021, respectively), with significant rate differences among the regional units and years. The highest maximum likelihood estimation for WNV infection rates calculated per 1000 mosquitoes for 2019 and 2020 were 1.89 and 3.84 in Serres, and 7.08 for 2021 in Pella regional unit. Sixteen whole genome sequences were taken by applying a recently described PCR-based next generation sequencing protocol. Phylogenetic analysis showed that the sequences belonged to the Central European clade of WNV lineage 2, and that a virus strain introduced in Greece in 2019 continued to circulate and spread further during 2020-2021. The data are useful for public health and mosquito control programs' operational scheduling, while the whole genome sequences are an added value for molecular epidemiology and evolutionary studies.

Seasonal Phenological Patterns and Flavivirus Vectorial Capacity of Medically Important Mosquito Species in a Wetland and an Urban Area of Attica, Greece

Seasonal patterns of mosquito population density and their vectorial capacity constitute major elements to understand the epidemiology of mosquito-borne diseases. Using adult mosquito traps, we compared the population dynamics of major mosquito species (Culex pipiens, Aedes albopictus, Anopheles spp.) in an urban and a wetland rural area of Attica Greece. Pools of the captured Cx. pipiens were analyzed to determine infection rates of the West Nile virus (WNV) and the Usutu virus (USUV). The data provided were collected under the frame of the surveillance program carried out in two regional units (RUs) of the Attica region (East Attica and South Sector of Attica), during the period 2017-2018. The entomological surveillance of adult mosquitoes was performed on a weekly basis using a network of BG-sentinel traps (BGs), baited with CO₂ and BG-Lure, in selected, fixed sampling sites. A total of 46,726 adult mosquitoes were collected, with larger variety and number of species in East Attica (n = 37,810), followed by the South Sector of Attica (n = 8916). The collected mosquitoes were morphologically identified to species level and evaluated for their public health importance. Collected Cx. pipiens adults were pooled and tested for West Nile virus (WNV) and Usutu virus (USUV) presence by implementation of a targeted molecular methodology (real-time PCR). A total of 366 mosquito pools were analyzed for WNV and USUV, respectively, and 38 (10.4%) positive samples were recorded for WNV, while no positive pool was detected for USUV. The majority of positive samples for WNV were detected in the East Attica region, followed by the South Sector of Attica, respectively. The findings of the current study highlight the WNV circulation in the region of Attica and the concomitant risk for the country, rendering mosquito surveillance actions and integrated mosquito management programs as imperative public health interventions.

West Nile virus lineage 2 in humans and mosquitoes in Bulgaria, 2018-2019

BACKGROUND

West Nile virus (WNV) lineage 2, and especially the Hungarian clade, predominates in Europe. Most of the Hungarian clade strains cluster into 2 groups: Central/South-West European and Balkan.

OBJECTIVES

Since there was not any study on WNV in mosquitoes in Bulgaria, the present study was designed to test Culex spp. mosquitoes in areas near the Danube river. The aim of the study was to gain an insight into the recent molecular epidemiology of WNV in Bulgaria.

STUDY DESIGN

A total of 1871 Culex pipiens mosquitoes collected in 2018 and clinical samples from 23 patients with West Nile neuroinavsive disease observed in 2018 and 2019 were tested by TaqMan RT-PCR and RT-nested PCR and PCR products were sequenced.

RESULTS

WNV RNA was detected in clinical samples from 10 patients and in five (12.2 %) of 41 pools of Cx. pipiens mosquitos by realtime RT-PCR, resulting in a minimum infection rate of mosquitoes of 0.27 %. Phylogenetic analysis based on partial NS3 gene sequences from one clinical sample and four mosquito pools showed that all sequences clustered into the Hungarian clade of WNV lineage 2 and all but one were identical to respective sequences from Romania. Whole genome sequences of one mosquito pool belong to the Hungarian group of WNV lineage 2 and cluster in a separate subclade from the Bulgarian strain from 2015, suggesting that at least two different introductions occurred in Bulgaria.

CONCLUSIONS

The current study provides insights into the geographic distribution of WNV in Bulgaria.