Tularemia on the rise in Switzerland? A one health approach is needed!

In the last 10 years, an increase in tularemia cases has been observed in both humans and animals in Switzerland. In these, infection with Francisella tularensis, the causative agent of the zoonotic disease tularemia, can occur through arthropod vectors or contact to infected animals or exposure to contaminated environmental sources. Currently, we are only able to postulate potential aetiologies: (i) behavioral changes of humans with more exposure to endemic habitats of infected arthropod vectors; (ii) an increased rate of tularemia infected ticks; (iii) increasing number and geographical regions of tick biotopes; (iv) increasing and/or more diverse reservoir populations; (v) increasing presence of bacteria in the environment; (vi) raised awareness and increased testing among physicians; (vii) improved laboratory techniques including molecular testing. To approach these questions, a one-health strategy is necessary. A functioning collaboration between public health, human medicine, and diagnostic and veterinary units for the control of tularemia must be established. Furthermore, the public should be included within citizen-supported-science-projects.

First Detection of West Nile Virus Lineage 2 in Mosquitoes in Switzerland, 2022.

West Nile virus (WNV) is one of the most widespread flaviviruses in the world, and in recent years, it has been frequently present in many Mediterranean and Eastern European countries. A combination of different conditions, such as a favourable climate and higher seasonal average temperatures, probably allowed its introduction and spread to new territories. In Switzerland, autochthonous cases of WNV have never been reported, and the virus was not detected in mosquito vectors until 2022, despite an entomological surveillance in place in Canton Ticino, southern Switzerland, since 2010. In 2022, 12 sites were monitored from July to October, using BOX gravid mosquito traps coupled with honey-baited FTA cards. For the first time, we could detect the presence of WNV in FTA cards and mosquitoes in 8 out of the 12 sampling sites monitored, indicating an unexpectedly widespread circulation of the virus throughout the territory. Positive findings were recorded from the beginning of August until mid-October 2022, and whole genome sequencing analysis identified a lineage 2 virus closely related to strains circulating in Northern Italy. The entomological surveillance has proved useful in identifying viral circulation in advance of possible cases of WNV infection in humans or horses.

Estimating the Impact of Consecutive Blood Meals on Vector Competence of Aedes albopictus for Chikungunya Virus.

The continuous expansion of Aedes albopictus in Europe and the increases in autochthonous arboviruses transmissions in the region urge a better understanding of the virus transmission dynamic. Recent work described enhanced chikungunya virus (CHIKV) dissemination in Aedes aegypti mosquitoes exposed to a virus-free blood meal three days after their infection with CHIKV. Our study investigated the impact of a second blood meal on the vector competence of Ae. albopictus from southern Switzerland infected with CHIKV. Seven-day-old Ae. albopictus females were exposed to CHIKV-spiked blood and incubated at constant (27 °C) and fluctuating (14-28 °C) temperatures. Four days post-infection (dpi), some of these females were re-fed with a non-infectious blood meal. Virus infectivity, dissemination, transmission rate, and efficiency were investigated at seven and ten dpi. No enhanced dissemination rate was observed among females fed a second time; however, re-fed females have shown higher transmission efficiency than those fed only once after seven days post-infection and incubated under a fluctuating temperature regime. Vector competence for CHIKV was confirmed in Ae. albopictus from southern Switzerland. We did not observe an increase in dissemination rates among mosquitoes fed a second time (second blood meal), regardless of the temperature regime.

Risk-Based Mapping Tools for Surveillance and Control of the Invasive Mosquito Aedes albopictus in Switzerland.

BACKGROUND: In Switzerland, Aedes albopictus is well established in Ticino, south of the Alps, where surveillance and control are implemented. The mosquito has also been observed in Swiss cities north of the Alps. Decision-making tools are urgently needed by the local authorities in order to optimize surveillance and control. METHODS: A regularized logistic regression was used to link the long-term dataset of Ae. albopictus occurrence in Ticino with socioenvironmental predictors. The probability of establishment of Ae. albopictus was extrapolated to Switzerland and more finely to the cities of Basel and Zurich. RESULTS: The model performed well, with an AUC of 0.86. Ten socio-environmental predictors were selected as informative, including the road-based distance in minutes of travel by car from the nearest cell established in the previous year. The risk maps showed high suitability for Ae. albopictus establishment in the Central Plateau, the area of Basel, and the lower Rhone Valley in the Canton of Valais. CONCLUSIONS: The areas identified as suitable for Ae. albopictus establishment are consistent with the actual current findings of tiger mosquito. Our approach provides a useful tool to prompt authorities' intervention in the areas where there is higher risk of introduction and establishment of Ae. albopictus.

Surveillance of invasive Aedes mosquitoes along Swiss traffic axes reveals different dispersal modes for Aedes albopictus and Ae. japonicus.

Over the past three decades, Europe has witnessed an increased spread of invasive aedine mosquito species, most notably Aedes albopictus, a key vector of chikungunya, dengue and Zika virus. While its distribution in southern Europe is well documented, its dispersal modes across the Alps remain poorly investigated, preventing a projection of future scenarios beyond its current range in order to target mosquito control. To monitor the presence and frequency of invasive Aedes mosquitoes across and beyond the Alps we set oviposition and BG-Sentinel traps at potential points of entry with a focus on motorway service areas across Switzerland. We placed the traps from June to September and controlled them for the presence of mosquitoes every other week between 2013 and 2018. Over the six years of surveillance we identified three invasive Aedes species, including Ae. albopictus, Ae. japonicus and Ae. koreicus. Based on the frequency and distribution patterns we conclude that Ae. albopictus and Ae. koreicus are being passively spread primarily along the European route E35 from Italy to Germany, crossing the Alps, while Ae. japonicus has been expanding its range from northern Switzerland across the country most likely through active dispersal.

Viral Metagenomic Analysis of Aedes albopictus Mosquitos from Southern Switzerland.

A metagenomic study was performed on 498 female and 40 male Aedes albopictus mosquitos collected in August and September 2019 in Ticino, a region in southern Switzerland, to address the question regarding the risk of the local transmission of zoonotic viruses. A total of 13 viruses from seven different virus families and several unclassified viral taxa were identified. Reads of insect-specific flaviviruses were present in all pools, and a complete genome of aedes flavivirus was assembled and phylogenetically analysed. The most abundant virus was Wenzhou sobemo-like virus, assembled from 1.3 × 105 to 3.6 × 106 reads in each pool. In a pool of male mosquitos, a complete genome of aedes Iflavi-like virus was detected and phylogenetically analysed. Most importantly, genomes of human pathogenic viruses were not found. This is the first study to determine the virome of Ae. albopictus from Switzerland and forms a baseline for future longitudinal investigations concerning the potential role of Ae. albopictus as a vector of clinically relevant viruses.

Emerging Aedes-borne infections in southern Switzerland: Preparedness planning for surveillance and intervention.

BACKGROUND: The Preparedness Plan for Surveillance and Interventions on Emerging Vector-Borne Diseases (VBDs) in Southern Switzerland outlines the strategy for preventing and managing potential outbreaks, as well as the surveillance and control activities with a specific focus on Aedes-borne diseases transmitted by Aedes albopictus mosquitoes. The objective of the plan is to provide Public Health Authorities with a framework of preventive and control measures according to the situation and level of epidemic risks. MATERIAL AND METHODS: The plan is divided into various phases representing the different steps for all potential situations, ranging from no vectors and no transmission risk to epidemic levels with multiple autochthonous/local cases of hospitalization (and deaths) until the end of the epidemic. An algorithm presents how decisions are taken to move from one phase of the plan to another, with detailed activities for different partners and strategies for each specific phase. RESULTS: The different phases of the plan include activities on disease surveillance and clinical case management, on vector surveillance and control, communication and coordination of activities. The plan is divided into five phases of activities and decision levels. From phase 0 (no cases) to phase 1 (low number of local cases, less than 5), phase 2 (small outbreak with more than 5 local cases), phase 3 (epidemic) and phase 4 (return to no more cases). CONCLUSION: The plan has been approved by the cantonal authorities and will be submitted to federal authorities. The required implementation tests will begin shortly.

Evaluation of honey-baited FTA cards in combination with different mosquito traps in an area of low arbovirus prevalence.

BACKGROUND: The threat of mosquito-borne diseases is increasing in continental Europe as demonstrated by several autochthonous chikungunya, dengue and West Nile virus outbreaks. In Switzerland, despite the presence of competent vectors, routine surveillance of arboviruses in mosquitoes is not being carried out, mainly due to the high costs associated with the need of a constant cold chain and laborious processing of thousands of mosquitoes. An alternative approach is using honey-baited nucleic acid preserving cards (FTA cards) to collect mosquito saliva that may be analysed for arboviruses. Here, we evaluate whether FTA cards could be used to detect potentially emerging viruses in an area of low virus prevalence in combination with an effective mosquito trap. METHODS: In a field trial in southern Switzerland we measured side-by-side the efficacy of the BG-Sentinel 2, the BG-GAT and the Box gravid trap to catch Aedes and Culex mosquitoes in combination with honey-baited FTA cards during 80 trapping sessions of 48 hours. We then screened both the mosquitoes and the FTA cards for the presence of arboviruses using reverse-transcription PCR. The efficacy of the compared trap types was evaluated using generalized linear mixed models. RESULTS: The Box gravid trap collected over 11 times more mosquitoes than the BG-GAT and BG-Sentinel 2 trap. On average 75.9% of the specimens fed on the honey-bait with no significant difference in feeding rates between the three trap types. From the total of 1401 collected mosquitoes, we screened 507 Aedes and 500 Culex females for the presence of arboviruses. A pool of six Cx. pipiens/Cx. torrentium mosquitoes and also the FTA card from the same Box gravid trap were positive for Usutu virus. Remarkably, only two of the six Culex mosquitoes fed on the honey-bait, emphasising the high sensitivity of the method. In addition, two Ae. albopictus collections but no FTA cards were positive for mosquito-only flaviviruses. CONCLUSIONS: Based on our results we conclude that honey-baited FTA cards, in combination with the Box gravid trap, are an effective method for arbovirus surveillance in areas of low prevalence, particularly where resources are limited for preservation and screening of individual mosquitoes.

Investigation of temperature conditions in Swiss urban and suburban microhabitats for the overwintering suitability of diapausing Aedes albopictus eggs.

BACKGROUND: In Switzerland, the invasive Asian tiger mosquito, Aedes albopictus, is firmly established in the Canton of Ticino, south of the Alps. According to a large-scale distribution model developed in 2013, suitable climatic conditions for the establishment of Ae. albopictus north of the Alps are found in Basel and Geneva while Zurich appears to be characterized by winters currently being too cold for survival of diapausing eggs. However, the spatial resolution of large-scale distribution models might not be sufficient to detect particular climatic conditions existing in urban settings, such as the presence of microclimatic temperatures, which may positively influence the probability of diapausing eggs to overwinter. In order to investigate this, microclimatic monitoring of potential diapausing sites (i.e. catch basins) and external controls was performed in January 2017 in Ticino and within the cities of Basel, Geneva and Zurich. RESULTS: Mean January temperatures in catch basins of Basel, Geneva and Zurich were always higher than the -1 °C temperature threshold previously set for survival probability of diapausing eggs, while mean January temperatures were below -1 °C in several catch basins south of the Alps, where Ae. albopictus eggs currently overwinter. The catch basin absolute January daily minimum temperatures both south and north of the Alps were in general higher than the external control temperatures. Absolute January daily minimum temperatures in catch basins in Basel, Geneva and Zurich were always above -10 °C, indicating that diapausing Ae. albopictus eggs could potentially survive winter nights in urban areas north of the Alps. CONCLUSIONS: The findings confirmed previous conclusions that urban catch basins can provide favourable conditions for overwintering of diapausing eggs compared to more cold-exposed sites. The results confirmed the presence of suitable winter conditions for the establishment of Ae. albopictus in the cities of Basel and Geneva. In addition, the microclimate-scale analysis added new information compared to the previous large-scale prevision model by showing that also the city of Zurich could provide winter conditions suitable for the establishment of Ae. albopictus. This illustrates the importance of the resolution of climate data in using models to predict Ae. albopictus distribution.

Is Switzerland suitable for the invasion of Aedes albopictus [corrected]?

BACKGROUND: Over the last 30 years, the Asian tiger mosquito, Aedes albopictus, has rapidly spread around the world. The European distribution comprises the Mediterranean basin with a first appearance in Switzerland in 2003. Early identification of the most suitable areas in Switzerland allowing progressive invasion by this species is considered crucial to suggest adequate surveillance and control plans. METHODOLOGY/PRINCIPAL FINDINGS: We identified the most suitable areas for invasion and establishment of Ae. albopictus in Switzerland. The potential distribution areas linked to the current climatic suitability were assessed using remotely sensed land surface temperature data recorded by the MODIS satellite sensors. Suitable areas for adult survival and overwintering of diapausing eggs were also identified for future climatic conditions, considering two different climate change scenarios (A1B, A2) for the periods 2020-2049 and 2045-2074. At present, the areas around Lake Geneva in western Switzerland provide suitable climatic conditions for Ae. albopictus. In northern Switzerland, parts of the Rhine valley, around Lake Constance, as well as the surroundings of Lake Neuchâtel, appear to be suitable for the survival at least of adult Ae. albopictus. However, these areas are characterized by winters currently being too cold for survival and development of diapausing eggs. In southern Switzerland, Ae. albopictus is already well-established, especially in the Canton of Ticino. For the years 2020-2049, the predicted possible spread of the tiger mosquito does not differ significantly from its potential current distribution. However, important expansions are obtained if the period is extended to the years 2045-2074, when Ae. albopictus may invade large new areas. CONCLUSIONS/SIGNIFICANCE: Several parts of Switzerland provide suitable climatic conditions for invasion and establishment of Ae. albopictus. The current distribution and rapid spread in other European countries suggest that the tiger mosquito will colonize new areas in Switzerland in the near future.

Comparison between diflubenzuron and a Bacillus thuringiensis israelensis- and Lysinibacillus sphaericus-based formulation for the control of mosquito larvae in urban catch basins in Switzerland.

A field test was conducted to evaluate a commercial biolarvicide based on Bacillus thuringiensis var. israelensis and Lysinibacillus sphaericus to control mosquitoes breeding in catch basins in southern Switzerland. The efficacy and residual activity of the microbial mosquito larvicide applied at the recommended rate of 10 g per catch basin was compared to the currently used larvicide diflubenzuron. Both products provided a very good control activity (> 97% of reduction) of late instars (3rd and 4th instars) and pupae for 4 wk. However, only the microbial formulation controlled immature stages during the whole period of the trial, with > 98% of larval reduction. A single application of the microbial larvicide applied at 10 g per catch basin significantly reduced the number of immature mosquitoes for at least 70 days. The quantity of rainfall in the 48-h period before each sampling and the water temperature did not influence the efficacy of the treatments. Under the environmental conditions encountered in southern Switzerland, the larvicide tested may be a valid alternative to diflubenzuron to control mosquitoes in urban catch basins. The long-lasting control by the microbial larvicide further reduces the number of treatments required to keep the population of mosquitoes at low levels.

Dynamics of Bacillus thuringiensis var. israelensis and Lysinibacillus sphaericus spores in urban catch basins after simultaneous application against mosquito larvae.

Bacillus thuringiensis var. israelensis (Bti) and Lysinibacillus sphaericus (Lsph) are extensively used in mosquito control programs. These biocides are the active ingredients of a commercial larvicide. Quantitative data on the fate of both Bti and Lsph applied together for the control of mosquitoes in urban drainage structures such as catch basins are lacking. We evaluated the dynamics and persistence of Bti and Lsph spores released through their concomitant application in urban catch basins in southern Switzerland. Detection and quantification of spores over time in water and sludge samples from catch basins were carried out using quantitative real-time PCR targeting both cry4A and cry4B toxin genes for Bti and the binA gene for Lsph. After treatment, Bti and Lsph spores attained concentrations of 3.76 (± 0.08) and 4.13 (± 0.09) log ml(-1) in water, then decreased progressively over time, reaching baseline values. For both Bti and Lsph, spore levels in the order of 10(5) g(-1) were observed in the bottom sludge two days after the treatment and remained constant for the whole test period (275 days). Indigenous Lsph strains were isolated from previously untreated catch basins. A selection of those was genotyped using pulsed field gel electrophoresis of SmaI-digested chromosomal DNA, revealing that a subset of isolates were members of the clonal population of strain 2362. No safety issues related to the use of this biopesticide in the environment have been observed during this study, because no significant increase in the number of spores was seen during the long observation period. The isolation of native Lysinibacillus sphaericus strains belonging to the same clonal population as strain 2362 from catch basins never treated with Lsph-based products indicates that the use of a combination of Bti and Lsph for the control of mosquitoes does not introduce non-indigenous microorganisms in this area.

Distribution of Bacillus thuringiensis subsp. israelensis in Soil of a Swiss Wetland reserve after 22 years of mosquito control.

Recurrent treatments with Bacillus thuringiensis subsp. israelensis are required to control the floodwater mosquito Aedes vexans that breeds in large numbers in the wetlands of the Bolle di Magadino Reserve in Canton Ticino, Switzerland. Interventions have been carried out since 1988. In the present study, the spatial distribution of resting B. thuringiensis subsp. israelensis spores in the soil was measured. The B. thuringiensis subsp. israelensis concentration was determined in soil samples collected along six transects covering different elevations within the periodically flooded zones. A total of 258 samples were processed and analyzed by quantitative PCR that targeted an identical fragment of 159 bp for the B. thuringiensis subsp. israelensis cry4Aa and cry4Ba genes. B. thuringiensis subsp. israelensis spores were found to persist in soils of the wetland reserve at concentrations of up to 6.8 log per gram of soil. Continuous accumulation due to regular treatments could be excluded, as the decrease in spores amounted to 95.8% (95% confidence interval, 93.9 to 97.7%). The distribution of spores was correlated to the number of B. thuringiensis subsp. israelensis treatments, the elevation of the sampling point, and the duration of the flooding periods. The number of B. thuringiensis subsp. israelensis treatments was the major factor influencing the distribution of spores in the different topographic zones (P < 0.0001). These findings indicated that B. thuringiensis subsp. israelensis spores are rather immobile after their introduction into the environment.