Tackling mosquito-borne disease risk through storm drain modifications in Barcelona

Climate scenarios for Barcelona project a rise in temperatures and periods of excessive heat, with the average number of days above 30°C likely to increase from 22 to 50 hot days under a moderate warming scenario of RCP4.5, and to 80 hot days under RCP8.5 (Barcelona City Council, 2018). Warming temperatures are anticipated to lengthen the transmission season for vector-borne diseases by up to 2 months, as well as significantly shorten the time a mosquito needs to become infectious (European Environment Agency, 2024: p. 154; Semenza and Paz, 2021). The city is also projected to experience an increase in sea storms and flooding, placing wastewater infrastructure, including storm drains, at a higher risk of malfunctioning and creating areas of stagnant water – prime breeding ground for mosquitoes (Barcelona City Council, 2018; European Centre for Disease Prevention and Control, 2021). The strength of this rainfall–breeding link is visible even across single years. In the wetter 2018 season, sewers held standing water and larvae more often, and needed more frequent treatment, than in the drier 2020 season (Montalvo et al., 2022).

As Barcelona already hosts established populations of Aedes albopictus and Culex pipiens mosquitoes, both competent vectors of dengue, chikungunya, Zika and West Nile virus (among others), the anticipated local impacts of climate change are likely to increase the risk for vector-borne diseases in the city. This pattern is already evident in Spain, which in 2024 recorded around 138 locally acquired West Nile virus cases, alongside a small autochthonous dengue outbreak of eight cases in Vila-seca, Tarragona (around 100 km from Barcelona) (European Centre for Disease Prevention and Control, 2024; Nadal-Barón et al, 2024). In Barcelona itself, imported cases of mosquito-borne disease are detected regularly; wherever viraemic travellers and competent vectors coincide, each such case is a potential catalyst for local transmission.

As a primary European touristic destination, local infectious disease risk in Barcelona is affected not only by rising temperatures, but equally by the high numbers of visitors received yearly: the city has 1.7 million inhabitants but receives an estimated 9 million tourists each year (Barcelona City Council, 2018). Frequent movements of people in and out of the city, together with high rates of human mobility more broadly, can lead to the dispersal of vector-borne pathogens capable of fuelling local outbreaks (Monge et al., 2020; Montalvo et al, 2022). Indeed, Barcelona routinely records a high number of imported arboviral infections – during the 2024 mosquito season alone, the Barcelona Public Health Agency was notified of 137 imported arbovirosis cases, demanding entomological surveillance and vector control interventions. The public health risk presented by an increase in vector-borne diseases therefore renders the local healthcare infrastructure vulnerable to stresses, an issue that increased visitor numbers could be expected to compound (European Environment Agency, 2024).

As any large city, Barcelona’s urban infrastructure hosts typical landscape and structural elements that can allow stagnant water build-up, thus facilitating mosquito breeding (ibid; Treskova et al., 2025). Sand sewers, in particular, are abundant throughout Barcelona: each comprises of a small basin and an overflow pipe for excess water, and helps to capture sand washed away by rain, in so reducing sedimentation in the sewerage system. However, along with this sand, a small volume of water is also retained, in which mosquitoes can breed. These spaces are dark and offer suitable temperatures and sufficient organic matter, making them a prime breeding ground for mosquitoes. Additionally, sand sewers are typically installed near parks, placing these potential breeding sites in close proximity to the public.

To address local mosquito-borne diseases, a control programme was established in 2004, consisting of population surveillance and control in the city's streets and sewage system and application of biocides at potential breeding sites (over 12,000 sewers and 300 fountains). Given the number of sewers, the regular application of biocide – a biological larvicide (Bacillus thuringiensis var. israelensis (Bti.)) without significant impacts on human health, pets or urban biodiversity – comes at significant economic cost to the municipality (Michaelakis et al., 2021; Montalvo et al., 2022).

Historically, Barcelona’s public health authorities primarily managed mosquito populations in this manner. However, local outbreaks of dengue and West Nile virus have increasingly occurred in other parts of Spain, and imported cases of mosquito-borne disease in Barcelona are regularly detected (Treskova et al., 2025). Given the heightened risk that accompanies the impacts of climate change on the city, without enhanced prevention and management of mosquito breeding beyond biocide application, public health risks and the associated economic burden are expected to increase.

The intervention was developed in alignment with Barcelona City Council’s “Pla Clima”, its Climate Action Plan for 2030, which promotes the adoption of climate adaptation measures. The Action Plan describes how climate change is expected to cause an alteration of the geographic and seasonal distributions of vectors and the diseases they transmit. In response to these threats, the Plan lists specific Actions that will be undertaken to respond to increasing risks, namely:

- “to consolidate the control programmes for arboviruses and other diseases transmitted by vectors and the mosquito control protocols;” and

- “to step up comprehensive pest control (cockroaches, tiger mosquitoes, etc.,) with minimum use of insecticides and biocides” (Barcelona City Council, 2021).

The Plan also describes a Line of Action focusing on resident’s health and wellbeing in the context of climate change, that recognises the need to adapt public spaces and municipal facilities to the climate realities of each neighbourhood and aims to prevent climate change from rendering the city’s natural areas a threat to health.

The main objectives of the intervention implemented by the Barcelona Public Health Agency (ASPB) were to:

(i) reduce mosquito reproduction in Barcelona by modifying the structures of sewers, (ii) cut the costs of mosquito control by reducing the need for biocide applications and agency resources, (iii) explore a lower-cost, long-term solution to mosquito breeding that could be expanded to more storm drains around the city, and (iv) reduce the nuisance and the risks of vector-borne disease transmission caused by mosquitoes for local populations.

In 2005, the Barcelona Public Health Administration (ASPB) began implementing the Mosquito Surveillance and Control Programme, a prevention strategy aimed at reducing mosquito populations, eliminating possible breeding sites and preventing their proliferation in urban areas. As part of the mosquito surveillance and control programme, sewers are visited regularly, (every four weeks), inspected for signs of risk and treated with larvicide where larval activity is detected. Understandably, the demand this places on city resources, both for inspections and larvicide applications, is significant, particularly during the months of most intense vector activity, between April and November.

In 2019, in view of the continued presence of mosquitoes and the increasing risk of vector-borne diseases, an experimental modification of sand sewers was carried out as part of this programme, to determine whether structural changes could reduce the accumulation of stagnant water and thus prevent mosquitoes from breeding there.

With the aim of reducing both mosquito breeding and this financial burden, 39 sand sewers with a history of Aedes albopictus and Culex pipiens larval activity were selected and monitored through 2018. During 2019, 19 of these were modified by adding a sloping layer of concrete rising to the level of the discharge pipe, directing water towards the pipe to prevent accumulation; the remaining 20 were retained as unmodified controls. In 2020, following the modifications, the sewers were visited at roughly two-week intervals between late April and November, with stagnant water and mosquito activity recorded.

The results of the trial were positive. Before the modifications, no meaningful difference in mosquito breeding was observed between the sewers later selected for modification and the unmodified control sewers. Once the structural changes had been made, however, larval activity in the modified sewers fell considerably compared to the unmodified sewers, with the former recording no mosquito larvae at all over the following breeding season, offering observational evidence that these drain modifications effectively reduced water accumulation, and, in turn, mosquito larval activity.

It was noted, however, that heavier rainfall in the year before the modifications meant that water, and consequently mosquito larvae, had been more frequently present in the sewers than in the year following the changes (and required more frequent larvicide treatment). To strengthen confidence in these findings, the trial was repeated in 2023, with the additional support of Barcelona Cicle de l'Aigua (BCASA), the city's water infrastructure authority.

This time, 22 sand sewers were fitted with honeycomb bricks at the base to provide structural support, with concrete then added to create a gradient. This directed water down towards the discharge pipe, preventing it from accumulating and thereby rendering it unsuitable for mosquito reproduction. A further 22 sewers were left unmodified as a comparison group, with no larvicide applied to either group during the trial.

Since this second trial, the city has continued to upgrade the drains identified as most problematic each year, helping to systematically reduce breeding sites, control mosquito presence and decrease the risk of vector-borne disease. Further evaluation is planned to better establish the long-term impact of these modifications on local mosquito populations and on the associated economic burden.

With concern for securing and improving the health and well-being of all the city's residents, specifically those of low-income individuals, the ASPB chooses to prioritise storm drain modifications in lower-income areas with fewer resources. This approach also extends to the agency's ongoing vector surveillance efforts, as lower-income groups are more likely to have limited resources to mitigate the disease risk posed by mosquitoes, for example, through purchasing repellents or installing mosquito screens in homes and businesses.

Effort is equally taken to avoid unintended trade-offs of the interventions: ongoing mosquito activity monitoring generates a list of storm drains with elevated activity, which are then prioritised for intervention each year. However, the list includes not only elements with detected activity, but also neighbouring elements that could potentially harbour breeding grounds, since a modification in one location could push mosquitoes to relocate to nearby spaces. This broader targeting allows the ASPB to pursue a more integrated intervention creating a consistent impact across the local area.

Furthermore, thanks to modern low-friction materials and high-water flow velocities facilitating the continuous transport of sediments, no operational trade-offs related to sediment accumulation have been detected in the sections of the sewer network where the sand box structures have been modified.

As BCASA modifies not only the storm drains identified as most problematic by SVIPLA but also adjacent structures in the surrounding area, the true number of drains modified thus far is higher than that recorded in public health databases – however, an estimated 100 sand box drains are modified each year.

The intervention was developed and carried out by SVIPLA, the Urban Epidemiological Surveillance and Control Service, (Servei de Vigilància i Control de Plagues Urbanes), part of Barcelona's Public Health Agency (Agència de Salut Pública de Barcelona), in coordination with Barcelona Cicle de l'Aigua (BCASA), the city's water infrastructure authority. The intervention was implemented with the support of the Barcelona City Council, which also contributed funding. Once the Horizon Europe project ‘IDAlert’ incorporated the intervention into its work as a case study, numerous scientific institutions contributed to the continued development and evaluation of the measure, including the CIBER Epidemiología y Salud Pública (CIBERESP), the Estación Biológica de Doñana, the Universitat Pompeu Fabra (UPF), the Agencia Estatal Consejo Superior de Investigaciones Cientificas (CSIC), and Heidelberg University (UKHD). 

Although an evaluation of the long-term effects of the storm drain modifications will take time, principal successes to date have been the physical elimination of several breeding sites in public areas. This has reduced both the nuisance caused by mosquitoes and the associated risk of vector-borne disease transmission for residents.

Much of the project's success can be attributed to the momentum generated by Pla Clima. As a cross-cutting framework, the City Council's Action Plan enabled the project to engage and align with a range of Barcelona agencies and services. The coordination between the different stakeholders involved is described by the Epidemiological Surveillance Service (SVIPLA) as one of the primary reasons for its concrete impact on the city. The fact that the intervention met the plan's cross-cutting criteria was also instrumental in securing Council support and justifying funding. The project equally benefited from not being a wholly new initiative, but rather an extension of a surveillance programme running since 2004, (and aimed specifically at reducing that programme's costs). This provided it with both continuity and demonstrable experience, and allowed the ASPB to draw on existing relationships with the Council and other agencies. Furthermore, as a grey infrastructure measure, its costs are relatively straightforward to calculate on a per-unit basis. Finally, joint oversight of the project's technical, design and monitoring aspects by the ASPB and BCASA ensured that the resulting product met high technical and scientific performance standards.

One limitation is that the storm drain modifications are not distributed evenly across the city. This reflects the fact that only "sand box" drains, typically installed near parks to prevent sand carried by rainwater from entering the sewer system, can be modified using this method, and such drains are thus more concentrated in park areas. However, modifications have been distributed as evenly as possible across the city, with priority given to lower-income areas to offset a potentially more limited access to relevant resources, such as repellent and protective equipment.

A final limitation is that the drains require ongoing monitoring and evaluation rather than functioning as a ‘hands-off’ solution, thereby placing a continuous demand on agency manpower and resources. However, this monitoring is itself integral to the surveillance programme and is conducted annually within existing budgets. The initial storm drain modifications established the scientific basis for the intervention, but continued monitoring and evaluation were always part of the programme's design.

Quantifying the cost savings of the storm drain modifications remains difficult, beyond a calculation of the direct savings on alternative control methods, i.e., biocides. Specifically, estimating the long-term savings in individual and public healthcare costs resulting from reduced mosquito burden will be a complex undertaking. Nevertheless, a comprehensive scientific evaluation and cost-benefit analysis of the storm drain modifications is currently underway, with the support of partners from the Horizon Europe project IDAlert and the Centre for Advanced Studies of Blanes (CEAB-CSIC).

As a cross-cutting measure that engages multiple actors and addresses both climate and health concerns, the intervention provides a breadth of potential benefits and trade-offs.

The intended benefits of the modifications are a reduction in mosquito breeding and a decrease in the operational costs of vector control, through a reduced need for biocide applications and agency resources. The scale of these savings depends on how widely the modification is rolled out. Montalvo et al. (2022) estimate that if all of Barcelona's roughly 8,000 sand sewers were modified, the city could lower the annual costs of its surveillance and control programme and of structural maintenance, and reduce the amount of biocide used across the sand-sewer network by around 50 kg per year. The same study costs each modification at around EUR 350 per sewer. The per-unit cost is modest, but the large number of eligible structures means the total budget, and hence the pace of rollout, is the binding constraint. These are modelled, whole-network potentials rather than savings realised to date. In practice, only a small proportion of sand sewers require treatment in any given year, and the modification is applied progressively to the drains identified as most problematic. A full cost-benefit analysis is needed to establish the actual savings (ibid). However, by lowering overall operational costs and freeing up manpower for other vector surveillance and control tasks, this has already enabled the Urban Epidemiological Surveillance and Control Service to prioritise surveillance in lower-income areas, in which residents are more likely to have limited resources to mitigate the health risk posed by mosquitoes, for example, through purchasing repellents or installing mosquito screens in homes and businesses.

The health benefits of the measure, while harder to quantify, are clear in principle. These include a reduced vulnerability to the growing threat of climate-sensitive mosquito-borne diseases across the city, with lower-income areas prioritised to ensure that these benefits are distributed as comprehensively as possible. This is expected to translate into avoided costs, both for individuals and for the public healthcare system, in terms of mosquito protection and the treatment of vector-borne diseases, at both the individual and epidemiological level.

The reduction in biocide use also carries potential environmental benefits, including increased ecological safety and support for urban biodiversity (Brühl et al., 2020), and a reduction in the amount of toxic chemicals entering the water system (Hernández-Moreno et al., 2019).

From an economic standpoint, reduced mosquito nuisance, particularly near public parks, may help protect the city's investment in green spaces, preserving their value as places of wellbeing rather than sources of annoyance or disease risk. This, in turn, helps maintain the broader attractiveness of the city for both residents and visitors, along with the economic activity this generates.

Finally, it remains to be seen whether the reduction in stagnant water achieved by the sand box modifications could also contribute to reducing flood risk across the wider Barcelona sewer system; while not the intended goal of these modifications, installing a gradient in these sewers could support their functionality during periods of sea storms and intense precipitation.

Storm drain interventions are carried out yearly, with screening, infrastructure modification, data collection and evaluation taking place at different times of the year, both preceding and following the breeding season. Each year, between November and January, the ASPB carries out an evaluation of the city’s drains, before modifications begin on selected drains, starting in February and continuing until April or May (Specifically, BCASA, the city’s water infrastructure authority, maintains the sewer network and implements the storm-drain modifications, while SVIPLA, the municipality’s epidemiological service carries out monitoring and evaluation of the drains). An estimated 100 sand box drains are modified each year. (In 2027, a citywide assessment of all storm drains will be carried out to determine which units have already been modified, to provide a more accurate figure).

While the initial experiment, taking place during the 2020 and 2023 mosquito seasons provided a scientific basis for the measure, it was adopted as a permanent part of the city’s vector surveillance and control programme, and thus has no envisioned end in sight. The modifications are created with concrete and each is intended as a permanent addition to the city’s sewer infrastructure. Continued support from the city council, combined with the ongoing scientific evaluation of the project, contribute to its long-term sustainability.

Tomás Montalvo, Head of the Urban Epidemiological Surveillance and Control Service (SVIPLA)

Agència de Salut Pública de Barcelona (ASPB)

Email: tmontal@aspb.cat

Barcelona City Council (Ajuntament de Barcelona). 2018. Barcelona climate plan 2018–2030. Barcelona City Council. Retrieved 2026, from https://www.barcelona.cat/barcelona-pel-clima/sites/default/files/climate_plan_maig.pdf

Barcelona City Council (Ajuntament de Barcelona). 2021. Climate Emergency Action Plan for 2030. Barcelona City Council (Pla Clima). Retrieved 2026, from https://www.barcelona.cat/barcelona-pel-clima/sites/default/files/plaaccioemergenciaclimatica_2021nov_en.pdf  

Brühl, C. A., Després, L., Frör, O., Patil, C. D., Poulin, B., Tetreau, G., & Allgeier, S. 2020. Environmental and socioeconomic effects of mosquito control in Europe using the biocide Bacillus thuringiensis subsp. israelensis (Bti). Science of The Total Environment, 724, 137800. https://doi.org/10.1016/j.scitotenv.2020.137800

European Centre for Disease Prevention and Control. 2021. Organisation of vector surveillance and control in Europe. Publications Office of the European Union. https://data.europa.eu/doi/10.2900/853486 

European Centre for Disease Prevention and Control. 2024. Historical data on local transmission in Europe for West Nile virus – Surveillance and Updates. European Centre for Disease Prevention and Control. Retrieved 2026, from https://www.ecdc.europa.eu/en/west-nile-fever/surveillance-and-disease-data/historical.

European Environment Agency, 2024. European climate risk assessment. Publications Office of the European Union. https://data.europa.eu/doi/10.2800/8671471

Hernández-Moreno, D., Blázquez, M., Andreu-Sánchez, O., Bermejo-Nogales, A., & Fernández-Cruz, M. L. 2019. Acute hazard of biocides for the aquatic environmental compartment from a life-cycle perspective. Science of The Total Environment, 658, 416–423. https://doi.org/10.1016/j.scitotenv.2018.12.186

Michaelakis, A., Balestrino, F., Becker, N., Bellini, R., Caputo, B., della Torre, A., Figuerola, J., L’Ambert, G., Petric, D., Robert, V., Roiz, D., Saratsis, A., Sousa, C. A., Wint, W. G. R., & Papadopoulos, N. T. 2021. A case for systematic quality management in mosquito control programmes in Europe. International Journal of Environmental Research and Public Health, 18(7), 3478. https://doi.org/10.3390/ijerph18073478

Monge, S., García-Ortúzar, V., López Hernández, B., Lopaz Pérez, M. A., Delacour-Estrella, S., Sánchez-Seco, M. P., Fernández Martinez, B., García San Miguel, L. García-Fulgueiras, A., Sierra Morros, M. J. 2020. Characterization of the first autochthonous dengue outbreak in Spain (August–September 2018). Acta Tropica, Volume 205. https://doi.org/10.1016/j.actatropica.2020.105402

Montalvo, T., Higueros, A., Valsecchi, A., Realp, E., Vila, C., Ortiz, A., Peracho, V., & Figuerola, J. 2022. Effectiveness of the Modification of Sewers to Reduce the Reproduction of Culex pipiens and Aedes albopictus in Barcelona, Spain. Pathogens, 11(4), 423. https://doi.org/10.3390/pathogens11040423

Nadal-Barón, P., Bou-Monclus, M. A., Serra, G. F., Granell, S. P., Vergés, S. M., López, S. C., Sanz, A. B., Mateo, A. I. M., Calleja, I. M., Vallverdú, R. M. V., Navas, B. V., Bueno, E. G., Corbella, I., Navero-Castillejos, J., Andrés, C., Antón, A., Chacón, C., Mendioroz, J., Muñoz, E. F., Martínez, M. J., Sulleiro, E. 2026. An autochthonous outbreak of dengue in Spain in 2024. Journal of Travel Medicine, 33(4). https://doi.org/10.1093/jtm/ta

Semenza J.C., and Paz S. 2021. Climate change and infectious disease in Europe: Impact, projection and adaptation. The Lancet Regional Health. Europe, 9, 100230. https://doi.org/10.1016/j.lanepe.2021.100230

Treskova, M., Montalvo, T., Rocklöv, J., Hatfield, C., Bartumeus, F., Dasgupta, S., Encarnação, J., Lowe, R., Semenza, J. C., Stiles, P., Noya, J., Valsecchi, A., Bärnighausen, T., Palmer, J. R. B., Bunker, A. 2025. Effects of mosquito-proofing storm drains on adult and larval mosquito abundance: Protocol of the IDAlErt storm drAin randomiSed controlled trial (IDEAS). MethodsX, Volume 14. https://doi.org/10.1016/j.mex.2024.103102