Understanding dengue transmission in Europe

Dengue is caused by the dengue virus and transmitted by mosquito bites. Traditionally endemic in tropical regions such as Latin America and the Caribbean, as well as Southeast Asia, local transmission across several countries in mainland Europe has been documented since 2010. In particular, the mosquito species Aedes albopictus, which is now well established in southern Europe, can act as a dengue vector even though the main dengue vector is Aedes aegypti.

In their article published in Eurosurveillance, a team has analyzed data from a dengue outbreak across three municipalities in Italy between July and October 2024. The outbreak resulted in 296 probable and confirmed locally acquired infections with dengue virus serotype 2, of which 286 occurred among residents in the municipalities of Fano, Cavezzo and Ortona.

The authors aimed to characterize possible transmission patterns of this outbreak to understand how the virus can spread in specific settings and surroundings in Europe. Even though the extensive investigations could not identify the primary case that initiated local transmission, the study found that 270 of the reported cases were probably exposed near someone who was already infected. For the remaining 26 cases, no likely exposure site could be identified.

Transmission influenced by spatial proximity

Based on their data analysis, the researchers found that transmission during this dengue outbreak was largely localized: More than half of linked transmission events involved people whose probable exposure sites were located less than 100 meters (328 feet) apart. Among people for whom a likely source of infection could be identified, 18% were linked to transmission within the same household and 41% to transmission in the immediate neighborhood.

The data imply that the likelihood of transmission went down with increasing distance, as 27% of transmission events were associated with exposure sites located between 100 and 200 meters (328 and 656 feet) apart, while only 10% happened between 200 and 300 meters (656 and 984 feet) apart. Transmission beyond 400 meters (1,312 feet) was rare, accounting for less than 1% of identified links. According to the authors, these findings are consistent with the limited flight range of Aedes albopictus mosquitoes.

The study estimates that 15% of cases were acquired from an infectious household member, while more than two-thirds (68%) were linked to someone with dengue close to their likely exposure site. For the remaining 16%, the infection was likely caused by viral circulation within one of the main transmission clusters but could not be attributed to a specific case.

Impact of control measures and temperatures

The researchers describe that before the outbreak was detected, each person with dengue symptoms infected—on average—more than one other person. Following outbreak detection and the implementation of mosquito control measures within a 200-meter radius (656-foot radius) around identified exposure sites, this average fell to below one secondary infection per infected person, indicating a decline in transmission.

In addition, the data analysis showed that a drop in temperatures during the autumn of 2024 likely contributed to reducing transmission. In turn, the authors estimate that each 1° C increase in temperature was associated with an approximate 20% rise in transmission.

The authors conclude, “The high proportion of focal transmission suggests that when cases are diagnosed promptly, vector control and active case finding within a buffer of ca. 400 m around detected cases may substantially reduce onward transmission. However, when delays in case detection occur, extending mosquito control to nearby surrounding areas or at the municipality level might be advisable to cope with the undetected transmission spread.”

Provided by
European Centre for Disease Prevention and Control (ECDC)

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