Epidemiological and Entomological Study After the Possible Re-Emergence of Dengue Fever in Croatia, 2024

Autochthonous dengue cases have been continuously recorded in Europe in the past two decades. The first autochthonous dengue case in Croatia was reported in 2010 on the Pelješac Peninsula, while imported cases were recorded continuously thereafter. In 2024, dengue re-emerged in Croatia. An epidemiological and entomological study was conducted after receiving information on dengue virus (DENV) infection in a German tourist probably acquired on Dugi Otok Island in Croatia in May 2024. Serum samples were collected from 30 residents of the Veli Rat region where the patient had stayed. In addition, mosquitoes were collected in the same area. Human samples were tested for the presence of DENV antibodies (ELISA and IFA) and DENV RNA (RT-qPCR), while mosquito samples were tested for DENV RNA (RT-qPCR). DENV IgM or IgG antibodies were found in 8 serum samples, while no one sample was RT-qPCR positive. No cross-reactivity with flaviviruses was detected in seropositive samples, supporting DENV infection. One patient was classified as a confirmed dengue case (IgG seroconversion in paired serum samples) and five as probable cases (IgM detection in a single serum sample). One additional patient, sampled only once, was IgG seropositive. Two of the seropositive individuals reported fever and rash three weeks before testing. The re-emergence of dengue in Croatia highlights the need for continuous monitoring of DENV circulation in both humans and vectors.