Dengue is a mosquito-borne viral disease caused by infection with any of four antigenically distinct dengue virus serotypes (DENV-1 to DENV-4) (1). The growing global importance of dengue is reflected in its geographic distribution and dramatic increase in dengue incidence in recent decades. The disease is now endemic in over 100 countries, putting a significant portion of the world’s population at risk (2, 3). The global burden remains high, with hundreds of millions of estimated infections annually (1). Due to population migration and climate change, previously only sporadic, imported cases of dengue were recorded, and in recent years autochthonous occurrence of infection has been reported in Southern Europe. In Croatia, locally acquired dengue was recorded in 2010 on Pelješac, and during 2022 and 2023, several cases of dengue were recorded in France, Italy, and Spain (4, 5).

The female mosquito of the genus Aedes aegypti plays an important role in the transmission of the dengue virus, although other species such as Aedes albopictus can also act as vectors. The mosquito becomes infected by ingesting the blood of an infected person during the viremic phase, and after a replicative phase in the midgut and an incubation period, the virus spreads to the salivary glands. Then, they can infect new human hosts through subsequent bites (6, 7). The efficiency of this transmission cycle is influenced by multiple factors, including vector competence, mosquito biology and behaviour, ambient environmental conditions, and human-environment interactions such as urbanization (8).

Upon human infection, DENV causes a wide clinical spectrum. Many infections remain asymptomatic or manifest as a nonspecific febrile illness, frequently escaping recognition. Classic symptomatic presentation, referred to as dengue fever (DF), typically presents after an incubation period of several days, with high-grade fever, severe headache, myalgia, arthralgia, and sometimes rash, nausea, or vomiting (9). In a subset of patients, especially during secondary infections with a different DENV serotype, disease may progress to severe dengue, a term classified by the WHO, which includes clinical manifestations previously called dengue hemorrhagic fever (DHF) and dengue shock syndrome (DSS) (10). Antibodies from the first infection can bind but not neutralize the new serotype, facilitating viral entry into immune cells and causing higher viral loads and stronger inflammation (11). These severe manifestations typically involve increased vascular permeability, plasma leakage, hemorrhagic phenomena, thrombocytopenia, leukopenia, coagulopathy, and in severe cases, shock or multi-organ dysfunction (1).

The diagnostic approach during the first 1–7 days of infection, when viremia is present, is that DENV can be detected by RT-qPCR or conventional PCR, with qPCR also enabling viral-load quantification (12). Serological diagnostics are based on the detection of NS1 antigen or IgM antibodies using ELISA assays or rapid immunochromatographic tests. More precise serological methods include MAC-ELISA, indirect IgG ELISA, haemagglutination inhibition (HI), complement fixation (CF), and neutralization tests (NT). However, NS1 and IgM tests may show limitations due to cross-reactivity with other flaviviruses (13).

Considering the expanding distribution of competent vectors, changing climatic conditions, population growth, and increasing urbanization, all of which contribute to mosquito breeding and human–vector contact, the global burden of dengue continues to escalate. The presence of Aedes albopictus, a competent dengue vector, has been established in Serbia, raising concerns about the potential for local transmission if viremic imported cases coincide with high vector densities (14). In this context, detailed case reports remain crucial for better understanding the clinical variability, risk factors for disease severity, diagnostic challenges, and treatment needs of dengue virus infection in different epidemiological settings.

In this report, we present a series of cases of dengue virus infection encountered in Serbia, illustrating the clinical picture, diagnostic approach, and treatment, thus placing our findings in the context of current epidemiological and transmission trends. Written informed consent was obtained from the patients as well as their legal representatives, in the case of minors.

An 11-year-old boy presented to the pediatric clinic with a 3-day history of fever, myalgia, headache, and watery diarrhea.

The illness began with temperatures up to 39 °C, diffuse muscle aches, and headache. Epidemiological history revealed recent travel to the Maldives, where the family experienced multiple mosquito exposures. The patient’s mother had been diagnosed with dengue virus infection and was hospitalized at the Infectious Diseases Clinic at the time of the boy’s evaluation.

On examination, the child was conscious, oriented, afebrile, and eupneic with oxygen saturation of 98% on room air and in good overall condition. Numerous older insect-bite lesions were noted on the skin. The remainder of the physical examination across all systems was unremarkable.

Initial laboratory testing showed leukopenia, while biochemical analyses demonstrated elevated LDH with normal serum transaminases (Table 1). Abdominal ultrasound was within normal limits. Testing for dengue virus was performed, and the NS1 antigen returned positive. Symptomatic treatment and rest were advised.

At follow-up after three days, the child was afebrile and clinically well, with normalized leukocyte count. Serologic testing confirmed acute dengue virus infection (positive IgM antibodies).

A 29-year-old female patient presented to her primary care physician with a 4-day history of fever, watery stools, myalgia, and arthralgia. The illness began abruptly with a fever of up to 38.9 °C occurring daily, accompanied by muscle and joint pain. Due to persistent symptoms, she was referred to an infectious disease specialist after two days. Chest X-ray and abdominal ultrasound findings were unremarkable. Laboratory analyses revealed leukopenia and thrombocytopenia (Table 1).

She reported having recently returned from a two-week stay in Brazil, during which she sustained multiple mosquito bites. There was another traveler with her who developed a fever and sore throat.

On admission, the patient was conscious, oriented, afebrile, eupneic, normotensive, moderately dehydrated, and intoxicated, without signs of hemorrhagic syndrome. Physical examination across all organ systems was unremarkable.

Symptomatic treatment was initiated. Blood samples were obtained for serologic testing for dengue virus. On the second day of hospitalization, due to worsening neutropenia and thrombocytopenia, rising serum transaminase levels, and the appearance of a petechial rash, a hematology consultation was requested. Peripheral blood smear revealed no atypical cells. Serologic testing detected IgM antibodies to dengue virus, confirming an acute dengue virus infection.

During the remainder of hospitalization, clinical and laboratory improvement was achieved, after which the patient was discharged in good general condition.

A 27-year-old male patient presented to an infectious disease specialist with a 5-day history of fever and fatigue. The illness began abruptly with a temperature of 38.6 °C. During febrile episodes, he reported chills and malaise while denying symptoms related to other organ systems.

In the socio-epidemiologic interview, he stated that he had recently traveled to the Maldives, where he sustained multiple mosquito bites.

Initial laboratory analyses revealed leukopenia with neutropenia and thrombocytopenia. Abdominal ultrasound showed a mildly enlarged spleen, while findings on other parenchymal organs were unremarkable. Due to fever and neutropenia, the patient was admitted to the Infectious Diseases Clinic.

On admission, the sclerae and conjunctivae were mildly injected. The rest of the physical examination was normal. On the second day of hospitalization, purpuric macules appeared on the lower legs and forearms, accompanied by conjunctival suffusion. Laboratory tests showed a further decrease in platelet count, low fibrinogen, and rising serum transaminase and creatinine levels (Table 1). Clinically and laboratorially, the patient developed a hemorrhagic syndrome while maintaining normal arterial blood pressure (Figure 1).

A hematology consultation was obtained upon admission. Bone marrow aspiration and peripheral blood smear revealed hypocellularity, few mature granulocytes, and reduced platelet numbers, without atypical cells. Treatment was initiated with granulocyte colony-stimulating factor (G-CSF), along with other supportive therapy.

Given his recent travel to tropical regions, serum samples were sent to a reference laboratory. The dengue virus genome was detected by PCR, and serologic testing confirmed the presence of IgM and IgG antibodies, establishing the diagnosis of acute dengue virus infection.

Treatment was continued with symptomatic and substitution therapy (including cryoprecipitate), along with hepatoprotective agents. In the following days, both clinical and laboratory improvements were observed, after which the patient was discharged in good general condition.

The three confirmed cases of dengue infections reported in Serbia exemplify the increasing epidemiological importance of travel-associated dengue in non-endemic European countries. All patients had recently returned from highly endemic regions – the Maldives and Brazil – and became symptomatic shortly after arrival. This pattern is consistent with global surveillance data showing that international travel is a key driver of dengue importation into Europe (15).

Although autochthonous transmission has not been documented in Serbia yet, the widespread presence of Aedes albopictus represents a potential ecological bridge enabling virus establishment should viremic travelers be bitten during the mosquito-active season (14). Several European countries, including France, Italy, Spain, Portugal, and Croatia (4, 16), have already recorded locally acquired dengue cases in the last decade, indicating that climatic conditions and vector distribution increasingly favor sporadic outbreaks. These trends emphasize the need for strengthened surveillance and preparedness in Serbia (15).

The clinical manifestations in the above-mentioned patients illustrate the broad spectrum of dengue severity, from asymptomatic form, febrile illness, hemorrhagic fever to shock syndrome. The pediatric patient presented with a mild febrile illness accompanied by leukopenia and gastrointestinal symptoms, which was consistent with typical dengue in children. Conversely, the two adult patients demonstrated more pronounced hematological and biochemical disturbances, including marked leukopenia, thrombocytopenia, elevated transaminases, and – in one patient – development of hemorrhagic features (10).

The most severe case, involving a 27-year-old male, evolved toward dengue hemorrhagic manifestations, as evidenced by purpuric lesions, conjunctival suffusion, declining platelet count, reduced fibrinogen levels, elevated transaminases and creatinine. Bone marrow findings of hypocellularity and suppressed granulopoiesis further support the recognized myelosuppressive effects of acute dengue infection. Such hematological patterns underscore the importance of early monitoring of complete blood counts, liver enzymes, and coagulation parameters in suspected cases (17).

Diagnosis in all three patients was established using a combination of NS1 antigen testing, serology, and PCR – the latter serving as the most reliable method during the viremic phase. NS1 antigen positivity in the early stage, followed by IgM seroconversion, was clearly demonstrated in the pediatric case. In the adult patients, PCR and IgM/IgG positivity provided definitive evidence of acute infection, which was particularly important in the cases complicated by bicytopenia or symptoms mimicking other viral or hematologic conditions (11–13).

Characteristic laboratory abnormalities – leukopenia, neutropenia, thrombocytopenia, elevated LDH, and significant transaminase elevations – were consistently present and aligned with reported hematological profiles in dengue patients globally. These findings highlight the importance of clinicians maintaining a high index of suspicion for dengue in travelers presenting with fever and cytopenias (1).

The detection of these imported infections holds important public health implications. The combination of increasing international travel to endemic regions and established populations of Aedes albopictus creates a realistic risk environment for sporadic local transmission. Seasonal increases in mosquito density, particularly during summer and early autumn, may coincide with travel peaks, creating the windows of vulnerability for virus introduction into local vector populations. Strengthening entomological surveillance, timely reporting systems, and clinician awareness is, therefore, essential to prevent its introduction and spread (14).

Management strategies in these cases were consistent with international guidelines emphasizing supportive care, hydration, monitoring of hematological parameters, and escalation of treatment when necessary. Although there is no specific antiviral drug for dengue, various natural and synthetic compounds have been studied for their effects on the course of the disease, including potential antiviral activity (18–22). Currently, the most severe cases required substitution therapy, G-CSF, and hepatoprotective measures, emphasizing the potential for clinical deterioration even in otherwise healthy individuals (1).

Despite the severity observed in the adult cases, all patients ultimately achieved full clinical recovery, illustrating that favorable outcomes are achievable with timely recognition, appropriate diagnostics, and a structured follow-up.

Unlike previously confirmed dengue cases in Serbia approximately 10 years ago (23, 24), which did not raise concern due to the absence of competent vectors at the time, the occurrence of dengue in recent years necessitates appropriate public health responses and surveillance.

The three presented cases demonstrate the increasing importance of imported dengue infections in Serbia, simultaneously highlighting the necessity of integrating clinical vigilance with robust diagnostic capabilities (15). Although all these cases originated abroad, the combination of expanding international travel and the established presence of Aedes albopictus creates ecological conditions conducive to potential local transmission (14).

This case series underscores several key points:

In summary, these cases reinforce the need for ongoing awareness, timely diagnostic evaluation, and coordinated public health measures for the purpose of mitigating the risk of dengue introduction and its potential autochthonous transmission in Serbia. The findings contribute meaningful clinical and epidemiological insight to the limited but growing body of regional literature on dengue in Southeastern Europe.