Seroepidemiology of Herpes Simplex Viruses Type 1 and 2 in Pregnant Women in Croatia.

Background and Objectives: Herpes simplex viruses (HSV-1 and HSV-2) are one of the most widespread causes of human viral infections. In Croatia, only two published studies have analyzed the seroprevalence of HSV infections in childbearing-aged and pregnant women (2005-2010), while more recent data are lacking. This study aimed to analyze the prevalence and risk factors for HSV-1 and HSV-2 infections among pregnant women in Croatia in the period from 2011 to 2021. Materials and Methods: This study included 667 pregnant women aged 16-45 years submitted for HSV-1 and HSV-2 serology testing. Serum samples were initially screened for HSV-1 and HSV-2 IgM and IgG antibodies using a commercial ELISA test with a confirmation of HSV-2-positive samples using an immunoblot assay. Results: The overall IgG seroprevalence rates were 69.9% for HSV-1 and 3.8% for HSV-2. A significant gradual increase in the HSV-2 seroprevalence with age was observed from 0.5% in participants under 30 years to 8.3% in participants above 40 years. The HSV-1 seroprevalence was stable up to 40 years (70.0 and 68.3%, respectively), with an increase to 86.1%, but this difference did not reach statistical significance. Area of residence (urban or suburban/rural), geographic region (continental or coastal), and obstetric history (normal pregnancy or unfavorable obstetric history) were not associated with HSV-1 and HSV-2 seroprevalence. Older age was found to be a significant risk factor for HSV-2 seropositivity in both univariate and multivariate risk analysis. Conclusions: HSV-1 infection is widely prevalent among pregnant women with a stable trend over time. However, a declining trend in the HSV-2 seroprevalence was observed compared to 2005-2010. Serological screening in pregnant women is important in identifying seronegative women who are susceptible to HSV infection as well as seropositive women who are at risk for genital herpes recurrence during delivery.

Prevalence of JC Polyomavirus in Patients with Neuroinvasive Disease of Unknown Etiology in Croatia.

Background and Objectives: John Cunningham polyomavirus (JCPyV) is a highly prevalent virus in the human population. The prevalence of JCPyV in patients with central nervous system disorders has not been examined extensively. The aim of this study was to analyze the prevalence of JCPyV DNA/antibodies in patients with neuroinvasive diseases (NID) of unknown etiology. Materials and Methods: The study included 132 patients with NID (febrile headache, meningitis, encephalitis) tested from January 2021 to December 2022. The control group consisted of 47 asymptomatic individuals. In patients with NID, serum and cerebrospinal fluid (CSF) samples were collected in the acute phase of the disease. CSF samples were tested for JCPyV DNA (PCR), while serum samples were tested for JCPyV IgG antibodies (ELISA). In controls, serum samples were tested for JCPyV IgG antibodies (ELISA). Results: JCPyV DNA was not detected in any of the CSF samples from patients with NID. JCPyV IgG antibodies were detected in 88.6% of patients and 74.5% of controls (p < 0.001). In the patients' group, a significant difference in the IgG prevalence was observed between males (94.6%) and females (81.0%). In addition, significant differences in the seropositivity between age groups were found. The lowest seroprevalence (28.6%) was in patients less than 20 years, followed by a sharp increase in the 20-29-year group (69.2%), after which the seroprevalence remained stable (90.0-94.1%) in patients up to 69 years. All patients older than 70 years were JCPyV IgG-seropositive. No significant difference in the seroprevalence was found in patients presenting with febrile headache (81.6%), meningitis (93.3%), or meningoencephalitis (91.3%). No difference in the seropositivity between genders was found in controls. Although the seropositivity steadily increased in older participants, these differences were not significant. Analyzing the JCPyV antibody levels in patients with NID, the median antibody titers differed significantly between groups, ranging from 248 AU/mL (younger age groups) to 400 AU/mL (older age groups). Conclusions: Higher seroprevalence in the patients' group highlights the need to further investigate the possible association of JCPyV and NID.

Measles cases in Split-Dalmatia County (a Croatian tourist region), in May-July 2019: outbreak report and lessons learnt.

BACKGROUND: Measles elimination was accomplished in Croatia in 2016. Split-Dalmatia County, with population of ca. 425 000 inhabitants, is among the most important Croatian tourist areas with numerous seasonal workers coming during summer months. In both 2018 and 2019, more than 3 million tourists visited this county. In 2000-2018, there were no measles cases in this county, or their number was low (1-3 cases per year). METHODS: After measles was clinically suspected, all contacts were traced and contacted. Detection of specific IgM/IgG antibodies and real-time reverse transcription-polymerase chain reaction detection of viral RNA were used for laboratory confirmation. Sequencing and genotyping were performed for strains' molecular epidemiology analysis. RESULTS: Six epidemiologically unlinked measles virus occurrences happened in Split-Dalmatia County in 15 May-19 July 2019. Causative viral strains belonged to genotypes B3 and D8. Four were single imported cases. Ten patients belonged to two separate clusters within domicile population. Multiple individual and public health measures were implemented. In total, 483 contacts were identified, 64.2% within healthcare system where two persons contracted the disease. CONCLUSIONS: Besides the importance of timely vaccination of children, the lessons learned from this outbreak point to the need of stricter implementation of other aspects of Croatian measles prevention programme, such as checking of vaccination status in early adulthood. Despite the fact that measles elimination within domicile population in this tourist region has been accomplished and maintained for years, continuous public health workers' efforts are still necessary for identification and diminishment of populational pockets of susceptibility.

COMMON VARIABLE IMMUNODEFICIENCY: PREDISPOSING OR PROTECTIVE FACTOR FOR SEVERE COMPLICATIONS OF COVID-19?

Coronavirus disease 2019 (COVID-19) is an emerging infectious disease caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The usual presentation of the disease is a common cold-like illness but it can present with more severe and sometimes fatal manifestations. Immunocompromised patients such as those with common variable immunodeficiency (CVID) also are among the infected population. A limited number of reports have been published concerning CVID patients with COVID-19. The main reported symptoms were fever, cough, dyspnea and fatigue while the median duration of illness was 19 (interquartile range 14-26.5) days. Total recovery rate was 88.4%. It is still unknown whether primary immunodeficiency interacts as a predisposing or protective factor against the severe forms of COVID-19. Substitute immunoglobulin (IG) therapy is the only treatment option for CVID. Some reports suggest that early administration of intravenous IGs or convalescent plasma infusion may positively influence the outcome of COVID-19 in these patients.

Liver graft harbouring hydatid disease: how far can we extend our donor pool?

As the current demand for liver transplantation exceeds our donor pool, the donor search is shifted towards the extended donor criteria. The livers harbouring hydatid disease are a controversial source of grafts. We report the use of a liver graft harbouring hydatid disease in urgent liver transplantation in a patient with autoimmune hepatitis. Corroborated with previous experiences, we show that the liver grafts harbouring hydatid cysts provide a rare but valuable source of organs.

Waning immunity six months after BioNTech/Pfizer COVID-19 vaccination among nursing home residents in Zagreb, Croatia.

AIM: To assess the humoral immunity to COVID-19 in nursing home residents six months after vaccination. METHODS: This seroepidemiological research enrolled 118 residents of one nursing home in Zagreb. All participants received two doses of BioNTech/Pfizer COVID-19 and had no previously detected SARS-CoV-2 infection. The samples were tested for the presence of neutralizing antibodies using a virus neutralization test. A SARS-CoV-2 strain isolated in Vero E6 cells from a Croatian COVID-19 patient was used as a stock virus. Neutralizing antibody titer was defined as the reciprocal of the highest serum dilution that showed at least 50% neutralization. Neutralizing antibody titer ≥8 was considered positive. RESULTS: Sixty-four (54%) participants had a positive neutralizing antibody titer, 27 (23%) had a low positive titer (titer 8), and 27 (23%) had a negative titer. Women had a significantly higher median titer than men (16 [interquartile range, IQR 24] vs 8 [IQR 12], Mann-Whitney U=1033, P=0.003). Age was negatively but not significantly correlated with neutralizing antibody titer (Spearman's rho -0.132, P=0.155). CONCLUSION: Almost half of the participants (46%) had a negative or low positive titer six months after having been fully vaccinated. This study suggests that humoral immunity among nursing home residents considerably wanes six months after BioNTech/Pfizer COVID-19 vaccination. Our results could contribute to the discussion about the need for a booster dose.

Parvovirus B19 in Croatia: A Large-Scale Seroprevalence Study.

Background and Objectives: Seroepidemiological studies indicate that parvovirus B19 circulates in all areas of the world, although with some differences. The aim of this study is to analyze the seroprevalence of parvovirus B19 in the Croatian population. Materials and Methods: From 2010 to 2021, 1538 serum samples from different populations were tested for the presence of parvovirus B19 IgM/IgG antibodies. Serological tests were performed using a commercial enzyme-linked immunosorbent assay. Results: IgG antibodies were detected in 986/64.1% of participants with differences (p < 0.001) among the following population groups: 42.4% of children and adolescents, 67.1% of the adult general population, 66.7% of hemodialysis patients, and 65.6% of liver transplant recipients. Seroprevalence increased with age, from 30.0% in the 6 months-9 years age group to 69.0% in the 40-49 years age group, and remained stable thereafter (68.8-73.3%). There was no difference in the seropositivity among males (66.1%) and females (63.1%), as well as the place of residence (suburban/rural 63.9%, urban 64.1%). IgM antibodies (current/recent infection) were found in 61/4.0% of participants with the highest seropositivity in the youngest age group (11.1%). In pregnant women, seroprevalence was higher in women with an unfavorable obstetric history compared with a normal pregnancy (IgG 71.0% vs. 62.6%; IgM 6.5% vs. 2.4%), but these differences were not significant. Logistic regression showed that the adult population had almost three times higher risk of IgG seropositivity compared to children/adolescents (general population OR = 2.777, 95% CI = 2.023-3.812; hemodialysis patients OR = 2.586, 95% CI = 1.531-4.367; and transplant patients OR = 2.717, 95% CI = 1.604-4.603). A one-year increase in age increased the risk of IgG seroprevalence (OR = 1.017; 95% CI = 1.011-1.022). Conclusions: Older age was the main risk factor for IgG seropositivity. Hemodialysis and organ transplantation seem unrelated to the increased parvovirus B19 seroprevalence. The role of parvovirus B19 in the etiology of TORCH infections needs to be studied further.

Simultaneous Seroprevalence to Toxoplasma gondii, Cytomegalovirus and Rubella Virus in Childbearing Women from Western Romania.

Background and Objectives: Toxoplasma gondii, cytomegalovirus (CMV) and rubella virus, besides other agents, belong to a group named the TORCH complex. Research on the epidemiology of these agents in women is of particular interest, as primary infection during pregnancy could cause severe damage to the fetus. Women who had contracted infection before pregnancy develop IgG antibodies, so the fetus is protected in case of contact with the same agent. Our scope was to identify the childbearing women simultaneously protected or susceptible to a primary infection to two or three agents mentioned above. Materials and Methods: A cross-sectional study was performed on 6961 fertile Caucasian women from Western Romania, to analyze the simultaneous seroprevalence to two or three of the pathogens from the TORCH complex: Toxoplasma gondii, CMV, and rubella virus. Sampling was conducted at two time points: 2008-2010 (group 1; 1461 participants) and 2015-2018 (group 2; 5500 participants). Results: The percentage of women simultaneously seropositive to IgG-anti-Toxoplasma gondii/IgG-anti-CMV, IgG-anti-Toxoplasma gondii/IgG-anti-rubella, IgG-anti-CMV/IgG-anti-rubella or IgG-anti-Toxoplasma gondii and IgG-anti-CMV/IgG-anti-rubella antibodies decreased between the two groups (2008-2010 vs. 2015-2018): 41.4% vs. 36.1%, OR = 0.79, p = 0.0002; 41.8% vs. 35.7%, OR = 0.77, p < 0.0001; 88.9% vs. 83.6%, OR = 0.63, p < 0.0001; 39.6% vs. 33.2%, OR = 0.75, p < 0.0001. When comparing women from urban and rural areas, the simultaneous seroprevalence was higher in rural areas. In women tested 2008-2010 (group 1) the simultaneous seroprevalence (urban vs. rural) was: 38.4% vs. 49.1%, OR = 1.54, p = 0.0002; 38.4% vs. 50.6%, OR = 1.64, p < 0.0001; 88.8% vs. 89.2%, OR = 1.04, NS; 36.4% vs. 47.7%, OR = 1.58, p = 0.0001. A similar trend was found in women tested in group 2. Conclusions: The rate of simultaneous seropositivity to Toxoplasma gondii, CMV and rubella virus among Romanian women of reproductive age decreased significantly between 2008-2010 and 2015-2018 and the susceptibility to infections increased. It is necessary to apply increased prevention measures among susceptible pregnant women.

Measles outbreak in Dubrovnik-Neretva County, Croatia, May to June 2018.

In May 2018, measles was introduced in the Dubrovnik region by an adult who recently travelled to Kosovo*. Control measures and an outbreak investigation were implemented: 15 epidemiologically-linked cases met the outbreak case definition of a visitor/resident of Dubrovnik-Neretva County with laboratory-confirmed measles and symptom onset beginning on May 19. New cases were identified through hospitals and primary care physicians. Throat swabs, urine and/or serum samples were collected from outbreak cases. RT-PCR detection of viral RNA and IgM/IgG was used to confirm infection. The median age of cases was 33 years, with one 8 month-old infant. Vaccination status was unknown for 9 cases, three were unvaccinated, one case had history of one dose and two cases reported receiving two doses of measles-containing vaccine. There were 11 hospitalisations and one person developed pneumonia. Control teams undertook an extensive search of contacts and implemented a range of control measures. Despite the outbreak occurring at the beginning of the summer tourism season, it was contained and did not spread to neighbouring regions. With continuing measles transmission in Europe, even small outbreaks create a burden on the health system in countries which have eliminated measles, and illustrate the importance of maintaining high immunisation coverage.

Fatal case of West Nile encephalitis associated with acute anteroseptal ST elevation myocardial infarction (STEMI): a case report.

Cardiac involvement has rarely been reported in West Nile (WNV) infection. We report a fatal case of WNV encephalitis associated with an acute anteroseptal ST elevation myocardial infarction. The patient was hospitalized with a fever, headache, nausea and vomiting. The physical examination revealed positive meningeal signs and an altered level of consciousness. High levels of cardiac enzymes (creatine phosphokinase/MB fraction, lactate dehydrogenase, myoglobin and cardiac troponin I) and ST elevation on electrocardiogram were found. Both CSF and urine samples were positive for WNV RNA. This case highlights the need of awareness of the possibility of a WNV-related myocardial infection, including myocardial infarction.

Comparative genomics of human rubulavirus 2.

Although human rubulavirus 2 (HPIV2) is an important respiratory pathogen, little is known about its molecular epidemiology. We performed a comparative analysis of the full-length genomes of fourteen HPIV2 isolates belonging to different genotypes. Additionally, evolutionary analyses (phylogenetic reconstruction, sequence identity, detection of recombination and adaptive evolution) were conducted. Our study presents a systematic comparative genetic analysis that complements prior analyses and utilizes full-length HPIV2 genomes to provide a basis for future work on the clinical significance, molecular variation and conservation, and evolution of HPIV2.

Seroepidemiological Study of Epstein-Barr Virus in Different Population Groups in Croatia.

BACKGROUND: The Epstein-Barr virus (EBV) is one of the most common viruses found in humans, causing lifelong infection in up to 95% of the world population. OBJECTIVES: To analyze the seroprevalence of EBV infection in different population groups in Croatia. METHODS: During a 2 year period (2015-2016), a total of 2022 consecutive serum samples collected from Croatian residents were tested for the presence of EBV-specific viral capsid antigen (VCA) immunoglobulin M (IgM) and IgG antibodies using an enzyme-linked immunoassay. IgM/IgG-positive samples were further tested for IgG avidity. RESULTS: The overall prevalence of EBV IgG antibodies was 91.4%. Females had significantly higher IgG seroprevalence than males (93.1% vs. 89.9%, P = 0.008). According to age, IgG seropositivity increased progressively from 59.6% in children age < 9 years to 98.3% in 30-39 year olds, and remained stable thereafter (P < 0.001). The IgG seroprevalence differed significantly among groups: 68.1% in children/adolescents and 95.9% in adults; multiple sclerosis (100%), hemodialysis patients (97.7%), heart transplant recipients (93.8%), hematological malignancies (91.2%), and Crohn's disease (88.5%), P < 0.001. IgM antibodies were detected in 9% of participants. Using IgG avidity, recent primary EBV infection was documented in 83.8% of IgM-positive subjects < 9 years old, 69.2% age 10-19, 33.3% age 20-29, and 3.6-4.2% > 40. All IgM positive participants > 40 years showed high IgG avidity. Logistic regression showed that age is associated with EBV IgG seropositivity. CONCLUSIONS: EBV is widespread in the Croatian population. Older age appears to be the main risk factor for EBV seropositivity.

Genetic Variability and Sequence Relatedness of Matrix Protein in Viruses of the Families Paramyxoviridae and Pneumoviridae.

BACKGROUND: The families Paramyxoviridae and Pneumoviridae comprise a broad spectrum of viral pathogens that affect human health. The matrix (M) protein of these viruses has a central role in their life cycle. In line with this, molecular characteristics of the M proteins from variable viruses that circulated in Croatia were investigated. METHODS: Sequences of the M proteins of human parainfluenza virus (HPIV) 1-3 within the family Paramyxoviridae, human metapneumovirus (HMPV), and human respiratory syncytial virus from the family Pneumoviridae were obtained and analyzed. RESULTS: M proteins were very diverse among HPIVs, but highly conserved within each virus. More variability was seen in nucleotide sequences of M proteins from the Pneumoviridae family. An insertion of 8 nucleotides in the 3' untranslated region in 1 HMPV M gene sequence was discovered (HR347-12). As there are no samples with such an insertion in the database, this insertion is of interest and requires further research. CONCLUSION: While we have confirmed that M proteins were conserved among individual viruses, any changes that are observed should be given attention and further researched. Of special interest is inclusion of HPIV2 M proteins in this analysis, as these proteins have not been studied to the same extent as other paramyxoviruses.

IgG Avidity: an Important Serologic Marker for the Diagnosis of Tick-Borne Encephalitis Virus Infection.

A total of 52 serum samples from patients with symptoms suggestive of tick-borne encephalitis virus (TBEV) infection and positive IgM and/or IgG antibodies were tested for IgG avidity. Acute/recent TBEV infection was confirmed by low/borderline avidity index (AI) in 94.8% IgM positive/IgG positive samples, while in 5.2% high AI was found indicating persisting IgM antibodies. Majority of IgM negative/IgG positive samples (78.6%) showed high AI consistent with past TBEV infection. However, in 21.3% patients without measurable IgM antibodies current/recent infection was confirmed by AI. IgG avidity represents an additional serologic marker that improves diagnosis of TBEV, especially in cases of atypical antibody response.

First cases of human Usutu virus neuroinvasive infection in Croatia, August-September 2013: clinical and laboratory features.

Few reports of human Usutu virus (USUV) infection have been reported to date. We describe the first three patients with USUV neuroinvasive infection in Zagreb and its surroundings from 30 August to 7 September 2013 during a West Nile virus (WNV) outbreak. Patients were aged 29, 56, and 61 years. The two older patients had several comorbidities (arterial hypertension, hyperlipidemia, and diabetes mellitus). All patients presented with meningitis and meningoencephalitis closely resembling WNV neuroinvasive disease. The main clinical features in all patients were headache, fever, nuchal rigidity, hand tremor, and hyperreflexia. Neuroimaging studies were normal and electroencephalography (EEG) revealed diffusely slow activity. The 29 years old, a previously healthy female patient, was deeply somnolent and disoriented for 4 days. Her recovery was slow and even 10 weeks after disease onset, she had memory and speech-fluency difficulties. The other two patients recovered promptly. USUV IgG antibodies were detected in all patients by ELISA with seroconversion documented in two of them. Titers of USUV-neutralizing antibodies were 10, 80, and 10, respectively. Because USUV and WNV share many clinical characteristics, USUV infection could be misdiagnosed as WNV. Testing for USUV should be considered in all suspected cases of meningoencephalitis, especially in areas where both viruses cocirculate.

Dengue virus infection in Croatia: seroprevalence and entomological study.

During 2011-2012, a total of 1,180 sera samples were collected from residents of seven Croatian counties located on the Adriatic Coast and four counties in northeastern Croatia and tested for the presence of dengue virus (DENV) IgG antibodies using ELISA. Reactive samples were further tested by IFA. Seven samples (0.59%) tested positive for DENV antibodies. Seroprevalence rates by county varied from 0-2.21%. The highest seropositivity rate (2.21%) was found in the Dubrovnik-Neretva County where autochthonous dengue cases were recorded in 2010. Additionally,3,699 mosquitoes were collected from 126 localities along the Adriatic coast in August-September, 2011. Aedes albopictus was the most prevalent species (81.37%). No evidence of DENV RNA was detected by RT-PCR among 1,748 female mosquitoes.

[Usutu virus: a novel flavivirus in Croatia]. / Usutski virus: novi flavivirus u hrvatskoj.

Usutu virus (USUV) belongs to the family Flaviviridae, genus Flavivirus, Japanese encephalitis serocomplex. The virus was discovered in 1959 in South Africa and has emerged since 1996 causing epizootics with high avian mortality in Europe. The importance of USUV in humans is not fully understood. However, several human clinical cases of USUV infection described so far indicate the role of this virus as an antropozoonotic agent. In Croatia, serologic evidence of USUV was first documented in 2011 in two horses from Zagreb and Sisak-Moslavina County. In 2012, USUV neutralizing antibodies were found in one human sample from a resident of a Vukovar-Srijem County. Human clinical cases of USUV infection were detected for the first time during the West Nile virus outbreak from July to September 2013. Three patients with USUV neuroinvasive disease were detected in the City of Zagreb and Zagreb County. Our results indicate USUV circulation in Croatia. Further human cases could be expected in the next transmission seasons.

Prevalence of viral hepatitis in Croatian adult population undergoing routine check-up, 2010-2011.

Seroprevalence data on viral hepatitis in the general population vary widely. The aim of this study was to determine the prevalence of hepatitis A (HAV), hepatitis B (HBV) and hepatitis C (HCV) viruses in the general Croatian adult population undergoing routine check-ups. The seroprevalence of anti-HAV, anti-HBc and anti-HCV was 40.5%, 7.0% and 0.9%, respectively. HBsAg was found in 0.7% and anti-HBs antibodies in 24.4% of participants. Gender was not associated with HAV, HBV or HCV seropositivity. HAV and HBV seropositivity increased progressively with age (HAV from 11.7% to 90.4%, p < 0.001; HBV from 1.7% to 15.8%, p < 0.001). Participants from rural areas showed a significantly higher HBV seroprevalence rate than those from urban areas (10.7% vs. 6.1%, p = 0.007). Results of univariate and multiple logistic regression showed that older age was a significant predictor for both HAV and HBV seropositivity while rural place of residence was a significant predictor for HBV seropositivity.

[POLIOMYELITIS ERADICATION--ONE STEP TO ACHIEVE THE GOAL]. / ERADIKACIJA POLIOMIJELITISA--KORAK DO CILJA.

Poliomyelitis is a very old disease of humans, caused by poliovirus. With appearance of the epidemics in the 20th century, poliomyelitis became a global public health issue. In 1988, the World Health Organization started a campaign for global eradication of poliomyelitis and till now poliomyelitis cases have been reduced by more than 99%. In Croatia, the introduction of vaccination in 1961 resulted in dramatic reduction of paralytic disease. The European region, including Croatia was certified polio free in 2002. However, the final goal of the "polio-free world" has not yet been reached. To reinforce the campaign, the global polio eradication initiative has come up with the Polio Eradication & Endgame Strategic Plan 2013-2018 with detailed program how to resolve the main challenges: (a) continued transmission of wild polioviruses in endemic reservoirs; (b) reinfection of polio-free areas; and (c) outbreaks due to the circulating vaccine-derived polioviruses (cVDPV). Global oral polio vaccine cessation will follow, with the introduction of universal use of inactivated polio vaccine.

[TICK-BORNE ENCEPHALITIS VIRUS: EPIDEMIOLOGICAL AND CLINICAL PICTURE, DIAGNOSIS AND PREVENTION]. / VIRUS KRPELJNOG ENCEFALITISA: EPIDEMIOLOSKA I KLINICKA SLIKA, DIJAGNOSTIKA I PREVENCIJA.

Tick-borne encephalitis virus (TBEV) is a small, enveloped virus that belongs to the family Flaviviridae, genus Flavivirus, tick-borne encephalitis serocomplex. There are three subtypes of TBEV: European, Far-Eastern and Siberian subtypes, which differ in geographical distribution, tick vector and clinical manifestation of disease in humans. TBEV is endemic in a wide geographic area ranging from Central Europe and the Scandinavian Peninsula to Japan. The virus is maintained in nature in so-called natural foci in cycles involving ticks and wild vertebrate hosts (mainly small rodents). The principal vector for the European subtype is Ixodes (I.) ricinus tick, whereas for Far-Eastern and Siberian subtypes it is I. persulcatus. In the Baltic States and Finland, co-circulation of two or all three subtypes was documented. Several animals, principally small rodents, serve as virus reservoirs. In the tick population, TBEV is transmitted by feeding/co-feed ing on the same host, transovarially (from infected females to their eggs) and trans-stadially (from one development stage to the next). An infected tick remains infected for life. While most TBE infections in humans occur following a tick bite, alimentary routes of TBEV transmission (consumption of unpasteurized milk/milk products from infected livestock) have also been described. All three tick stages can transmit the infection to humans. In the last decade, an increase of TBE incidence has been observed in some endemic areas. This could be due to a number of interacting factors such as changes in the climatic conditions affecting tick habitats, improvements in the quality of epidemiological surveillance systems and diagnostics, in landscape resources and their utilization and more outdoor recreation activity. In addition, the endemic area of TBEV has expanded to higher altitudes (up to 1500 m), apparently influenced by climatic changes. The typical clinical picture of infection with European subtype TBEV is characterized by a biphasic course (50%-77%). The first phase is characterized by nonspecific, flu-like symptoms followed by an asymptomatic interval of about one week. In 20%-30% of persons who develop symptoms, the second phase occurs with symptoms of central nervous system involvement (meningitis, encephalitis, myelitis, radiculitis). The mortality rate for European subtype is 1%-2%. Diagnosis is usually based on detection of specific antibodies (enzyme immunoassay, indirect immunofluorescent assay, plaque reduction neutralization test). From 1993 to 2013, a total of 777 cases of TBE were reported in Croatia. Endemicity is highest in north-western counties (mean incidence 3.61-6.78/100,000 inhabitants). The majority of patients were older than 20 years (88%). Most cases (73%) were reported from May to July.