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Background: Since sensitivity and specificity vary widely between tests, SARS-CoV-2 serology results should be interpreted with caution., Methods: The study included serum samples from patients who had recovered from COVID-19 ( n = 71), individuals vaccinated against SARS-CoV-2 ( n = 84), and asymptomatic individuals ( n = 33). All samples were tested for the presence of binding antibodies (enzyme immunoassay; EIA), neutralizing (NT) antibodies (virus neutralization test; VNT), and surrogate NT (sNT) antibodies (surrogate virus neutralization test; sVNT) of SARS-CoV-2., Results: SARS-CoV-2-binding antibodies were detected in 71 (100%) COVID-19 patients, 77 (91.6%) vaccinated individuals, and 4 (12.1%) control subjects. Among EIA-positive samples, VNT was positive (titer ≥ 8) in 100% of COVID-19 patients and 63 (75.0%) of the vaccinated individuals, while sVNT was positive (>30% inhibition) in 62 (87.3%) patients and 59 (70.2%) vaccinated individuals. The analysis of antibody levels showed a significant moderate positive correlation between EIA and VNT, a moderate positive correlation between EIA and sVNT, and a strong positive correlation between VNT and sVNT. The proportion of positive sVNT detection rate was associated with VNT titer. The lowest positivity (72.4%/70.8%) was detected in samples with low NT titers (8/16) and increased progressively from 88.2% in samples with titer 32 to 100% in samples with titer 256., Conclusions: sVNT appeared to be a reliable method for the assessment COVID-19 serology in patients with high antibody levels, while false-negative results were frequently observed in patients with low NT titers.

Background: Tahyna orthobunyavirus (TAHV) is widely distributed in continental Europe. Very few studies have analyzed TAHV seroprevalence in Croatia. We analyzed the prevalence of TAHV RNA and antibodies in Croatian patients with neuroinvasive disease (NID)., Methods: A total of 218 patients with unsolved NID detected during five consecutive arbovirus transmission seasons (April 2017-October 2021) were tested. Cerebrospinal fluid (CSF) and urine samples were tested for TAHV RNA using RT-PCR. In addition, CSF and serum samples were tested for TAHV antibodies using a virus neutralization test (VNT)., Results: Clinical presentations in patients with NID were meningitis (141/64.7%), meningoencephalitis (56/25.7%), myelitis (8/3.7%), and 'febrile headache' (13/5.9%). TAHV RNA was not detected in any of the tested CSF or urine samples; however, TAHV-neutralizing (NT) antibodies were detected in 22/10.1% of patients. Detection of NT antibodies in the CSF of two patients presenting with meningitis suggested recent TAHV infection. TAHV seropositivity increased significantly with age, from 1.8% to 24.4%. There was no difference in seroprevalence between genders or areas of residence (urban, suburban/rural). The majority of seropositive patients (90.9%) resided in floodplains along the rivers in continental Croatia., Conclusions: The presented results confirm that TAHV is present in Croatia. The prevalence and clinical significance of TAHV infection in the Croatian population have yet to be determined.

Background: The global outbreak of coronavirus disease 2019 (COVID-19) leads to the development of accessible and cost-effective rapid antigen-detection tests (RATs), as quick and accurate diagnosis is crucial to curb the pandemic., Aim: To evaluate the Humasis COVID-19 Ag Test (Humasis Co., Ltd., Gyeonggi-do, Republic of Korea) in the diagnosis of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2)., Methods: This retrospective study was carried out at the Croatian Institute of Public Health and included patients with clinical symptoms of COVID-19 lasting no longer than 5 d prior to testing, whose nasopharyngeal swabs were primarily tested with RAT. Negative RAT samples underwent confirmatory real-time reverse transcription-polymerase chain reaction (RT-PCR). Diagnostic efficacy was determined compared to RT-PCR. The patients were divided into three age groups (< 18, 19-65, > 65 years). Statistical analysis was performed with the significance level set at P < 0.05., Results: In total, 2490 symptomatic patients were tested; 953 samples were positive on RAT, and 1537 were negative. All negative RAT samples were subjected to RT-PCR; 266 samples were positive and marked as false-negative results on RAT. The calculated negative predictive value as a measure of RAT efficacy was 82.69%. The χ 2 test and Kruskal-Wallis test showed a significant difference in the proportion of false negatives ( P < 0.001) and RT-PCR cycle (Ct) values for false-negative RATs ( P = 0.012) among the age groups. The young age group was significantly less likely to be false negative, whereas the false negatives from the elderly group experienced significantly lower Ct values than the other two age groups., Conclusion: Evaluated RAT demonstrated satisfactory performance with more reliable results in younger patients. Humasis COVID-19 Ag RAT is potentially a valuable tool in areas where access to molecular methods is limited; however, RT-PCR remains a gold standard for SARS-CoV-2 detection., Competing Interests: Conflict-of-interest statement: The authors declare no conflict of interest., (©The Author(s) 2022. Published by Baishideng Publishing Group Inc. All rights reserved.)

During the four pandemic waves, a total of 560,504 cases and 10,178 deaths due to COVID-19 were reported in Croatia. The Alpha variant, dominant from March 2021 (>50% of positive samples), was rapidly replaced by Delta variants (>90%) by August 2021. Several seroprevalence studies were conducted in different populations (general population, children/adolescents, professional athletes, healthcare workers, veterinarians) and in immunocompromised patients (hemodialysis patients, liver/kidney transplant recipients). After the first pandemic wave, seroprevalence rates of neutralizing (NT) antibodies were reported to be 0.2-5.5%. Significantly higher seropositivity was detected during/after the second wave, 2.6-18.7%. Two studies conducted in pet animals (February-June 2020/July-December 2020) reported SARS-CoV-2 NT antibodies in 0.76% of cats and 0.31-14.69% of dogs, respectively. SARS-CoV-2 NT antibodies were not detected in wildlife. Environmental samples taken in the households of COVID-19 patients showed high-touch personal objects as most frequently contaminated (17.3%), followed by surfaces in patients' rooms (14.6%), kitchens (13.3%) and bathrooms (8.3%). SARS-CoV-2 RNA was also detected in 96.8% affluent water samples, while all effluent water samples tested negative. Detection of SARS-CoV-2 in humans, animals and the environment suggests that the 'One Health' approach is critical to controlling COVID-19 and future pandemics.

West Nile virus (WNV) is one of the most widely distributed (re-)emerging arboviruses. In Croatia, acute WNV infections as well as seropositivity were detected in humans, horses, birds and poultry. Although serologic evidence of WNV human infections dates back to the 1970s, no clinical cases were reported until 2012. WNV outbreaks, as well as sporadic infections, were continuously recorded in continental Croatian counties from 2012 to 2018. In addition, acute asymptomatic infections (IgM antibodies) in horses have been regularly notified in continental regions since 2012, while seropositive horses (seroprevalence rates 3.7-21.4%) were detected in both continental and coastal regions. Moreover, WNV seropositivity in poultry (1.8-22.9%) was reported from 2013 to 2020. During the largest WNV outbreak in 2018, WNV RNA was detected for the first time in two dead goshawks ( Accipiter gentilis ) from the same aviary in North-West Croatia, while WNV antibodies were found in one buzzard ( Butteo butteo ) from the same region. In addition, WNV RNA was detected in a dead blackbird ( Turdus merula ) at the Croatian littoral. The phylogenetic analysis of 11 strains detected in urine samples of patients with neuroinvasive disease and 1 strain detected in a goshawk showed circulation of WNV lineage 2. Thus far, WNV has not been detected in mosquitoes in Croatia.

Over a year into the COVID-19 pandemic, there is growing evidence that SARS-CoV-2 infections among dogs are more common than previously thought. In this study, the prevalence of SARS-CoV-2 antibodies was investigated in two dog populations. The first group was comprised of 1069 dogs admitted to the Veterinary Teaching Hospital for any given reason. The second group included dogs that shared households with confirmed COVID-19 cases in humans. This study group numbered 78 dogs. In COVID-19 infected households, 43.9% tested ELISA positive, and neutralising antibodies were detected in 25.64% of dogs. Those data are comparable with the secondary attack rate in the human population. With 14.69% of dogs in the general population testing ELISA positive, there was a surge of SARS-CoV-2 infections within the dog population amid the second wave of the pandemic. Noticeably seroprevalence of SARS-CoV-2 in the dog and the human population did not differ at the end of the study period. Male sex, breed and age were identified as significant risk factors. This study gives strong evidence that while acute dog infections are mostly asymptomatic, they can pose a significant risk to dog health. Due to the retrospective nature of this study, samples for viral isolation and PCR were unavailable. Still, seropositive dogs had a 1.97 times greater risk for developing central nervous symptoms.

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a novel coronavirus with a pandemic spread. So far, a total of 349,910 SARS-CoV-2 cases and 7687 deaths were reported in Croatia. We analyzed the seroprevalence and neutralizing (NT) antibody response in the Croatian general population after the first (May-July 2020) and second (December 2020-February 2021) pandemic wave. Initial serological testing was performed using a commercial ELISA, with confirmation of reactive samples by a virus neutralization test (VNT). A significant difference in the overall seroprevalence rate was found after the first (ELISA 2.2%, VNT 0.2%) and second waves (ELISA 25.1%, VNT 18.7%). Seropositive individuals were detected in all age groups, with significant differences according to age. The lowest prevalence of NT antibodies was documented in the youngest (<10 years; 16.1%) and the oldest (60-69/70+ years; 16.0% and 12.8%, respectively) age groups. However, these age groups showed the highest median NT titers (32-64). In other groups, seropositivity varied from 19.3% to 21.5%. A significant weak positive correlation between binding antibody level as detected by ELISA and VNT titer (rho = 0.439, p < 0.001) was observed. SARS-CoV-2 NT antibody titers seem to be age-related, with the highest NT activity in children under 10 years and individuals above 50 years.

Lymphocytic choriomeningitis virus (LCMV) is a neglected rodent-borne zoonotic virus distributed worldwide. Since serologic assays are limited to several laboratories, the disease has been underreported, often making it difficult to determine incidence and seroprevalence rates. Although human clinical cases are rarely recorded, LCMV remains an important cause of meningitis in humans. In addition, a fatal donor-derived LCMV infection in several clusters of solid organ transplant recipients further highlighted a pathogenic potential and clinical significance of this virus. In the transplant populations, abnormalities of the central nervous system were also found, but were overshadowed by the systemic illness resembling the Lassa hemorrhagic fever. LCMV is also an emerging fetal teratogen. Hydrocephalus, periventricular calcifications and chorioretinitis are the predominant characteristics of congenital LCMV infection, occurring in 87.5% of cases. Mortality in congenitally infected children is about 35%, while 70% of them show long-term neurologic sequelae. Clinicians should be aware of the risks posed by LCMV and should consider the virus in the differential diagnosis of aseptic meningitis, especially in patients who reported contact with rodents. Furthermore, LCMV should be considered in infants and children with unexplained hydrocephalus, intracerebral calcifications and chorioretinitis. Despite intensive interdisciplinary research efforts, efficient antiviral therapy for LCMV infection is still not available.

Several arboviruses have emerged in Croatia in recent years. Tick-borne encephalitis is endemic in continental counties; however, new natural micro-foci have been detected. Two autochthonous dengue cases were reported in 2010. West Nile virus emerged in 2012, followed by emergence of Usutu virus in 2013. Although high seroprevalence rates of Toscana virus have been detected among residents of Croatian littoral, the virus remains neglected, with only a few clinical cases of neuroinvasive infections reported. Lymphocytic choriomeningitis virus is a neglected neuroinvasive rodent-borne virus. So far, there are no reports on human clinical cases; however, the seroprevalence studies indicate the virus presence in the Croatian mainland. Puumala and Dobrava hantaviruses are widely distributing rodent-borne viruses with sporadic and epidemic occurrence. Hepatitis E virus is an emerging food-borne virus in Croatia. After the emergence in 2012, cases were regularly recorded. Seropositivity varies greatly by region and population group. Rotaviruses represent a significant healthcare burden since rotavirus vaccination is not included in the Croatian national immunization program. Additionally, rotaviruses are widely distributed in the Croatian ecosystem. A novel coronavirus, SARS-CoV-2, emerged in February 2020 and spread rapidly throughout the country. This review focuses on emerging and neglected viruses of zoonotic importance detected in Croatia.