Epidemiological and Clinical Characteristics of Tick-Borne Encephalitis

Abstract

Background: Tick-borne encephalitis is a disease with natural foci caused by Flavivirus genes of the Togaviridae family. It is transmitted by Ixodes ticks and characterized by fever and central nervous system complications. The disease can be transmitted by infected ticks as well as through the consumption of unpasteurized milk and raw meat from infected animals. Since 2005, our country has ofÏcially reported and diagnosed cases of tick-borne encephalitis. It is considered an emerging infectious disease in our country. Therefore, there is a need to study the more detailed characteristics of the disease’s epidemiology and clinical aspects. Aim: The aim of the study was to characterize the epidemiology and clinical presentation of tick-borne encephalitis. Materials and Methods: All cases of tick-borne encephalitis diagnosed in NCCD between 2013 and 2023 were included in the study based on specific criteria. These criteria encompassed a history of inhabiting, visiting, or working in natural foci known for tick-borne encephalitis, being bitten by Ixodes persulcatus ticks, presenting clinical manifestations, testing positive for IgM and IgG antibodies related to tick-borne encephalitis, and being admitted to the hospital at the NCCD. A descriptive study was conducted using epidemiological, clinical, and certain serological testing data. These data were collected from epidemiological investigation forms and the disease history records of diagnosed tick-borne encephalitis cases, entered into Microsoft Excel 2010. The study data underwent analysis using STATA 12.1 and ArcGIS. Results: The study included a total of 90 newly diagnosed cases of tick-borne encephalitis at the NCCD between 2013 and 2022. Among all diagnosed cases at the NCCD, 44.4% comprised the population in UB, while 55.6% belonged to 12 provinces. The average age of these cases was 33±16.8 years (with a median and mode of 30 years), ranging from 11 months to 81 years old. Of all diagnosed cases, 70.7% were female and unemployed, with 37.8% falling within the 20-34 age group. A significant proportion (75.6%) of diagnosed cases reported tick bites in 9 soums of Selenge province, while 24.4% reported tick bites in 12 soums across Bulgan, Arkhangai, Orkhon, Darkhan-Uul, Tuv, and Khuvsgul provinces, as well as the ‘Goodoin’ camp, ‘Gunt’, and ‘Bogd’ mountain in the suburban area of Ulaanbaatar. Regarding seasonal distribution, 41.1% of diagnosed cases were bitten by I.persulcatus ticks in May, while 30.0% were bitten in June (p=0.001). Clinical manifestations across diagnosed cases included fever (72.2% [95% CI 57.0-74.0]), meningo-encephalitis (13.3% [95% CI 6.0-19.0]), meningitis (7.8% [95% CI 2.0-13.0]), and encephalomyelitis (5.6% [95% CI 1.0-10.0]). Symptoms such as severe headache and fatigue were prevalent across all clinical types. Nervous system manifestations varied depending on the type of disease. The average estimated incubation period was 10 days (standard deviation ±4.1 days, ranging from 5 to 21 days, with Q2-9 days). The average day of antibody detection was 10.5 days. Conclusion: Males were predominantly affected by tick-borne encephalitis while engaging in activities such as picking cedar nuts, berries, and medicinal plants, preparing woods, and working in natural foci of the infection. In clinical observations, fever was the predominant manifestation of tick-borne encephalitis.