Tick-borne viral encephalitis has a high prevalence among the population in our country currently, especially in endemic areas. Ixodid ticks transmit the virus through bites, as they are carriers of the infection. The incidence of tick-borne infection increases annually in spring-summer because of natural tick activity during this period. In terms of the prevalence of natural focal and zoonotic infections, tick-borne viral encephalitis ranks the 4th place in the Primorsky territory in 2020. Low level of preventive education among the population and refusals of citizens that visit forests to vaccinate explain the high prevalence of this infection. It is known that the consequences of encephalitis, caused by degenerative changes in brain nerve cells, inevitably lead to disability or to death [1].

Objectives:

1. To estimate the incidence of tick-borne encephalitis in the Primorsky territory in 2005 — 2020.
2. To determine the main pathophysiological mechanisms, that occur in the infected with tick-borne encephalitis human body, based on the analysis of scientific articles.

Materials and methods:

We conducted a retrospective epidemiological analysis of forms of state statistical observation «Information on patients with certain infectious and parasitic diseases» in the Primorsky territory from 2005 to 2020 [2] to assess the prevalence of tick-borne encephalitis among the patients in the Primorsky territory (Pic. 1). We conducted a review of scientific articles to identify general pathophysiological mechanisms of development of tick-borne viral encephalitis.

Pic. 1. Prevalence of tick-borne encephalitis in the Primorsky territory
(https://rosstat/gov/ru/storage/mediabank/Zdravoohran-2021.pdf)

Results:

1. An analysis of statistical data of the prevalence of tick-borne encephalitis in the Primorsky territory in 2005 — 2020 revealed a decrease of the incidence from 4,6 thousand people to 1 thousand people. The analysis of scientific literature on this topic showed that the positive dynamics is caused by the following factors [3]:

— preventive education of the population about the necessity of vaccination, individual protection, the use of special protective clothing and repellents

— self-examination every 2 hours while spending time in the forest for detecting ticks on clothes and on the body

— immediate visit to the clinic for injection of tick-borne encephalitis immunoglobulin if an attached tick is found, as the period of incubation usually lasts from 10 to 14 days, with variations from 1 to 60 days

— submitting the extracted tick to the laboratory for the analysis

— urgent seeking for medical help if symptoms of tick-borne encephalitis are detected

— avoiding self-medication during the initial phase of tick-borne encephalitis, as the signs of it are similar to symptoms of acute respiratory viral infections and the flu. The acute start of the disease begins with strong headache, chills, nausea, muscle pain. Temperature rises to 39 degrees

— acaricide treatment for ticks in gardens and suburban areas

2. The analysis of literature on the topic of the study showed that most authors consider the following aspects as leading pathogenetic mechanisms:
3. The host’s weakened immune resistance, which is typical for children, old-aged people and individuals with cancer or HIV-infection, is a common condition for the pathogenetically significant activation of the tick-borne encephalitis virus. In this case, the virus affects lymphocytes and modulates the host’s immune response. This leads to the development of secondary immunodeficiency, which is characterized by immune function imbalance [4].
4. The disease affects the nervous system, so the consequences of tick-borne encephalitis infection includes limb paralysis, impaired neck muscle function and loss of cognitive abilities (Pic. 2).

Pic. 2. Pathogenesis of tick-borne encephalitis (Z. I. Vozianova, 2001)

In some cases, the manifestations of the past disease are detected several months later in the form of progressive epileptic seizures. The clinical manifestations of these processes are associated with damage of motor neurons of the brain and spinal cord by the flavivirus. Apoptosis is the major component in immunopathogenesis of tick-borne encephalitis. In the acute form of neuroinfection, an increase in the number of apoptotic lymphocytes is observed. Normally, the launch of specific antiviral immunity occurs due to activation of T-lymphocytes, that destroy virus-infected cells. However, the body’s defense systems are violated because of the presence of a persistent infectious agent in the cells [5]. This is explained by the tropism of tick-borne encephalitis virus for lymphoid tissue and its ability to interrupt and disrupt the functionality of immunocompetent cells [6]. Cytokines, xenobiotics, hormones, and genetic characteristics of cells are among the regulators of apoptosis. However, the massive death of the cells involved in the implementation of antiviral immunity is a result of imbalance exactly in regulatory cytokines. It explains the inadequate immune response that contributes to the transition of the infection into a chronic course [7].

Conclusions:

1. A persistent decrease in the prevalence of tick-borne encephalitis in the Primorsky territory has been observed from 2005 to 2020.
2. The majority of authors consider that the state of the human immune system is the leading factor in the pathogenesis of tick-borne encephalitis.
3. Apoptosis is a key component of tick-borne encephalitis immunopathogenesis.