West nile fever in the Southern federal District: an overview epizootic and epidemiological situation

Introduction

The European part of Russia indispensably requires close monitoring by the sanitary and epidemiological service due to the diversity of registered natural focal infectious diseases, the risk of threats associated with the expansion of the natural area of carriers of relevant arboviruses, and the intensively developing tourism industry, which potentiates the risks of complications of the epidemiological situation [1–3]. West Nile fever (WNF) holds a special place among natural focal infections in the south of Russia [2][4][5]. The area of this infection covers all subjects of the Southern Federal District (SFD), and the number of disease cases is the highest in Russia, namely 81.7% of all registered cases for the period from 1997 to 2023.

The epidemic activity of the foci of the disease that has persisted for more than 25 years necessitates continuous and systematic epidemiological surveillance on WNF. Determining the current state of natural foci of WNF in the territory of the SFD and promptly assessing their epidemic potential are of great practical importance for ensuring the epidemiological well-being of the population, not only in this strategically important region but also in the Russian Federation as a whole.

The purpose of the study was to investigate the characteristics of epizootic and epidemiological manifestations of WNF in the subjects of the SFD.

Materials and methods

The work was carried out using complex epidemiological and statistical methods. The sources of information were official data on the registration of disease cases and reporting forms on the results of epidemiological surveillance on WNF, submitted to the Reference Center for Monitoring the WNF pathogen based at the Volgograd Anti-Plague Research Institute of Rospotrebnadzor by the Rospotrebnadzor Directorates in the constituent entities of the Russian Federation from 2009 to 2023; archival data for the period 1999–2008; as well as information contained in the electronic databases “Manifestations of West Nile Fever in the Russian Federation” (state registration number 2021621282 dated June 16, 2021) and “Main Carriers of West Nile Virus on the Territory of the Russian Federation” (state registration number 2022622592 dated July 9, 2022). A comprehensive assessment of the epizootological situation on WNF in the subjects of the SFD was carried out on the basis of information from practical institutions of Rospotrebnadzor and research organizations, including the results of the Reference Center’s own investigations of field material, materials from scientific publications and dissertations.

Statistical data processing was performed using generally accepted methods of variation statistics, including calculation of arithmetic means, standard error, and confidence intervals (CI) with a confidence probability of 95%. Calculations were performed using software Microsoft Excel 2016 (Microsoft Corporation, USA).

Results

The subjects of the SFD are located within the southern zone of continental, moderate continental, and sharply continental climates, partially including a humid subtropical area in the Krasnodar Territory to the south of Tuapse¹. Average summer air temperatures in this area vary from 22 °C to 24.8 ℃, which contributes to the existence of local foci of WNF regardless of the climatic features of the epidemic season. The species composition of blood-sucking mosquitoes includes over 30 species in the Volgograd Region, over 40 species in the Krasnodar Territory, the Republic of Crimea, and the city of Sevastopol, and over 20 species in the Rostov Region. The mosquito fauna in the Republic of Kalmykia and the Astrakhan Region is less diverse and includes 10 and 15 species, respectively. According to phenological observations, the season of possible transmission of the West Nile virus (WNV) in the Republics of Kalmykia, Adygea, Crimea, the city of Sevastopol, and the Krasnodar Territory begins in the first ten days of March; in the Astrakhan, Volgograd, and Rostov Regions, it starts in the second ten days of March and continues through the second or third ten days of September, or until the end of October if the autumn is warm. The most favorable temperature conditions and high numbers of mosquitoes carrying WNV are observed from June to August. Besides, the Mediterranean-Black Sea, West African, Central Asian, and East European flyways of migratory birds pass across these territories. Thus, the subjects of the SFD have optimal climatic and biological conditions for the formation and functioning of WNF foci.

Over 1997–2023, 2,652 cases of WNF were registered in the subjects of the SFD with an average value of 98.2 cases per year. The dynamics of WNF incidence in the long-term aspect tends to decrease (Fig. 1). During the period under analysis, 4 risings of the incidence of a disease were recorded, namely 475 cases in 1999, 488 cases in 2010, 338 cases in 2012, and 317 cases in 2019. The lowest absolute number of cases was detected in 2017 (2 cases).

Among the subjects of the SFD, the highest proportion of WNF cases accrued to the Volgograd Region (Fig. 2). Summarizing the data on the identified cases, we noted that the disease was more often registered in males (56.9%; 95% CI: 54.43‒59.31), in the age group of 60 years and older (35.5%; 95% CI: 33.52‒37.50) and occurred without injury to the central nervous system (CNS) (21.4%; 95% CI: 19.47‒23.37) with a moderate disease course (73.43%; 95% CI: 71.36‒75.51).

Taking into account the landscape features inherent to this region’s territories, the number and species composition of carriers and transmitters of WNV, as well as the number and density of the population at risk of infection, it seems relevant to conduct an analysis in the context of individual subjects of the SFD.

Astrakhan Region. The circulation of WNV within this territory, as well as in Russia as a whole, was first established in 1963 by a group of specialists from the Institute of Poliomyelitis and Viral Encephalitis of the Academy of Medical Sciences USSR [6–8]. Until 2011, studies on the ecology of WNV have been conducted with varying intensity, and the information about them available in the press does not allow for a high-quality spatial and temporal analysis. In general, over a long-term observation period, the presence of persistent foci of WNF has been shown in the Astrakhan Region with pronounced epizootic activity in the middle and lower zones of the Volga Delta [6][8][9]. For 2011‒2023, enzooticity on WNF was revealed in 6 out of 11 (54.5%) districts of the region (Yenotaevsky, Ikryaninsky, Krasnoyarsky, Limansky, Kharabalinsky, Chernoyarsky Districts, and the city of Astrakhan). Manifestations of the epizootic process of WNF were recorded annually, with the exception of 2014, 2017, and 2018, and acquired the most intense character in 2013, 2016, and 2020 [10]. The total detection rate of WNV markers in field material in the Astrakhan region amounted to 0.70±0.33%. During this period, the infestation rate of mosquitos averaged 0.76±0.33%, and the infection rate of ticks was 3.28%, once in 2022. A high frequency of positive samples from blood-sucking arthropods was noted in the city of Astrakhan, as well as in the Limansky and Kharabalinsky districts. No positive findings were found in biological material from birds and small mammals.

The incidence of WNF in the population of the Astrakhan Region has been recorded since 1967 [9]. Since 1997, the disease has been detected annually except in 2021. Cases of WNF have been diagnosed in all administrative territories of the region. Risings in the incidence were observed in 1999, 2005, 2012, 2013, and 2019; the largest number of cases was identified in 1999 (95 patients, 5 of which were fatal) [7–9]. Thus, cyclical fluctuations in the incidence of WNF with an interval of 5 to 6 years have been revealed in the Astrakhan Region. A rise in the incidence, which occurred against the background of the intensification of the epizootic process, was noted only in 2013. In the seasonal aspect, the maximum incidence occurred in August (51.4%; 95% CI: 45.95‒56.85). The average long-term incidence rate in 1997–2023 was 2.57 (95% CI: 1.50‒3.64) cases per 100 thousand population. The dynamics of morbidity in general tended to decrease (y = ‑0.0817x + 3.7152). The total number of deaths amounted to 18 (2.6%; 95% CI: 1.42‒3.78); the last case of death from WNF was confirmed in 2019. Among the features of the morbidity structure, it is necessary to accentuate the almost equal involvement of the urban (49.1%; 95% CI: 43.64‒54.56) and rural population (50.9%; 95% CI: 45.44‒56.36) in the epidemic process; the high proportion of infected children (12.3%; 95% CI: 8.73‒15.87), exceeding the values for other subjects of the SFD with a long-term registration of WNV transmission, and the unemployed population of working age (22.%; 95% CI: 18.15‒27.25); the predominant infection of identified patients at their place of residence in rural areas (43.8%; 95% CI: 38.39‒49.20); and an extremely low proportion of WNF cases with a mild clinical course (0.6%; 95% CI: 0.24‒1.46). According to other indicators, the structure of morbidity did not differ essentially from that in the SFD.

Volgograd Region. The enzooticity of WNF in the Volgograd Region was first demonstrated in the early 1980s according to the results of exploratory monitoring investigations conducted by specialists from the D.I. Ivanovsky Institute of Virology [11]. Epizootological monitoring of WNF in the region has been carried out on an ongoing basis since 1999, which was associated with the registration of the first epidemic outbreak here [8]. Summarizing the data obtained by scientific and practical institutions for the period from 1999 to 2023, the circulation of WNV in the epizootic cycle has been confirmed in 30 out of 39 (76.9%) administrative-territorial entities of the region: 27 districts and 3 towns of regional subordination, namely Volgograd, Volzhsky, and Uryupinsk. Positive findings on WNV in zooentomological material were detected annually (except for 2003) with an average frequency of 2.38±0.39%. Concurrently, the involvement of all links of the parasitic system, including accidental carriers, in the circulation of WNV was revealed. The average infection rate on WNV was 2.34±0.62% among birds, 2.03±1.24% among small mammals, 1.08±0.44% among ticks, and 2.83±0.61% among mosquitoes. The most intense manifestations of the epizootic process of WNF were detected in the city of Volgograd and the town of Volzhsky, as well as in the Gorodishchensky, Kamyshinsky, and Sredneakhtubinsky Districts. The detection rate of WNV markers was higher in periods of 2000–2002, 2006–2008, 2018–2019, and 2021–2022; herein, in years of incidence rises, a low activity of the epizootic process was noted.

The incidence of WNF in the Volgograd Region was not recorded only in 2003, 2004, 2015, 2017, and 2020 over the course of 25 years under observation. Cyclic increases in incidence were revealed with intervals of 1 to 7 years; epidemic outbreaks were identified in 1999, 2007, 2010, and 2012 [12]. After 2013, the intensity of the WNF epidemic process corresponded to a sporadic level, which might indicate the effect of random factors against the background of the recorded periodicity of incidence fluctuations. The average long-term incidence rate was 2.03 (95% CI: 0.35‒3.71) cases per 100 thousand population. In long-term dynamics, we identified a tendency towards a decrease in the number of registered patients (y = -0.1373x + 3.8172). The overall mortality rate was at the level of 4.3% (58 fatal outcomes, 95% CI: 3.23‒5.41). It is worth noting that a large number of scientific studies are devoted to the analysis of the epidemiological situation on WNF in the Volgograd Region [4][8][12–14]. In this regard, we highlighted only the features of the epidemic process and clinical course of WNF, including a significant contribution of such social groups as pensioners (33.2%; 95% CI: 29.78‒36.22) and employees (21.7%; 95% CI: 19.16‒24.84) to the incidence; the predominant infection of identified patients during their stay in summer cottages (38.3%; 95% CI: 34.9‒41.6), and a high detection rate of mild forms of WNF (22.1%; 95% CI: 19.21‒24.89). As for the territories, cases of WNF were diagnosed in 32 of 39 (82%) administrative units of the Volgograd Region [12]. The seasonality of the WNF epidemic process was also characterized by the maximal number of WNF cases revealed in August (58.6%; 95% CI: 55.97‒61.23).

Rostov Region. Manifestations of the epizootic process of WNF in the Rostov Region have been registered annually since 2001, with the exception of 2004, 2008, 2009, and 2018. For the period 2001‒2023, enzooticity on WNF was established in 25 out of 55 (45.5%) administrative-territorial entities of the Rostov Region. The overall detection rate of WNV markers amounted to 0.62±0.17%. Circulation of the pathogen has been identified in all components of the parasitic system in natural and anthropogenic foci of WNF [1][8][15]. According to long-term monitoring studies, the WNV infection rate was 1.13±0.56% in birds, 2.89±1.35% in small mammal populations, 0.62±0.18% in mosquitoes, 0.54±0.33% in ticks, and 2.5% in other blood-sucking arthropods (bloodsucker flies), once in 2021. The highest frequency of detection of positive findings from field material was observed in 2001, 2010, 2012, and 2022. An intensive epizootic process has been registered in the Don River delta (Azovsky, Myasnikovsky, and Neklinovsky Districts) [16], as well as in the Salsky, Remontnensky Districts, and the city of Rostov-on-Don.

The incidence of the population in the Rostov Region has been registered annually since 2000 [1][15], with the exception of 2002 and 2020. Cyclic increases in incidence were revealed in 2005, 2007, 2010, 2012, 2019, and 2023, that is, with an interval of 1 to 6 years. A general upward trend was found in the dynamics of the incidence of WNF (y = 0.0144x + 0.1649); the average long-term indicator amounted to 0.36 (95% CI: 0.14‒0.56) cases per 100 thousand population. In total, 5 cases of death associated with WNF were confirmed for the period 2000‒2023 (1.4%; 95% CI: 0.19‒2.61). Among the features of the WNF epidemic process, it is worth noting the low detection rate of cases among children, whose proportion in the morbidity structure was 1.0% (95% CI: 0.13‒2.13); active involvement of pensioners (29.5%; 95% CI: 23.64–34.36) and the unemployed population of working age (25.1%; 95% CI: 19.88–30.12) in the epidemic process; and a high frequency of registration of WNF with a mild clinical course (24.3%; 95% CI: 19.75‒28.91). Cases of WNF were registered in 33 (60%) administrative-territorial entities of the Rostov Region; the main areas of infection for patients with WNF were cities and towns (37.4%; 95% CI: 31.79‒43.01). In contrast to the Astrakhan and Volgograd Regions, in the Rostov Region, there was an even distribution of disease cases in August (46.6%; 95% CI: 41.6‒51.6) and September (45.0%; 95% CI: 40.01‒49.99) [12].

Krasnodar Territory. Circulation of WNV in the Krasnodar Territory was revealed in 1987 [8][17][18]. The most active investigation period on foci of WNF and other arbovirus infections in the Krasnodar Territory was the 1990s and early 2000s, but information on the conducted studies is limited in the available sources. Based on the monitoring results, contact with the WNF pathogen was confirmed in more than 30 species of birds, 8 species of rodents, cattle, as well as infection of mosquitoes with WNV in the Otradnensky, Temryuk, and Slavyansky Districts and the city of Sochi [17]. The available official data on the results of zoological and entomological monitoring for the period 2010–2023 indicated an extremely low detection rate of WNV markers (0.12±0.05%). Positive findings were identified in 2012, 2014–2017, 2019, 2021, and 2022 in the cities of Sochi, Novorossiysk, Goryachiy Klyuch, as well as in the Temryuk, Beloglinsky, and Vyselkovsky Districts. Thus, modern data on the circulation of WNV in the enzootic cycle were absent in the majority (84.9%) of the areas of Krasnodar Territory, namely in 45 of 53 administrative entities. During the period under analysis, the infection rate of WNV averaged 0.21±0.11% in mosquitoes and 0.08±0.01% in ticks. Other animal species were examined without positive findings.

The first cases of WNV in the Krasnodar Territory were confirmed retrospectively in 1999 (85 cases), and subsequently in 2000 and 2007, but they were not included in the federal statistical observation forms. Official registration of WNF cases in the Krasnodar Territory has been carried out since 2010 [8][18]. From that period to the present, mainly sporadic WNF incidence has been found, with the exception of 2 large epidemic outbreaks in 2019 (120 patients) and 2023 (93 cases, 13 deaths) [1]. It is noteworthy that during the years of the epidemic rise in morbidity, the epizootic activity of the focus was not detected or was recorded at a low level. The average long-term incidence rate of WNF in the Krasnodar Territory was 0.3 (95% CI: 0.11‒0.48) individuals per 100 thousand population. In the long-term aspect, a general trend of increasing incidence was established (y = 0.0632x ­– 127.07). Apparently, the frequency of contact of the population with the WNF pathogen in the Krasnodar Territory was significantly higher, as attested by the repeated registration of imported cases of WNF in other subjects of Russia. In particular, imported cases, associated with visiting the Krasnodar Territory, were officially reported in the following regions: Ulyanovsk (2009, 2012), Penza (2013), Lipetsk (2019), and Rostov Regions (2010, 2019), Stavropol Territory (2012), and Khanty-Mansi Autonomous Area – Yugra (2022 and 2023). The total number of deaths amounted to 13; the mortality rate was 5.5% (95% CI: 2.59‒8.38). Compared to other subjects of the SFD, the Krasnodar Territory had the highest proportion of WNF forms occurring with CNS injury (40.1%; 95% CI: 33.85‒46.32), and no cases with a mild clinical course were diagnosed here during the entire observation period. When analyzing the supposed areas of WNV infection, we noted an almost equal contribution of patients living in cities (33.8%; 95% CI: 27.73‒39.78) and staying in summer cottages (30.0%; 95% CI: 24.12‒35.79) in the incidence of the disease. The disease cases were identified in 23 (43.4%) administrative-territorial entities of the region.

Republic of Adygea. Information on the circulation of WNV in the Republic of Adygea was first obtained in the 1980s, simultaneously with its detection in the Krasnodar Territory. Until 2010, isolated cases of detection of pathogen markers were brought to light among the birds and small mammals, which are the major and minor carriers, respectively, in the Maykopsky District [17]. Systematic epizootological monitoring of WNF has been conducted since 2010. During the period under analysis, WNV markers were detected in zoological and entomological material only in 2010 and 2016, which does not allow assessing the intensity of the epizootic process. The detection rate of WNV markers averaged 0.01±0.008% including their indication among mosquitoes (0.05±0.03%) and large mammals (0.17±0.12%). Enzootic transmission of the WNV pathogen was established for 2 of 9 (22.2%) administrative-territorial units, namely in the Maykopsky and Giaginsky Districts.

Cases of WNF among the population as of the beginning of 2024 were officially registered only in 2012 (2 cases, 1 of which was fatal). Moreover, 2 cases of WNF importation from the Republic of Adygea to the area of Krasnodar Territory were confirmed in 2023. The average long-term incidence rate in the Republic of Adygea was 0.04 cases (95% CI: 0–0.08) per 100 thousand population, which was the lowest value among the subjects of the SFD. The disease cases were diagnosed in the town of Maykop and the Giaginsky District, where WNV circulation in the enzootic cycle was established. It is noteworthy that both identified cases of WNF were registered among children, while the high-risk group for the disease is people aged 60 years and older. The mortality rate, which reached 50%, differs significantly from the data presented in the literature. The average mortality among patients with WNF in mild, moderate, and severe forms was no more than 0.1%, while among cases with a severe course, it reached up to 20% [19][20]. All the above suggests that WNF patients were not being effectively identified.

The Republic of Crimea and the city of Sevastopol. Information on the manifestations of the epizootic process of WNF on the Crimean Peninsula is scarce. The first data on the epizootic activity of foci of the disease date back to 2018 [21]. Circulation of WNV was also confirmed in 2019 and 2022². To date, enzooticity for WNF has been established only in 4 of 25 (16%) administrative entities of Crimea, namely in the cities of Sevastopol and Feodosia, as well as in Belogorsky and Krasnoperekopsky Districts, the latter of which accounted for the largest proportion of WNV marker findings (70%). The overall detectability of WNV markers in zoological and entomological material amounted to 0.34±0.19%. All positive findings were detected only in mosquitoes; the infection rate was 0.54±0.30%.

The incidence of the disease among the population in these territories has been officially registered since 2018; however, according to literature data, the first case of WNF was described in 2013. Over the period 2018–2023, a total of 13 patients with WNF were identified in Crimea [1]; the average long-term indicator was 0.11 (95% CI: 0.02–0.20) cases per 100 thousand population. Nevertheless, the latent epidemic process was confirmed by repeated registration of imported cases of WNF in other constituent entities of the Russian Federation associated with visiting the Crimean Peninsula. In particular, in 2019, information was received from the Tula, Voronezh, and Kursk Regions, the Republics of Adygea and Mordovia, in 2022 from the Rostov Region, and in 2023 from the Omsk Region. Considering the short period of observation of the epidemiological situation, it is not possible to analyze the dynamics of morbidity. The structure of morbidity did not differ from that in the SFD taken as a whole. The territorial distribution of cases was characterized by the detection of patients in the southern part of the Crimean Peninsula, including the resort cities of Feodosia and Sevastopol.

Republic of Kalmykia. Circulation of WNV in the Republic of Kalmykia was confirmed in 2000 [6][8]. Manifestations of the epizootic process of WNF were registered annually, with the exception of 2007–2009 and 2021, with the highest frequency of detection of positive samples in 2006 and 2018. Enzootic transmission of WNV has been confirmed in 12 out of 14 (85.7%) administrative entities of the Republic of Kalmykia, excluding only the Yashaltinsky and Yustinsky Districts. The overall detection rate of WNV markers in field material collected in 2000–2023 was 4.39±1.17% including 5.08±1.72% in birds, 4.44±1.37% in mosquitoes, 1.75±0.51% in ticks, and 5.3% in small mammals, once in 2015.

However, along with such an intensive transmission of WNV in the foci, the incidence among the population was recorded at a sporadic level. In total, 5 patients with WNF were identified in the Republic of Kalmykia during the observation period, namely 1 case in 2010 and 2011 each, and 3 cases in 2012; the average long-term incidence rate was 0.12 (95% CI: 0.07–0.18) cases per 100 thousand population. Cases were diagnosed in the city of Elista (3 patients) and the town of Lagan (2 patients). Children predominated in the morbidity structure; individuals of the age group of 1–14 years amounted to 20.0% of all cases, and in the 15–19 years group, the indicator reached 40.0%. All identified cases had a moderate course of the disease. In addition, there is information about 1 imported case of WNF from the Republic of Kalmykia (Chernozemly District) to the Stavropol Territory in 2018.

Discussion

The generalized results of the analysis of the epidemiological and epizootological situation on WNF in the subjects of the SFD indicate that persistent foci of WNV circulation with a permanent risk of the infection of the population have been formed in the studied territories. The recorded nature of epizootic manifestations of WNV does not always reflect the real activity of foci of the disease. The circulation of WNV in the epizootic cycle has been confirmed almost annually in the Volgograd, Astrakhan, Rostov regions, and the Republic of Kalmykia. Apparently, the high detection rate of WNV markers in these regions is stipulated by the active and systematic participation of anti-plague institutions, including the Reference Center, in conducting the monitoring studies. However, in the first three subjects, both epizootic and epidemic processes of WNF were registered in these circumstances, while in the Republic of Kalmykia, with intensive circulation of WNV in natural and natural-anthropological foci, cases of human disease were not diagnosed. The predominant detection of positive WNV findings in certain territories of the subjects, including regional centers and adjacent areas, can probably be explained not only by the presence of the most favorable factors that contribute to the transmission of WNV but also by the high frequency of their examination.

Over a long-term observation period, isolated findings of WNV markers have been detected in the Krasnodar Territory, the Republics of Adygea, Crimea, and the city of Sevastopol; in this regard, it can be assumed that the biocenotic and spatial structures of WNV foci, as well as their epidemic potential, remain poorly studied. In our opinion, the low detection rate of WNV markers in these territories is not an objective indicator, since the landscape and natural-climatic conditions ensure the formation and functioning of persistent WNF foci. In this regard, it is unjustified to state the presence of more and less active WNF foci in the subjects of the SFD before conducting high-quality monitoring studies.

The main reasons for the low frequency of positive findings in zoological and entomological material or the lack of registration of signs of intensification of the epizootic process in years of epidemic morbidity rises may be stipulated by insufficient volumes of studies of zoological and entomological material, selection of non-target species of carriers and transmitters, disorders in temperature conditions during storage and transportation of material, etc. We have previously summarized the analysis data on indicators and factors affecting the quality and efficacy of zoological and entomological monitoring, as well as pathways to optimize it, using the example of the Volgograd Region [22].

Based on the manifestations of the epidemic process of WNF, the subjects of the SFD can be divided into three groups. The first group includes the Volgograd, Astrakhan, and Rostov Regions, which are characterized by the longest period of registration of morbidity, with almost annual detection of WNF cases. In the territories of the Astrakhan and Rostov Regions, the epidemic process has pronounced cyclical rises in morbidity. However, in the Volgograd Region, over the past 11 years, no regular changes in the dynamics of morbidity have been revealed, which, apparently, may indicate “shortcomings” of the epidemic outbreak detection due to a decrease in the quality of diagnostic work of medical specialists.

The Krasnodar Territory should be included in a separate group. The epidemic process in dynamics (2010–2023) was represented by only 2 epidemic outbreaks in the region, which occurred with a 3-year interval; in the remaining years, disease cases were not diagnosed or the incidence was recorded at a sporadic level. It must therefore be concluded that a longer observation period is required to assess the cyclical nature of the manifestations of the epidemic process in this territory.

In the long-term aspect, the general trend of growth in the number of cases has been established in the Krasnodar Territory and Rostov Region, while in the Astrakhan and Volgograd Regions, a decrease in the number of registered patients was revealed. This circumstance, while maintaining the epidemiological tension of natural foci, indicates insufficiently effective monitoring of the epidemiological situation. The age, gender, and social structure of cases in all the above-mentioned entities have similar features and are characterized by the predominance of males, city residents, and older age groups among the identified cases. The clinical picture of WNF is dominated by cases of the flu-like variant of the disease course and moderate form. The observed individual differences in the structure of morbidity or clinical manifestations of WNF are associated with both the peculiarities of the organization of the epidemiological surveillance system (in particular, the ability of medical specialists to detect mild forms of the disease) and the influence of territorial social factors (lifestyle of the population, etc.).

The third group of subjects of the SFD includes the Republics of Kalmykia, Adygea, Crimea, and the city of Sevastopol, where only isolated disease cases were identified, which does not allow assessing the dynamics and structure of the incidence of WNF at present. In order to obtain objective data on the manifestations of the epidemic process of WNF in these areas, it is necessary to ensure increased readiness of health care bodies and institutions for the prompt detection and diagnosis of WNF patients, as well as to strengthen the control function of the Rospotrebnadzor Directorates in these subjects of the Russian Federation.

Conclusion

The obtained results can be used in planning epizootological surveillance in the subjects of the SFD with the inclusion in the plan of monitoring studies of territories with an undetermined circulation of WNV in the epizootic cycle, conducting the zoning of territories according to the risk of infecting with WNV, optimizing approaches to identifying cases of diseases, developing and implementing management decisions.