Naturally occurring infections (NOIs) are diseases that affect both humans and animals. The pathogens are able to persist for long periods under certain climatic conditions within a limited geographical system, forming a natural reservoir of infection.

The prevalence and often severe clinical course of NOIs make them a major concern. It should be noted that the current sanitary rules and regulations 3.3686-21 Sanitary and Epidemiological Requirements for the Prevention of Infectious Diseases consider some NOIs to be particularly dangerous infections, which require increased attention from epidemiologists and infectious disease specialists.

NOIs are monitored locally, regionally, and nationally to respond quickly to epidemiological challenges [1][2][3][4][5].

The Russian Federal Service for the Oversight of Consumer Protection and Welfare (Rospotrebnadzor) maintains constant surveillance of infectious diseases such as plague, tularemia, hemorrhagic fever with renal syndrome (HFRS), Crimean-Congo hemorrhagic fever (CCHF), West Nile fever (WNF), Ixodes tick-borne borreliosis (ITBB), rabies, etc. [6][7][8][9][10][11][12].

At present, tularemia, WNF, HFRS, and coxiellosis are endemic in all districts of the Volgograd Region, while CCHF and ITBB are endemic in 25 and 11 districts, respectively [13]. The incidence of NOIs in the population has been sporadic for a long time and is consistent with national data, with the exception of WNF and CCHF [14].

The aim of the study was to analyze the epizootologic and epidemiologic impact of NOIs (plague, tularemia, HFRS, WNF, CCHF, ITBB, coxiellosis, leptospirosis, and rabies) in the Volgograd Region in the first 10 months of 2023.

The study was based on the official statistics reported by the Volgograd Department of the Federal Service for the Oversight of Consumer Protection and Welfare, the data reviews provided by the Volgograd Center of Hygiene and Epidemiology, and the 2023 NOI reports published by the Elista and Astrakhan Plague Control Stations. The data were analyzed using the epidemiological method.

The analysis of the epizootologic and epidemiologic patterns of NOIs in 2023 revealed a varying activity of natural foci in the Volgograd Region.

The status of carrier and vector populations is an essential measure to assess the up-to-date epidemiological scenario.

The Volgograd Center of Hygiene and Epidemiology has confirmed that the number of small mammals has risen to 5.6 per 100 trap nights in the winter and spring period of 2023. This is above the number recorded in the same period of the previous year (4.7), but below the long-term annual average of 6.9. There was a 2.8-fold decrease in the number of small mammals in winter. The mean number of small mammals in the summer and autumn of 2023 was 11.8 per 100 trap days, a reduction from the previous year (15.6) and below the long-term annual average of 11.9 (Fig. 1).

The most prevalent species remains common field mice, accounting for 36.0% of the total in 2023 vs. 41.2% recorded in the previous year. House mice, the second most prevalent species, have seen a notable increase, rising from 12.9% in 2022 to 25.8% in 2023. Noteworthy is a 3.6% increase in the proportion of common voles and a concomitant decrease in the proportions of field mice by 10.4%, yellow-necked mice by 2.3%, and red-backed mice by 2.2% when compared to the previous year (Fig. 2).

The seasonal activity of Ixodes ticks was recorded in natural biotopes in the central Volgograd Region on March 13, 2023 (compared to April 1, 2022). The mean number of Ixodes ticks in the natural environment per flag was higher than over the previous year, reaching 21.4 ticks per 1 flag hour (in comparison to 16.0 ticks per 1 flag hour in 2022). The results of the animal surveys indicated that the abundance index was 6.5 Ixodes ticks per 1 host animal. In the first half of 2023, there was a 1.8- and 1.3-fold increase in the number of ticks for animals and natural biotopes, respectively, in comparison to the previous year. In the central and southern areas, there has been a notable increase in the Rh. rossicus dominance index, which was likely attributable to the rise in the number of feeders observed last year. The primary carriers of the CCHF virus, H. marginatum, were identified not only in the southern districts but also in two northern ones (Rudnyansky and Zhirnovsky).

In the autumn period, the number of ticks was found to be significantly lower than in the previous year (4.1 vs. 45.3 in 2022). This was due to a sharp decrease in the number of Dermacentor ticks. The animal testing demonstrated a reduction in the abundance index of ticks to 2.7 (in comparison to 3.0 in the previous year).

The first generation of adult mosquitoes was observed in the second decade of May 2023. Their peak number was observed on May 15, 2023, a month earlier than in 2022 (June 13, 2022). The 20-minute observation yielded 1,024 imagos (798 in the previous year). The emergence was considerable in number, influenced by an early spring season with high temperatures and the favorable warming of coastal waters.

Two natural plague foci are partially located on the territory of the Volgograd Region. These include the Caspian Northwest steppe zone in the Oktyabrsky and Svetloyarsky Districts and the Volga-Ural steppe zone in the Leninsky and Pallasovsky Districts. For a considerable period, natural foci of plague epizootics have not been documented in the Volgograd Region. Monitoring conducted by the Elista and Astrakhan Plague Control Stations in the focal territories of the Volgograd Region indicates that the number of little ground squirrels, the primary vector for plague transmission, is low at less than 2.8 animals per 1 ha. Furthermore, plague epizootics in the 2023 season have not been identified.

The Volgograd Region is endemic for tularemia, with all 33 districts and the cities of Volgograd and Volzhsky reporting cases. Over the years, the proportion of positive tests for environmental objects exhibited considerable variability. The proportion of positive tularemia markers in 2023 (19%) was lower than that observed in 2022 (22%).

It is noteworthy that no cases of tularemia have been reported in the region since 2008. This is attributed, at least in part, to the efficacy of targeted infection prevention measures. In 2023, 21,701 individuals were vaccinated against tularemia, representing 87.44% of the planned target population.

Given the endemicity of tularemia in the region and the prevalence of carriers and vectors of infection, there is the potential for local epizootics of tularemia in areas where small mammals congregate, as well as the possibility of sporadic human morbidity.

The incidence of HFRS in the Volgograd Region is relatively low, placing it among regions with a low incidence of the disease. Nevertheless, there has been a slight increase in the incidence of HFRS from one case in 2021 to four cases in 2022. Of these, three cases were reported in the Rudnyansky District and one case in the Zhirnovsky District.

In 2023, six cases of HFRS were identified, including two imported cases. The remaining four cases occurred in different districts of the region and were not epidemiologically linked. All patients with HFRS were adults between the ages of 19 and 55. The majority of them were males (67%).

Two individuals were infected during open-air recreation. In one case, the individual infected was a resident of a private house with a rodent infestation, while the location of the infection in the remaining case could not be determined. There is no clear seasonal pattern to the occurrence of infections, as infections occurred throughout three seasons, including spring, summer, and winter. All patients were admitted to infectious disease hospitals with symptoms of moderate severity. The clinical diagnosis was subsequently corroborated by laboratory tests.

The epidemiological status of HFRS in the Volgograd Region is relatively stable. In 2023, the proportion of positive samples from small mammals, the primary carriers of hantaviruses, decreased compared to the previous year (from 3.4% in 2022 to 1.8% in 2023).

At screening, 4.5% of samples from the risk group (agricultural workers from four districts within the region) tested positive for antibodies, representing a 0.5% increase from the previous year (4.0%).

The rising proportion of seropositive samples and the increasing incidence of HFRS in recent years indicate the need for additional measures, including a reduction in the number of small mammals in natural foci.

A total of 33 districts in the Volgograd Region have been identified as current foci of coxiellosis, with cases of the disease having been reported since 1957. The natural reservoirs of Coxiella burnetii are small warm-blooded animals, primarily rodents. Anthropogenic reservoirs include domestic animals, such as cattle and small cattle. The vectors are Ixodes ticks.

The epidemiological status of coxiellosis in the Volgograd Region remains relatively stable. Over the past five years, two cases of the disease have been documented (one each in 2019 and 2022).

In 2023, the proportion of positive samples among Coxiella carrier and vector populations remained almost unchanged in comparison to the previous year (from 8.3% in 2022 to 8.5% in 2023).

A total of two hundred livestock workers from four districts of the region were screened, with 33 blood serum samples (16.5%) tested positive for anti-Coxiella burnetii antibodies. This represented a significantly higher prevalence compared to the previous year (2.4%). This evidence suggests that cases of coxiellosis may have been unreported. Over the 10-month period of 2023, no cases of coxiellosis were identified.

Given the relatively high percentage of positive samples among carriers and vectors of coxiellosis in the Volgograd Region, sporadic human cases cannot be ruled out.

The natural foci and initial cases of ITBB in the Volgograd Region were identified in 1999. At present, 11 administrative areas are known to be endemic for ITBB.

In the analysis of environmental objects in 2023, the proportion of positive samples was 0.6%.

A screening of 200 residents from four districts of the region revealed that eight individuals exhibited positive anti-Borrelia burgdorferi antibodies in their blood serum samples (4.0% vs. 0.9% in the previous year).

The annual epidemiological analysis of ITBB indicates a prevalence of sporadic cases. In 2020 and 2021, a total of two cases of ITBB were reported. In 2022, there was a further increase in the number of patients, with a total of four cases (including two cases imported from other regions).

Within the 10-month period of 2023, a total of 10 individuals were diagnosed with ITBB in the region. Three cases of infection occurred outside the region, while the remaining seven were identified in the districts within the region. The local cases were related to open-air recreation, with three cases in the suburban area and one case in a country house. The occurrence of imported cases was observed during open-air recreation activities, with children aged under 18 years accounting for 50% (5 cases) of all cases. Three patients were adults aged between 18 and 60 years of age; the remaining two individuals were over 60.

The majority of patients were treated as outpatients (six individuals). Of the remaining patients, four received inpatient care in infectious diseases hospitals.

The first case of ITBB was identified in April. A total of seven cases of ITBB were diagnosed in May, while one case was identified in each of June and August. All patients had a history of tick bite. Two patients exhibited typical annular erythema, while the remaining patients demonstrated hyperemia and tenderness at the site of the tick bite. All clinical cases of ITBB were confirmed by positive laboratory tests.

The number of cases of ITBB observed in 2023 has increased significantly, necessitating the implementation of additional measures to reduce the number of Ixodes ticks, the primary vector of Borrelia burgdorferi, in endemic territories.

In the 2023 epidemic season in Russia, there was a notable increase in the incidence of WNF cases. A total of 209 cases were identified, representing a 6.3-fold increase in incidence compared to the previous epidemic season (in 2022, 33 patients with WNF were identified).

In 2023, there was an increase in the number of WNF cases in the Volgograd Region, as compared to previous years.

In 2023, a total of 1,039 individuals were tested for anti-West Nile virus (WNV) IgG antibodies. A total of 222 cases were positive, representing a prevalence of 21.4% compared to 15.9% in 2022. These findings confirm the widespread prevalence of WNF among the population of the region, with a significant proportion of unreported cases.

A total of 55 cases of the disease were identified among the region's population over the past 5 years. These included 28 cases in 2018, 12 cases in 2019, no cases in 2020, 13 cases in 2021, and 1 case in 2022.

In 2023, 18 cases of WNF were confirmed. A total of 56% of cases of infection occurred in country houses (10 cases), while three individuals became infected on a private farmstead in rural areas, three more in a recreational area, and two at their place of residence in cities. The majority of infected patients were individuals over the age of 60 (nine patients) and adults under the age of 60 (eight patients). One patient was a 15-year-old adolescent. The most of them were males (67%). Fifteen patients exhibited moderate symptoms, while three cases demonstrated severe disease. Nine patients experienced neuroinvasive symptoms, including two cases of meningoencephalitis and seven cases of meningitis. A total of 17 patients were admitted to hospital, and one patient received outpatient treatment. One case of West Nile virus meningoencephalitis was fatal. A total of 16 out of 18 cases of infection were observed in August, which coincides with the peak activity of the primary WNV vectors (Cx. pipiens mosquitoes). In two cases, the onset of the disease occurred in July and September.

A notable proportion of cases classified as moderate to severe may be attributable to a low detection rate of WNF of mild severity.

In 2023, the sample testing of the primary WNV vectors revealed that 3.0% of the samples were positive for WNF RNA, which was slightly lower than in 2022 (3.4%).

A comprehensive molecular genetic analysis demonstrated that the WNV isolates obtained from bird, mosquito, and tick samples exhibited genotype 1, whereas the samples derived from lake frogs demonstrated genotype 4.

In the Volgograd Region, Crimean-Congo hemorrhagic fever has been the most prevalent tick-borne infection since the outbreak of 2000. The current epidemiological situation in the region reveals that 25 districts have been identified as endemic for CCHF, while the focal boundaries are gradually shifting to the north.

In 2022, 2 cases of CCHF were reported in the southern Volgograd Region, where H. marginatum ticks are the primary carriers of the CCHF virus. In 2023, no CCHF cases were reported in the Volgograd Region.

The proportion of positive samples among CCHF virus carriers and vectors was 2.8% in 2023.

In 2023, the Reference Center for CCHF Monitoring (the Stavropol Scientific Research Plague Control Institute of the Federal Service for the Oversight of Consumer Protection and Welfare) conducted CCHF RNA genotyping of tick pool homogenates collected in 2022 in the Volgograd Region. The isolates were identified as the Va subgroup (Stavropol-Rostov-Astrakhan) and Vb subgroup (Volgograd-Rostov-Stavropol) of the Europa-1 gene variant, which is characteristic of the Volgograd region.

For many years, the Volgograd Region has demonstrated a relatively low incidence of leptospirosis. Furthermore, both agricultural and natural foci of infection were documented.

The systematic and comprehensive implementation of epidemic control and preventive measures has enabled the significant improvement of the number of economic (anthropogenic) foci over the past decade. Moreover, the incidence of leptospirosis infection among the population living in the enzootic territory has been successfully eliminated.

The most recent case of leptospirosis was diagnosed in 2019. The screening testing of environmental objects revealed low activity of natural foci of infection. In 2023, 1.1% of samples obtained from leptospira carriers and vectors were found to be positive.

A total of 200 individuals (livestock breeders and veterinary workers) from four districts of the region were tested for antibodies to the causative agent of leptospirosis. Only two individuals (1.0%) were found to be positive. In 2023, there were no reported cases of leptospirosis, as there had been in the previous three years.

Following the challenging epidemiological situation in the Volgograd Region regarding rabies in 2021, with two cases of human disease out of six in Russia, the situation improved significantly in 2022. The number of laboratory-confirmed cases of animal rabies decreased to 11, and no human cases were reported.

In the first 10 months of 2023, 22 cases of laboratory-confirmed rabies among animals were identified (compared to 11 cases in the previous year). These cases involved dogs (8), followed by cats (4), cattle (1), small cattle (3), horses (2), foxes (3), and wolves (1).

In the Volgograd Region, a total of 9,876 individuals (vs. 8,516 in the previous year) sought medical assistance for animal attacks (bites and salivation), including 278 individuals who were victims of wild animal attacks (vs. 336 in the previous year). In 2023, no cases of rabies were reported.

The epidemiological situation with regard to rabies in the region remains challenging and requires a comprehensive approach to reduce the potential danger of human infection from sick animals.

It is important to highlight the ongoing efforts to prevent NOIs. In 2023, disinsection treatments were conducted on various objects in the Volgograd Region, including water bodies (2,814.60 ha), coastal vegetation (1,165.00 ha), mass-attendance places (96.658 ha), children's summer camps (197.3475 ha), and industrial facilities (1,867.10 m²). Acaricide treatments covered 259.3916 ha of mass-attendance places, 654.0675 ha of children's summer camps, and 12.9811 ha of kindergartens. The total deratization area was 104.31 thousand m² of indoor spaces in children's summer camps, 130.725 ha of open area, and 59.77778 ha in mass-attendance places.

In 2023, epidemiological data indicated a relatively stable prevalence of NOIs in the Volgograd Region. The number of cases and infection rates among the primary carriers and vectors of NOIs were comparable to those observed in the previous year. There has been an increase in the incidence of West Nile fever, Ixodes tick-borne borreliosis, and hemorrhagic fever with renal syndrome. However, no cases of plague, tularemia, coxiellosis, leptospirosis, Crimean-Congo hemorrhagic fever, and rabies have been reported.

The epidemiological analysis of the Volgograd Region in 2023 revealed the continued presence of various degrees of activity of natural foci of dangerous infectious diseases. This necessitates the implementation of a constant monitoring program and the development of an appropriate response strategy to address any changes that may arise, to prevent the potential complications of the NOIs-related epidemiological situation.