One of the key priorities of the Federal State Healthcare Institution Rostov-on-Don Anti-Plague Institute and the Federal State Healthcare Institution North Caucasian Anti-Plague Station of Rospotrebnadzor, which became part of the Rostov Anti-Plague Institute in April 2021, is epizootological monitoring within the Rostov Region natural foci of dangerous infectious diseases to identify epizootic activity and the subsequent organization of several activities to prevent the spread of these diseases among the regional population.

Zoonotic diseases are a group of infectious diseases common to humans and animals with evolutionary foci in nature. A hot (autonomous) spot of an infectious disease is a geographically or ecologically localized area within which the infectious agent circulates under current conditions without introducing it from the outside for an indefinite period (dozens of epizootic cycles following one another). Each natural focus is characterized by a historically developed parasitic system (biocenosis, in which the infectious agent is naturally built), including the pathogen, hosts (warm-blooded animals), carriers (blood-sucking arthropods). When a person appears in natural foci during epizootic activity, contact with the source or via carriers may result in human infection with a naturally occurring infection whose pathogen circulates in the natural foci1 2.

Within the Rostov Region, taking into account natural and climatic factors, landscape, and geographical zones, there are areas where persistent natural foci of such hazardous infectious diseases as tularemia, West Nile fever (WNF), Crimean hemorrhagic fever (CHF), ixodic Lyme borreliosis (ILB), and some others were formed. The presence of a natural plague spot within two districts of the region should be particularly noted.

The study purpose is to analyze the results of epizootological monitoring within the Rostov region to identify the epizootic activity of natural foci of dangerous infectious diseases common to humans and animals and assess of epidemic manifestations in such outbreak as part of epidemiological surveillance of these infections.

Epizootological monitoring of natural foci for identifying epizootic activity included epizootological examination of natural foci with catching living small mammals which are carriers of dangerous infection agents and blood-sucking arthropods (carriers of infection agents) with subsequent determination of species composition. The species, sex, approximate age, and generative state were determined in captured small mammals. The obtained biological material (samples of visceras) was sent to the bacteriological laboratory to test the infectious agents. Focus epizootological examination and laboratory analysis of the obtained material was performed strictly under the current regulatory documents.

Monitoring to search for infectious agent antigen were performed by ELISA and PCR.  

Given the fact that an essential aspect of comprehensive epidemiological surveillance and a criterion for assessing the status of natural infection focus is the screening study of blood sera of healthy people living within natural foci, immunological and serological monitoring (examination of blood sera of healthy donors) was selectively conducted to detect antibodies to pathogens of natural foci of infectious diseases.

Screening of donor sera for the specific antibodies was performed using an ELISA.

It presents data on epizootological monitoring of natural foci of dangerous infectious diseases common to humans and animals within the Rostov Region and a brief description of some infections of the highest epidemiological significance.

Plague is a zoonotic natural focal dangerous bacterial infectious disease with predominantly transmission of the pathogen. Each case of human plague is grounds for declaring an emergency in the sanitary and epidemiological well-being of the population of the Russian Federation [1]. At present, plague remains one of the most significant epidemic threats in the world, capable of causing a public health epidemiological emergency3.

For many years, specialists of the Federal Public Health Institution North Caucasian Anti-Plague Station of Rospotrebnadzor have carried out plague epizootological monitoring within the Pre-Caspian Northwest Natural Steppe plague-spot located in the central and eastern parts of the Rostov Region in the Remontnensky District.

The natural plague spot on the territory of the Remontnensky District, assigned to the North Caucasus Plague Control Station, was 12 sectors of primary accounting (1200 km2). 

In their work, zoologists considered the differentiation of territories according to the level of potential epidemic danger4.

In the Caspian Northwest Natural Steppe Plague Spot, the main carrier of plague is a little ground squirrel (Citellus pygmaeus). Within the spot, 14 species of fleas were recorded in the little ground squirrel microbiotope. Its specific parasites are Neopsilla setosa, Citellophilus tesquorum, Frontopsilla setosa, Ctenophthalmus pollex.

In the second half of the 20th century, areas of virgin and fallow lands within the focus steppe part in the Salsko-Donsky steppe were plowed, which led to the deterioration of the little ground squirrel habitat and sharply reduced the epizootic potential of this territory5.

At present, the natural plague spot within the Remontnensky District is in a deep inter-epizootic period.

In 2020, an epizootological survey and collection of field material were conducted in the Remontnensky District. The results of biological material research were negative, which indicates the absence of epizootic activity of the natural focus. It can be assumed that epizootic manifestations of plague are unlikely in 2021-20225.

Tularemia is a natural focal bacterial infection with a febrile syndrome, specific lymphadenitis, and polymorphic manifestations due to the site of entry.

With a large number of natural foci of tularemia in the Russian Federation, there is a relative epidemiological safety for this infection, which has recently developed as a result of specific prophylaxis for the population living within enzootic areas [2][3].     

In Rostov Region, 37 of 43 administrative districts are enzootic for tularemia.

In the last 3 years (2018-2020), no cases of tularemia were registered among the region's population.

From 2013 to 2020, more than 9000 rodents and ixodic ticks were tested by laboratory methods for the presence of tularemia pathogen.

Six strains of tularemia pathogen were isolated from rodents by biological sampling during monitoring of natural tularemia foci in 2020. It was shown that all strains are typical representatives of Francisella tularensis subsp. holarctica biovar EryR according to their biological properties and the data of PCR-analysis and INDEL typing by canonical markers. The strain isolated in 2020 in the natural focus of the Salsky region was identical to the strain isolated in the same area in 1989. Based on whole-genome sequencing of the two strains, it was found that they were most similar to the cultures isolated in Turkey (2009, 2012) and Khanty-Mansiisk (2013) by the studied set of SNP-markers. Thus, it is found that both identical (or closely related) clones of the tularemia pathogen can circulate for a long time in natural foci of the Rostov region, and new strains with unique genotypes not previously described for the Rostov Region can appear [4].

Epizootological monitoring data testify to epizootic activity in tularemia foci, which is confirmed by detecting tularemia pathogen markers in biological material from small mammals in Salsky (2012), Orlovsky (2013, 2020), Remontny (2019, 2020), Egorlyksky (2019), and Sholokhovsky (2020) District.

The high landscape-ecological similarity of natural foci of tularemia and ixodic tick-borreliosis located in the border territories of Rostov Region on the one hand and in Donetsk and Lugansk regions of Ukraine, on the other hand, allows us to speak about actually common transboundary foci of these bacterial infections. Since the state border of the Russian Federation and Ukraine is not impossible for the movement of rodents and insectivores (carriers of tularemia and ixodic tick-borreliosis pathogens) across it, the introduction of these pathogens into the border areas of Rostov Region from the territory of the neighboring state during migration of small mammals is a real threat that requires constant epizootological monitoring of these areas [5].

Considering the monitoring results, we can confidently assume that immunoprophylaxis of the region's population provides a real opportunity to prevent tularemia among people living in enzootic areas.  

Crimean-Congo hemorrhagic fever (CCHF) is a zoonotic natural focal arbovirus infectious disease, the germ of which is transmitted to humans by blood-sucking arthropods (ticks). CCHF belongs to the group of tick-borne hemorrhagic fevers and is a particularly dangerous infectious disease with a mortality rate of 10-40%.

According to specialists of the Stavropol Anti-Plague Institute-Research Center for monitoring CCHF, the Russian Federation has a large natural focus of CCHF, which occupies a vast territory in the south of European Russia, including a large part of the Rostov region [6]. In the region, Aksaisky, Belokalitvinsky, Volgodonsky, Dubovsky, Egorlyksky, Zavetinsky, Zimovnikovsky, Kamensky, Konstantinovsky, Krasnosulinsky, Millerovsky, Morozovsky, Myasnikovsky, Oblivsky, Oktyabrsky, Orlovsky, Peschanokopsky, Proletarsky, Remontnensky, Salsky, Semikarakorsky, Tarasovsky, Tatsinsky, Ust-Donetsky, Tselinsky, and Tsimlyansky District is enzootic for CCHF.

In recent years, the incidence of CCHF has been registered among the population of the region. In particular, in 2019, there will be 48 cases of CCHF (incidence rate per 100,000 people — 1.14), in 2020 — 16 cases of CCHF (incidence rate 0.38), without lethal outcomes.

During the monitoring, markers of the CCHF pathogen were detected in samples obtained from small mammals caught in the following districts: Remontnensky (2012, 2013, 2016, 2020), Orlovsky (2012, 2015, 2017, 2018, 2019), Salsky (2014), Zimovnikovsky (2018).

Antibodies to the CCHF virus were detected in the blood of healthy donors living in the Oktyabrsky, Salsky, Kamensk-Shakhtinsky District, and Taganrog (2012).

Thus, the results obtained testify to the significant activity of CCHF natural foci.  

West Nile fever (WNF) is an acute viral zooanthroponotic natural focal disease with a transmission mechanism characterized by polyadenopathy, erythema, and meningial inflammation, occurring against a background of febrile intoxication syndrome.

According to the European Center for Disease Prevention and Control, WNF is currently one of the topical natural focal infections for some countries [7]. West Nile virus (WN) has a high epidemic potential and can cause outbreaks both in hot tropical countries and in temperate latitudes, including some regions of the Russian Federation.

Considering natural and climatic factors, landscape, and geographic zones on the territory of Rostov Region, there are ecosystems favorable for the formation of persistent natural foci of WNF, namely river floodplains and deltas, frequently flooded areas where wild migratory waterfowl nest — the main virus reservoirs in nature. Ornithophilic mosquitoes, carriers of the WNF virus, are also widespread in the region.

Considering all the facts mentioned above, zoologists have been conducting zoological-entomological and immuno-serological monitoring in the territory of natural WNF foci from 2009 up to the present time.

During the past ten years (2011 to 2020), more than 9,500 samples of field material have been examined. These are mosquitoes, ixodic mites, the brain tissue of waterfowl, and organ suspensions of small rodents obtained in suspected natural foci of WNF.

Monitoring studies revealed markers of WN virus in suspension samples of small rodents harvested in open stations in Remontnensky, Orlovsky, Proletarsky, Zimovnikovsky, Zavetinsky, Matveevo-Kurganovsky District, and in Rostov-on-Don.

Given the fact that an essential aspect of comprehensive epidemiological surveillance and a criterion for assessing the status of arbovirus natural infection focus is the screening study of blood sera of healthy people living within natural foci; since 2008, laboratory testing of blood sera from healthy donors for antibodies to the ST virus has been conducted in Salsky, Oktyabrsky, Kamensky District, and Taganrog.

During the observation period, positive findings were found throughout the monitoring period, with positive findings ranging from 0.3% (2016) to 8.5% (2014), with the highest percentage of findings detected among donors in the Salsky District.

The detection of anamnestic antibodies indicates contact of the population with the pathogen and, as a result, previously suffered from the disease asymptomatic or mild form.

Detection of WN virus antigens in samples of small rodents and specific antibodies in donors' blood confirms the presence of natural foci of WNF with pronounced activity, where the contact of the population with the pathogen takes place.  

Ixodic tick-borreliosis (ITB) (systemic tick-borreliosis, Lyme disease, Lyme borreliosis) are natural focal transmissible polyethiological diseases with predominant lesions of the skin, nervous system, cardiovascular system, and musculoskeletal system caused by spirochaetes and transmitted by ticks. ITB leads to morbidity and socio-economic damage among transmissible natural focal infections in Russia [8]. They are also a current problem for the Rostov region [9].

In 2018, in the Rostov region, among the population, there will be 30 cases of ITB (incidence rate per 100,000 people — 0.71), in 2019 — 20 cases of ITB (incidence rate 100), without lethal outcomes. In 2020, there was no incidence of ITB.

According to the monitoring data, the pathogen markers were detected in ticks caught within the territory of Ust'-Donetsky District (1918) and Rostov-on-Don (2020).

Antibodies to the ITB pathogen were detected in healthy donors living in Salsk (2015).

Monitoring results showed the presence of natural ITB foci.

Hemorrhagic fever with renal syndrome (HRSF) is an acute viral naturally occurring zoonotic infection with intoxication syndrome, development of universal Schonlein purpura with hemorrhagic manifestations and kidney damage [6]. In Russia, endemic areas are the Far East, Eastern Siberia, Transbaikalia, and the European territory, where the infection has persistent natural foci. In some periods (2009-2018) in the Russian Federation, HRSF accounted for up to 90% of natural focal viral infectious diseases [10].

In the Rostov Region, there are isolated cases of HRSF among the population. Thus, 2 cases of HRSF were detected in 2019 (rate per 100,000 population – 0.05) (Rostov-on-Don, Myasnikovsky District), no cases of HRSF were registered in 2020.

As the monitoring results showed, the antigen of HRSF pathogen was detected from samples of biological material of small mammals in Orlovsky District in 2019, which confirms the circulation of HRSF pathogen in the specified administrative territory.

Separately, it is necessary to highlight data on the findings of markers of infectious agents in natural foci of infectious diseases, which were not registered among the population. Such natural foci, taking into account their epizootic activity, pose a specific potential epidemic danger to the region's population.

According to the results of epizootological monitoring, in natural habitats were identified markers of causative agents of Batay fever, Sindbis fever, Q fever and fever caused by viruses of the California encephalitis serogroup (Inko, Tahyna, and snowshoe hare virus).

Markers of the Batai fever pathogen were detected in biological material obtained from small mammals in Orlovsky (2013) and Remontnensky District (2019) and from mosquitoes in Aksai District (2017).

According to the results of epizootological monitoring, Sindbis fever pathogen antigen detections were found in biological material from small mammals in Zavetinsky, Remontnensky, Rodionovo-Nesvetaysky District (2016), Aksaysky, Zimovnikovsky, Matveevo-Kurgansky, Rodionovo-Nesvetaysky District (2017), Remontnensky district (2018), Ust-Donetsky, Orlovsky District (2020). In 2016, the antigen was detected in blood-sucking arthropods caught in Salsky District.

The monitoring managed to detect the antigen of the Q fever pathogen in biological material obtained from small mammals in Remontnensky (2017) and Sholokhovsky District (2020), as well as from samples of ixodic ticks caught in Azovsky District (2017).

Antigens of California encephalitis serogroup viruses (Tahyna, Inko, and snowshoe hare virus) were detected in biological material from small mammals in Orlovsky and Remontninsky District (2019) and Matveevo-Kurgansky and Orlovsky District (2020). 

Information about all detected positive results confirming the epizootic activity of natural foci of infectious diseases was sent to Rospotrebnadzor institutions to organize and conduct a set of anti-epidemic and preventive measures.

These studies indicate the presence of natural foci of dangerous infectious diseases common to humans and animals on the territory of the Rostov Region. At the same time, if in recent years the natural plague focus has been in a depressive state and shows no activity, then in foci of tularemia, WNF, CCHF, ITB, HRSF, Q fever, Batay, and Sindbis fevers, viruses of California encephalitis serogroup (Tahyna, Inko, and snowshoe hare virus) markers of these pathogens were revealed, which confirms the assumption of epizootic activity of these foci. Epizootic activity resulted in epidemic manifestations in natural foci, where patients with WNF, CCHF, ITB, and HRSF were registered.

The results of epizootological monitoring emphasize the relevance and the need to continue these studies in natural foci of dangerous infectious diseases common to humans and animals within the Rostov Region, expanding the monitoring area.

The team of authors expresses sincere gratitude to Yuri Georgievich Kireev, a candidate of medical sciences who headed for many years (until October 2020) the Federal State Institution of Health Care North Caucasus Plague Control Station of Rospotrebnadzor, for help in collecting and summarizing materials and scientific advice in preparing this paper.

1. Kucheruk V.V. Current thinking on the prospects for developing the natural-focal infections doctrine // Methodological. aspects of program-targeted planning of scientific research in the infectious pathology scope. 1981. Moscow., P. 46–54. 2. Kucheruk V.V., Rositsky B. Natural focality of infections – basic terms and concepts // Med. parasitology and parasitic diseases. 1984. No. 2. P. 7–16. 3. Guidelines MU 3.1.3.2355-08 Organization and Implementation of Epidemiological Surveillance in Natural Plague Spot in the Russian Federation. 4. Cadastre of epidemic and epizootic manifestations of plague in the territory of the Russian Federation and neighboring countries (1876 to 2016) / Edited by Academician of RAS V. V. Kutyrev, Professor A.Yu. Popova. – Saratov: OOO Amirite, 2016. 248 p. 5. Kireev Yu.G., Balakhnova V.V., Kuznetsov M.V. et al. Current epizootological features of the Precaspian Northwestern steppe plague spot (within Rostov Region). Major aspects in the study of hazardous and natural focal diseases. Collected papers of the research and practical conference. Novosibirsk, 2019. P 255-258.