Introduction. According to the modern understanding, trematodes are parasitic organisms that had appeared in the Cambrian period and currently consist of more than 2 700 nominal genera and approximately 18 000 species [1] and parasitize in fish, amphibians, reptiles, birds, mammals, and humans. Most of them are characterised by dixenous or trixenous life cycles, and involve gastropods as intermediate hosts. Many of them are the causative agents of diseases in agricultural and domestic animals, as well as in humans, and, therefore, are being actively and comprehensively studied. Particular attention is usually paid to such diseases as opisthorchiasis, fascioliasis, clonorchiasis, paragonimiasis, schistosomiasis, etc., however, some infestations remain insufficiently studied.

One of such poorly studied infestations are notocotylidoses — the trematodiases in animals caused by parasitism of the representatives of the family Notocotylidae, which are registered in the intestines of many bird species, as well as in some mammals [2–6]. From the economic point of view, notocotylidoses cause significant damage to poultry reared at the farms and to some species of game birds. Parasitic worms cause mechanical damage to the intestinal mucosa and facilitate penetration of microorganisms into the intestinal wall. Despite the fact that the morphology, life cycles and molecular analysis of notocotylids attract the attention of specialists of the relevant fields [7][8], pathoanatomical changes caused by Notocotylidae are poorly described. The Manual on Veterinary Parasitology specifies that in trematodiases cause the catarrhal or catarrhal-hemorrhagic inflammation of the cecum and rectum mucous membrane; at the sites of parasite attachment, the epithelium is destroyed and ulcers appear; in the infested mollusks, the reaction is formed in response to the invasion and development of trematode larval stages [9]. However, no likewise information could be found in the literature with regard to infestation by the representatives of the family Notocotylidae.

Notocotylidoses had been widely spread in Russia since long ago. In earlier publications, the infestations were reported to be revealed in Western Siberia [10], in the Caucasus [11], in the European North of the country [2][12] and in some other regions. In the Perm Territory, there are more than 29 000 rivers with a total length of about 90 000 km, about 115 lakes (with an area of more than 10 hectares), 18 reservoirs, including the large ones such as Kama and Votkinsk reservoirs, and numerous ponds1. The malacofauna in the Upper and Middle Kama basin alone includes 97 species of 30 genera and 10 families of the classes Bivalvia and Gastropoda. The most diverse are the families Sphaeriidae (29 species), Lymnaeidae and Planorbidae (21 species each) [13]. The avifauna of the region is also rich [14][15]: according to official data of the Ministry of Natural Resources and Environment for 2024, there were 71 846 ducks and 4 665 geese registered in the Perm Territory2.

Despite the abundance of water resources, wide spreading of specific hosts, and possibility of infestation to circulate in natural foci, no targeted study of the prevalence of notocotylidoses in the Perm Territory has been conducted, which served the rationale for the present work.

Materials and Methods. The research was conducted at the Department of Infectious Diseases of Perm State Agro-Technological University named after Academician D.N. Pryanishnikov. Due to the notocotylids’ having dixenous life cycle, the following materials were taken for the study: freshwater mollusks (n=340); cadavers of wild waterfowl that died for various reasons (n=16); droppings samples (n=139) collected in mallard aggregation areas. 39 locations of various water bodies in several districts of Perm Territory and the city of Perm were explored during the warm seasons of 2024 and 2025 to collect the material: Raiskii sad (Motovilikha Pond), Danilikha River, Zolotye Peski, Parkovyi and Lipovaya gora districts. All locations are characterized by free access to water and abundant fauna and flora. Mollusks were collected manually, and their species membership was determined by the authors themselves based on conchological features using open-access species identification key [16–18]. Shell height and width, and shell mouth width were measured using a TOPEX callipers (Poland). Compressor microscopy was then performed using a MIS-7 compressor (“Petrolazer”, Russia) and a Mikromed MS-1 stereomicroscope (“Micromed”, Russia). The shells were additionally examined with a binocular magnifying glass to detect larval forms of trematodes.

Until being provided to ornithologists for autopsy, the wild duck cadavers were stored at –18°C. Autopsy was performed according to the standard technique.

The locations of collecting the material and amounts of material collected for examination are presented in Table 1.

For histological examination, mollusk tissues were placed in the formalin solution with 4% formaldehyde content; birds’ autopsy specimens were placed in the 10% formaldehyde solution. Hematoxylin-eosin-stained specimens were scanned and viewed using the Vision Assist automated system (West Medica, Austria) and Vision microscopy automation software (“Medica Product”, Russia).

Droppings samples were analysed using sequential washing technique. Helminth eggs were identified by morphological characteristics [19].

Research Results. The study of 15 specimens of Whirlpool ramshorn snail (Anisus vortex) from the Motovilikha Pond (Perm), collected in the first ten days of June, resulted in detecting the cercariae with the morphological characteristics of Notocotylus sp. in one of the cases (Fig. 1). The infestation extensity (IE) in Planorbidae from the Motovilikha Pond was 6.67%. The distribution of A. vortex from the Motovilikha Pond by shell size, taking into account their infestation, is presented in Fig. 2.

Histological examination of the infested mollusk revealed that the tissue between the mantle and muscular layer was frayed and edematous, with a high concentration of mucin in the cell cytoplasm. In the walls of capsules consisting of a series of cells, single cercariae were found, which had a visible pigmented outer layer and underlying bundles of muscle fibers, as well as parenchyma and tubular structures located closer to the tail and representing the digestive system organs (Fig. 3). In the tissues surrounding the parasites no inflammatory reaction was observed.

Of 16 duck specimens, only the intestines of the long-tailed duck — a rarely found in the Perm Territory northern migratory marine species that winters on the shores of the Black and Azov Seas — was found to be infested with N. attenuates (Fig. 4). The examination revealed the thickened intestinal wall, mucous membrane of grey colour, abundantly covered with thick non-transparent mucus and grey-yellow creamy substance. Helminths were found in the mucus. Histological examination carried out with preservation of intestinal wall structure, the villi, and the crypts, revealed the dystrophic changes in the epithelial cells. Dystrophic processes were also observed in the intermuscular ganglia. A moderate macrophage response was observed in the medulla of the lymph nodes. The pronounced edematisation of submucosa was observed (Fig. 5). The above changes describe the chronic toxic effect of N. attenuates at the place of its localization.

Thus, the examination of freshwater mollusks in the Perm Territory resulted in acknowledging the presence of parthenites of notocotylids only in the Motovilikha Pond. Whereas, the examination of faeces of ducks from this location, had revealed no eggs characteristic for N. attenuates in any of the cases, even though the concentration of mallards in this pond was significant, and the number of droppings samples examined at different time of the year equalled 53. The discovery of N. attenuatus in a single migratory duck is likely to indicate its infestation in nesting or wintering areas.

Discussion and Conclusion. Analysis of shell size features, made it possible to divide the sample into two groups: one group — represented by the larger and older specimens (n=7), which included a specimen infested with notocotylids; the second group (n=8) consisted of younger and smaller specimens. No information concerning the age of mollusks susceptible to infestation by notocotylids could be found in the scientific literature; however, it is acknowledged that Bithyniidae become in maximum infested with trematodes at the age of 2–3 years, and the timeline of infestation largely depends on the ambient temperature [17][18].

Compared to similar indicators in other regions, the IE in mollusks of the family Bithyniidae in the estuary of the Kargat River (Western Siberia) during many years of observation was only 0.62%, with notocotylids being completely absent in the sample of mollusks in some of the years. The level of infestation with parthenites of notocotylids in mollusks of the family Lymnaeidae fluctuated between 0.1 and 0.2% [10]. In our sample, Lymnaeidae were not infested by notocotylids in any of the cases. As to Bithyniidae, they were not found in any of the locations.

According to the scientific data, mallards, garganeys, shovelers, gadwalls, common pochard, and other anseriformes are often infested by notocotylids with the Infestation Intensity (II) of up to 1118 specimens [10][21]; infestation with notocotylids is also found in many wild species worldwide [22–24]. Notocotylids are also known to parasitize not only in waterfowl, but in some micromammals too [5]. Thus, the Palearctic species N. noyeri had been recorded in 38.46% of voles of the species Microtus oeconomus near the Makhnevskaya cave (Aleksandrovsky District, Perm Territory) [26]. It could be assumed that the cercariae found in the ramshorn snails belong to notocotylids hosted by micromammals, although according to some data, some of the notocotylids, can be hosted both by birds and rodents [27].

The Motovilikha Pond is an artificial reservoir (Figs. 6–7) having the area of 0.8–0.9 km², created in 1736–1738 to serve the needs of the Motovilikha copper smelting plant. Since 1991, it got the status of a specially protected natural area of regional importance. Currently, its banks are primarily occupied by private low-rise buildings, as well as recreational areas where micromammals apparently find suitable habitat.

The study conducted in Perm and the Perm Territory has revealed the infestation of the mollusks of the family Planorbidae with invasive forms of notocotylids, which were not detected in the faeces of ducks from the same location. N. attenuatus was recorded to parasitize only in the long-tailed duck, which is a migratory species. Therefore, a conclusion about the absence of the natural foci of notocotylidosis in birds in the studied territory can be made.

1. According to the Data of the Ministry of Natural Resources and Environment of the Russian Federation (Perm Territory). https://priroda.permkrai.ru/?ysclid=mmoxx4z0we339984914 (accessed: 5.11.2025) 2. According to the Data of the Ministry of Natural Resources and Environment of the Russian Federation (Perm Territory). https://priroda.permkrai.ru/deyatelnost/okhotnichi-turizm/okhotnichi-resursy-i-okhotnichiturizm (accessed: 5.11.2025)