ECOLOGICAL AND PARASITOLOGICAL CHARACTERISTICS OF THE “ FASCIOLA HEPATICA L.– LYMNAEA ( GALBA ) SUBANGULATA ” SYSTEM

cercariae shows a stable uniform character in the form of a two­peak curve, with peaks in May (2.51 ± 0.88 %) and August (9.19 ± 1.78 %). The overall prevalence of L. ( G. ) subangulata invasion by fasciola cercariae in the studied water bodies in 2020 was significantly lower than in 2021. Conclusions . In modern climatic conditions, in the territory of the region, perma­ nent water bodies have become fasciolosis foci and common places for the settlement of L. ( G. ) subangulata , infected with F. hepatica cercariae. The most favorable environmen­ tal conditions for fasciola and its intermediate hosts develop in spring; accordingly; for the definitive hosts, this period is quite dangerous in terms of infection of animals.


INTRODUCTION
The trematode Fasciola hepatica is the causative agent of one of the world's most common zoonoses -fasciolosis. This parasite is widespread in 81 countries, it causes significant economic damage to livestock, leads to the death of animals and reduces their productivity. In addition, F. hepatica poses a direct threat to human health and wellbeing of the population in general (Vázquez et al., 2018;Alba et al., 2019;Beesley et al., 2019;Leka, 2019;Malatji & Mukaratirwa, 2019;Zhang et al., 2019;Caravedo, & Cabada, 2020;Huang et al., 2020;Coelho et al., 2021;Cwiklinski, Robinson, Donnelly, & Dalton, 2021;Lalor et al., 2021). According to the literature (Caravedo, & Cabada, 2020), there is an expansion of endemic zones of F. hepatica. An estimated number of people infected byfasciola worldwide is over 17 million and the number of infected humans is constantly growing.
The ability of fasciola to infect and reproduce in their intermediate mollusk hosts, as well as their adaptation to a wide range of final mammalian hosts, contribute to their high transmission and spread.
Researchers note (Zhang et al., 2019) that today fasciolosis, which is one мсссof the important and dangerous trematodoses, does not attract enough scholarly atten tion in connection with climate change towards warming, whichwill possibly affect the spread of F. hepatica.
In recent years in Ukraine, the intensity of research on the infection of freshwater mol lusks by larvae of the trematode F. hepatica has decreased. Currently, there are isolated data concerning parasitocenoses of cattle in the Central region of Ukraine, in particular a study of the epizootiology of fasciolosis of these animals in Zhytomyr region (Hud & Dovhiy, 2021). Therefore, despite the existing publications on the spread of fasciolosis in Ukraine, studies of some aspects of the biology of F. hepatica under modern envi ronmental conditions are rather topical, particularly because they can be useful for the prediction of fasciolosis outbreaks in certain areas. Given the lack of data in recent years on the infection of freshwater mollusks by partenites and larvae of F. hepatica in Zhytomyr region and its foreststeppe zone, the aim of our research was to clarify some aspects of F. hepatica biology in Berdychiv district. During the collection of mollusks, their population density was determined. For this purpose, standard wooden frames with a surface area of 1 m 2 were used (Stadnichenko, 2006).
Water sampling was performed at mollusk collection sites. The analysis was per formed on the same day in the laboratory of the Polissia National University using stan dard methods (Yatsyk, Denisova, Chernyavskaya, & Zimina, 1995). Identification of mollusks was carried out by conchological features, taking into account their anatomical data (Stadnichenko, 2004).
Parasitological studies of mollusks were performed by conventional methods (Chernogorenko, 1983). The morphology of cercariae was studied using vital dyes: neu tral red and sulphate Nile blue. Live and fixed cercariae were measured. In the course of parasitological study of the mollusks, bioethical norms were not violated.
The advanced analytics software package Statistica 6.0 was used for statistical processing of the primary data.

RESULTS AND DISCUSSION
Conditions necessary for the successful existence of intermediate hosts -mol lusks of the subgenus Galba -are vital for the circulation of fasciola in a particular area. Today, due to climate change towards warming, it is the permanent water bodies that ensure the successful overwintering of pond snails (lymnaea), which is necessary for the completion of the development cycle of F. hepatica in early spring. At present, tem porary reservoirs do not play an important role in the spread of fasciolosis due to their drying up.
The exploration of the area under study revealed four water bodies inhabited by pondsnails of the subgenus Galba, namely Lymnaea (G.) subangulata and L. (G.) truncatula. The typical habitats of these pondsnails in Berdychiv district are ponds and canals that occur on pastures. Mollusks L. (G.) subangulata and L. (G.) truncatula were not found in permanently darkand poorly heated water bodies.
The population density of mollusks depends on the water body type, the season of the year, the composition of dissolved minerals and oxygen content in water. Thus, in spring, the population density of L. (G.) subangulata in the studied reservoirs ranged from 2 to 18 specimens/m 2 , while at the end of summer it increased to 26 specimens/m 2 . For L. (G.) truncatula, the values of this indicator were 1-3 specimens/m 2 in spring and 15 specimens/m 2 in summer ( Table 1).
It was found that L. (G.) subangulata and L. (G.) truncatula play the main role in the spread of fasciolosis, the incidence of the former species is 88 %, that of the latteronly 12 %.

Habitat
Mollusk species Pond snails, infected by the trematode F. hepatica, inhabit reservoirs with a similar chemical composition of water and rather different quantitative ratios of substances dis solved in water ( Table 2). In the water of the examined reservoirs inhabited by L. (G.) subangulata, as well as the pastures of the district, an increased content of nitrogen nitrates (2.8-70.80 mg/L) was detected. This is probably due to the pollution of water bodies with agricultural and household waste as well as decomposition of organic residues.
Mass emergence of the trematode F. hepatica intermediate hosts -mollusks L. (G.) subangulata and L. (G.) truncatula -was observed in the second half of April and in early May. The active life of these mollusks begins at water temperature of about 10-14 °C.
Collected in May 2020-2021, L. (G.) subangulata and L. (G.) truncatula, which suc cessfully overwintered, belonged mainly to two previous year's generations -mature (spring) and young (autumn) ones. With the onset of spring, young mollusks of the autumn generation continue their development. After reaching sexual maturity, these mollusks begin to reproduce. In one of the studied reservoirs, some individuals with a shell height of more than 8.2 mm were found. This indicates the ability of L. (G.) subangulata to survive two winters.
Temperature is the main factor influencing the growth and development of fasciola partenites and larvae in mollusks. In winter, the process of hatching larvae of F. hepatica is suspended and resumes only in spring, with the release of mollusks from overwinter ing. In recent years (2020-2021), there has been an increase in average daily tempera ture in spring and summer, which accelerated the development of F. hepatica from the beginning of embryogenesis in eggs released into the environment to the formation of adolescents after cercariae emerge from mollusks. During the study period (2020-2021), the infestation of mollusks by fasciola parteni tes and cercariae in water bodies of Berdychiv district pastures reached 2.08-18.75 % (4.98 ± 0.47 % on average). The low level of infection 0.8-4.0 % (2.28 ± 0.68 % on ave rage) of L. (G.) subangulata by partenites and cercariae of F. hepatica was recorded in May and reached 0.8-4.0 % 2.28 ± 0.68 % on average), while the maximum, recorded in August, was 4.44-18.75 % (8.07 ± 1.99 % on average) ( Table 3).
The generalized results indicate that the dynamics of the prevalence of L. (G.) subangulata invasion has a stable onetype nature represented by a twovertex curve, with peaks in May (2.51 ± 0.88 %) and August (9.19 ± 1.78 %) (Fig. 2). Compared with the results of previous studies [3] on the dynamics of infection of (G.) subangulata by the trematode F. hepatica in Zhytomyr region, the peak of fasciola invasion of this species of mollusks shifted from September to August. In our opinion, this is due to the increase in average daily temperatures in recent years.  It should be noted that fewer L. (G.) subangulata were infected by F. hepatica after winter season than in autumn due to the death of some of the fasciolainfested lymnaeid snails in winter under the influence of low temperatures.
Part of the mollusks that overwintered is made up of individuals that were infected in late autumn; thus, the development of parthenogenetic generations was not com pleted before winter.
Further development of partenites occurs in mollusks after overwintering.
In L. (G.) subangulata, which were infected in spring, the release of F. hepatica cercariae occurs no later than June. In midsummer (July) there was a decline in mol lusk infection (2.49 ± 0.82 % on average), which in our opinion is due to a decrease in their population density caused by the death of old mollusks, a large proportion of which were infected by F. hepatica. From July, a rapid increase in invasion begins. In August and early September, cercariae of F. hepaticawere detected mainly in mollusks of the spring generation of the current year, their infestation level being rather high. Due to the death of old individuals that overwintered, the number of the latter in the population was insignificant; thus, the cases of invasion among them were rare.
According to the results of the study, the overall extent of invasion of L. (G.) subangulata by fasciola cercariae in the examined water bodies in 2020 was significantly lower than in 2021 (P ≤ 0.05), which is probably due to weather conditions ( Table 4). In the spring and summer period of 2020 in Berdychiv district, the average daily air temperature was 11 °C, the average precipitation -56.6 mm, the average relative humidity -70 %, while in 2021 the values of these indicators reached 12 °C , 72 mm and 74 %, respectively.
In the autumn period, late October -early November, when water temperature drops to +5-8 °C, the mollusks of the autumn and partly of the spring generations of the current year prepare for overwintering by immersing in muddy soil and becoming sedentary.
The study has proved the dependence of the seasonal dynamics of L. (G.) subangulata invasion on both the season of the year and the proportion of the number of individuals of different ages in the mollusk population. Infection of individuals with a shell height of 4.0 to 8.2 mm by F. hepatica cercariae was detected. At the same time, fasciola cercariae were not found in mollusks with a shell height of 0.8-3.8 mm.
The analysis of the size and age (2020-2021) of individuals of the studied sample of L. (G.) subangulata showed that populations of these mollusks included individuals of several generations. The main size and age groups were as follows: 1) mollusks of the previous year's spring generation with a shell height of 4.3-6.3 mm in spring, and 5.7-7.0 mm in autumn; 2) mollusks of the previous year's autumn generation with a shell height of 3.6 to 5.6 mm; 3) mollusks of the spring generation of the current year, which reached a shell height of 2.9-4.9 mm before overwintering; 4) mollusks of the autumn generation of the current year whose shell did not exceed 3 mm in height before overwintering (the range of shell height 0.8-2.8 mm).
The general sample under study was dominated by individuals with a shell height of 0.8-4.9 mm (68 % of the total number), which means that mollusks of the current year's generations made up the larger part of the population (Fig. 3). This is evidenced by the distribution of mollusks infected by cercariae of F. hepatica by size and age groups (Fig. 4). In summer (June-July), individuals with a shell height of 5.0-5.6 mm make up the most infected group, which accounts for 80 % of all the infected mollusks. In August, the overall picture changes significantly. Among the infected L. (G.) subangulata, the dominant group is mollusks with a shell height of 4.34.9 mm (61 % infected, their share reaches 19.7 % of the total sample of mollusks). In September and October, it is over taken by a group with a shell height of 3.6-4.2 mm (44 % infected, 19.4 % of the total sample of mollusks). Both groups are represented by individuals of the spring genera tion of the current year, which are dominant in the population. These mollusks overwin ter and remain infected until spring. However, in spring the extent of their invasion is significantly lower, due to the mortality of the infected L. (G.) subangulata in winter.
The generalized results of our study show that the potential intermediate hosts for It is known (Bennema et al., 2011;Beesley et al., 2017) that the life cycle of F. hepatica depends on environmental and climatic factors. In particular, the presence of vegetation, wetlands, water bodies, high animal density, as well as the lack of pre vention and treatment of fasciolosis contribute significantly to the spread of fasciolosis. On the pastures of Berdychiv district, there are permanent water bodies with stagnant water. These habitats with relatively stable water levels, shores overgrown with grassy vegetation, and a large number of cattle from individual farms provide favorable condi tions for the successful circulation of F. hepatica.

CONCLUSIONS
Based on the obtained results, we can conclude that under modern climatic condi tions, in the territory of Berdychiv district of Zhytomyr region, permanent water bodies have become foci of fasciolosis and common habitats for L. (G.) subangulata and L. (G.) truncatula. The most common and numerous species L. (G.) subangulata (occurrence 88 %) plays the leading role in the spread of fasciolosis. In these mollusks, infection by F. hepatica was registered at a shell height of 4.0 mm or more. Therefore, during the epizootological assessment of water bodies for fasciolosis, small L. (G.) subangulata do not need to be examined, which significantly improves and facilitates the effectiveness of work. The most favorable environmental conditions for F. hepatica and its interme diate hosts develop in the spring, accordingly, for the definitive hosts, this period is quite dangerous in terms of the possibility of their infection by fasciola.
Thus, elucidation of some issues in the biology of the trematodes F. hepatica in Berdychiv district will enable scientists to predict the infection of animals with fasciolosis and stop outbreaks of this disease in farms by preventing contact between fascioles and their definitive hosts.

Conflict of Interest:
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Human Rights: This article does not contain any studies with human subjects per formed by any of the authors.