Our studies were aimed at analyzing biodiversity of ecto- and endoparasites of sheep, cattle, rabbits, domestic birds and dogs in the lowland zone of the Republic, with semidesert climate and height of 850 m above the sea level. This is one of the warm regions of the Republic. The studies performed have established moderate and high degree of animal infection with parasites. We identified 33 parasite species, mostly helminths (15 species), including 10 species of nematodes (Trichocephalus ovis, Nematodirus spathiger, Haemonchus contortus, Chabertia ovina, Dictyocaulus filaria, Muellerius capillaris, Protostrongylus spp., Passalurus ambiqus, Ascaridia galli, Heterakis gallinarum), 4 species of trematodes (Fasciola hepatica, F. gigantica, Dicrocoelium dendriticum, Paramphistomum cervi), 1 species of cestodes (Moniezia expansa), 9 protozoan species (Piroplasma bigeminum, Babesia ovis, Eimeria perforans, E. magna, E. irresidua, E. tenella, E. acervulina), 7 species of ticks, 1 species of insects, and 1 species of leishmaniasis vector, the mosquitos from the genus Phlebotomus. We registered 5 species of freshwater molluscs (Lymnaea auricularia, L. stagnalis, L. ovata, L. truncatula, Planorbis planorbis), and 2 species of terrestrial molluscs (Helicella derbentina, Napaeopsis hohenackeri).

The work provides data on the presence and localization in the nervous system of the causative agent of dicroceliosis, Dicrocoelium lanceatum serotonergic and FMRFaminergic structures. Using the immunocytochemical method and confocal scanning laser microscopy, the test substances were found in the central and peripheral parts of the parasite's nervous system — in the head ganglia, the central nervous commissure, the longitudinal nerve trunks and the transverse commissures connecting them. The innervation of the nerve fibers of the attachment organs of trematodes by immunoreactive to serotonin and neuropeptide is shown.Trematode Dicrocoelium lanceatum (Stiles and Hassal, 1896) (Dicrocoeliidae) is the causative agent of Dicroceliosis, a widespread helminthiasis of domestic and wild animals. Dwelling in the bile ducts of the liver and in the gallbladder, dicroceliums cause irreversible pathological changes in the host organism, chronic inflammation of the bile ducts, cirrhosis of the liver, and metabolic processes in animals occur. As a result of the disease, animals are depleted, their productivity decreases, growth and development lag, and the nutritional and caloric value of meat decrease.In the cycle of development of the causative agent of dicroceliasis, the intermediate hosts are land mollusks, while the additional hosts are ants. The eggs of the parasite along with the bile enter the host intestine, and then with the feces into the external environment. Mollusks swallow eggs, miracidia come out from the eggs in the intestines of the mollusk, and penetrate the liver, where daughter sporocysts develop, in which cercariae form. Of the daughter sporocysts, cercariae are introduced into the respiratory cavity of the mollusk, where they are covered with sticky mucous substance. The formed mucous lumps are pushed by the respiratory movements of the mollusk into the external environment. Infection of ants occurs when eating these mucous balls. Once in the intestinal tract, cercariae enter the ant's abdominal cavity, where they turn into metacercariae. The definitive owners infect on pastures, swallowing invasive ants along with grass.

Original long-term studies of the authors on the sheep infestation by D. filaria in conditions of pronounced vertical zonal character of climate and landscape belts (300–2000 m above sea level) have shown a presence of wide infestation among all age groups of the animals: in lambs up to 60%, in young sheep up to 57% and in adult sheep up to 45%. Dictyocaulus is also present in moufflons and bezoar goats in Armenia. The main reasons for such a wide infection are the following: • a monoxenous character of D. filaria life cycle, i.e. the parasite being a geohelminth; • its survivability in wide range of biotic conditions; • an insufficient volume of planned prophylaxis measures against dictyocaulosis. A study of seasonal and age dynamics on author's own and literature data was also performed. Dynamics of infestation of lambs with Dictyocaulus is in both lowlands and mountain zones characterized with 2 peaks: summer and autumn ones (prevalence reaching 29.0% for lowlands, 42.0% for highlands in contrast to 5.7% and 2.8% respectively in the spring) with no invasion at start of the year. For young and adult sheep, dynamics of invasion with Dictyocaulus is characterized with high prevalence in spring and autumn periods (start of the year prevalence up to 45.0%, decrease to 15–25.0% in summer, rising to 40.0% and higher in October-December).

Rabbits’ eimeriosis is widespread in Armenia. In 2015–2018 we found 7 species of coccidia of the genus Eimeria: E. magna, E. media, E. perforans, E. irresidua, E. coecicola, E. exigua and E. stiedae in the mountainous, foothill and flat zones. However, in addition to the above species, the giant form of E. sp. (size of oocysts: length – 62.5 microns, width – 31.25 microns) was found in the winter in the mountainous zone, and in the spring, in the private farms of the plain zone. An analysis of our research has shown that 3-month-old rabbits become infected with oocysts of ameri within 3 days. At the same time, they are infected with 9 species of oocyst of ameri – E. magna, E. media, E. perforans, E. irresidua, E. coecicola, E. exigua, and two sizes of E. sp. of a giant form (form 1 – 28.13х53.13 microns and form 2 – 31.25х62.50 microns). The species E. coecicola was found in one individual on the first day of the study, and the species E. exigua in two individuals on the fifth day of the study. The highest level of infection with the species E. magna, E. perforans, E. irresidua, E. sp.-1 (giant form-1) was registered on the 3rd day of research. Type E. media is an optional pathogen. The highest infection level of this species was recorded on the 2nd day of research. Despite the fact that in the giant second form of E. sp.-2, infection ranged by day, invasion was weak throughout the study days.

The fauna of parasites of domestic animals (cows, sheep, rabbits, hens), ixodes ticks, as well as paths of circulation of some biohelminths in conditions of foothills ecosystem of Razdan-Marmarik region of Armenia have been studied. It's been found that fauna of animal parasites in the region consists of 33 species, including 17 species of helminths: 2 trematodes (Dicrocoelim lanceatum, Fasciola hepatica); 4 cestodes (Moniezia expansa, M. benedeni, Echinococcus granulosus larvae, Cysticercus taenicollis); 11 nematodes (Chabertia ovina, Haemonchus contortus, Nematodirus spathiger, Trichocephalus ovis, Dictyocaulus filaria, Cystocaulus nigrescens, Muellerius capillaries, Protostrongylus sp., Ascaridia galli, Heterakis gallinarum, Passalurus ambigus); 3 species of blood parasites (Piroplasma bigeminum, P. ovis and Theileria ovis); 8 species of Protistae (Eimeria tenella, E. maxima, E. acervulina, E. irresidua, E. media, E. magna, E. perforans and E. exigua) and 4 species of ticks (Hyalomma sp., Rhipicephalus bursa, Boophilus calcaratus, Dermacentor marginatum). Fauna of land mollusks is able to be intermediate hosts of Dicrocoelim and Protostrongylushelminths includes the single species Napaeopsis hohenackeri while freshwater mollusks include Lymmnaea truncatula and Planorbis planorbis, with L. truncatulaable to be an intermediate host of trematode Fasciola hepatica