The article investigates into the species composition of agents causing gastrointestinal tract nematodes in sheep. It determines the forms of nematodes communities. For this purpose, 710 sheep were examined, 79.58% of which were infected. The fauna of nematodes was represented by 15 species of helminths from 12 genera: Bunostomum, Oesophagostomum, Nematodirus, Chabertia, Cooperia, Trichostrongylus, Ostertagia, Haemonchus, Тrichuris, Aonchotheca, Strongyloides, and Skrjabinema. The indicators of infestation intensity in sheep ranged from 1 to 93 specimens per animal, and the abundance index ranged from 0.01 to 16.96 specimens per animal. The most common nematodes are H. contortus (prevalence, 61.97%), O. circumcincta (59.58%), T. colubriformis (57.35%), Т. оvis (55.21%), N. spathiger (49.01%), Oe. venulosum (42.54%), S. ovis (41.13%), Ch. оvina (36.76%), and T. skrjabini (26.34%). Gastrointestinal nematodes in the body of 99.12% of the infected sheep proceeded as parasitic communities. Most frequently, coinvasions were represented by parasitic communities which consisted of five (prevalence, 14.51%), six (19.44%), or seven (15.92%) species. The study revealed 361 varieties of mixed coinvasions. The most frequent comembers were H. contortus, O. circumcincta, T. colubriformis, Т. оvis, N. spathiger, S. ovis, and Ch. оvina. The data obtained provide an opportunity to increase the effectiveness of measures to combat and prevent gastrointestinal tract nematodes in sheep farms.

The study of biodiversity, morphological and biological peculiarities of pathogens of invasive animal diseases in natural and anthropogenically transformed ecosystems is an important task of modern population and applied ecological parasitology. The dominant position among helminthiasis of the digestive organs in ruminants occupy strongylatoses of the gastrointestinal tract. One of the components of the strongylates fauna, which parasitizes on sheep, is Oesophagostomum venulosum Rudolphi, 1809.In order to study the morphological features of the structure of imaginal forms of nematodes of the species Oe. venulosum and determination of their differential metric characteristics, the study of sexually mature oesophagostomes isolated from the intestines of sheep was conducted. Collection of helminths was carried out by the method of complete helminthology of the thick intestine of animals. The congenital morphological features of oesophagostomes are the presence of a clearly separated main vesicle, well-developed major papillae, the outer and inner radial corona around the mouth. Species Oe. venulosum in females is characterized by peculiarities in the structure of the caudal end, vulva, oviduct, vagina. It was found that the length of the body and main vesicle in females was 27.73 and 9.3% higher than that of males and averaged 18.75 ± 0.58 mm and 0.43 ± 0.01 mm, respectively. Indices of body width and major vesicles of mature males and females ranged 0,30–0,64 mm, 0,34–0,43 mm and did not differ significantly. New data on morphological and metric parameters of females Oe. venulosum, which increase the effectiveness of differentiation of helminths to the species. The parameters of the esophagus width in different sites, the width of the body in the vulva and the anus, the width and length of the spherical protrusion in the vulva area are determined. It was found that the length of the eggs in the femoral cavity and in the secretion it secretes significantly changes, the eggs are thickened, decreasing by 13.97%. The length and width of the eggs allocated by the female Oe. venulosum, on average, is 82.51 ± 2.37 and 53.51 ± 1.17 μm, respectively, and have a structure characteristic of the strongylidsc type eggs.