Tehran University of Medical Sciences Iranian Journal of Parasitology 1735-7020 17 2 2022 05 28 214 222 Abbas Jadidoleslami Department of Pathobiology, School of Veterinary Medicine, Ferdowsi University of Mashhad, Mashhad, Iran Abolghasem Siyadatpanah Ferdows Paramedical School, Birjand University of Medical Sciences, Birjand, Iran Hassan Borji Department of Pathobiology, School of Veterinary Medicine, Ferdowsi University of Mashhad, Mashhad, Iran Mehdi Zarean Department of Parasitology and Mycology, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran Lida Jarahi Department of Community Medicine, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran Elham Moghaddas Department of Parasitology and Mycology, Faculty of Medicine, Mashhad University of Medical Sciences, Mashhad, Iran Christine Budke Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX, USA 2021 03 31 2021 11 02 Background: This study was conducted to determine the prevalence and intensity of nematode infections in sheep located in northeastern Iran from Apr 2018 to Mar 2019. Methods: Gastrointestinal nematodes of 300 sheep were inspected. The season of slaughter, anatomic location where the parasite was located, the animal’s sex, infection prevalence and intensity were recorded. Seasonal differences in arrested larvae numbers also were assessed using Cochran's Q test. Results: Overall, 4,331 adult nematode specimens were collected. Among the examined sheep, 53% (159/300) were infected with one or more nematode species. Among infected sheep, 42.8% were infected with a single species of nematode, 26.4% were infected with two species of nematodes, and 30.8% were infected with three or more species of nematodes. Marshallagia marshalli (13.3%) was the most common nematode recovered from the abomasums of infected sheep, while Trichostrongylus vitrinus (4.6%) was commonly recovered from the small intestines, and Trichuris ovis (25.6%) was commonly recovered from the large intestines. In total, 463 arrested larvae were found in the abomasums of 7.5% of infected sheep and 104 arrested larvae were found in the small intestines of 8.8% of infected sheep. A significantly higher numbers of arrested larvae were found in summer compared to autumn (P<0.001). Conclusion: Intestinal parasites continue to be a problem for sheep in northeastern Iran and additional control measures need to be explored. https://ijpa.tums.ac.ir/index.php/ijpa/article/view/3310 https://ijpa.tums.ac.ir/index.php/ijpa/article/download/3310/1214