Trichostrongylus nematodes are highly prevalent and considered as gastrointestinal parasitic pathogens among ruminants with worldwide distribution (Roberts and Janovy, 2009, Sharifdini et al., 2017b). Clinical symptoms of humans are mild although in some patients gastrointestinal signs and eosinophilia may occur (Ghanbarzadeh et al., 2018, Wall et al., 2011). These nematodes are major health challenges, causing reduced animal products or even death among the infected animals in severe cases (da Rocha et al., 2020, McLeod, 1995). Several species of the parasite have been reported from herbivores with approximately 12 species identified in humans (Phosuk et al., 2013, Sharifdini et al., 2017b). Also, the frequency of Trichostrongylus spp. in human and animal hosts has been repeatedly reported in Iran (Ghadirian, 1977, Ghadirian et al., 1974, Ghasemikhah et al., 2011, Shahbazi et al., 2012). Ruminant infection was reported from Isfahan, Khuzestan, Mazandaran, Kermanshah, Hormozgan, and West Azerbaijan provinces, with human infections found in Khuzestan, Isfahan, Tehran, Hormozgan, Kermanshah, Mazandaran, Guilan, Sistan & Baluchestan, and West Azerbaijan provinces (Ashrafi et al., 2020, Ashrafi et al., 2015, Ghadirian and Arfaa, 1975, Sharifdini et al., 2020, Sharifdini et al., 2017b).
According to the morphological features reported in previous studies from Iran, several species of nematodes have been identified in human including T. orientalis, T. vitrinus, T. axei, T. colubriformis, T. probolurus, T. skrjabini, T. capricola, and T. lerouxi (Ghadirian, 1977, Ghadirian and Arfaa, 1975, Ghadirian et al., 1974). In recent years, some studies clarified the human infections with T. vitrinus, T. axei, T. colubriformis, and T. longispicularis species in endemic areas of northern Iran with T. colubriformis considered as the predominant species (Sharifdini et al., 2017b, Sharifdini et al., 2017a, Gholami et al., 2015, Ashrafi et al., 2020). Infection with various species of Trichostrongylus including T. colubriformis (Anvari-Tafti et al., 2013, Borji et al., 2010, Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011, Shahbazi et al., 2012), T. vitrinus (Anvari-Tafti et al., 2013, Borji et al., 2010, Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011, Shahbazi et al., 2012), T. axei (Ghadirian and Arfaa, 1975), T. capricola (Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011), T. probolurus (Anvari-Tafti et al., 2013, Borji et al., 2010, Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011, Shahbazi et al., 2012), T. longispicularis (Ghasemikhah et al., 2011), T. orientalis (Ghadirian and Arfaa, 1975), T. lerouxi (Biocca et al., 1974), T. skrjabini (Ghadirian and Arfaa, 1975), and T. hamatus (Anvari-Tafti et al., 2013) were reported in different herbivores such as sheep (Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011, Shahbazi et al., 2012), goats (Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011), cattle (Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011), buffalos (Ghadirian and Arfaa, 1975, Ghasemikhah et al., 2011), and camels (Anvari-Tafti et al., 2013, Borji et al., 2010) in most parts of Iran. The predominant species of Trichostrongylus among different herbivores are T. colubriformis, T. vitrinus, and T. axei found in most parts of the country (Ghadirian and Arfaa, 1975).
There is a tremendous diversity of the nematodes in the country (Ghasemikhah et al., 2011, Ghasemikhah et al., 2012) however, the molecular approaches, currently available and easily applicable, could accurately identify these species. Molecular studies based on ITS and 28S regions of ribosomal DNA were applied for genetic variation and phylogenetic analysis of Trichostrongylina (de Bellocq et al., 2001, Hoberg et al., 1999, Sharifdini et al., 2017a, von Samson-Himmelstjerna et al., 2002, Pandi et al., 2021). Although, numerous number of studies have focused on ITS2 for analysis of the Trichostrongylidae family in genetic variation, species detection, and phylogenetic relationships (Ghasemikhah et al., 2012, Sharifdini et al., 2017a, Sharifdini et al., 2017b), yet mitochondrial (mt) genomes have the potential to present valuable information. Mt genomes are conserved and present large amounts of sequence data in the organisms, therefore mtDNA are used for evolutionary analyses, taxonomy, population genetics, and systematics studies (dos Santos et al., 2017, Hu et al., 2004, Saccone et al., 1999). There are few studies that have investigated the mitochondrial gene of the Trichostrongylidae family, in which the mtDNA of Marshallagia marshalli, Haemonchus placei, Haemonchus contortus, T. vitrinus, T. axei, Ostertagia trifucata, and Teladorsagia circumcincta species were evaluated for phylogenetic relationship and species identification (Ahmad et al., 2019, Archie and Ezenwa, 2011, dos Santos et al., 2017, Jex et al., 2010, Kuchboev et al., 2020, Palevich et al., 2020, Sun et al., 2018). Taxonomy studies of the nematodes based on sequences of coding mitochondrial genes are more accurate than non-coding ribosomal genes. While mitochondrial genomes are considered as suitable markers for population evolution studies (Kuchboev et al., 2020, Palevich et al., 2020), the studies targeting the mtDNA for identification of Trichostrongylidae family are very limited worldwide with even no single report on mitochondrial gene of the nematodes from Iran. Therefore, the present study focused on molecular phylogenetic analysis based on cytochrome c oxidase subunit I (Cox1) from mitochondrial gene of Trichostrongylus species in northern Iran.