Ticks are significant vectors of infectious diseases, and they are recognized for their ability to sojourn on a variety of host species and transmit a variety of pathogens that can infect various vertebrate hosts, including humans. Anaplasmosis is the causative agent of tick-borne diseases, which have a significant impact on human and animal health (Buysse et al., 2024). The impact of anaplasmosis on the health and productivity of domestic animals has been well documented for over a century, and it remains a significant contributor to economic losses in the livestock farming industry. A minimum of seven species have been identified, including A. marginale, A. centrale, A. ovis, A. phagocytophilum, A. bovis, A. capra, and A. platys. It has been established through documented evidence that A. ovis, A. phagocytophilum, and A. capra have the potential to infect humans (Chochlakis et al., 2008; Lee et al., 2020; Li et al., 2015). Furthermore, novel Anaplasma species have been identified. In Japan, a potentially novel Anaplasma spp. was identified in a sika deer, exhibiting genetic divergence in the 16S rRNA, gltA and groEL genes from all known Anaplasma spp (Ybañez et al., 2012). The genus Rickettsia is an important vector-borne disease that has emerged or re-emerged globally and has increasingly posed a challenge to public health services. Rickettsia species are classified internationally into four groups: the spotted fever group (SFG), the transitional group (TRG), the ancestral group (AG), and the typhus group (TG). The SFG is the most diverse and geographically widespread group of known Rickettsiae. In China, a considerable proportion of SFG rickettsiosis have been identified as belonging to the R. sibirica group. Furthermore, additional Rickettsia species that are known to cause SFG rickettsiosis have also been identified, including R. heilongjiangensis, R. sibirica, R. raoultii, R. slovaca, R. felis, R. aeschlimannii and R. massiliae (Wei et al., 2015; Guo et al., 2016). The pathogens belonging to the genus Theileria and Babesia are among the most extensively researched parasites, due to the factors of their extensive geographical distribution, wide host range, and significant impact on public and animal health. Transmission occurs via the primary vectors, ixodid ticks, with disease outbreaks resulting in mortality, damage to hides, and poor production (Lempereur et al., 2017). Up to now, Theileria is only found in animals. In contrast to Theileria, three Babesia species have been identified as the causative agents of disease. These include B. divergens, B. venatorum, and B. microti with asymptomatic or mild but severe disease being predominantly observed in asplenic or immunocompromised individuals.

Nevertheless, there is a paucity of literature on the prevalence of these pathogens and their vectors in the eastern Tibetan Plateau, particularly in Zoige County (Wang et al., 2012). The Zoige region is home to the largest population of local livestock, namely yaks (Bos grunniens), which number approximately 800,000 individuals. These yaks represent the primary economic source for the local residents, providing dairy products, meat, and other by-products (Tang et al., 2019). The traditional lifestyle of the local population has resulted in a lack of timely deworming, which has led to the observation of severe tick infestation in yaks (Tang et al., 2019). Consequently, the objective of this study is to investigate the prevalence of Anaplasma spp., Rickettsia spp., Theileria spp. and Babesia spp. in ticks and yaks in Zoige County, providing preliminary data for the further control of transmission of diseases.

A total of 957 ticks infesting yaks were collected from the villages of Jiangzha, Qiuji, Hongxing, and Baxi in Zoige County, Sichuan Province, China. The combination of morphological and molecular identification techniques facilitates more precise identification of ticks, representing the most prevalent approach to parasite identification in current practice (Estrada-Pena et al., 2017). In this study, two species of ticks were identified: D. silvarum and H. longicornis. Dermacentor silvarum was the dominant species in four villages, whereas H. longicornis was only observed in Jiangzha and Qiuji. Previously, D. silvarum was documented in the northern hemisphere, with a range extending from 22° N to 57° N latitude (Guo et al., 2021). Dermacentor silvarum has been identified in 11 provinces, three autonomous regions, and one municipality of China, with a total of 290 counties and 34 prefectures (Guo et al., 2021). The initial report of the detection of D. silvarum in Sichuan province was published (Liu, 2021). As has been previously documented, D. silvarum is typically found near mountain ranges and exhibits a preference for deciduous and coniferous forests, as well as cultivated and shrubby vegetations (Guo et al., 2021). In this study, D. silvarum was identified at altitudes ranging from 2673 to 3513 meters and was observed to be located in grassland areas devoid of mountainous terrain in the vicinity. This constitutes a valuable addition to the known habitat of D. silvarum. Conversely, H. longicornis has been identified across a range of latitudes, from 18° to 53° in the Northern Hemisphere and from 16° to 45° in the Southern Hemisphere (Zhao et al., 2020). The species has been identified in all provinces of China (Zhao et al., 2020). In contrast to D. silvarum, H. longicornis was observed only in Qiuji and Jiazhang at altitudes below 3, 000 m. It was hypothesized that the most suitable habitat for H. longicornis would be coastal areas, with eastern North America identified as a particularly promising location (Namgyal et al., 2020). Nevertheless, our research has identified grassland as a suitable habitat for H. longicornis. This also represents a valuable addition to the habitat of H. longicornis. The primary hosts of D. silvarum were primarily domestic animals, including cattle, goats, and sheep (Guo et al., 2021). In contrast, a total of 77 species of animals have been identified as potential hosts of H. longicornis (Zhao et al., 2020). The findings of this research indicate that the yak is the optimal host for two tick species, with severe tick infestation frequently observed in Zoige. The Zoige County region is home to a considerable number of yaks, sheep, horses, Marmota himalayana, Lepus spp., Myospalax spp. and Ochotona spp., with a wide geographical range and a notable distribution (Zhou et al., 2021). To obtain more detailed information on tick hosts, it is recommended that tick samples should be collected from a variety of sources, including other animals, vegetation, and even the local community. The infection risk for humans and animals can be estimated by dragging blankets across grasslands to capture and calculate the numbers of unfed ticks in a given area (Edwards et al., 2022). The paucity of clinical cases reported or recorded can be attributed to the traditional lifestyle of the local community, particularly the practice of herding (Tang et al., 2019). This lifestyle renders them less likely to seek medical attention from a hospital unless they have a serious issue.

Ticks play a significant role in the transmission and propagation of a diverse array of serious zoonotic diseases, acting as vectors and reservoirs for a multitude of pathogens (Athni et al., 2021). A total of four genera of pathogens were identified in ticks and yaks: Anaplasma spp., Rickettsia spp., Theileria spp., and Babesia spp., with at least two species detected in each genus, which represents a significant public health concern (Livengood et al., 2020). The extant literature indicates that Anaplasma spp. can be classified into several distinct categories, including A. ovis, A. phagocytophilum, A. capra, A. marginale, A. platys, A. bovis and A. centrale. Of these, A. ovis, A. phagocytophilum, and A. capra have been reported to infect humans (Chochlakis et al., 2008; Lee et al., 2020; Li et al., 2015). In this study, five species were identified in infected ticks and yaks: A. capra, A. bovis, A. ovis, A. phagocytophilum, and A. spp. The prevalence of Rickettsiae spp. in D. silvarum was higher than that in H. longicornis. The highest prevalence was observed in Baxi, followed by Jiangzha, Hongxing, and Qiuji. In accordance with the established criteria for determining the Rickettsiae spp., three species (R. massiliae, R. raoultii, and Candidatus R. longicornii) were identified in infected ticks and yaks. The first isolation of R. raoultii from ticks was reported in Russia (Rydkina et al., 1999). Subsequent studies have identified the presence of this bacterium in at least 26 tick species belonging to seven genera, including Dermacentor (Jia et al., 2014), Haemaphysalis (Li et al., 2018; Zheng et al., 2018), Amblyomma (Parola et al., 2013), Rhipicephalus (Liu et al., 2018), Ixodes (Rar et al., 2017; Shpynov et al., 2009), and Hyalomma (Yin et al., 2018). Rickettsia raoultii has been predominantly identified in Dermacentor spp. ticks across multiple countries in Europe (Mediannikov et al., 2008; Wang et al., 2012; Foldvari et al., 2013; Spitalska et al., 2012). Furthermore, R. massiliae was identified in ticks collected from yaks, indicating that the local area is a risk area for R. massiliae infection and that prevention and control of local ticks should be strengthened. Theileria sinensis was initially identified in 1995 by Bai Qi from cattle in the northwestern region of China (Bai et al., 1997). Theileria sinensis exhibits relatively weak pathogenicity, and is primarily reported in Asia, including China, Japan, and the Korean Peninsula. In 2020, the DNA of T. sinensis was identified in yaks in the neighboring regions of Hongyuan and Aba in Sichuan Province, China (Hao et al., 2020). Theileria luwenshuni was initially identified in sheep and goats, exhibiting high pathogenicity. It is widely distributed throughout most parts of China and is primarily transmitted by both H. longicornis and H. qinghaiensis (Li et al., 2007). Of the two Babesia species detected, B. bigemina is a globally distributed agent of bovine babesiosis. The current literature indicates that B. bigemina is present in at least five tick species, including R. microplus, R. decoloratus, R. annulatus, R. geigyi and R. evertsi. In China, B. bigemina has been predominantly documented in ticks (R. microplus) and domestic animals across numerous provinces including Qinghai, Gansu, Guangxi, Chongqing, Liaoning, Yunnan, Shandong, Henan, Hubei and Xinjiang (He et al., 2021). B. caballi is the pathogen responsible for equine babesiosis, which affects horses, donkeys and mules (Cui et al., 2024). It is primarily transmitted by ticks including D. silvarum, D. ralbipictus, D. nitens, D. reticulates, and H. truncatum (Zhang et al., 2021). The subsequent step is to investigate local horses, donkeys, mules, and other equine animals to ascertain the prevalence of B. caballi in the area.

In the course of this study, two species of ticks (D. silvarum and H. longicornis) were identified through the application of morphological and molecular identification techniques. Furthermore, the phylogeny of these pathogens was explored encompassing Anaplasma spp., Rickettsia spp., Theileria spp., and Babesia spp. To gain a more comprehensive understanding of the infection risk for humans and animals, it would be beneficial to conduct a more detailed study of pathogens in other livestock and wildlife hosts from Zoige County in the future.