**3. Discussion**

Camels are well known as the ships of the desert and are famous as the beasts of the burden. Camels provide wool, milk, meat, leather, and even dung as fuel for the people in many semi-arid and arid zones, mainly in Africa and Asia [21]. Currently, camel husbandry has been transforming from nomadism to intensive production, resulting in the increase of the total population of camels, with an estimated global population of 35 million [21]. This intensive farming practice of camels has been posing an increased risk for zoonotic disease transmission to humans [22]. Many zoonotic parasites are reported to be transmitted from camels to humans globally [21]. However, there is scarce knowledge regarding camel parasites and their zoonotic importance in China. In this study, the overall *Cryptosporidium* prevalence was 7.6% (36/476), and six species of *Cryptosporidium* (*C. andersoni*, *C. parvum*, *C. hominis*, *C. ubiquitim*, *C. occultus*, and *C. bovis*) were identified, which indicated the genetic diversity of *Cryptosporidium* in Bactrian camels from Xinjiang, China.

From previously published studies, *C. andersoni*, *C. parvum*, *C. muris*, *C. bovis*, *Cryptosporidium* rat genotype IV, and camel genotype have been detected in camels [19]. Among them, only two species of *Cryptosporidium* have been reported in China, namely *C. andersoni* and *C. bovis* [10,12–14]. In the present study, both *C. andersoni* and *C. bovis* were identified, and *C. andersoni* was the dominant genotype detected in Bactrian camels. Although *C. andersoni* and *C. bovis* are commonly seen in calves and sheep, *C. andersoni* has also been found in several human cases [23,24].

Perhaps unsurprisingly, the most important zoonotic *Cryptosporidium* species., *C. parvum*, was previously reported in Dromedary camels in Algeria, Australia, and Egypt [11,15,19]. According to sequence analysis of the *gp60* gene, two subtypes of *C. parvum* were identified: IIdA15G1 and If-like-A15G2. In China, *C. parvum* isolates, including IIdA14G1, IIdA15G1, IIdA17G1, IIdA18G1, and IIdA19G1, mostly belong to the IId subtype family in goats, humans, cattle, donkeys, horses, rodents, monkeys, Golden takins, and yaks [1]. Previous studies have shown that IIdA15G1 was the predominant subtype in dairy calves and yaks in northwestern China [25,26]. In the present study, IIdA15G1 was identified in Bactrian camels in northwestern China, further confirming the dominance of the IIdA15G1 subtype in western China.

A unique *C. parvum* subtype If-like-A15G2 isolate was identified in Bactrian camels in the current research, which was similar to a previously observed If-like-A22G2 isolate found in Dromedary camels in Algeria [11]. Moreover, subtypes IIaA17G2R1, IIaA15G1R1, and IIdA19G1 were also identified in Dromedary camels in Australia and Egypt [15,19]. The *gp60* gene is highly polymorphic and can be used to categorize *C. parvum* and *C. hominis* into multiple subtypes according to nucleotide sequence differences [27]. However, it seems that *gp60* polymorphisms are ineffective for *C. parvum* subtype identification in camels. In the phylogenetic analysis of *gp60* sequences, *C. parvum* If-like genetically related to the *C. hominis* If subfamily and all If and If-like sequences formed a large clade (Figure 2). More extensive genetic characterization is needed to improve our understanding of the genetic similarity between *C. parvum* and *C. hominis* within the *gp60* gene.

Using *gp60* sequence analysis, *C. hominis* subtype IkA19G1 appeared to belong to subfamily Ik. Family Ik is commonly found in horses and donkeys [28,29] and has been also isolated from patients in Sweden and squirrel monkeys in China [30,31]. This is the first report of *C. hominis* in camels. Further studies should be carried out to expand the biological characterization of *C. hominis* subtype family Ik due to its potential for zoonotic transmission.

*C. ubiquitum* has a worldwide distribution, and six subtypes/families (XIIa–XIIf) have been identified [32]. Among these subtypes, subtype XIIa has been commonly observed in humans and a wide range of animals, especially domestic and wild ruminants [33,34]. In this study, *C. ubiquitum* and subtype XIIa were detected in Bactrian camels, which indicated that *C. ubiquitum* has a broad host range and high significance for zoonotic infection in this region. *C. occultus*, previously described as *Cryptosporidium* suis-like, was recognized as a valid species in 2018 and has been identified in cattle, yaks, alpacas, and wild rats in China [35–38]. Moreover, cases of human infection with *C. occultus* have also been found in Canada, China, and the UK [39–41]. The present study is the first report of *C. ubiquitum* and *C. occultus* in camels. Further studies into the epidemiology of *Cryptosporidium* infection in both human and livestock is essential.

#### **4. Materials and Methods**

#### *4.1. Ethics Approval*

The study was designed and conducted in accordance with the Guide for the Care and Use of Laboratory Animals of the Ministry of Health in China. The Research Ethics Committee of Tarim University critically reviewed this research protocol (approval no. ECTU 2016-0007) and then cleared it for performing. Finally, before fecal sample collection from Bactrian camels, appropriate permission was obtained from the farm owners.
