Almost all countries in the world except Antarctica has been recorded to have a high incidence of cryptosporidiosis in humans [12]. The African nations, central and south American countries, Asian nations, European countries, and North American countries have been reported to have 2.6%-21.3%, 3.2%-31.5%, 1.3%-13.1%, 0.1%-14.1%, 0.3%-4.3% cases of Cryptosporidium infection, respectively, resulting in a pooled prevalence of 7.6% [9, 13]. Statistics have shown that up to 2018 in China, approximately 200,054 individuals participated in Cryptosporidium related studies, of which 5,933 (3.0%) were diagnosed with Cryptosporidium infection/cryptosporidiosis. The disease was found to be less prevalent in adults (1.9%, 402/21316) than in children < 5 years old (2.6%, 269/10491) in China (P < 0.01) [9].
Other than age, sensitivity and specificity of the detection techniques, host health, and living criteria might also affect disease prevalence [14]. The estimated prevalence was high in those from low-income countries, individuals with gastrointestinal symptoms and residents not living in urban areas [12]. In developing countries, the prevalence of Cryptosporidium in children under five years with diarrhea was 27.4% in India (ELISA) [15], 27.8% and 32.0% in Ghana and Guatemala (microscopic analysis), respectively [16, 17], and 25.0% and 10.4% in Uganda and Tanzania (PCR), respectively [18, 19]. In China, significantly higher prevalence of Cryptosporidium was observed in rural population (1.8%-12.9%) than in urban population (0-3.7%) [9]. In the USA, cryptosporidiosis was mainly observed in children between 1–9 y of age; its high incidence was associated with recreational water use and communal swimming venues, resulting in peak infection in the summer [20].
There was a limited of information on the prevalence of asymptomatic infection. Cryptosporidium was detected in 20 (7.2%) of 276 asymptomatic aboriginal children living in villages in Malaysia [21]. In Jeddah, South Africa, almost 4.7% Cryptosporidium-positive cases were asymptomatic compared with 32.0% cases with diarrhea from pediatric clinics [22]. This study is the first study to explore the infection of Cryptosporidium in asymptomatic children in China with low infection rates (1.3%; 8/609).
Three Cryptosporidium species were identified, including C. felis (37.5%; 3/8), C. hominis (37.5%; 3/8) and C. parvum (25.0%; 2/8). C. felis oocysts were first identified in cat feces and were therefore considered a host-adapted species [23]. Human C. felis infections have shown more recent prevalence in developing countries including China, where it was found to cause Cryptosporidium infection in minimum eight human cases [7]. In addition to cats and humans, C. felis has also been found in other animals including non-human primates, calves, horses, and foxes, suggestive of a possible risk of zoonotic transmission [24]. C. hominis is a pathogenic species commonly found in humans and natural infections have been reported in nonhuman primates, cattle, dugong, marsupials, and goats [25]. The major Cryptosporidium species in Chinese population is C. hominis, which has been shown to cause approximately 48.3% (127/263) of human cases [9]. Recent studies have identified C. hominis as the dominant Cryptosporidium species in other species in China, such as horses and donkeys [26–28]. This supports the theory that these animals have been infected by human feces. C. parvum has been shown to have the highest zoonotic potential among the Cryptosporidium species. In China, 16.7% (44/263) of human cases are caused by C. parvum [9]. In this study, we identified C. parvum in yaks, sheep, goats, golden takins, horses, cattle, and donkeys, indicating zoonotic potential.
Subtyping tools based on the analysis of the gp60 gene have been developed for human-pathogenic Cryptosporidium spp. to track potential sources of infection [7]. In this study, three subtype families (Ib, Id, and If) were identified for C. hominis, composed of IdA14, IfA12G1 and IbA9G3. Elevated frequency of all three subtype families (Ib, Id, and If) were identified in rhesus monkeys in Guizhou, where humans and animals are known to closely interact [29]. This study is the first work that is performed on humans with the discovery that those subtypes can spread between humans and monkeys.
There is a limited availability of data on the subtypes of C. parvum in humans in China compared to C. hominis. Only two subtypes (IIa and IId) have been identified and only IIdA19G1 was found [30]. In this study, subtyping successfully identified two C. parvum isolates of the IId family; IIdA15G1 and IIdA14G1. These were the dominant groups in animals in China indicating that these animals may represent an important source of zoonotic Cryptosporidium in China [7].