Cryptosporidium spp. are apicomplexan parasites that cause cryptosporidiosis in both humans and animals (Checkley et al., 2015). To date, 45 Cryptosporidium species and more than 120 genotypes have been described in mammals, birds, reptiles, amphibians and fish (Ryan et al., 2021). These species and genotypes differ significantly in host specificity (Feng et al., 2018). Some Cryptosporidium species have a narrow host range, such as Cryptosporidium hominis, Cryptosporidium bovis, Cryptosporidium xiaoi, Cryptosporidium suis, Cryptosporidium tyzzeri, and Cryptosporidium erinacei, which are found mostly in humans, bovine animals, ovine animals, pigs, mice, and hedgehogs, respectively (Ryan et al., 2014). However, others such as Cryptosporidium parvum and Cryptosporidium ubiquitum have a broad host range, infecting multiple species of mammals including humans (Feng et al., 2018).

Equine cryptosporidiosis was first reported in 1978 in Arabian foals with combined immunodeficiency (Snyder et al., 1978). Since then, cryptosporidiosis has been reported in horses and donkeys worldwide, with Cryptosporidium horse genotype, C. parvum and C. hominis being the major causes (Burton et al., 2010, Jian et al., 2016, Couso-Pérez et al., 2020, Xu et al., 2023).

Despite its common occurrence, Cryptosporidium horse genotype is the only major Cryptosporidium sp. in domesticated animals without formal species recognition (Ryan et al., 2021). It was initially identified in a wild Prezewalski’s horse in the Czech Republic (Ryan et al., 2003). Subsequently, it has been commonly found in horses, donkeys and hedgehogs (Ryan et al., 2003, Caffara et al., 2013, Mirhashemi et al., 2016, Li et al., 2019). Based on sequence analysis of the 60 kDa glycoprotein (gp60) gene, three subtype families, including VIa, VIb and VIc, have been identified within the horse genotype (Qi et al., 2015, Lebbad et al., 2021). Among them, VIa subtypes are mainly detected in horses and donkeys (Burton et al., 2010, Jian et al., 2016, Li et al., 2019), VIb subtypes have been detected in hedgehogs, squirrels and humans (Xiao et al., 2009, Abe and Matsubara, 2015, Takaki et al., 2020), while VIc has been detected in one human patient (Lebbad et al., 2021).

The present study was undertaken to characterise the morphology and biology of the Cryptosporidium horse genotype. The data from experimental infections and comparative genomics analysis support its species status. Based on the host-adapted nature of the organism and apparent preference of using horses and other equine animals as the host, we propose the name Cryptosporidium equi n. sp. for the horse genotype.