Species in the genus Klossiella Smith and Johnson, 1902 are unique among the suborder Adeleorina because they are monoxenous in mammals exclusively, whereas all other reported members of the Adeleorina use invertebrates as definitive hosts. Unlike other coccidia, all members of the Adeleorina undergo syzygy, the association of microgamonts and macrogamonts before maturation to gametes and syngamy. After fertilization, many members of the Adeleorina produce thin-walled polysporocystic oocysts. Despite being biologically similar to other members of the Adeleorina, the phylogenetic placement of the genus Klossiella has been questioned based on its unique host affinity. In the present study, 2 cases of Klossiella equi were reported from the kidneys of horses in Ontario. Details of the life cycle as well as mitochondrial and nuclear 18S ribosomal DNA (18S rDNA) sequences were analyzed to provide both morphological and molecular evidence for the phylogenetic placement of K. equi. Initially, various stages of the life cycle were identified in histological slides prepared from the kidney tissue, and DNA was isolated from the infected tissue. Polymerase chain reaction and Sanger sequencing were used to generate a complete mitochondrial genome sequence (6,569 bp) and a partial 18S rDNA sequence (1,443 bp). The K. equi 18S rDNA sequence was aligned with various publicly available apicomplexan 18S rDNA sequences. This alignment was used to generate a phylogenetic tree based on Bayesian inference. Multiple K. equi stages were identified including meronts, microgamonts, and macrogamonts associating in syzygy as well as thin-walled oocysts in various stages of sporogonic development. The 18S rDNA sequence of K. equi positioned within the monophyletic Adeleorina clade. The mitochondrial genome of K. equi contained 3 coding sequences for cytochrome c oxidase I, cytochrome c oxidase III, and cytochrome b as well as various fragmented ribosomal sequences. These components were arranged in a unique order that has not been observed in other apicomplexan mitochondrial genomes sequenced to date. Overall, it was concluded that there were sufficient morphological and molecular data to confirm the placement of K. equi and the genus Klossiella among the Adeleorina. The biological and molecular data obtained from these cases may assist with future studies evaluating the prevalence and life history of this seemingly underreported parasite and better define the impact of K. equi on the health of domestic and wild equids.

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