Helicosporidium sp. infection in a California kingsnake (Lampropeltis californiae): Spillover of a pathogen of invertebrates to a vertebrate host.

Helicosporidium is a genus of nonphotosynthetic, green algae in the family Chlorellaceae, closely related to Prototheca. It is a known pathogen of invertebrates, and its occurrence in vertebrates has not been documented. A captive, 10-month-old, male, albino California kingsnake (Lampropeltis californiae) was submitted for necropsy. Gross examination revealed hemorrhagic laryngitis and a red mottled liver. Histologically, intravascular, intramonocytic/macrophagic and extracellular, eukaryotic organisms were observed in all tissues. These organisms stained positive with Grocott-Gomori methenamine silver and periodic acid-Schiff and were variably acid-fast and gram-positive. Ultrastructural analysis revealed approximately 4 µm vegetative multiplication forms and cysts with 3 parallel ovoid cells and a helically coiled filamentous cell. A polymerase chain reaction with primers targeting Prototheca, amplicon sequencing, and Bayesian phylogenetic analysis confirmed it clustered within Helicosporidium sp. with 100% posterior probability. The genus Helicosporidium was found to nest within the genus Prototheca, forming a clade with Prototheca wickerhamii with 80% posterior probability.