Fatal hepatic sarcocystosis in three captive and one free-ranging pinniped

Abstract

Fatal hepatic sarcocystosis was diagnosed as the cause of death in four pinnipeds: two captive Hawaiian monk seals (Monachus schauinslandi), a captive, and a free-ranging California sea lion (Zalophus californianus). Based on necropsy, histopathology, electron microscopy and DNA sequencing, intralesional protozoal schizonts were determined to have caused the necrotizing hepatitis observed. Transmission Electron Microscopy (TEM) revealed schizonts similar to Sarcocystis canis in hepatocytes. PCR-DNA sequencing and phylogenetic analysis at the conserved 18S rRNA and variable ITS1 gene markers within the nuclear rRNA gene array from schizont-laden tissue established that the parasites were indistinguishable from Sarcocystis canis at the 18S rRNA locus. However, six distinct single nucleotide polymorphisms (SNPs) were resolved at ITS1 suggesting that the parasites infecting pinnipeds were distinct from S. canis, which commonly infects bears and dogs. We hypothesize that the parasite represents a novel Sarcocystis variant that we refer to as S. canis-like that infects pinnipeds. The definitive host of S. canis is enigmatic and its life cycle incomplete. These findings document a critical need to identify the life cycle(s), definitive host(s), and all susceptible marine and terrestrial intermediate hosts of S. canis and the S. canis-like variant infecting pinnipeds.