Gastric cryptosporidiosis in farmed Australian Murray cod, Maccullochella peelii peelii

Protozoa morphologically consistent with Cryptosporidium sp. were histologically-evident in the stomach of three cohorts of Murray cod (Maccullochella peelii peelii) from a single aquaculture facility in Australia. One cohort was asymptomatic. A second cohort had shown low-level mortalities, reduced appetite, and slowed growth, and fluid distension of the stomach and small intestine. Fish of the third cohort, sampled a year following diagnosis of the second cohort, were of stunted size and had displayed abnormal swimming behaviour. Microscopically, infection was associated with mild to moderate gastric mucosal mononuclear leukocyte infiltrate and occasional gastric mucosal epithelial cell necrosis and sloughing. Cryptosporidium molnari was confirmed in one cohort using SSU rDNA phylogenetic reconstruction with all available Cryptosporidium genotypes in fish. This study is the first to report Cryptosporidium sp. infection in Murray cod. Associated pathology and clinical signs highlight the possibility of production losses for the affected industry.