The disease can be caused by any one of about 20 distinct species of unicellular eukaryotes known as scuticociliates, which are free-living marine microorganisms that are opportunistic or facultative parasites.
The mortality rate is particularly high among flatfish, possibly due to their sedentary lifestyle involving skin-to-skin contact between individuals.
Histological postmortem examination of affected fish usually reveals ciliates in the skin and gills, blood, and internal organs, with significant damage to the brain and nervous system, which is likely responsible for behaviors such as abnormal swimming in infected individuals.
[6] Infections by U. marinum show a less severe disease course, possibly restricted to the skin surface; it has been suggested that this ciliate may be only a secondary pathogen.
[7] Scuticociliatosis due to Miamiensis avidus infection is believed to be responsible for a 2017 die-off of fish and leopard sharks found in the San Francisco Bay.