The scuticociliate Miamiensis avidus was isolated from olive flounder Paralichthys olivaceus showing typical symptoms of ulceration and hemorrhages in skeletal muscle and fins. In an infection experiment, olive flounder (mean length: 14.9 cm; mean weight: 26.8 g) were immersion challenged with 2.0 x 10(3), 2.0 x 10(4) and 2.0 x 10(5) ciliates ml-1 of the cloned YS1 strain of M. avidus. Cumulative mortalities were 85% in the 2.0 x 10(3) cells ml-1 treatment group and 100% in the other 2 infection groups. Many ciliates, containing red blood cells in the cytoplasm, were observed in the gills, skeletal muscle, skin, fins and brains of infected fish, which showed accompanying hemorrhagic and necrotic lesions. Ciliates were also observed in the lamina propria of the digestive tract, pharynx and cornea. The fixed ciliates were 31.5+/-3.87 pm in length and 18.5+/-3.04 microm in width, and were ovoid and slightly elongated in shape, with a pointed anterior and a rounded posterior, presenting a caudal cilium. Other morphological characteristics were as follows: 13 to 14 somatic kineties, oral ciliature comprising membranelles M1, M2, M3, and paroral membranes PM1 and PM2, contractile vacuole at the posterior end of kinety 2, shortened last somatic kinety and a buccal field to body length ratio of 0.47+/-0.03. In addition, continuous PM1 and PM2, lack of M3 and variable kinetosome numbers in M2 and M3 were frequently observed. Specimens in the current study were compared with previous reports on M. avidus and Philasterides dicentrarchi and confirmed consistently that these 2 taxa are conspecific.

The scuticociliates Miamiensis avidus (syn. Philasterides dicentrarchi), Pseudocohnilembus persalinus, Pseudocohnilembus hargisi and Uronema marinum were cloned and identified using morphological characteristics and the small subunit ribosomal RNA gene (SSU rRNA). M. avidus strains YS1, WS1, YK1 and JJ3 from southern coastal areas and Jeju Island in Korea were pathogenic to olive flounder Paralichthys olivaceus (80 to 100% mortality in 8 to 10 g fish) when inoculated intraperitoneally (i.p.) with 1.0 to 1.4 x 10(6) ciliates fish(-1). Mortality was lower (10 to 45%) when the inoculum was 1.0 to 1.4 x 10(4) ciliates fish(-1) in the i.p.-injected group. The M. avidus strains of YS1, WS1, YK1 and JJ3 caused 60 to 100% mortality by immersion infection with 3.2 to 4.2 x 10(3) ml(-1) in 8 to 10 g fish and 3.0 to 4.0 x 10(3) ml(-1) in 30 to 40 g fish. M. avidus strain Mie0301 from the Mie prefecture in Japan caused 70% mortality by immersion infection with 4.4 x 10(3) ml(-1) in 30 to 40 g fish. The predominant sign was severe abdominal distension in i.p.-injected fish, and extensive ulcer lesions in the skeletal muscle in immersion-infected fish. Numerous ciliates were observed in the ascetic fluid, ulcers, haemorrhagic lesions, gills and brain of infected fish. However, P. persalinus (strain SCL-A), P. hargisi (strain SCL-B) and U. marinum (strain JK3) showed less than 30% mortality from both i.p. and immersion challenges, with no ciliate invasion in the skin, gills or brain. M. avidus-infected fish showed many ciliates in gills, fins, skin muscle, brain and intestine accompanied by necrosis and haemorrhages. However, no histological changes were observed in P. persalinus-, P. hargisi- or U. marinum-infected fish.