Growth kinetics, protease activity and histophagous capability of Uronema sp. infesting cultured silver pomfret Pampus argenteus in Kuwait.

Laboratory-produced and -reared sub-adults of silver pomfret (zobaidy) Pampus argenteus affected by severe scuticociliatosis during April and May 2005 were investigated and the causative was a scuticociliate, presumptively identified as Uronema sp. The parasite was capable of producing highly necrotic skin lesions. Mortalities started with a rise in the seawater temperature from 20 to 21.5 degrees C, coinciding with an increase in the total bacterial load of the rearing tank water from 10(3) to 10(5) CFU ml(-1). The parasite was successfully cultured in vitro (85 passages), using 10% brain heart infusion broth inoculated simultaneously with an aquatic Vibrio sp. The parasite reached peak cell density (1.82 x 10(5) cells ml(-1)) on the third day of inoculation. The parasite cell count was found to be inversely related (r = -0.45) to the bacterial count in the incubation medium. The ciliate body dimensions measured, on average, 34.05 +/- 0.99 microm in length and 16.70 +/- 0.74 microm in width (n = 300). Silver staining revealed morphological characteristics of Uronema sp., including a truncated anterior end, a buccal apparatus, caudal cilium and 3 oral polykinetids, with the first one appearing as a single row. Average number of kineties was 12 +/- 3. There was an inverse relationship (R2 = 0.84) between the proteolytic activity and the number of in vitro passages of the scuticociliate. However, the parasite from a freshly infected fish (using inoculum from the tenth passage) showed higher proteolytic activity (31.2%) compared with that produced by the inoculum from in vitro cultures (9.75%). The ciliate also showed a distinct histophagous capability when tested in vitro using host muscle tissue.