Abstract:

Streptococcus sp. was isolated from dead and moribund hybrid tilapia, (Oreochromis niloticus x O. aureus) in Saudi Arabia. Hybrid tilapia (O. niloticus x O. aureus), Taiwanese red tilapia (O. mossambocus x O. niloticus), Nile tilapia (O. niloticus), blue tilapia (O. aureus) and Mossambique tilapia (O. mossambicus) were tested for susceptibility to Streptococcus sp. at concentrations of 104-108 cells per 0.1 ml. Total mortalities at termination of the experiment were significantly different among the five species with Nile tilapia demonstrating slightly less susceptibility to the pathogen than the other four species. The LD₅₀ of Streptococcus sp. was 2.8 x 105 cells for Mossambique tilapia, 3.5 x 10⁵ cells for red tilapia, 1.8 x 10⁶ cells for hybrid tilapia, 2 x 10⁷ cells for blue tilapia and 9.2 x 10⁷ for Nile tilapia.
Streptococcus sp. was not isolated from the peritoneal cavity, liver or kidney from any of the five tilapia species that survived challenge (12-d post-infection). Moribund fish of all five species displayed similar clinical symptoms, i.e., erratic swimming, exophthalmia, melanosis (with exception in red tilapia), hemorrhagic areas around the jaws and the base of the pectoral and dorsal fins, and on the tail, and the presence of ascetic fluid in the abdominal cavity.

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Date 1996/12/01

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