| Studies were conducted to determine the causative agent/s of Epizootic Ulcerative Syndrome (EUS) in snakeheads (Ophicephalus striatus) and catfish (Clarias batrachus). Results showed that EUS could be transmitted to naive snakeheads; or catfish by cohabitation with EUS-diseased and apparently healthy snakeheads.; Severely EUS-lesioned snakeheads showed a mean bacterial load of 1.14 x 107 colony forming units (cfu) per gram of lesioned tissue, predominantly Aeromonas hydrophila. This bacterium induced dermomuscular lesions upon intramuscular (IM) injection of at least 105 cfu per catfish or snakehead.; Viruses were isolated from 35% of 71 EUS-affected fishes, predominantly from moderately lesioned fish. A virus titer of 3.02 x 106 TCID50 per ml was attained in snakehead spleen (SHS) cells at 25°C with optimum replication at 15°C. The virus was inactivated at 56°C for 30 min, resistant to IUDR and sensitive to chloroform and low pH. Freeze-thaw and storage at --10 and 8°C for 10 days did not significantly affect titer. Humoral viral neutralizing antibodies of snakeheads showed a decreasing trend in mean viral titer with severity of the disease.; The virus induced dermal lesions (+ to +++) 3 to 10 days following IM injection of at least 103 TCID50 per fish to naive snakehead juveniles at 20--22.5°C but not at 28--32°C. Naive snakehead fry and fingerlings exposed to the virus by bath manifested significant mortalities (p < 0.01) on days 4 and 5 with no apparent lesions. When the 95--96 virus isolate, A. hydrophila and Aphanomyces sp. were inoculated individually into the muscles of naive snakehead juveniles, +++, ++++ and + to ++ lesions were induced, respectively. Addition of A. hydrophila to a previously virus-injected fish, however, exacerbated the virus-induced lesion to ++++. Treatments with virus plus bacteria, virus plus fungus and all three pathogens together resulted in histopathologies consistent with those induced by virus/bacteria.; Virus isolated and primarily, propagated in SHS cells (91--97) was non-pathogenic while that isolated and primarily propagated in channel catfish ovary (CCO) cells (95--96) was pathogenic to naive snakeheads. This difference appears related to the generation of defective interfering (DI) viral particles by the former. Both isolates showed typical rhabdoviral morphology; dimensions of 65 x 175 nm; similarity of SDS-Page protein bands except for an additional band (approximate molecular weight = 29 kD) in 95--96; and antigen sharing, hence, are nearly related rhabdoviruses. The name EUS rhabdovirus (ERV) is proposed.; Taken together the data indicate that the rhabdovirus described here is likely the primary pathogen of EUS with A. hydrophila/ Aphanomyces sp. as secondary infectious agents. |
