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Studies were conducted to determine the potential pathogenicity of Edwardsiella ictaluri to economically important nonictalurid fishes in California, USA. White sturgeon Ac~penser transmontanus, striped bass Morone saxatilis, and chinook salmon Oncorhynchus tshawytscha were immersionchallenged in parallel with channel catfish Ictalurus punctatus. During a 14 d period, chlnook salmon and channel catfish succumbed to ~nfechons with E, ictalun but the other species did not. An immersion exposure to 4.0 and 7.9 X 10' cfu rill-' of E. ictalun for 30 S resulted in a 92 and 48 % mortality among chinook salmon and rainbow trout 0 . mykiss, respectively. A Gram-negahve septicemia occurred m infected fishes, and pure cultures of E. ictaluri were recovered from dead and surviving fish. There was a moderate to severe necrosis of the Liver and kidney in both salmonids and channel catfish. Intracellular bacteria occurred within mononuclear Inflammatory cells and hepatocytes. These results suggest that E. ictaluri is a potential pathogen of salmonid fishes.

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Tetracycline (TC) and oxytetracycline (OTC) caused a dose‐dependent suppression of the chemiluminescence (CL) emitted by phagocytes from the kidney of rainbow trout, Salmo gairdneri, using zymosan or latex beads as the stimulus. Compared to the control response without antibiotics, partial but significant suppression was found after exposing the cells to OTC concentrations of 0.1–50.0 μg ml−1. Cells exposed to 100 or 500 μg ml−1 OTC showed CL responses below the base levels elicited by control cells. Comparable results were obtained with cells exposed to TC and stimulated by the same stimuli. The kinetics of the CL response and the suppressive effects of the antibiotics were similar in cells from individual fish but the magnitude of responses varied. No acclimation occurred following extended exposure to the drugs.

The effect of in vitro exposure to tributyltin (TBT) on the chemiluminescent (CL) responses of kidney macrophages was examined in oyster toadfish (Opsanus tau), hogchoker (Trinectes maculatus) and Atlantic croaker (Micropogonias undulatus). Phagocytic activity was evaluated using a luminol-amplified chemiluminescent (CL) assay with zymosan as the stimulus. Following brief exposure to selected doses of TBT, the CL response of toadfish and hogchoker phagocytes was found to be significantly decreased at 400 micrograms/L TBT, while the croaker phagocytic activity was significantly decreased at 40 micrograms/L TBT. With 18 hr-exposure to TBT, the effect on the CL response was noticeable at lower doses (40 micrograms/L TBT for toadfish and 4 micrograms/L TBT for hogchoker).

Three myxosporeans were encountered in the cranial tissues of a California population of rainbow trout Oncorhynchus mykiss examined for the presence of Myxobolus cerebralis, the causative agent of whirling disease. Typical spores of M. cerebralis and a previously undescribed species ofMyxobolus were found in the cranial tissues prepared by the pepsin HCl-trypsin digestion method. Henneguya zschokkei was also detected in digest preparations of cranial tissues, but was more numerous when branchial cartilage was included in the preparations. Microscopic examinations of tissues of individual rainbow trout showed occasional infections with both myxobolid species. Myxobolus cerebralis trophozoites and spores were found in the cranial and gill cartilage, and Myxobolus sp. was found in the brain and spinal cord. Henneguya zschokkei was also found within granulomas in the connective tissues below the gill arch. Both M. cerebralis and H. zschokkei were associated with a chronic inflammatory response in their respective tissues. In contrast, the Myxobolus sp. spores were found in pockets within the nervous tissues with no detectable host response. The spore measurements, calculated from fresh digests of infected tissues for the three myxosporeans (N -20), for length x width x thickness in micrometers (SD) were 11.7 (0.6) without tails and 42.6 (5.2) with tails x 7.7 (0.8) x 7.0 (0.1) for H. zschokkei, 9.9 (0.4) x 8.4 (0.1) x 6.5 (0.3) for M. cerebralis, and 12.7 (0.7) x 10.5 (1.0) x 9.5 (0.8) for Myxobolus sp. Examined under scanning electron microscopy, the latter two species were morphologically similar although distinctive in size.

Typical primary antibody responses were found after injecting carp maintained at 25°C with formalin-killed (Fk) Vibrio anguillarwn bacteria, but not in fish injected with the same antigen held at 12" C. There were no significant increases over the primary responses in fish receiving a second injection 60 days after the first injection.The differences in the results between fish groups receiving three V. unguillururn antigens at two dosages were found to be insignificant in most of the experiments. A comparison of the antibody titres of fish groups after challenge with Fk bacteria showed no difference from control fish receiving a single injection similar to the challenge dose or fish receiving a previous injection with the antigens emulsified in complete Freund's adjuvant (CFA) at 25" C. However, significant immunosuppression was found in fish that were given the primary injection intracardially at 25" C or 12" C, or with CFA emulsion at 12" C.