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Some aspects of the interrelationship of bacterial kidney disease infection and sodium pentachlorophenate exposure in juvenile chinook salmon (Oncorhynchus tshawytscha)

University of British Columbia

The interrelationship of bacterial kidney disease, a chronic disease of cultured salmonids, and an environmental toxicant, sodium pentachlorophenate, in juvenile chinook salmon, over time, was studied. This was carried out by monitoring various haematological parameters in the fish exposed to the two factors, singularly and in combination. An evaluation of these parameters as useful indices of stressful states imposed by the treatment was also attempted. Furthermore, external and internal physical examinations, gram stained kidney smears and the occurrence of mortalities supplemented the blood changes in evaluating the response of the fish to these factors. Approximately 41 x 10⁶ viable kidney disease bacteria, isolated from moribund adult pink salmon, were injected intraperitoneally into the fish after anaesthetization with neutralized tricane methanesulphonate. Control fish were similarly sham injected. All injections were carried out on the same day. Both experimental and control groups of fish were then exposed to clean water, intermediate and high levels of sodium pentachlorophenate based on the incipient 96 h LC₅₀ value. The three levels were: 0 x 96 h LC₅₀, 0.05 x 96 h LC₅₀ and 0.50 x 96 h LC₅₀ respectively. Four days after the beginning of toxicant exposure, haematocrit, haemoglobin, red and total white blood cell counts, mean cell volume, mean corpuscular haemoglobin concentration, mean cellular haemoglobin, blood urea nitrogen, total protein and plasma glucose values were determined for experimental and control groups of fish at each toxicant level. Subsequently, these determinations were made for each group of fish every four days. This was carried out for 36 days after beginning toxicant exposure unless mortalities due to the treatments prevented sampling. Pooled (two fish per measurement; 12 measurements per sample) blood samples from the severed caudal peduncle were used for these determinations. A descriptive code was developed to categorize the progression of the bacterial infection based on the symptoms of this disease. This code was also used to quantitatively compare' the physical condition of each sample fish among the three toxicant levels. Generally, a synergistic effect was observed between the bacterial infection and toxicant exposure in the measured blood parameters, physical characteristics and the occurrence of mortalities. This synergistic effect was indicated in the measured blood parameters (blood urea nitrogen, mean cell volume and white blood cell count) by an earlier deviation of the infected fish value from the control fish value with toxicant exposure, an increase in the difference between infected and control fish values with toxicant exposure, or both at any one time or for the entire sampling period. The advanced state of physical debilitation at both levels of toxicant exposed infected fish relative to the respective control fish also indicated synergism between the two factors. Furthermore, a catastrophic mortality occurred in the infected fish at the high toxicant level on the second sampling day that was interpreted as a result of a synergistic effect of the pathogen and the toxicant. In response to the infection, depressed haematocrit, haemoglobin, red blood cell count, blood urea nitrogen, total protein and glucose values were observed in infected fish relative to control fish over the sampling period. Haemodilution due to pathogenic destruction of osmo-regulatory tissues was considered the primary cause for the observed results. Inhibition of erythropoiesis, leakage of proteins through open lesions, depletion of glycogen stores by the multiplying bacteria and cessation of feeding were also considered as additional factors that may have contributed to these results. Mean cell volume was observed to increase as a result of the infection. Erythrocytic swelling and inhibition of erythropoiesis, resulting in fewer, smaller immature cells, were considered as the causal factors for this result. Total white blood cell counts in infected fish were initially lower than respective control fish but showed an increasing trend with time. This result was seen as an initial streee-mediated leucopenia followed by a neutrophilia in response to the increase in tissue damage as a result of the disease progression. In response to the toxicant exposure uninfected control fish showed decreased haematocrits at the high level of toxicant exposure. Elevated blood urea nitrogen and glucose values were observed in the intermediate level of toxicant exposed uninfected fish relative to the uninfected controls in clean water. The increased glucose values were interpreted as being caused by an increase in the secretion of "stress hormones" as a general stress response. These results led to the conclusion that sodium pentachlorophenate exposure reduced the resistance of the fish to the effects of the kidney disease bacterial infection. It was also concluded that some of the measured blood parameters are sensitive indicators of stressful states caused by these factors. Haematocrit, red blood cell count, mean cell volume and total/differential white blood cell count measurements are recommended for routine monitoring of the physiological conditions of fish stocks for the purposes of stress detection. The careful evaluation of the physiological condition of fish stocks is recommended as a part of bioassay procedures for the purpose of making meaningful comparisons between test results.

Text

2010-02-16

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http://hdl.handle.net/2429/20269

Zoology

University of British Columbia

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