Vibriosis in channel catfish, Ictalurus punctatus (Rafinesque)

Abstract. An epizootic due to Vibrio anguillarum was observed in channel catfish, Ictalurus punctatus (Rafinesque). Lesions in infected fish included ulceration and petechiae on body surfaces, vent and caudal peduncle. Internally, haemorrhages in liver and kidney were present and the intestinal tract was filled with a clear viscous fluid. Using an isolant recovered during the epizootic, laboratory studies were conducted to determine (a) bacterial dynamics of the agent in blood, kidney and liver and (b) clinical haematologic and biochemical parameters in infected fish. Under conditions of the study, vibrios were apparently sequestered in kidney and liver during initial stages of infection (8–12 h after exposure). Later, bacterial numbers in blood were comparable to those in kidney and liver. Clinical parameters of infected catfish were suggestive of cellular and tissue destruction and renal dysfunction. Based upon data of the present study and those of others, vibriosis appears to be a disease in which the agent is localized in select tissues. Secondary septicaemia may be incidental to factors which compromise host defences.     

Detection of channel catfish virus in asymptomatic adult channel catfish,Ictalurus punctatus (Rafinesque)*

Abstract. Two populations of channel catfish were examined for the presence of channel catfish virus (CCV) by use of a nucleic acid probe. In one population of 22 fish with no history of CCV, viral DNA was found in every liver. These fish had previously been examined by a technique involving co-cultivation of their leucocytes with catfish tissue culture cells. The co-cultivation method had identified virus in 10 of these fish. The second fish population consisted of 14 adults that had survived a CCV outbreak in 1980. Of the 14 fish, 11 showed positive indication of CCV DNA. The tissue distribution of the CCV differed from fish to fish. All fish from the first group and one fish from the second group showed some alterations in the DNA banding patterns expected from pure CCV DNA. This might be indicative of modifications in the genomic structure of the CCV DNA when the virus is latent in a fish.     

Branchiomyces infection in farm-reared channel catfish, Ictalurus punctatus (Rafinesque)

Four cases (representing outbreaks in four different ponds on three farms) of branchial mycosis caused by Branchiomyces spp. were identified in channel catfish fry during the summer of 1996. Mortalities ranged from a few hundred to several thousand fish per pond. Significant gross and histopathological findings from these four cases were limited to the gills. All fry examined had fungal mycelia that were mainly but not entirely confined to the base of the primary lamellae and the gill arches. These fungal hyphae were intravascular and occluded vessels in the gill tissues. The present paper describes the fungal characteristics and pathology of branchial mycosis caused by Branchiomyces spp. in channel catfish.     

Cutaneous antibodies in excised skin from channel catfish, Ictalurus punctatus Rafinesque, immune to Ichthyophthirius multifiliis

This study determined whether cutaneous antibodies were present in the excised skin of channel catfish, Ictalurus punctatus Rafinesque, immune to Ichthyophthirius multifiliis Fouquet (Ich). Theronts were immobilized on or near the excised skin from immune fish. The survival of immobilized theronts was significantly reduced after exposure for 8 h to the culture of excised skin from immune fish. Culture fluids from excised skin of immune fish immobilized theronts with a peak in the immobilization titre at 24 h post-exposure. Immobility of theronts in the culture fluid from immune skin was removed after immunoabsorption with theronts. Indirect immunofluorescent staining of theronts treated with culture fluid from excised skin of immune fish revealed strong and uniform fluorescence on the cilia and cell surface of theronts. Western blot analysis of the culture fluid from immune fish revealed a 70-kDa band which corresponded to the molecular weight of catfish immunoglobulin heavy chain. The results of this study show that cutaneous antibodies to Ich theronts were present in and released from the excised skin from fish immune to Ich. Immobilization and killing of the theronts are two characteristics of the antibody response that appear to prevent the successful invasion of theronts into excised skin.     

Ultrastructural changes associated with carbon tetrachloride hepatotoxicity in channel catfish, Ictalurus punctatus Rafinesque

Abstract. Ultrastructural changes occurring in juvenile channel catfish, Ictalurus punctatus Ratinesque, hepatocytes were examined following exposure to carbon tetrachloride (CCl₄). Juvenile catfish were intraperitoneally injected with a sublethal dose of undiluted CCl₄and controls, used for comparisons, received physiological saline. The rough endoplasmic rcticulum (RER) was slightly dilated by 1 h and was significantly dilated by 4 and 24 h post-CCl₄-exposure. At 4 h, the RER became degranulated, mitochondrial eristae indistinct with electron dense flocculant matrieal deposits, and nuclear membranes indented. Autophagic vacuoles were present in some hepatocytes. Within 48 h post-exposure, the hepatocytes were indistinguishable from control hepatocytes. CCl₄ can induce reversible or irreversible cell damage which appears dose dependent.     

Molecular cloning and sequencing of hemoglobin-β gene of channel catfish, Ictalurus punctatus Rafinesque

The hemoglobin-β gene of channel catfish, Ictalurus punctatus, was cloned and sequenced. Total RNA from head kidneys was isolated, reverse transcribed and amplified. The sequence of the channel catfish hemoglobin-β gene consists of 600 nucleotides. Analysis of the nucleotide sequence reveals one open reading frame and 5′- as well as 3′-untranslated regions. The open reading frame of the sequence potentially encodes 148 amino acids with a calculated molecular mass of 16.3 kDa. The pI and charge at pH 7.0 of the deduced hemoglobin-β protein were 7.28 and 0.47, respectively. Overall, 22 amino acid residues were conserved throughout the sequences, including His64 and His93, the sites for heme-binding. Unlike the counterpart of other common cultured fish such as Salmo salar, Oncorhynchus nerka, Oncorhynchus mykiss, Cyprinus carpio and Ctenopharyngodon idella, the hemoglobin-β of channel catfish did not have cysteine. The amino acid sequence of channel catfish hemoglobin-β shows 84% homology with that of Silurus asotus (both are in the order Siluriformes). However, comparison with those of other fish species shows homology ranging from 53 to 68%. Structural analysis by the 3D-PSSM program displays that channel catfish hemoglobin-β has eight α-helices, A–H.     

Logit models for evaluating spawning performance of channel catfish, Ictalurus punctatus (Rafinesque)

Broodstock evaluations are often measured by variables such as spawning success, fecundity, fertilization and hatching rates, usually expressed as percentage values. Outcomes are generally analysed as continuous random variables, assuming that they follow a normal distribution. Ordinary linear regression models (e.g. analysis of variance) as well as χ² analysis are typically applied. However, these models may not be the most appropriate as a number of test criteria may not be met. For example, spawning success outcomes are inherently discrete and non‐negative data and hence their distribution is not likely to be normal. As these models may not be the most appropriate, a case study using logit analysis as an alternative method for the evaluation of this type of data is presented by considering the response as binary data (spawned versus did not spawn). An exact version of logit analysis was performed due to the sparseness of the data. The results demonstrate that appropriate statistical models provide better insight into the cause–effect relationships that exist between control variables and the dependent variable (likelihood of spawning in this case). As would be expected, each strain of fish responded somewhat differently to the test variables. Changing the protein level of the diet from 32% to 42% or increasing the feeding frequency from three to six times per week either did not influence spawning or negatively affected spawning respectively. Additionally, older fish performed better than younger fish and the early spawning period was better than the later spawning period, regardless of strain. These responses, however, were only detected using logit analysis, which is a more sensitive test and would thus be recommended for this type of data.     

Histological effects of prolonged sublethal hypoxia on channel catfish Ictalurus punctatus (Rafinesque)

Abstract. A vacuum degassing apparatus was used to subject channel catfish Ictalurus punctatus (Rafinesque) to sublethal hypoxia for 24, 48 or 72 h. Spleen, liver, gills, anterior kidney and posterior kidney from the catfish exhibited histopathological changes that included necrosis, hyperaemia, oedema, haemorrhage, hyperplasia and hypertrophy. Of the organs examined, gills were most affected by the hypoxic conditions. After 5 days of reacclimatization to normoxic conditions, channel catfish continued to exhibit histopathological effects of prolonged hypoxia.     

Antibody mediated immune response against Ichthyophthirius multifiliis using excised skin from channel catfish, Ictalurus punctatus (Rafinesque), immune to Ichthyophthirius

This study investigated antibody mediated immune response against Ichthyophthirius multifiliis (Ich) by determining whether theronts would retain the potential for reinfection, both in vitro and in vivo, after treatment with the culture fluid of excised skin from channel catfish, Ictalurus punctatus , immune to Ich. The invasion was reduced significantly ( P  < 0.05) for theronts treated with the immune culture fluid compared with those treated with the culture fluid from naive fish. The treatment of theronts with the immune culture fluid greatly reduced the size and survival of trophonts compared with those treated with the culture fluid from naive fish. Fewer fish were infected and the infection density was less for fish exposed to theronts treated with immune culture fluid. The infection was severe for fish invaded by theronts treated with the culture fluid from naive fish, with a high number of infected fish and heavy density of trophonts per fish. All fish were infected by Ich when exposed to the theronts treated with the immunoadsorbed culture fluid. In summary, results of this study show that cutaneous antibodies in the culture fluid of excised skin from immune fish significantly reduces theront infectivity by immobilizing or weakening theronts.     

Environmental factors affecting outbreaks of winter saprolegniosis in channel catfish, Ictalurus punctatus (Rafinesque)

Abstract. Previous studies have led to the development of a laboratory model for a disease termed 'winter saprolegniosis', locally referred to as 'winter kill', occurring in channel catfish, Ictalurus punctatus Rafinesque, raised in commercial ponds in the southeastern United States. In the laboratory, the onset of disease was routinely brought about by a combination of two related factors: (1) a rapid drop in water temperature which induces immunosuppression in catfish; and (2) maintenance of low water temperatures (∼10°C), which favour the proliferation of an ubiquitous fungus of the genus Saprolegnia to produce high levels of fungal zoospores (≥5 spores ml⁻¹). In this study, two commercial catfish ponds were monitored for over one year to determine if the above factors occurred in the field and could be correlated with outbreaks of winter saprolegniosis. It was noted that passages of severe cold weather fronts were able to drop pond water temperatures ≥10°C within 24h and that such decreases in water temperature were associated with immunosuppression of the catfish in the ponds. Furthermore, when Saprolegnia sp. zoospore levels were ≥5 spores ml^-1, the immunocompromised catfish exhibited overt signs of winter saprolegniosis. If one of the factors implicated in the induction of disease was missing, fish in the ponds remained healthy. In addition, the onset of disease in the ponds appeared independent of pH and oxygen, total ammonia nitrogen and un-ionized ammonia levels. Collectively, these field results confirm the laboratory-based hypothesis concerning the aetiology of winter saprolegniosis in channel catfish.     

Protective effect of cutaneous antibody produced by channel catfish,Ictalurus punctatus (Rafinesque), immune to Ichthyophthirius multifiliis Fouquet on cohabited non-immune catfish

Fish which survive a sublethal ichthyophthiriasis acquire protective immunity against Ichthyophthirius multifiliis Fouquet (Ich). This study evaluated the protective effect of cutaneous antibody secreted by channel catfish, Ictalurus punctatus (Rafinesque), immune to Ich on cohabited non-immune catfish. Non-immune and immune fish controls were separately maintained and infected with theronts. The Ich infection was assessed by scoring 0, < 50, 50-100, and > 100 trophonts fish(-1) at 5 days post-infection. The results of infection showed that cohabited fish at the ratio of 15 non-immune to two immune fish had < 50 trophonts fish(-1). Eighty per cent of the cohabited fish at the ratio of 10 non-immune to two immune fish showed 0 or < 50 trophonts fish(-1). The 76% of control non-immune fish had more than 100 trophonts fish(-1). The control immune fish had 0 trophonts fish(-1). Anti-Ich antibody was detected using enzyme-linked immunosorbent assay in water samples taken from tanks containing immune fish after the water samples were concentrated 40-fold. The study suggests that immune fish cohabited with non-immune fish may protect non-immune fish against Ich infection.     

Detection of channel catfish virus in channel catfish,Ictalurus punctatus (Rafinesque): use of a nucleic acid probe*

A nucleic acid probe for channel catfish virus (CCV) was constructed using recombinant DNA techniques. This probe consisted of a specific viral DNA fragment generated by digestion of CCV DNA with the restriction enzyme EcoRI. The probe was used to examine DNA isolated from tissues of fish that had been injected with CCV. Viral DNA was detected in some tissues of various injected fish. The sensitivity limit of detection was determined to be one viral DNA per cell.