Enteric Septicemia Resistance in Blue Catfish and Three Channel Catfish Strains

Abstract

Seven full-sib families in each of three strains of channel catfish Ictalurus punctatus (mean weight, 10.7 g; SD, 2.6 g) and one family of blue catfish I. furcatus (mean, 10.9 g; SD, 0.4 g) were challenged by immersion with the bacterium Edwardsiella ictaluri, the causative agent of enteric septicemia, to evaluate variation in disease resistance. Blue catfish were resistant to infection and had only 0.7% mortality. Significant (P < 0.001) variation in channel catfish resistance was found among strains, families, and body weights. Red River strain channel catfish were the most resistant (14.9% mortality), followed by Mississippi-select fish (67.1%) and Mississippi-normal fish (72.3%). Mean family mortality of channel catfish ranged from 1.3% in a Red River family to 95.3% in a Mississippi-select family. Although weight had a significant effect on survival, the regression slope of survival on weight was ?0.10 (SE, 0.02). Sources of variation in channel catfish mortality adjusted for weight were 67.3% from strains, 29.7% from families, and 3.0% from replicate tanks. The mechanism for differential resistance presently is unknown. Results indicate considerable potential for reducing disease losses by using E. ictaluri-resistant catfish strains in aquaculture and for genetic improvement in resistance via selective breeding programs.     

Experimental Induction of Proliferative Gill Disease in Specific-Pathogen-Free Channel Catfish

Abstract

Specific-pathogen-free channel catfish Ictalurus punctatus were exposed to sediment and mud from a pond containing channel catfish with proliferative gill disease. In one experiment, fish were to exposed to mud and sediment for 2 months in water maintained at 19°C. Fish were necropsied weekly, and certain tissues were examined histologically and ultrastructurally. Four trials were conducted with sediment samples from different epizootics of proliferative gill disease. In a second experiment, fish were exposed to sediment for 7 d in water maintained at 16, 19, or 26°C; the fish were then moved to clean water held at 16, 19, or 26°C. Fish were necropsied before transfer to clean water and weekly thereafter for 2 months. Channel catfish held at 19°C developed proliferative gill disease within 2 d of exposure to sediment. Primary cells of a uninucleate myxosporean parasite were present in the gills at the base of lamellae. These developed into plasmodia with numerous secondary cells, and some primary cells disintegrated, releasing their internal secondary cells. Similar development was observed in internal organs 1 week after appearance of the parasite in gills. Complete sporogony did not occur over the 2 months of this study. Plasmodia became necrotic and were not detected after 60 d. In fish exposed to sediment for 7 d at 16, 19, and 21°C, similar organisms were detected, but clinical disease occurred only at 19 and 26°C. Proliferative gill disease may be attributed to extrasporogonic stages of a myxosporean resembling Sphaerospora spp.     

Channel Catfish Response to Ultraviolet-B Radiation

Abstract

Fingerling channel catfish Ictalurus punctatus exposed to simulated ultraviolet-B radiation at an average daily dose of 2.9 J/cm² were quite sensitive to the radiation. After a 24-h exposure, thinning of the most dorsal epidermis frequently was accompanied by edema. Compared with epidermis of unexposed fish, mucous cells in exposed fish were less superficial and club cells were less numerous both dorsally and high on the lateral surface of the body. Sunburn cells with pyknotic nuclei were evident in the epidermis of exposed fish. Among fish exposed for 48 h, focal necrosis and sloughing of the outer epidermal layer were widespread. A methanol-extractable skin substance that is associated with resistance to sunburn in other fish species was not detected in channel catfish.     

Efficacy of Potassium Permanganate in Treating Ichthyophthiriasis in Channel Catfish

Abstract

Epizootics of ichthyophthiriasis can be controlled with potassium permanganate (KMnO₄), but its effectiveness has not been confirmed by controlled studies. The purpose of this study was to determine the concentration of KMnO₄ needed to halt an active Ichthyophthirius multifiliis infestation in channel catfish Ictalurus punctatus. Juvenile channel catfish were exposed to fish infested with I. multifiliis until they developed immature trophonts. They were then moved to individual static containers with 2 L of filtered well water, where they were treated with KMnO₄ daily for 10 d. The lowest effective dose of KMnO₄ required to eliminate theronts was 1.25 mg/L. The results indicate that KMnO₄ is effective for controlling I. multifiliis epizootics at low concentrations in clean water. However, effective treatment in ponds will be strongly influenced by detoxication of KMnO₄ depending on the concentration of easily oxidizable substances in the water.     

Longevity of Bolbophorus damnificus Infections in Channel Catfish

Abstract

The digenean Bolbophorus damnificus infects commercial channel catfish Ictalurus punctatus, causing mortality, lower feed consumption, and reduced growth in surviving fish. The purpose of this study was to determine the length of time for which B. damnificus prodiplostomulum metacercariae (juvenile trematode stage that infects fish) would remain viable (parasite appearing to be intact or exhibiting movement) in channel catfish. Fish (n = 210) were infected with molecularly confirmed B. damnificus cercariae harvested from naturally infected marsh rams-horn snails Planorbella trivolvis. During the first sampling (at 20 d postinfection), 8.3 ± 3.6 metacercariae/fish (mean ± SD) were found in the host muscle and visceral organs. The channel catfish were then acclimated to a water temperature of either 18°C or 28°C. After 11 months, 6.8 ± 3.5 and 5.9 ± 3.0 metacercariae/fish were found in groups held at 18°C and 28°C, respectively. The mean number of parasites per fish did not significantly differ between fish held at the two temperatures and did not significantly decline over time at either temperature. Fish examined from 13 to 30 months postinfection all contained viable metacercariae that were morphologically and molecularly identified as B. damnificus. At 18 months, 12 metacercariae (of which 11 were intact and 10 displayed movement) were found in the one fish sampled; at 30 months, the last fish sampled contained three intact metacercariae (one displayed slight movement). Our results indicate that B. damnificus metacercariae can remain viable in channel catfish for at least an 18–30-month production cycle during which they have the potential to affect fish growth; in addition, infected fish may serve as intermediate hosts for these metacercariae for at least 2.5 years postinfection.

Received July 14, 2010; accepted March 6, 2011     

Effect of Temperature and Dissolved Oxygen Concentration on Edwardsiella ictaluri in Experimentally Infected Channel Catfish

Abstract

Channel catfish Ictalurus punctatus were experimentally infected with Edwardsiella ictaluri by immersion exposure. After clinical disease ran its course for 52 d, the surviving fish were exposed to one of the following environmental regimes in troughs: 25°C with aeration, 25°C with no aeration, or variable temperature (18–23°C) with no aeration. After 29 d of exposure to the environmental regimes, various organs and tissues of the fish were assayed to determine the effects of these conditions on E. ictaluri concentrations (colony-forming units/mL of tissue sample). The concentrations of this pathogen were significantly (P < 0.05) higher in all tissues (trunk kidney, liver, head kidney, blood, spleen, gallbladder, muscle, brain, and gonad) 52 d postinfection than 29 d after exposure to any of the environmental regimes (81 d postinfection). Fish exposed to a near-normal concentration of dissolved oxygen (6.4 mg/L) and a constant temperature of 25°C had E. ictaluri concentrations that were significantly (P < 0.01) lower than those offish exposed to a low oxygen concentration (2.6 or 1.8 mg/L) and either a constant or a variable temperature.     

Efficacy of Florfenicol for Control of Mortality Associated with Edwardsiella ictaluri in Three Species of Catfish

The efficacy of florfenicol for control of mortality associated with Edwardsiella icatluri was studied in fingerlings of Channel Catfish Ictalurus puntatus (Delta strain), Blue Catfish I. furcatus (D&B strain), and a hybrid catfish (Delta strain Channel Catfish × D&B strain Blue Catfish). On day 0, fish were immersion challenged in 65-L aquaria. For each of the three species of catfish, 10 aquaria were randomly assigned to two treatment groups, either treated with florfenicol at 0 mg/kg of body weight (unmedicated feed) or at 10 mg/kg (medicated feed). Fish were treated for 10 consecutive days, monitored for mortality during this treatment period, and observed for 14 d afterwards. Post observation, all survivors were humanely euthanized in tricaine methanesulfonate, cultured for E. ictaluri, and examined for gross pathology. The mean cumulative percent mortality from enteric septicemia of catfish (ESC) challenge among the three genotypes of catfish did not differ between Blue Catfish, hybrid, and Channel Catfish in treated or control groups. However, the florfenicol-treated fish had a significantly lower mean cumulative mortality (6%) than the controls (78%). All genotypes of catfish tested were responsive to treatment with florfenicol-medicated feed for control of mortality associated with ESC. There were no significant differences in mortality associated with hybrid catfish, blue catfish, and Channel Catfish (Delta strain).

Received July 20, 2014; accepted October 10, 2014     

Early Changes in Pigmented Macrophages in Head Kidney of Channel Catfish Infected with Aeromonas hydrophila

Abstract

Channel catfish Ictalurus punctatus with scarified skin were exposed to the bacterial pathogen Aeromonas hydrophila. Systemic infections developed in 80% of the exposed fish, and the remaining exposed fish had infections limited to the cutaneous lesion. Skin of control fish was injured in the same manner as for the exposed fish, but control fish were not exposed to A. hydrophila nor was A. hydrophila recovered from them. None of the fish died during the 3-d experiment, and gross lesions in head kidneys, including changes in the size or relative weight, were not found in infected fish. Macrophage aggregates in head kidney of systemically infected fish increased 68% in mean area, 28% in number, and 111% in total relative volume. Volume of lipofuscin, the predominate pigment found in channel catfish head kidney, in systemically infected fish was about twice that of control fish. Volume of hemosiderin was about four times as great and macrophages containing hemosiderin were about three times as numerous in systemically infected fish as in control fish. The amount of melanin in macrophages did not change in infected fish. Significant differences in macrophage aggregates and pigments were not found between controls and fish with only superficial infections. Microscopic lesions, other than changes in macrophage aggregates and pigments, were not found in the head kidney. Increases in macrophage aggregates, lipofuscin, and hemosiderin in head kidney of channel catfish were useful for quantification of injury caused by a systemic bacterial infection.     

Comparative Susceptibility of Channel Catfish,Blue Catfish,and their Hybrid Cross to Experimental Challenge with Bolbophorus damnificus (Digenea: Bolbophoridae) Cercariae

Abstract

The digenetic trematode Bolbophorus damnificus has been implicated in significant losses in catfish aquaculture since the late 1990s. The complex life cycle sequentially involves the American white pelican Pelecanus erythrorhynchos, the marsh rams horn snail Planorbella trivolvis, and Channel Catfish Ictalurus punctatus. Research supports anecdotal reports from the industry, suggesting that the hybrid of Channel Catfish×Blue Catfish I. furcatus is less susceptible to disease agents that have been historically prohibitive to Channel Catfish production, namely the gram-negative bacteria Edwardsiella ictaluri and Flavobacterium columnare, as well as the myxozoan parasite Henneguya ictaluri. This current research compared the susceptibility of Channel Catfish, Blue Catfish, and their hybrid cross to an experimental challenge by B. damnificus. Fish were exposed to 0, 100, 200, and 400 B. damnificus cercariae per fish, and the numbers of metacercariae per fish were determined 14 d postchallenge. Metacercariae were recovered from all challenged fish. There were no significant differences among fish groups challenged with the same dose, suggesting Channel and Blue Catfish and their hybrid are comparably susceptible to B. damnificus infection. As such, it is recommended that producers raising hybrid catfish remain diligent in controlling populations of the snail intermediate host to prevent production losses attributed to B. damnificus, especially when loafing pelicans have been observed at the aquaculture operation.

Received October 22, 2013; accepted January 5, 2014     

Survival and Antibody Response of Channel Catfish,Blue Catfish,and Channel Catfish Female × Blue Catfish Male Hybrids after Exposure to Edwardsiella ictaluri

Abstract

Juvenile Norris strain channel catfish Ictalurus punctatus, blue catfish I. furcatus, and Norris strain channel catfish female × blue catfish male hybrids were challenged with Edwardsiella ictaluri by bath immersion or intraperitoneal injection (high or low dose) in aquaria. Survival (%) after bath immersion was highest for blue catfish (89.5 ± 2.8), intermediate for hybrids (73.8 ± 6.7), and lowest for channel catfish (62.0 ± 4.2). Prechallenge antibody levels to E. ictaluri, measured by enzyme-linked immunosorbent assay, were negative (mean ± SE optical density [OD] = 0.010 ± 0.003). Postchallenge antibody response for blue catfish (OD = 0.132 ± 0.045) was significantly lower than that of channel catfish (OD = 0.350 ± 0.045), whereas the response of the channel × blue catfish F₁ hybrids (OD = 0.263 ± 0.051) was intermediate and not significantly different from either parental species. Intraperitoneal injections of E. ictaluri resulted in significant mortality only in channel catfish (88.3 ± 2.6% survival) and were sublethal to hybrid catfish and blue catfish with 100.0% and 99.3 ± 0.4% survival, respectively. Antibody responses after the injection challenge were significantly different among catfish groups and injection dose with no group × dose interaction. Antibody responses after the injection challenge were consistent with the immersion challenge, and means of high and low challenge doses were lowest in blue catfish (OD = 0.061 ± 0.014), intermediate in hybrids (OD = 0.187 ± 0.014), and highest in channel catfish (OD = 0.272 ± 0.014). For all fish groups combined, the high injection challenge dose resulted in higher antibody levels (OD = 0.206 ± 0.011) than low injection challenge dose (OD = 0.140 ± 0.012). Overall results indicate greater resistance to E. ictaluri and lower antibody response in blue catfish, and show the potential to identify molecular markers linked with disease resistance and introgression of resistance genes from blue catfish into channel catfish.     

Efficacy of Copper Sulfate for the Treatment of Ichthyophthiriasis in Channel Catfish

Abstract

Ichthyophthirius multifiliis is a protozoan that may infest and significantly damage cultured fish species. The purpose of this study was to measure the efficacy of copper sulfate in treating ichthyophthiriasis. Fingerling channel catfish Ictalurus punctatus exposed to at least 2,000 theronts of I. multifiliis per liter of water developed consistent infestations of I. multifiliis (20 or more trophonts on the dorsal surface of the head of the fish). Infestation was observed in untreated controls at day 5 after exposure and mortality occurred after day 10. Coexposure studies with theronts and different concentrations of copper sulfate revealed that all theronts were killed at concentrations greater than 0.05 mg/L. To determine the effect of copper sulfate in the treatment of ichthyophthiriasis, fish were exposed to the parasite until trophonts were observed (day 5), and they were subsequently treated with copper sulfate. The lowest effective concentration of copper sulfate for treatment of ichthyophthiriasis (i.e., after infestation was observed in the fish) was 0.4 mg/L. To assess the effects of various water quality conditions on copper treatment, total suspended solids (TSS) and pH were varied during treatment of ichthyophthiriasis. Concentration of TSS was inversely correlated to the efficacy of copper sulfate for I. multifiliis infestations, whereas no relationship was observed between pH and efficacy of a single copper sulfate dose. The results indicated that copper sulfate can be used to treat ichthyophthiriasis at concentrations of 0.4 mg/L for at least 5 d under the specific water conditions used in this study (pH, 7.45 ± 0.27; temperature, 20.5 ± 0.7°C; alkalinity, 176.6 ± 28.1 mg/L as CaCO₃) and that efficacy of copper sulfate was affected more by TSS concentration than by pH.     

Transmission of Edwardsiella ictaluri from Infected,Dead to Noninfected Channel Catfish

Abstract

Enteric septicemia of catfish (ESC) was transmitted horizontally from channel catfish Icialurus punctatus that had died from Edwardsiella ictaluri infection to contact channel catfish during 2 d of habitation in a tank. The contact channel catfish became positive for E. ictaluri antibody, became infected with this bacterium, and had signs of ESC and died within 12 d postexposure. Edwardsiella ictaluri was recovered from 24 of the 30 contact channel catfish that died from ESC, as well as from 9 of the 25 tested contact survivors. The cannibalizing of E. ictaluri-infected fish, or the shedding of E. ictaluri from dead fish, or both, were shown to be mechanisms of horizontal transmission of ESC among channel catfish.     

Differences between Plasma and Serum Samples for the Evaluation of Blood Chemistry Values in Rainbow Trout,Channel Catfish,Hybrid Tilapias,and Hybrid Striped Bass

Abstract

Blood chemistry analytes are determined for fish from either serum or plasma samples. Serum and plasma are similar in that they both represent the fluid component of blood; however, plasma contains clotting factors that are not present in serum. This study was conducted to determine whether the type of sample—plasma or serum—had an effect on measured blood analytes in rainbow trout Oncorhynchus mykiss, channel catfish Ictalurus punctatus, hybrid tilapia Oreochromis spp., and hybrid striped bass (striped bass Morone saxatilis × white bass M. chrysops). Paired plasma and serum samples were analyzed for the following standard biochemical analytes: total protein, albumin, globulin, creatinine, total bilirubin, alkaline phosphatase, aspartate aminotransferase, sodium, potassium, chloride, calcium magnesium, phosphorus, glucose, and cholesterol. For all four taxa, values for potassium were lower in the serum and magnesium and phosphorus values were higher in the serum. Glucose values were lower in the serum from rainbow trout, hybrid striped bass, and channel catfish; whereas cholesterol values were higher in the serum of rainbow trout, channel catfish, and hybrid tilapias. The differences observed between serum and plasma were distinct from changes occurring with hemolysis and, therefore, do not represent release of erythrocyte constituents. The differences most likely represent metabolic utilization of blood constituents while the blood was clotting. This work indicates that plasma should be used preferentially to serum for biochemical analysis because analyte levels determined from serum may not accurately reflect those found in circulating blood.     

Subchronic Toxicity of Dietary Aflatoxin B1 to Channel Catfish

Abstract

Means for growth rate, hematocrit, hemoglobin concentration, and erythrocyte count of channel catfish Ictalurus punctatus fed 10,000 μg aflatoxin B₁ (AFB₁) per kilogram of feed for 10 weeks were significantly lower than those of fish fed 2,154 μg/kg or a lower concentration (P < 0.05). Mean leukocyte count was significantly higher in the fish fed the highest concentration of AFB₁ (P < 0.05). Gross appearance and behavior of all fish were normal. Histopathological effects were observed only in fish fed the highest concentration of AFB₁. These fish had foci of necrotic hepatocytes mixed with basophilic hepatocytes. Spaces, apparently resulting from hepatocellular necrosis, were present within the basophilic foci. Sinusoids in the head (hematopoietic) kidney were dilated and circular in profile. Increased hematopoietic activity of blood-forming tissues was apparent from the presence of numerous immature blood cells. The intestinal mucosal epithelium accumulated excessive amounts of iron pigments. Gastric glands in the stomach were necrotic and contained infiltrating macrophages.     

Communications: Specificity of the Channel Catfish Antibody to Edwardsiella ictaluri

Abstract

The specificity of channel catfish Ictalurus punctatus serum antibody to Edwardsiella ictaluri was characterized by microtiter agglutination assay. There was no correlation between antibody titer to Aeromonas hydrophila and antibody titer to E. ictaluri in wild or feral channel catfish. Anti-E. ictaluri antibodies in naturally infected channel catfish were not removed by adsorption by nine other species of bacteria found in the channel catfish intestine and fish ponds. Channel catfish immunized with nine other species of bacteria did not develop substantial antibody titer to E. ictaluri. The antibody response of channel catfish to E. ictaluri is highly specific, and the microtiter agglutination test is a specific indicator of previous exposure to E. ictaluri     

Effect of Dietary Lipid Sources on Production of Leukotriene B by Head Kidney of Channel Catfish Held at Different Water Temperatures

Abstract

Separate and combined effects of dietary fatty acids and water temperature on the production of leukotriene B (LTB) by the head kidney of channel catfish Ictalurus punctatus were investigated. Fish were fed semipurified diets containing 7% lipid as beef tallow, corn oil, linseed oil, menhaden oil, or a mixture (1:1:1) of menhaden oil, beef tallow, and corn oil. At 28°C, fish fed corn oil produced the greatest amount of LTB, and fish fed beef tallow produced the least. At 17°C, production of LTB by fish fed the beef tallow diet was again lowest; production of LTB by fish fed the corn oil diet was highest but was not significantly different from that obtained with the mixed-oil or menhaden oil diet. Production of LTB for the other diets was intermediate. This is the first report that the head kidney of channel catfish produces LTB; this production was affected by dietary lipid sources but not by the two water temperatures tested. Production of LTB was not related to weight gain. Moreover, the pattern of LTB production does not explain survival rates of channel catfish observed in a previous experiment in response to pathogen challenge. However, LTB concentration does appear to be roughly proportional to the amount of eicosanoid precursors (arachidonic acid and eicosapentaenoic acid) previously reported for liver phospholipids of channel catfish fed diets containing different amounts of essential fatty acids.     

Attempts to Control Flexibacter columnaris Epizootics in Pond-Reared Channel Catfish by Vaccination

Abstract

Evaluation of bacterins for the immunization of channel catfish Ictalurus punctatus against Flexibacter columnaris was carried out over 5 years. Groups of 60,000–100,000 pondreared channel catfish, weighing an average of 1.5–4.1 g, were vaccinated each year with formalininactivated Flexibacter columnaris bacterins by immersion. Bacterins were prepared from isolates of the preceding year's epizootic. Equal numbers of nonvaccinated channel catfish reared under similar conditions served as controls. Fish were vaccinated during May each year when water temperatures ranged from 16 to 18°C. Data were recorded daily from early June through midSeptember as water temperatures rose to 24–26°C. Deaths were recorded daily, and the number of hours required for antibiotic treatments were monitored. Analysis of the data indicated a general trend toward a beneficial effect of vaccination, particularly for larger fish. Mortality and hours of antibiotic treatment were significantly higher for most control groups than for vaccinated groups.     

Spleen Index and Mannose-Binding Lectin Levels in Four Channel Catfish Families Exhibiting Different Susceptibilities to Flavobacterium columnare and Edwardsiella ictaluri

Abstract

Edwardsiella ictaluri and Flavobacterium columnare are two bacterial pathogens that affect channel catfish Ictalurus punctatus aquaculture. At the Catfish Genetics Research Unit (U.S. Department of Agriculture, Agricultural Research Service), some progress has been made in selectively breeding for resistance to E. ictaluri; however, the susceptibility of these families to F. columnare is not known. Our objectives were to obtain baseline information on the susceptibility of channel catfish families (maintained as part of the selective breeding program) to E. ictaluri and F. columnare and to determine whether the spleen index and plasma levels of mannose-binding lectin (MBL) are predictive indicators of susceptibility to these pathogens. Four channel catfish families were used: family A was randomly chosen from spawns of fish that were not selectively bred for resistance; families B, C, and D were obtained after selection for resistance to E. ictaluri. All four families were immersion challenged with both bacterial pathogens; the spleen index and plasma MBL levels of unchallenged fish from each family were determined. Mean cumulative percent mortality (CPM) after E. ictaluri challenge ranged from 4% to 33% among families. Families A and B were more susceptible to F. columnare (mean CPM of three independent challenges = 95% and 93%) than families C and D (45% and 48%), demonstrating that there is genetic variation in resistance to F. columnare. Spleen index values and MBL levels were not significantly different, indicating that these metrics are not predictive indicators of F. columnare or E. ictaluri susceptibility in the four tested families. Interestingly, the two families that exhibited the highest CPM after F. columnare challenges had the lowest CPM after E. ictaluri challenge. Further research on larger numbers of families is needed to determine whether there is any genetic correlation between resistance to E. ictaluri and resistance to F. columnare.

Received November 18, 2011; accepted February 23, 2012     

Changes in Peripheral Blood Leukocyte Percentages and Function of Neutrophils in Stressed Channel Catfish

Abstract

Channel catfish Ictalurus punctatus were subjected to a simulated handling (including bleeding) and transport stress. Using flow cytometry in conjunction with monoclonal antibodies to channel catfish B and T lymphocytes (minimal cross-reaction with thrombocytes) and neutrophils coupled with routine hematological methods, we demonstrated significant decreases in the percentage of B lymphocytes and increases in T lymphocytes and neutrophils of transported fish, yet there was no change in neutrophil phagocytic function. These data suggest that the length of the stressing event or the duration of exposure of the neutrophils to cortisol could determine if immune responses become suppressed. These data prompted us to expose channel catfish in vivo, and isolated neutrophils in vitro, to cortisol at various concentrations and for various time periods. Exposure of neutrophils to cortisol concentrations of 100 μg/dL for 2 h did not suppress phagocytosis; however, significant decreases in the percent phagocytosis and bacterial killing and increases in total number of neutrophils isolated were obtained after repeated in vivo administration of cortisol. The results of these experiments indicate (1) that bleeding, handling, and transport of channel catfish induce a characteristic relative lymphopenia and neutrophilia, (2) that cortisol does not act alone to induce suppression of phagocytic function (based on the in vitro administration of cortisol), and (3) that high physiological concentration of cortisol in vivo can initiate phagocytic suppression. In general, the intraperitoneal cortisol injection experiments suggested that stress on channel catfish that produces a concomitant high serum concentration of cortisol can suppress neutrophil phagocytic function, perhaps providing an avenue for the onset of infection.     

Electron Microscopy of Infection by Saprolegnia spp. in Channel Catfish

Abstract

Saprolegnia sp. isolated from channel catfish Ictalurus punctatus grew slower than S. parasitica on cornmeal agar (CMA). Oogonia in Saprolegnia sp. appeared frequently, whereas oogonia were rarely seen in S. parasitica on CMA. In experimental exposures of injured channel catfish to fungal spores, infections were apparent after 3–4 d, but were most common after 7–9 d. Multiple lesions were usually seen in naturally infected fish, whereas a single lesion appeared at the injured site of experimentally infected fish. No obvious differences were found between lesions caused by S. parasitica and those caused by Saprolegnia sp. Most of the epidermal cells in fungusinfected lesions were necrotic. In some lesions, the epidermis was completely sloughed and the dermis was exposed. Both Saprolegnia parasitica and Saprolegnia sp. penetrated the dermis, causing damage to fibroblasts and collagen lamellae.