Effects of Increasing Stocking Density on the Performance and Ileal Microbiota of Broilers

This study was conducted to investigate the effects of increasing stocking density under suitable environmental conditions on the performance and ileal microbiota of broilers. A total of 108 Arbor Acres male broilers (28 days old) were allocated to a normal stocking density (NSD, normal stocking density; 31 kg/m²) and a maximum allowed stocking density group (MSD, maximum stocking density; 39 kg/m²). All birds were reared at a constant temperature of 21°C. At 42 days of age, bacterial DNA was extracted from ileal content, and the V3–4 hypervariable region of 16S rRNA was amplified. Increasing stocking density had no significant effect on average daily gain, average daily feed intake, and feed conversion ratio (P>0.05). The alpha and beta diversities of the ileal microbiomes did not differ significantly between the NSD and MSD groups; however, increasing stocking density altered the composition of ileal microbiota. The relative abundance of Lactobacillales, including Lactobacillus, Enterococcus, and Streptococcus, significantly decreased in MSD broilers, compared with NSD broilers. The present results suggest that even under suitable environmental conditions, an increase in stocking density to a level of 39 kg/m² may disturb the composition of ileal microbiota in broilers. Further studies are needed to determine the reasons and the potential consequences for animal health and physiology.     

Impact of stocking density on broiler growth performance,meat characteristics,behavioural components and indicators of physiological and oxidative stress

1. The study was designed to assess the impact of stocking density (6 and 13 birds/m² equivalent to 12·6 or 27·2?kg/m², respectively) on growth performance, meat quality, behaviour, and indicators of physiological and oxidative stress as measures of bird welfare.

2. The higher stocking density negatively affected final body weight and feed intake but not cumulative feed conversion rate. Muscle colour traits, pH₂₄, cooking loss and shear values were not affected. Birds reared at the lower density showed higher intramuscular fat, liver weight, liver NADP-isocitrate and NADP-malate dehydrogenase activity.

3. Higher stocking density was associated with decreased locomotor activity and increased physiological (H:L ratio and bursa weight) and oxidative (glutathione concentrations and reduced:oxidised glutathione ratios) stress indicators.

4. The results show that stocking density did not significantly affect broiler meat quality characteristics but higher density decreased growth performance, increased physiological and oxidative stress levels and decreased locomotor activity.     

Observations on the Occurrence of Bacterial Gill Disease and Amoeba Gill Infestation in Rainbow Trout Cultured in a Water Recirculation System

Abstract

Spontaneous outbreaks of bacterial gill disease (BGD) occurred in 17-44-g rainbow trout Oncorhynchus mykiss after they were placed in a water recirculation system. In each of five groups stocked from September 1991 through July 1992, BGD occurred within 6–8 d after stocking. In each instance, BGD was followed by a secondary amoeba infestation. The spontaneous BGD outbreaks did not occur among previously stocked groups that had recovered from earlier BGD disease outbreaks. Examination of gill tissue by Gram stain and indirect fluorescent antibody technique showed increased numbers of filamentous bacteria associated with BGD after the rainbow trout were stocked into the system. Bacterial numbers decreased after a 1-h treatment with chloramine-T at concentrations of 9–15 mg/L but increased within 2 d after treatment. Although the chloramine-T treatments controlled mortality related to BGD, the amoeba infestation persisted. Histological examination of gills showed some focal hyperplasia before the rainbow trout were placed into the recirculation system, but hyperplasia became more extensive and lamellar fusion and mild telangiectasis developed within a week after placement in the system. The density at which the fingerling rainbow trout were stocked and the suspended solids present in tank water may have contributed to the BGD outbreaks. Exposure of juvenile rainbow trout to tank water from the recirculation system before they were placed into the system did not afford them protection against BGD after stocking.     

Effects of stocking density on growth performance,blood parameters and immunity of growing pigs

This study was conducted to evaluate the effects of different stocking densities on growth performance, blood parameters, intestinal morphology and intestinal immunity of growing pigs. A total of 288 male pigs (44.35 ± 0.50 kg) were randomly assigned to groups with stocking densities of 2.46, 1.23 and 0.82 m²/pig for a month. The results showed that there was no significant difference on growth performance among groups. Pigs in the stocking density of 0.82 m²/pig had the lowest backfat thickness and spleen weight index among groups (P < 0.05). With increasing stocking density, the serum concentrations of blood urea nitrogen (BUN), transglutaminase (TGG), alkaline phosphatase (AKP) and immunoglobulin A (IgA) were increased, and albumin (ALB), albumin-to-globulin ratio (ALB:GLO), insulin-like growth factor-1 (IGF-1) and immunoglobulin (IgM) were decreased (P < 0.05), and cortisol tended to increase and glucose tended to decrease (0.05 < P < 0.1). Compared with the stocking density of 2.46 m²/pig, the ileal villus height and jejunal villus width decreased in stocking densities of 1.23 and 0.82 m²/pig (P < 0.05). The duodenal villus height and ileal villus width in the stocking density of 0.82 m²/pig were the lowest among 3 groups (P < 0.05). The content of immunoglobulin A in duodenum, jejunum and ileum mucosa increased along with increasing density (P < 0.05). The contents of interleukin (IL)-2 in the spleen or liver and IL-10 in the spleen were higher in the stocking density of 0.82 m²/pig than in other 2 groups. These results showed that stocking density could affect the metabolism, intestinal morphology, and immunity of growing pigs and 1.23 m²/pig may be the suitable stocking density for the growing pigs in the present study.     

Responses of male broiler chickens from two genetic groups to different stocking densities in a semi-arid subtropical environment

The effects of stocking densities of 30, 35, 40 and 45 kg body weight (BW)/m² on the growth performance of Ross 308 and Hubbard broiler chickens were studied in a 49-day production period. Ross 308 was superior to Hubbard in weight gain and market weight and consumed more feed (p < 0.05) but the strains did not differ significantly in feed efficiency, mortality rate and abdominal fatness. Overall, birds raised at stocking densities of 30, 35 and 40 kg BW/m² performed equally well in weight gain, feed consumption and 49-day live weight and outperformed those raised at 45 kg BW/m² (p < 0.05). It is suggested that for profitable broiler production under tropical conditions Ross 308 and stocking density of 40 kg BW/m² be considered.     

White Sturgeon as a Potential Vector of Infectious Hematopoietic Necrosis Virus

Abstract

Cell lines from white sturgeon Acipenser transmontanus were derived from peripheral blood cells, heart, and spleen. Incubated with infectious hematopoietic necrosis virus (IHNV) for 8 d at l5°C, these cell lines produced 0.7–53.2 plaque-forming units (PFU)/cell. Waterborne exposure of larval white sturgeons (60 d posthatch) to 10⁶ PFU/mL of IHNV resulted in 10% mortality 5–6 d postinfection, with virus concentrations consistently greater than 10⁵ PFU/g. A replicate group of larval white sturgeons that were sampled at different times post-IHNV exposure had no detectable virus at 24 h, but 72% of the fish had IHNV concentrations of 10²-10⁶ PFU/g when they were examined 2–9 d postinfection. Juvenile white sturgeons (mean weight, 35 g) immersed in or injected with IHNV exhibited no mortality, and virus was only detected immediately postexposure in just 25% of the fish tested. Juvenile white sturgeons fed either virus-free rainbow trout Oncorhynchus mykiss or dead IHNV-infected rainbow trout had no viable virus in their feces. Juvenile white sturgeons fed or exposed to IHNV failed to transmit the virus to cohabiting rainbow trout fry. These results suggest that IHNV can replicate in larval white sturgeons but presumably not in juveniles or adults. Virus neutralization activity was detected in serum from adult white sturgeons (4–6 years old) cultured with rainbow trout exposed to IHNV but not in white sturgeons kept in a pathogen-free environment and fed a manufactured diet. White sturgeon serum with IHNV-neutralizing activity was used to passively immunize rainbow trout, and it provided significant (P < 0.01) protection against IHNV challenge.     

The prevalence of proliferative kidney disease from the kidney and muscle of rainbow and brown trout in Aragón (Spain)

The prevalence and parasite density of PKX (the unknown myxosporean that causes proliferative kidney disease [PKD] of salmonids) were investigated in eight fishfarms in Aragon, Spain. Tissue sections stained with the biotynilated lectin GS-I revealed the presence of this protozoan in only one of the farms. In rainbow trout, the renal prevalence and parasite density peaked in July, but in brown trout the maximum renal prevalence and maximum renal parasite density were reached in May and in July, respectively. In rainbow trout, after the acute phase of the disease, the number of PKX decreases in the kidney but increases in the muscle. In this species of fish, the prevalence and parasite density are much higher in the muscles than in the kidney in October.     

Comparison of the susceptibility of brown trout (Salmo trutta) and four rainbow trout (Oncorhynchus mykiss) strains to the myxozoan Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease (PKD)

Differences in susceptibility to the myxozoan parasite Tetracapsuloides bryosalmonae, the causative agent of proliferative kidney disease (PKD), between four strains of rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were evaluated. Fish were exposed to water enzootic for the parasite in the field for 5 days and were subsequently transferred to the laboratory. Relative parasite load was determined after 2, 3 and 4 weeks post-exposure (wpe) by quantitative real-time PCR (qPCR) of kidney samples and number of parasite stages was determined in immunohistochemical stained sections of kidney, liver and spleen tissues. According to qPCR results, the highest amount of parasite DNA per equal amount of host tissue at all time points was measured in brown trout. Two of the rainbow trout strains showed lower relative parasite load than all other groups at the beginning of the experiment, but the parasite multiplied faster in these strains resulting in an equal level of relative parasite load for all rainbow trout strains at 4 wpe. A weak negative correlation of fish size and parasite load was detected. Only in samples of a few fish, single stages of T. bryosalmonae were found in sections stained by immunohistochemistry impeding quantitative evaluation of parasite numbers by this method. The results indicate a differential resistance to T. bryosalmonae between the rainbow trout strains investigated and between rainbow trout and brown trout.     

Performance and welfare of broilers as affected by stocking density and zinc bacitracin supplementation

The influence of stocking density (16, 20 and 24 birds/m²) and zinc bacitracin (0 or 100 mg/kg) on the performance, carcass characteristics and some indirect welfare indicators of broilers were examined in a 35‐day trial. The weight gain, feed intake, liveability and carcass characteristics of broilers grown at densities of 16, 20 and 24 birds per m² were similar (P > 0.05). Over the whole trial period, however, feed per gain of birds at the highest density (24 birds/m²) was higher (P < 0.05) than those at the other two densities. Litter quality was poorer at high population densities, but there were no welfare implications as indicated by the lack of effect of density on gait scores and the incidence of hock and fat pad burns. Inclusion of zinc bacitracin improved weight gains and feed intake of broilers over the 35‐day trial period at all three stocking densities tested. Zinc bacitracin supplementation had no influence on the feed per gain of birds reared at densities of 16 and 20 birds/m², but improvements were observed at a density of 24 birds/m². The relative weights of lymphoid organs (spleen and bursa) were lowered (P < 0.05) with increasing density, suggesting a greater degree of stress on the birds. Zinc bacitracin supplementation had no influence on bursa weights of birds reared at densities of 16 and 20 birds/m², but increased the bursa weights of birds reared at a density of 24 birds/m². Overall, these data indicate that, with the removal of in‐feed antibiotics in the future, care should be taken when considering population densities of greater than 20 birds/m².     

High dietary biotin levels affect the footpad and hock health of broiler chickens reared at different stocking densities and litter conditions

Responses to stocking density (SD), dietary biotin concentration and litter condition were evaluated on 2016 Ross 308 male broilers in the fattening period (day 22–day 42). The birds were placed in 48 pens with either dry or wet litter to simulate the final stocking density of 30 kg (12 broilers/m²; normal stocking density, NSD) and 40 kg (16 broilers/m²; high stocking density, HSD) of body weight (BW)/m² floor space. A corn–soybean meal‐based diet was supplemented with biotin to provide a normal (NB; 155 μg/kg) or high (HB, 1521 μg/kg) level of dietary biotin. There were six repetitions per treatment. The inappropriate moisture content of litter associated with HSD was avoided (p < 0.05) by good management (SD difference: dry litter, 6.65% vs. wet litter, 13.23%; 42 days), which made it advantageous (p < 0.01) for footpad (SD difference: dry litter, 0.118 vs. wet litter, 0.312; weekly average value) and hock health (SD difference: dry litter, 0.090 vs. wet litter, 0.303; weekly average value) of HSD birds, but not (p > 0.05) for growth and processing yield. In HSD, the biotin effect (gains, FCR) was significantly higher (p < 0.01) than in NSD. The similar response of HSD birds to supplemental biotin was observed (p < 0.05) for lesion scores of footpad and hock in particularly finishing chickens, and a significant interaction (p < 0.01) among stocking density, biotin supplementation and litter condition existed from 35 to 42 days of age. Taken together, increasing dietary biotin improves the performance and well‐being of broiler chickens stocked at high densities in litter‐independent and litter‐dependent manners respectively.     

Susceptibility of Selected Inland Salmonids to Experimentally Induced Infections with Myxobolus cerebralis,the Causative Agent of Whirling Disease

Abstract

Laboratory exposures to the infectious stages (triactinomyxons) of Myxobolus cerebralis demonstrated a range of susceptibility to whirling disease among four species of inland salmonids. Replicate groups of each species were exposed to two concentrations of triactinomyxons, a low dose (100–200 per fish) and a high dose (1,000–2,000 per fish). Exposed fish were evaluated for clinical signs, for severity of microscopic lesions at 35 d, 2 and 5 months, and for spore concentrations in the head cartilage at 5 months. A standard strain of rainbow trout Oncorhynchus mykiss matched for age served as a susceptible species control. Rainbow trout, westslope cutthroat trout O. clarki lewisi, Yellowstone cutthroat trout O. clarki bouvieri, and bull trout Salvelinus confluentus were susceptible to M. cerebralis infections. Clinical signs, including radical swimming (“whirling”) and black tails, were observed at 7 weeks postexposure among rainbow and cutthroat trout challenged at 3 weeks of age. Clinical signs were rare among bull trout exposed at an age of 4 weeks and absent among rainbow and cutthroat trout exposed at 3 months posthatch. Most rainbow, cutthroat, and bull trout were found to be infected when examined at 5 months postexposure. The most severe microscopic lesions among infected fish at 5 months postexposure were found among rainbow trout. Cutthroat trout had less severe lesions, bull trout had mild infections, and no evidence of infection was found among Arctic grayling Thymallus arcticus. Mean spore concentrations among infected fish correlated with the severity of microscopic lesion scores. Rainbow trout had mean concentrations of spores in head cartilage reaching 10⁶, whereas more resistant species such as bull trout had 10⁴ spores; no spores were found among Arctic grayling at 5 months postexposure.     

Role of dietary gamma-aminobutyric acid in broiler chickens raised under high stocking density

The present study was conducted to evaluate the effects of dietary gamma-aminobutyric acid (GABA) in broiler chickens raised in high stocking density (HSD) on performance and physiological responses. A total of 900 male broiler chicks (Ross 308) at 1 d old were assigned in a 2 × 2 factorial arrangement to 4 treatments (10 replicates per treatment) with stocking density, 7.5 birds/m² (low stocking density; LSD) or 15 birds/m² (HSD), and dietary GABA, 0 or 100 mg/kg. Chickens raised in HSD exhibited a decrease in body weight gain in all phases (P < 0.05) and feed intake in starter and whole phases (P < 0.01), and an increase in feed conversion ratio in the finisher phase (P < 0.01) compared with LSD-raised chickens. However, dietary GABA did not affect growth performance nor interacted with stocking density on production variables. The HSD vs. LSD increased relative liver weight on d 35 whereas dietary GABA increased relative liver weight and decreased relative bursa weight on d 21. Both stocking density and dietary GABA affected yield and quality of breast and leg muscles. Dietary GABA increased (P < 0.05) width of tibia on d 35 and interacted (P = 0.054) with stocking density on breaking stocking density on d 35. The HSD vs. LSD group lowered (P < 0.05) feather coverage scores. Significant interaction between stocking density and GABA on surface temperature of shank on d 21 was noted (P = 0.024). Dietary GABA exhibited an opposite effect on the concentrations of cecal short-chain fatty acids depending on stocking density leading to a moderate to significant interaction. Stocking density decreased alpha-1-acid glycoprotein whereas dietary GABA decreased heterophil-to-lymphocyte ratio and corticosterone in blood or serum samples. Serum biochemical parameters were altered by stocking density or dietary GABA. It is concluded that dietary GABA alleviated stress indices including corticosterone and heterophil-to-lymphocyte ratio, but failed to reverse stocking density-induced growth depression.     

Effect of dietary supplementation of medium-chain fatty acids on growth performance and prevalence of carcass defects in broiler chickens raised in different stocking densities

A 3 × 3 factorial experiment was conducted to examine the effect of medium-chain fatty acids (MCFA) on performance and occurrence of carcass defects in broiler chickens raised in stocking densities of 14, 16, and 18 birds/m². A total number of 720 Ross 308 broiler chicks that were 1 d old were reared up to 49 d of age. Corn- and soybean meal (SBM)–based starter and grower diets were supplemented with 0, 1.5, and 2 g/kg Aromabiotic and provided to the birds ad libitum. Inclusion of 2 g/kg Aromabiotic in the diet significantly improved weight gain of the birds during 1 to 21 d of age compared with control birds (P< 0.001). A significant reduction in mortality (P= 0.004) and a significant increase in European performance efficiency index (EPEI;P= 0.036) were noted in the birds that received diets containing 2 g/kg Aromabiotic compared with control birds at 49 d of age. The birds grown in a placement density of 18 birds/m²had significantly improved FCR along with reduced feed intake (FI) during 1 to 49 d of age compared with those reared in a density of 14 birds/m². The mean EPEI was significantly greater in the birds raised in a density of 16 birds/m²(P< 0.001). At 48 d of age, litter caking was significantly increased in the birds raised in a density of 18 birds/m²and decreased in the birds that received dietary supplementation of Aromabiotic (P< 0.05). The influence of dietary Aromabiotic was significant related to reduced incidence and severity of footpad lesions and hock burns (P< 0.01). Frequency of lesions with a score of 2 to 5 on footpad and hock significantly increased for the birds raised in densities higher than 14 birds/m². It was concluded that dietary supplementation of 2 g/kg Aromabiotic improved early productive performance of broiler chickens and enhanced the PEI mainly due to decreased mortality. Dietary Aromabiotic improved broiler welfare as indicated by reduced foot defects in the birds raised in placement densities of 16 and 18 birds/m².     

Effects of stocking density on growth performance,meat quality and tibia development of Pekin ducks

This study was performed to investigate the effects of stocking density on performance, meat quality and tibia development in Pekin ducks reared on a plastic wire floor. A total of 372 healthy, 21‐day‐old, male ducks with similar body weight (BW) were randomly allotted to stocking densities of five (low), eight (medium) and 11 (high) birds/m². Each group had six replicates. Results showed that compared with the low density group, medium and high stocking density caused a decrease in final BW at 42 days old, and in average daily gain, European performance index (p < .01) and meat pH at 45 min postmortem (p < .001), and an increase of meat drip loss (p < .01). High stocking density resulted in an increase of feed/gain ratio (p < .001), but a decrease of tibia calcium (p < .01) and phosphorus content (p < .05). Meat color, shear force values, tibia size (weight, length, and width) and breaking strength were not significantly influenced by stocking density. In conclusion, stocking density over eight birds/m² negatively affects growth performance, but meat quality and tibia development are not dramatically influenced. Based on this study, the stocking density of male Pekin ducks should be adjusted between five and eight birds/m².     

Effects of stocking density,light and perches on broiler growth

The aim of this study was to determine the effect of stocking density, light intensity and light color on broiler growth. The experiment consisted of four 35‐day phases during each of which 320 chickens were surveyed. The research was performed at stocking densities of four and six birds/m². Illuminances of 15 and 30 lx were obtained through commercial lamps with 4000 K and 6000 K color temperatures. Lighting was used 17 h a day, between 06.00 and 23.00 hours (17 L:7 D). The results showed a decrease in body, carcass, breast and thighs weight (P < 0.05) with the increase in stocking density. Body weight decreased by 10.5% and carcass weight decreased by 9.4% at six birds/m² stocking density. Contrastingly, no differences were found for the tested light colors. Increasing illuminance from 15 to 30 lx caused a 1.9% decrease in body weight. The analysis of the effect of perches revealed that using perches significantly increased body (2.5%) and breast weight (11.8%). The interactions between light intensity or color and stocking density and between light intensity and light color were analyzed.     

Assessment of serum protein fractions in rainbow trout using automated electrophoresis and densitometry

Background: Although rainbow trout (Oncorhynchus mykiss, Walbaum) are one of the most‐studied fish, electrophoretic techniques and classification of serum protein fractions have not been standardized, such that clinically useful values are lacking. Objective: The aim of the present study was to evaluate preliminarily the serum protein fractions of rainbow trout using automated cellulose acetate electrophoresis and densitometry. Methods: Serum samples from 25 rainbow trout (Oncorhynchus mykiss, Walbaum) were electrophoresed on cellulose acetate plates and quantified using densitometry. Results: A maximum of 6 fractions were identified and numbered, in order of decreasing mobility, as I, II, III, IV, V, and VI. In 3 of 25 (12%) samples, 6 fractions were identified; in 18 (72%) samples, 5 fractions were identified; and in 4 (16%) samples, 4 fractions were identified. Fractions I, V, and VI were always clearly identifiable, whereas fractions II and IV were frequently fused and indistinguishable from fraction III. The pattern with 5 fractions was the most probable type (χ², P<.01). The mean (±SEM) protein concentrations of the 6 fractions were I, 0.8±0.1 g/dL; II, 0.3±0.0 g/dL; III, 1.6±0.1 g/dL; IV, 0.3±0.1 g/dL; V, 0.6±0.0 g/dL; and VI, 0.2±0.0 g/dL. Based on comparison of serum and plasma electrophoretic patterns from 8 fish, fibrinogen was found in fraction V. Conclusion: Automated cellulose acetate electrophoresis and densitometry appear to be a practical method for estimation of serum protein fractions in rainbow trout.     

Accurate genomic predictions for BCWD resistance in rainbow trout are achieved using low‐density SNP panels: Evidence that long‐range LD is a major contributing factor

Previously accurate genomic predictions for Bacterial cold water disease (BCWD) resistance in rainbow trout were obtained using a medium‐density single nucleotide polymorphism (SNP) array. Here, the impact of lower‐density SNP panels on the accuracy of genomic predictions was investigated in a commercial rainbow trout breeding population. Using progeny performance data, the accuracy of genomic breeding values (GEBV) using 35K, 10K, 3K, 1K, 500, 300 and 200 SNP panels as well as a panel with 70 quantitative trait loci (QTL)‐flanking SNP was compared. The GEBVs were estimated using the Bayesian method BayesB, single‐step GBLUP (ssGBLUP) and weighted ssGBLUP (wssGBLUP). The accuracy of GEBVs remained high despite the sharp reductions in SNP density, and even with 500 SNP accuracy was higher than the pedigree‐based prediction (0.50–0.56 versus 0.36). Furthermore, the prediction accuracy with the 70 QTL‐flanking SNP (0.65–0.72) was similar to the panel with 35K SNP (0.65–0.71). Genomewide linkage disequilibrium (LD) analysis revealed strong LD (r² ≥ 0.25) spanning on average over 1 Mb across the rainbow trout genome. This long‐range LD likely contributed to the accurate genomic predictions with the low‐density SNP panels. Population structure analysis supported the hypothesis that long‐range LD in this population may be caused by admixture. Results suggest that lower‐cost, low‐density SNP panels can be used for implementing genomic selection for BCWD resistance in rainbow trout breeding programs.     

Effects of Salinity on Yersinia ruckeri Infection of Rainbow Trout and Brown Trout

Abstract

Juvenile rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta acclimated to freshwater or salinities of 9.0‰ or less were exposed to Yersinia ruckeri, the bacterial pathogen that causes enteric redmouth disease (ERM). Both species of fish were kept in the same recirculating systems after bacterial exposure. Rainbow trout mortality was significantly (P < 0.05) different in each salinity: 96.5% in freshwater, 89.5% in water of 1.1‰ salinity, 81.3% in 3.0‰ salinity, and 75.0% in 9.0‰ salinity (model SE = 1.0). All deaths occurred between 3 and 12 d after exposure to Y. ruckeri. Only 2.3% of brown trout in all salinities died, and differences among treatments were not significant. For both fish species, Y. ruckeri was isolated from liver, spleen, and trunk kidney of fish dying during this experiment, and lesions of rainbow trout were consistent with ERM. Yersinia ruckeri was not isolated from brown trout surviving for 21 d after bacterial exposure but was isolated from 3 of 24 surviving rainbow trout; a polymerase chain reaction assay detected the DNA of Y. ruckeri in 3 additional rainbow trout survivors. Neither the lesions of fish with ERM nor the percentage of surviving fish subclinically infected with Y. ruckeri was affected by salinity. Bacterial growth in vitro was not affected by low (≤9.0‰) salinity; however, bacterial adhesion to polystyrene was significantly reduced as salinity increased. Although mortality caused by Y. ruckeri was significantly lower for rainbow trout in water with slightly increased salinity, none of the salinities tested was effective in preventing serious losses caused by this pathogen in recirculating systems.     

Early Kinetics of Infectious Hematopoietic Necrosis Virus (IHNV) Infection in Rainbow Trout

Abstract

A series of experiments was carried out with infectious hematopoietic necrosis virus (IHNV; 193-110 isolate) in rainbow trout Oncorhynchus mykiss (weight, ～1.2 g) to determine the duration of the patent period and the timing of onset of the infectious periods. We first attempted to transmit IHNV to recipient fish from infected rainbow trout 2–3 d after they had been exposed. No infection transfer occurred despite high titers (10^4.79 to 10^4.91 plaque-forming units 5–8 d postexposure (dpe). To determine the number of secondary cases produced by one infectious individual, we exposed approximately 50 rainbow trout (weight, ～1.5 g) in each of seven replicate tanks to a donor fish that had been infected with virus by bath exposure 3 d earlier. The prevalence of infection in recipient fish rose from 0.84% at 2 dpe to 7.9% at 6 dpe. Maximum incidence (22 cases) occurred between 2 and 4 dpe. No disease-specific mortalities occurred in recipient fish during the experiment. The titer of virus in both recipient and donor fish increased from 2 to 4 dpe. There was a positive correlation between the level of infection among donors and prevalence values among recipient fish (r ² = 0.60). The level of challenge by one infectious fish under the conditions provided was enough for infection transfer from sick cohabitant to susceptible fish but was not enough for initiation of a full-scale epizootic among recipients.     

Influence of environmental temperature on experimental infection of redfin perch (Perca fluviatilis) and rainbow trout (Oncorhynchus mykiss) with epizootic haematopoietic necrosis virus, an Australian iridovirus

SUMMARY Experimental transmission of epizootic haematopoietic necrosis virus (EHNV) to adult redfin perch Perca fluviatilis and juvenile rainbow trout Oncorhynchus mykiss was undertaken at different water temperatures using intraperitoneal (IP) and bath inoculation. Redfin perch were highly susceptible to EHNV by both routes of infection. Bath inoculation with as few as 0.08 TCID₅₀. _mL^-1 was lethal. The incubation period in redfin perch was about 11 days at a water temperature of 19–21°C but was longer at colder temperatures and disease did not occur at temperatures below 12°C. The longest incubation period recorded in redfin perch was 28 days. Rainbow trout were not susceptible to infection by bath inoculation but the disease was reproduced after IP inoculation with 10^5.6 TCID₅₀ at water temperatures ranging from 8–21°C. The incubation period was 3–10 days at 19–21°C, but was up to 32 days at 8–10°C. Persistent infection with EHNV was detected by virus isolation in a clinically unaffected rainbow trout after 63 days. The implications of these findings in the understanding of the epidemiology of EHNV infection are discussed.