Mortality and pathology of hybrid catfish,Clarias macrocephalus (Günther) × Clarias gariepinus (Burchell), associated with Edwardsiella ictaluri infection in southern Thailand

Enteric septicaemia of catfish (ESC) caused by Edwardsiella ictaluri is becoming an increasing problem in aquaculture and has been reported worldwide in a variety of fish species. This study reports ESC in hybrid catfish, Clarias macrocephalus (Günther) × Clarias gariepinus (Burchell), cultured in southern Thailand. The bacteria were identified as E. ictaluri by conventional and rapid identification systems, as well as by genetic and phylogenetic characterization. Analysis of 16S rRNA indicated 100% homology to the 16S rRNA sequence of several E. ictaluri strains in GenBank. Plasmid profiles demonstrated 4.0‐ and 5.6‐kb plasmids, compared with the 4.8‐ and 5.6‐kb plasmids in the US isolates, and representative genes of three of the four known pathogenicity islands of US isolates were present. Serologically, lipopolysaccharide (LPS) purified from the Thai isolates was not recognized by a monoclonal antibody against the LPS of US isolates. Fish experimentally infected with E. ictaluri showed 23–100% mortality within 14 days with a 168‐h LD₅₀ of 6.92 × 10⁷ CFU mL^?1 by immersion and a 96‐h LD₅₀ of 1.58 × 10⁶ CFU fish^?1 by intraperitoneal injection. Examination of tissue sections obtained from both naturally and experimentally infected fish indicated that infection of hybrid catfish with E. ictaluri produced lesions in several organs including liver, kidney, spleen, heart and brain. Histopathology findings included cellular necrosis, focal haemorrhage, infiltration of lymphocytes and multifocal granulomatous inflammation in the infected organs.     

Antimicrobial susceptibility and enrofloxacin resistance of streptococcal bacteria from farmed Nile tilapia,Oreochromis niloticus (Linnaeus 1758) in Thailand

This study examined the antimicrobial susceptibility and mutation(s) in quinolone resistance‐determining regions (QRDRs) in streptococcal pathogens isolated from farmed Nile tilapia Oreochromis niloticus in Thailand. Surveillance of antimicrobial susceptibility in tilapia streptococcal pathogens reveals that Streptococcus agalactiae (n = 97) and Streptococcus iniae (n = 3) from diseased tilapia were susceptible to amoxicillin, florfenicol, sulfamethoxazole/trimethoprim and sulfadimethoxine/ormetoprim, however, only 78 isolates were susceptible to enrofloxacin. Twenty‐two enrofloxacin‐resistant S. agalactiae isolates were further examined for mutations in the QRDRs of gyrA, gyrB, parC and parE genes. Twenty isolates had single base pair changed in the gyrA sequence, C‐242‐T. Point mutations in gyrB, GC‐1135, 1136‐AA and T‐1466‐G, were identified in one isolate. All resistant isolates harboured a mutation in the parC gene, C‐236‐A, while no mutations were observed in the parE gene. The study represented mutations of gyrA and parC genes as marked modification of the enrofloxacin‐resistant S. agalactiae from farmed tilapia. This study is a primary report of the QRDRs mutations associated with fluoroquinolone resistance from streptococcal pathogen in the cultured fish. The phenotypic and genotypic characterization of enrofloxacin resistance S. agalactiae evident in this study has led to an improved regulation of antimicrobial use in Thai aquaculture.     

Efficacy of an experimentally inactivated Streptococcus agalactiae vaccine in Nile tilapia (Oreochromis niloticus) reared in Brazil

Tilapia aquaculture is one of the fastest‐growing segments of fish production in Brazil. Nile tilapia (Oreochromis niloticus) is largely cultivated in the state of Parana, where Streptococcus agalactiae is the cause of severe disease outbreaks. The objective of this paper was to evaluate an inactivated S. agalactiae vaccine in tilapia for the control of streptococcal disease outbreaks. Tilapia, weighing approximately 20 g each, were intraperitoneally (i.p.) inoculated with 0.1 mL of the vaccine at a dose of 2.0 × 10⁸ colony‐forming unit (CFU) mL^?1. One group of tilapia (treatment 1) received one vaccine dose, and the other group of tilapia (treatment 2) received two doses, with an interval of 21 days. The control group was i.p. inoculated with 0.1 mL tryptic soy broth fish^?1. Immunized and control tilapia were i.p. challenged with 0.1 mL of 3.0 × 10⁷ CFU mL^?1 at 30 days post vaccination. The fish were monitored daily for disease signs and for mortality for 16 days post challenge. A statistically significant difference (P=0.0045) was found between the mortality of treatments 1 and 2. The value of relative per cent of survival of 83.6% and 96.4%, respectively, indicate that this vaccine was efficient in Nile tilapia.     

Chemical composition of Lippia spp. essential oil and antimicrobial activity against Aeromonas hydrophila

Phytotherapy can replace antibiotic administration as an alternative to control Aeromonas hydrophila, one of the main bacteria involved in the aetiology of farmed fish diseases. Given that plants of the Lippia spp. genus show biological potential for antimicrobial activity, this study evaluated the chemical composition of essential oils extracted from Lippia alba, Lippia origanoides and Lippia sidoides and their activity against A. hydrophila. The oils were obtained by steam distillation in a Clevenger‐type apparatus and their composition determined by gas chromatography and mass spectrometry (CG/MS). Antibacterial activity was assessed by calculating the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) using broth microdilution method. The main compounds identified were geranial (25.4%) and neral (16.6%) in L. alba oil, carvacrol (40.4%) and p‐cymene (11.4%) in L. origanoides oil and thymol (76.6%) and ortho‐cymene (6.3%) in L. sidoides oil. The three Lippia species showed bacteriostatic and bactericidal action against A. hydrophila, with MICs and MBCs ranging from 1250 to 5000 μg mL^?1. Of the species tested, the best performance was obtained with essential oil of L. sidoides.     

A multi-epitope chimeric protein elicited a strong antibody response and partial protection against Edwardsiella ictaluri in Nile tilapia

Edwardsiella ictaluri infects several fish species and protection of the all the susceptible fish hosts from the pathogen using a monovalent vaccine is impossible because the species is composed of host-based genotypes that are genetic, serological and antigenic heterogenous. Here, immunoinformatic approach was employed to design a cross-immunogenic chimeric EiCh protein containing multi-epitopes. The chimeric EiCh protein is composed of 11 B-cell epitopes and 7 major histocompatibility complex class II epitopes identified from E. ictaluri immunogenic proteins previously reported. The 49.32 kDa recombinant EiCh protein was expressed in vitro in Escherichia coli BL-21 (DE3) after which inclusion bodies were successfully solubilized and refolded. Ab initio protein modelling revealed secondary and tertiary structures. Secondary structure was confirmed by circular dichroism spectroscopy. Antigenicity of the chimeric EiCh protein was exhibited by strong reactivity with serum from striped catfish and Nile tilapia experimentally infected with E. ictaluri. Furthermore, immunogenicity of the chimeric EiCh protein was investigated in vivo in Nile tilapia juveniles and it was found that the protein could strongly induce production of specific antibodies conferring agglutination activity and partially protected Nile tilapia juveniles with a relative survival percentage (RPS) of 42%. This study explored immunoinformatics as reverse vaccinology approach in vaccine design for aquaculture to manage E. ictaluri infections.     

Cathelicidins enhance protection of channel catfish,Ictalurus punctatus,and channel catfish ♀ × blue catfish,Ictalurus furcatus ♂ hybrid catfish against Edwarsiella ictaluri infection

Cathelicidins are a class of antimicrobial peptides (AMPs) known to possess rapid and direct antimicrobial activities against a variety of microorganisms. Recently identified cathelicidins derived from alligator and sea snake were found to be more effective in inhibiting microbial growth than other AMPs previously characterized. The ability of these two cathelicidins along with the peptides, cecropin and pleurocidin, to protect channel catfish (Ictalurus punctatus, Rafinesque) and hybrid catfish (I. punctatus ♀ × blue catfish, Ictalurus furcatus, Valenciennes ♂) against Edwardsiella ictaluri, one of the most prevalent pathogens affecting commercial catfish industry, was investigated. Cathelicidin-injected fish (50 µg ml⁻¹ fish⁻¹) that were simultaneously challenged with E. ictaluri through bath immersion at a concentration of ~1 × 10⁶ CFU/ml had increased survival rates compared with other peptide treatments and the infected control. Bacterial numbers were also reduced in the liver and kidney of channel catfish and hybrid catfish in the cathelicidin treatments 24 hr post-infection. After 8 days of challenge, serum was collected to determine immune-related parameters such as bactericidal activity, lysozyme, serum protein, albumin and globulin. These immune-related parameters were significantly elevated in fish injected with the two cathelicidins as compared to other peptide treatments. These results indicate that cathelicidins derived from alligator and sea snake can stimulate immunity and enhance the resistance to E. ictaluri infection in channel catfish and hybrid catfish.     

Effect of Sublethal Hypoxia on the Immune Response and Susceptibility of Channel Catfish, Ictalurus punctatus, to Enteric Septicemia

The effect of sublethal hypoxia exposure on stress and immune responses and susceptibility to Edwardsiella ictaluri infection in juvenile channel catfish, Ictalurus punctatus, was investigated. Fish were monitored for temporal changes in glucose and cortisol concentrations before, during, and after 2 h exposure to sublethal hypoxia (<2 mg/L dissolved oxygen [DO]) and when maintained under normoxic conditions (6.0 ± 0.3 mg/L DO). Both blood glucose and plasma cortisol increased significantly in response to hypoxic conditions. Fish exposed to hypoxic or normoxic conditions were challenged with a high dose (1.3 × 10⁷ colony‐forming units [CFU]/mL) or a low dose (1.3 × 10⁵ CFU/mL) of E. ictaluri or sterile culture broth by 30‐min immersion bath. Approximately 1% of fish in both the normoxic and the hypoxic groups died when challenged with the low dose of E. ictaluri. However, when challenged with the high dose of E. ictaluri, catfish exposed to hypoxic conditions had significantly higher cumulative mortality (36 ± 12.1%) than those maintained under normoxic conditions (12 ± 1.1%). Total hemolytic complement and bactericidal activities and antibody response were lower in hypoxia‐exposed channel catfish, indicating that increased susceptibility of channel catfish to E. ictaluri may be the result of the immunosuppressive effects of the stress response to hypoxia.     

Differential pathogenicity of five Streptococcus agalactiae isolates of diverse geographic origin in Nile tilapia (Oreochromis niloticus L.)

Streptococcus agalactiae is an emerging pathogen of fish and has caused significant morbidity and mortality worldwide. The main objective of this study is to assess whether pathogenic differences exist among isolates from different geographic locations. Nile tilapia (Oreochromis niloticus L.) were administered an intraperitoneal injection of suspension containing USA, Brazil, Honduras, Israel, or Kuwait S. agalactiae isolates at concentrations ranging from 10² to 10⁷ cfu mL^?1. The LD₅₀ values 7 days after challenge were as follows: USA (1.0 × 10² cfu mL^?1), Brazil (1.5 × 10³ cfu mL^?1), Honduras (6.8 × 10³ cfu mL^?1), Israel (1.0 × 10⁴ cfu mL^?1) and Kuwait (7.2 × 10⁵ cfu mL^?1). Fish from all groups exhibited lethargy, anorexia, exophthalmia, corneal opacity, erratic swimming, petechiae and mortality. Opercular clearing and ascites were only found after infection with certain geographic isolates. The findings in this study indicate that S. agalactiae isolates of different geographic origin can cause significant mortalities after experimental challenge and can have different pathogenic capacities. Isolates from the Americas (USA, Brazil and Honduras) were more pathogenic to Nile tilapia than isolates from the Middle East/Asia (Israel and Kuwait).     

Efficacy of feed‐based adjuvant vaccine against Streptococcus agalactiae in Oreochromis spp. in Malaysia

This study was conducted to determine the systemic, mucosal immunity and protective capacity of the feed‐based adjuvant vaccine (FAV) of Streptococcus agalactiae following oral vaccination against streptococcosis in tilapias. Two hundred and sixteen red tilapia fish were divided into three major groups. Each major group consisted eight tilapia kept in nine 2000 L glass aquaria. At day 0, all fish from the FAV group were fed with feed that had been incorporated with an adjuvant, while fish in the feed‐based vaccine (FNV) group were fed with vaccine incorporated into the pellet without adjuvant. Fish in the control‐unvaccinated group, FC, were fed with normal commercial pellet. Booster dose was performed on day 14 post immunization. Fish from each group were sacrificed on a weekly basis for the entire 7 weeks. Serum, body mucus and gut lavage fluid were evaluated for antibody responses by indirect ELISA, while histological examination was carried out on the gut following intraperitoneal challenge. The FAV group had a significantly higher protection (P < 0.05) following challenge with 3.4 × 10⁹ CFU mL^?1 of live S. agalactiae than FNV group. This level of protection may be due to high antibody responses, increase in size of gut‐associated lymphoid tissue and high number of lymphocytes in the FAV group.     

Efficacy of a Live‐attenuated Edwardsiella ictaluri Oral Vaccine in Channel and Hybrid Catfish

This study evaluated the efficacy of an oral live‐attenuated Edwardsiella ictaluri vaccine against enteric septicemia of catfish (ESC) in channel and hybrid catfish. The vaccine was delivered one time orally by feeding fish a diet coated with an attenuated E. ictaluri isolate at four doses to deliver between 4 × 10⁶ to 3.2 × 10⁷ viable vaccine cells/g wet feed. Thirty‐five days postvaccination, control and vaccinated fish were challenged with virulent E. ictaluri and mortality was examined for 30 d postchallenge. Mortality of nonvaccinated hybrids (85%) and nonvaccinated channel catfish (73%) was similar but significantly greater than all groups of vaccinated fish. In channel catfish, a trend toward increasing mortality with decreasing dose was observed. Mortality of channel catfish vaccinated with the lowest dose (26.6%) was significantly higher than fish vaccinated with the highest dose (14.1%) but similar to fish vaccinated at the intermediate doses (17.5 and 19.4%). In contrast, mortality of four doses of vaccinated hybrid catfish was similar and ranged between 10.4 and 14.0%. The data demonstrate that the attenuated E. ictaluri vaccine at all four doses tested is effective at reducing ESC‐related mortalities in hybrid and channel catfish.     

A Non-Nutritive,Ambient Temperature-Processed Floating Pellet as a Carrier of Antibiotics for Channel Catfish

A non-nutritive, ambient temperature-processed floating pellet (ATFP) was used as a medium to deliver apramycin (Eli Lilly and Co., Greenfield, Indiana), 52% active, and oxytetracycline hydrochloride (Pfizer, Inc., Lee's Summit, Missouri) (91% active) to channel catfish, Ictalurus punctatus, experimentally infected with Edwardsiella ictaluri. The drugs were fed at 50 mg of active antibiotic per kilogram of fish per day. Two medicated feeding trials were run for 10 consecutive days in troughs with channel catfish, beginning 24 hours after exposure to E. ictaluri. Mortality in the non-infected control population in Trial 1 was 7.6%, significantly (P < 0.05) lower than the 88.3% of the infected, non-medicated fish. Fish fed diets containing apramycin experienced 62.5% mortality, while fish fed diets containing oxytetracycline hydrochloride had 63.3% mortality. Mortality of the two medicated groups were significantly (P < 0.05) different from either control group. In Trial II, the 23% mortality in the non-infected group was significantly lower than the 48% mortality in the infected, non-medicated group and the 41% mortality in the oxytetricycline hydrochloride-medicated group, but not different than fish fed diets containing apramycin (28%). Mortality of the two medicated groups in Trial II were significantly different from each other. The presence of thc apramycin and oxytetracycline hydrochloride in the ATFP was demonstrated by the antibiotic diffusion method on agar plates swabbed with E. ictaluri.     

Establishing bacterial infectivity models in striped Catfish Pangasianodon hypophthalmus (Sauvage) with Edwardsiella ictaluri

A bacterial infectivity challenge model of Edwardsiella ictaluri in striped catfish was developed. All experiments were conducted using a bacterial isolate of E. ictaluri that had been recovered during a natural outbreak of bacillary necrosis of Pangasianodon (BNP) in farmed striped catfish Pangasianodon hypophthalmus in Vietnam. Time of immersion in 10⁷ CFU.ml⁻¹ had a significant effect on mortality. The immersion bacterial dose of 10⁷ CFU/ml for 30 s resulted in a cumulative percentage mortality of 63%. Three to four days post-bacterial challenge, fish showed gross clinical signs of natural BNP and E. ictaluri was recovered and identified from these fish. Moreover, a cohabitation challenge was evaluated as an alternative challenge method, although the mortalities among the infected fish were lower at around 15%–40%. This study confirmed the horizontal transmission of E. ictaluri in striped catfish and elucidated that cohabitation challenge could be used in reproducing the disease under controlled conditions.     

麻保沙星对主要海洋致病性弧菌的体外抗菌活性及抗菌后效应

采用牛津杯法和试管二倍稀释法分别测定麻保沙星对溶藻弧菌(Vibrio alginolyticus)、哈维氏弧菌(V.harveyi)、费氏弧菌(V.fischeri)及灿烂弧菌(V.splendidus)的敏感性、最小抑菌浓度(MIC)和最小杀菌浓度(MBC),并与盐酸二氟沙星、盐酸恩诺沙星、诺氟沙星及氟苯尼考进行比较;采用菌落计数法测定麻保沙星对溶藻弧菌、哈维氏弧菌和灿烂弧菌的抗菌后效应(PAE)。结果表明,麻保沙星对4种弧菌的体外抗菌活性与氟苯尼考相近,高于盐酸二氟沙星、盐酸恩诺沙星和诺氟沙星;麻保沙星在1×MIC、2×MIC和4×MIC浓度时,对溶藻弧菌、哈维氏弧菌和灿烂弧菌的PAE值分别为:0.49h、0.87h和1.17h;0.64h、0.98h和1.22h;0.75h、1.02h和1.25h,表明麻保沙星对溶藻弧菌、哈维氏弧菌和灿烂弧菌均有不同程度的抗菌后效应,且PAE值与药物浓度呈正相关。     

The Relationship Between Vaccine Dose and Efficacy in Channel Catfish Ictalurus punctatus Vaccinated as Fry with a Live Attenuated Strain of Edwardsiella ictaluri (RE‐33)1

Channel catfish Ictalurus punctatus were vaccinated at 12 d of age (post‐hatch) by a 2‐min bath immersion with attenuated Edwardsiella ictaluri RE‐33 at doses of 2.5 × 10⁵, 2.5 × 10⁶, and 2.4 × 10⁷ colony‐forming units CFU/mL of water. Following vaccination, RE‐33 was recovered from a greater percentage of fry that were vaccinated at the high and intermediate doses compared to fry vaccinated at the lowest dose. Independent of dose, the greatest percentage of RE‐33 positive fry occurred between 1 and 6 d post‐vaccination with a significant decrease in positive fry observed on day 12. A significant increase in mortality occurred 6 to 12 d post‐vaccination in fry vaccinated at the highest dose. No differences in post‐vaccination mortalities occurred between the other treatments. Following virulent E. ictaluri challenge, mortalities of fish vaccinated at doses of 2.5 × 10⁶ and 2.4 × 10⁷ CFU/mL were significantly less than those of fish vaccinated at 2.5 × 10⁵ CFU/mL and sham‐vaccinated control fish. These data show that vaccination with RE‐33 can offer protection against subsequent virulent E. ictaluri infection.     

A rapid bioassay for bactericides against the catfish pathogens Edwardsiella ictaluri and Flavobacterium columnare

The most common bacterial diseases in pond‐raised channel catfish Ictalurus punctatus (Rafinesque) are enteric septicemia of catfish and columnaris, caused by Edwardsiella ictaluri and Flavobacterium columnare respectively. Medicated feed containing antibiotics is one management approach that catfish producers use in the treatment of bacterial diseases. However, the future use of all types of medicated feed in catfish aquaculture is uncertain. To discover effective alternatives to antibiotics, a rapid 96‐well microplate bioassay utilizing E. ictaluri and F. columnare to evaluate natural compounds and extracts was developed. In this bioassay, bacterial growth is determined by absorbance measurements of microplate wells after 24 h incubation and then confirmed by detecting cell viability after the addition of 3‐(4,5‐dimethylthiazol‐2‐yl)‐2,5‐diphenyl tetrazolium bromide with additional incubation for 24 h. The minimum inhibitory concentration, minimum bactericidal concentration and 50% inhibition concentration (IC50) are determined by graphing the absorbance data. The 24 h IC50 results of test compounds are compared with the 24 h IC50 results of the drug controls oxytetracycline and florfenicol. Among the antibiotics evaluated, doxycycline and tetracycline appear more effective against E. ictaluri and F. columnare than either drug control. This bioassay is rapid, reproducible and economical for evaluating a large number of compounds and extracts.     

银离子对几种细菌的杀菌效应

由抗生素导致的耐药性问题日益严重,因此新型抗菌药物的研发迫在眉睫。银离子因具有抗菌作用,以及安全、无耐药性、稳定性高等优点而备受关注。为了探究银离子的抗菌机制,本实验选择几种常见水生细菌,研究其对银离子的耐受性和细菌种内、种间的"僵尸效应"(被银离子杀死的细菌可以杀死其他新鲜的细菌),以进一步明确银离子长效杀菌的机理。结果显示,银离子对5种细菌都表现出了明显的生长抑制。同种细菌和异种细菌之间都表现出明显的"僵尸效应",并且随着银离子浓度的提高,"僵尸效应"的效果越明显。为进一步研究银离子杀菌机制,通过透射电镜观察银离子处理后的嗜水气单胞菌和无乳链球菌,结果显示银离子处理后的细菌出现胞质皱缩,细胞膜呈弥散状态,甚至破裂,细胞内容物外流,最终致使细菌死亡。研究表明,银离子对5种细菌都有明显的生长抑制效应,并发现细菌之间存在"僵尸效应",可为研发新型的抗菌药物提供参考。     

Intra- and interspecific phenotypic characteristics of fish-pathogenic Edwardsiella ictaluri and E. tarda

Intra‐ and interspecific characteristics of fish‐pathogenic Edwardsiella ictaluri, and E. tarda were determined by numerical analysis of gel electrophoresed protein profiles, fatty acid methyl esters (FAMEs) and immunoblotting. The 18 E. ictaluri isolates revealed a high degree of homogeneity (70% similarity or higher) in their protein profiles and 95% similarity in their FAME, while the nine E. tarda isolates revealed 30% similarity in their protein profiles and 95% similarity in their FAME. Immunoblots probed for antigenic epitopes with goat antiserum produced against E. ictaluri and E. tarda, respectively, revealed that E. ictaluri were more homogeneous compared with the E. tarda isolates. Overall, there was a considerable degree of relatedness between the two species. Our findings suggest that phenotypically E. ictaluri represents a clonal bacterial population structure compared with the less monomorphic E. tarda.     

Ethnobotanical Uses and Antimicrobial Properties of Plants in Small‐Scale Tropical Fish Farms: The Case of Indonesian Fish Farmers in Java (Indonesia)

Among the most common plants used by fish farmers in Java, 18 were selected for deeper ethnobotanical and laboratory investigation into their antimicrobial activity. These plants are mainly used against fish diseases, but with no precise therapeutic indication, leading to low specificity of use. Leaves, which were the most commonly used part of plants, were generally placed directly in water or less frequently added to the feed. Disk diffusion tests showed that 15 plants of the 18 (83%) displayed varying degrees of antibacterial activity and that Streptococcus agalactiae was more sensitive than Aeromonas hydrophila. Crude ethanolic extracts (EE) were more active than water macerates (WM), and minimum inhibitory concentrations (MICs) ranged from 12.5 to 25 mg/mL. Only the EE of Piper betle had an inhibitory effect against A. hydrophila. No MIC was found for any of the WM studied here. Furthermore, both the EE and, to a lesser extent, the WM of active plants were able to reduce bacterial growth by more than 99%, even at doses below their MICs. These results suggest that these plant extracts have a potential for eco‐friendly prevention of bacterial fish diseases.     

Field Efficacy of Florfenicol for Control of Mortality in Channel Catfish, Ictalurus punctatus (Rafinesque), Caused by Infection With Edwardsiella ictaluri

A field study to assess the efficacy of florfenicol (FFC) against enteric septicemia of catfish (ESC) was conducted with pond‐reared channel catfish fingerlings held in 0.1‐acre earthen ponds. Fish were challenged with Edwardsiella ictaluri in a natural pond outbreak or by cohabitation with E. ictaluri‐infected fish held in netpens. Fourteen ponds were assigned in equal number to two treatment groups, that is, either treated (with 10 mg FFC/kg body weight in medicated feed) or not treated (control) for 10 consecutive d. The threshold for enrollment into the study was 0.3% cumulative mortality attributed to ESC. Treatment was initiated on different dates for each pond because each pond was enrolled when 33 fish/pond were diagnosed with ESC based on clinical signs, lesions, or positive cultures. Mortality was monitored during the 10‐d treatment period and during a 14‐d posttreatment observation period. At the end of the 14‐d posttreatment observation period, all fish were euthanized, and 20 fish from each pond were examined by gross necropsy and evaluated for the presence of E. ictaluri by bacterial culture. The odds of a mortality in the control group were 2.20 times the odds of a mortality in the FFC‐treated group. Significantly fewer (P≤ 0.05) FFC‐medicated catfish died in comparison to unmedicated catfish. The minimum inhibitory concentration of FFC for this strain of E. ictaluri was 0.25 μg/mL in all fish that were assayed. The mean zone of inhibition (Kirby Bauer) was 36.8 mm from E. ictaluri isolates of test fish. There were no FFC treatment‐related lesions seen on gross pathology. FFC was efficacious and safe for control of mortality from E. ictaluri infection in catfish.     

Isolation and molecular characterization of streptococcal species recovered from clinical infections in farmed Nile tilapia (Oreochromis niloticus) in the Philippines

Streptococcosis cause severe losses for global tilapia farming, especially in developing countries. The aim of this study was to identify and characterize streptococci recovered from Nile tilapia farmed in the Philippines. Moribund and apparently healthy fish were sampled from grow-out cages, ponds and hatcheries. Clinical signs observed included exophthalmia, eye opacity, ascites, lethargy, erratic swimming and haemorrhages. Results showed that both Streptococcus iniae and Streptococcus agalactiae were associated with disease in these sites. Consistent with global reports, including those from South-East Asia, S. agalactiae was more widespread than S. iniae. Molecular serotyping of the S. agalactiae isolates identified the serotype Ia and serotype Ib. Histopathological findings were meningitis, meningoencephalitis and septicaemia. Identical virulence profiles were found for all strains of S. iniae, while S. agalactiae strains were separated into virulence profile I and profile II. All strains were susceptible to the tested antibiotics and resistant to oxolinic acid. Only S. agalactiae serotype Ib showed resistance to sulphamethoxazole–trimethoprim. This is the first study from the Philippines to characterize the streptococci involved in disease outbreaks in tilapia aquaculture. Outputs from this study will promote the development of efficacious disease control strategies in tilapia farming for the Philippines and South-East Asia.