Aeromonas hydrophila in Aquaculture: Virulence Factors and Antimicrobial Resistance

Introduction

Aeromonas hydrophila is a Gram-negative, facultatively anaerobic rod-shaped bacterium belonging to the family Aeromonadaceae. It is a ubiquitous aquatic microorganism capable of causing significant disease outbreaks in freshwater and brackish water aquaculture systems worldwide. The bacterium is the primary etiological agent of Motile Aeromonas Septicemia (MAS), a hemorrhagic septicemic disease that affects a wide range of fish species including tilapia, catfish, carp, and salmonids. In addition to fish, A. hydrophila can infect amphibians, reptiles, and occasionally mammals, but this article focuses strictly on its relevance to aquaculture.

The economic impact of A. hydrophila infections is substantial, with mortality rates in affected farms often exceeding 50% during outbreaks. The pathogen is particularly problematic in high-density rearing systems where stress factors such as poor water quality, overcrowding, and temperature fluctuations predispose fish to infection. Over the past two decades, the emergence of multidrug-resistant (MDR) strains has further complicated disease management, driving the need for improved biosecurity protocols, rapid diagnostics, and effective vaccines.

This reference article provides an exhaustive examination of A. hydrophila in aquaculture, covering its microbiological characteristics, virulence factor repertoire, antimicrobial resistance mechanisms, diagnostic methodologies, and prevention strategies. Emphasis is placed on the molecular and biochemical basis of pathogenicity and the evolutionary dynamics of resistance acquisition.

Bacterial Characteristics

Aeromonas hydrophila is a motile, oxidase-positive, glucose-fermenting bacillus that typically grows at temperatures ranging from 4 degrees Celsius to 42 degrees Celsius, with optimal growth at 28 degrees Celsius to 30 degrees Celsius. The bacterium is readily cultured on standard media such as Tryptic Soy Agar (TSA) or Blood Agar, where it produces characteristic smooth, cream-colored colonies. Selective media such as Rimler-Shotts Agar or Aeromonas Agar supplemented with ampicillin are used for primary isolation from clinical and environmental samples.

The organism possesses a polar flagellum enabling motility in liquid environments, and some strains express lateral flagella for swarming on solid surfaces. The lipopolysaccharide (LPS) layer constitutes a major antigenic structure and contributes to serum resistance. The outer membrane proteins (OMPs) and S-layer proteins are critical for adhesion and immune evasion.

Epidemiology and Disease Outbreaks

Aeromonas hydrophila is considered an opportunistic pathogen; disease outbreaks typically occur when fish are immunocompromised due to environmental stress. Common predisposing factors include elevated water temperature (above 25 degrees Celsius), low dissolved oxygen, high ammonia levels, and handling or transport stress. Outbreaks have been reported in nearly all commercially important finfish species. In channel catfish (Ictalurus punctatus), for example, MAS presents as a highly lethal hemorrhagic septicemia with mortality rates exceeding 80% in fry and fingerlings. Similarly, in Nile tilapia (Oreochromis niloticus) and common carp (Cyprinus carpio), the disease manifests as exophthalmia, skin ulcers, fin rot, and internal organ congestion.

Coinfections with other aquatic pathogens are common. Notably, Streptococcosis in Farmed Tilapia: Streptococcus agalactiae and Streptococcus iniae Pathogenesis, Rapid Diagnostic Tests, and Vaccine Development can occur concurrently with A. hydrophila, complicating clinical diagnosis and treatment. Additionally, Ichthyophthirius multifiliis (White Spot Disease) in Farmed Fish: Advances in Molecular Detection and Treatment often predisposes fish to secondary bacterial infections including A. hydrophila.

The global distribution of A. hydrophila is reflected in its presence in both tropical and temperate aquaculture systems. However, the emergence of hypervirulent strains, particularly those carrying specific virulence gene combinations, has heightened the risk of severe epizootics in previously unaffected regions.

Virulence Factors

Aeromonas hydrophila possesses an extensive arsenal of virulence factors that enable colonization, host tissue damage, and immune evasion. These factors are encoded by both chromosomal and plasmid-borne genes. The major categories are summarized in Table 1 below.

The aerolysin (AerA) pore-forming toxin is considered one of the most potent virulence determinants. Upon binding to glycosylphosphatidylinositol (GPI)-anchored proteins on host cell membranes, aerolysin oligomerizes to form heptameric pores that disrupt ion gradients, leading to osmotic lysis and tissue necrosis. This toxin is responsible for the hemorrhagic lesions characteristic of MAS.

The Type III secretion system (T3SS) injects effector proteins directly into the cytosol of host cells, modulating signal transduction pathways. Effectors such as AexT act as ADP-ribosyltransferases that depolymerize actin filaments, causing cytoskeletal collapse and inhibition of phagocytosis. The T6SS, in contrast, functions primarily in bacterial competition but also contributes to virulence by delivering effectors that evade host immune responses.

Biofilm formation, regulated by quorum sensing via N-acyl-homoserine lactones (AHLs), allows A. hydrophila to persist in aquaculture environments, resist antimicrobial treatments, and serve as a reservoir for recurrent infections. The LuxI/LuxR-type quorum sensing system controls the expression of exoproteases, hemolysins, and biofilm matrix components.

Antimicrobial Resistance

The widespread use of antibiotic agents in aquaculture has exerted strong selective pressure on A. hydrophila populations, leading to the emergence and dissemination of resistance determinants. Resistance has been reported against nearly all commonly used antimicrobial classes including tetracyclines, quinolones, sulfonamides, beta-lactams, aminoglycosides, and phenicols. Multidrug resistance, defined as resistance to three or more classes, is now prevalent in many production regions.

Mechanisms of Resistance

Resistance in A. hydrophila is mediated by three primary mechanisms: enzymatic inactivation, target site modification, and efflux pump upregulation. A summary of resistance genes and their corresponding mechanisms is provided in Table 2.

Mobile genetic elements (MGEs) such as plasmids, transposons, and integrons are major drivers of resistance dissemination. Class 1 integrons, in particular, are frequently identified in A. hydrophila isolates from aquaculture settings. These genetic structures carry gene cassettes encoding resistance to aminoglycosides, trimethoprim, and chloramphenicol, and are often located on conjugative plasmids that can transfer across species boundaries.

Of increasing concern is the emergence of resistance to colistin, a polymyxin antibiotic considered a last-resort agent. The plasmid-mediated mcr genes (mcr-3, mcr-5) have been detected in A. hydrophila isolates from Asian and African aquaculture systems, posing a risk of transfer to other Gram-negative pathogens, including those affecting humans. Although this article does not discuss human clinical implications, the presence of such resistance genes in aquatic bacteria highlights the role of aquaculture reservoirs in the global dissemination of AMR.

Epidemiologic Trends

Regional surveillance studies indicate that resistance rates vary considerably. In Southeast Asian catfish farms, tetracycline resistance can exceed 60% of isolates, while quinolone resistance ranges from 30% to 50%. Beta-lactamase genes are nearly ubiquitous, though carbapenem resistance remains rare. The co-carriage of multiple resistance genes is common, often linked to large multidrug resistance plasmids harboring multiple integrons.

The use of antibiotic growth promoters in some jurisdictions has been linked to higher resistance prevalence. In contrast, countries that have implemented strict veterinary antibiotic oversight, such as Norway and Canada, report lower AMR rates in aquaculture bacteria, though A. hydrophila remains a persistent challenge.

Diagnostic Approaches

Accurate and rapid diagnosis of A. hydrophila infections is essential for timely disease management. Diagnostic methods range from conventional culture to molecular assays and point-of-care techniques.

Conventional Bacteriology

Isolation from kidney, spleen, or liver tissue on selective media provides a presumptive identification. Colonies suspected to be A. hydrophila are oxidase-positive, catalase-positive, and ferment glucose with gas production. Biochemical profiling using commercial test strips (e.g., API 20E) can distinguish A. hydrophila from other Aeromonas species, but misidentification with A. veronii or A. caviae is possible. Definitive identification requires molecular confirmation.

Molecular Diagnostics

Polymerase chain reaction (PCR) targeting the 16S rRNA gene, gyrB gene, or housekeeping genes such as rpoD provides species-level identification. Real-time PCR assays using probes targeting the aerA or hlyA virulence genes can simultaneously confirm pathogenicity. Multiplex PCR panels capable of distinguishing A. hydrophila from other aquatic bacterial pathogens, including Streptococcus iniae and Lactococcus garvieae Infections in Farmed Fish: Detection and Antimicrobial Stewardship, are available for syndromic diagnostic testing.

Whole genome sequencing (WGS) has become increasingly accessible for characterization of virulence and resistance profiles. High-throughput sequencing platforms allow for the detection of all known resistance and virulence genes, phylogenetic clustering, and identification of outbreak strains. However, WGS remains cost-prohibitive for routine surveillance in low-resource settings.

Serological Methods

Enzyme-linked immunosorbent assay (ELISA) using monoclonal antibodies against the LPS O-antigen or OMPs can detect A. hydrophila antigens in tissue homogenates or water samples. Lateral flow immunoassays, similar in principle to those described for Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus: p27 Antigen Detection and Diagnostic Interpretation, have been developed for fish-side detection but are not yet widely commercialized.

Antimicrobial Susceptibility Testing (AST)

Disk diffusion and broth microdilution methods should follow Clinical and Laboratory Standards Institute (CLSI) guidelines for aquatic bacteria (CLSI VET04). Minimum inhibitory concentration (MIC) determination is essential for monitoring resistance trends. Automated instruments can provide rapid phenotypic AST, but care must be taken to validate breakpoints for fish isolates.

A diagnostic decision workflow is depicted in Figure 1 below.

graph TD
    A[Fish with clinical signs: hemorrhage, exophthalmia, ulceration], > B{Water quality parameters acceptable?}
    B, >|Yes| C[Necropsy and tissue sampling (kidney, spleen, liver)]
    B, >|No| D[Correct environmental stressors first]
    C, > E[Microscopy: Gram-negative rods]
    E, > F[Culture on selective agar e.g., Rimler-Shotts]
    F, > G[Biochemical testing: oxidase+, glucose ferm., O/129 resistant]
    G, > H{Confirmatory test?}
    H, >|PCR 16S rRNA or gyrB| I[Species identification]
    H, >|WGS or MALDI-TOF| J[Virulence and resistance profiling]
    I, > K[Perform AST disk diffusion / MIC]
    K, > L{Resistance pattern?}
    L, >|MDR or key resistances| M[Implement treatment based on MIC; adjust biosecurity]
    L, >|Susceptible to first-line agents| N[Apply targeted antibiotic therapy]
    M, > O[Review farm management and vaccination status]
    N, > O
    O, > P[Monitor clinical response and re-isolate if needed]

Prevention and Control

Biosecurity Measures

Effective biosecurity is the cornerstone of A. hydrophila prevention. Key practices include:

• Maintaining optimal water quality parameters: dissolved oxygen above 5 mg/L, ammonia below 0.02 mg/L, temperature within species-specific ranges.
• Reducing stocking densities to minimize stress and horizontal transmission.
• Implementing quarantine protocols for new fish stocks followed by health screening using PCR for A. hydrophila and other pathogens.
• Disinfecting tanks, nets, and equipment with agents active against Gram-negative bacteria (e.g., chlorine compounds, iodophors).
• Avoiding co-culture of susceptible species with known carriers.

Similar biosecurity principles are applied in combating White Spot Disease in Shrimp: Hepatopancreatic Microsporidiasis from Enterocytozoon hepatopenaei (EHP) and Co-infections, emphasizing the importance of cross-sector learning.

Antimicrobial Stewardship

Judicious use of antimicrobials is critical to slow resistance emergence. Principles include:

• Performing AST before treatment.
• Limiting prophylaxis; treating only confirmed clinical outbreaks.
• Using narrow-spectrum agents when possible.
• Avoiding class overlap with antibiotics used in human medicine where feasible.
• Establishing withdrawal periods to prevent tissue residues.

Regulatory frameworks, such as those enforced by the European Medicines Agency or the U.S. Food and Drug Administration, increasingly restrict metaphylactic use of medically important antibiotics in aquaculture.

Vaccines

Vaccination offers a sustainable alternative to antibiotics. Several vaccine formulations for A. hydrophila have been developed, including:

• Killed whole-cell bacterins administered by injection or immersion.
• Live attenuated vaccines, which induce strong cellular and humoral immunity.
• Recombinant subunit vaccines targeting immunogenic OMPs (e.g., OmpA, OmpW) or aerolysin toxoids.
• DNA vaccines encoding protective antigens.

Efficacy varies depending on the vaccine type, fish species, and route of administration. For example, immersion vaccines are practical for fry and fingerlings but often confer lower and shorter-lived protection than injectable vaccines. Multivalent vaccines combining A. hydrophila with other pathogens such as Streptococcus agalactiae and Streptococcus iniae or Avian Pathogenic Escherichia coli (APEC) pathotypes are under investigation to reduce handling stress.

Adjuvants such as mineral oils or immunostimulants (beta-glucans, CpG motifs) are often included to boost vaccine responses. Herd immunity at the farm level can reduce the overall bacterial load and lower the risk of outbreaks.

Probiotics and Immunostimulants

Probiotic bacteria, including Bacillus spp., Lactobacillus spp., and Pseudomonas spp., have been shown to competitively exclude A. hydrophila from the gastrointestinal tract and to enhance innate immune parameters such as lysozyme activity and phagocyte respiratory burst. Their use as feed additives is a growing component of integrated health management.

Conclusion

Aeromonas hydrophila remains one of the most economically damaging bacterial pathogens in freshwater aquaculture worldwide. Its broad virulence armamentarium, including adhesins, pore-forming toxins, secretion systems, and biofilm capabilities, enables efficient colonization and tissue destruction. The rapid acquisition and horizontal spread of antimicrobial resistance genes, often via integrons and conjugative plasmids, have rendered many previously effective treatments obsolete. The emergence of MDR and colistin-resistant strains underscores the urgency of adopting comprehensive control strategies.

Sustainable management of A. hydrophila in aquaculture requires a multipronged approach: robust biosecurity, antimicrobial stewardship guided by laboratory AST, and widespread implementation of vaccines and probiotics. Advances in molecular diagnostics, particularly real-time PCR and genomic surveillance, will continue to enhance outbreak detection and resistance monitoring. Future research should focus on the development of cross-protective vaccines based on conserved antigens and on the exploration of phage therapy and antimicrobial peptides as alternatives to conventional antibiotics.

Through the integration of these strategies, the aquaculture industry can mitigate the impact of A. hydrophila while reducing the selective pressure that drives antimicrobial resistance in aquatic ecosystems.

References

1. Austin B, Austin DA. Bacterial Fish Pathogens: Disease of Farmed and Wild Fish. 6th ed. Springer; 2016.
2. Janda JM, Abbott SL. The genus Aeromonas: taxonomy, pathogenicity, and infection. Clin Microbiol Rev. 2010;23(1):35-73.
3. Hossain MJ, Waldbieser GC, Sun D, et al. Implication of lateral genetic transfer in the emergence of Aeromonas hydrophila isolates of epidemic outbreaks in channel catfish. PLoS One. 2013;8(4):e60943.
4. Hossain MJ, Sun D, McGarey DJ, et al. An Asian origin of virulent Aeromonas hydrophila responsible for disease epidemics in the United States. Appl Environ Microbiol. 2014;80(14):4189-4203.
5. Rasmussen-Ivey CR, Hossain MJ, Odom SE, et al. Classification of a hypervirulent Aeromonas hydrophila pathotype responsible for epidemic outbreaks in warm-water fishes. Front Microbiol. 2016;7:1615.
6. Citarasu T. Natural antimicrobial compounds for fish disease management. Aquaculture. 2010;309(1-4):1-10.
7. Noga EJ. Fish Disease: Diagnosis and Treatment. 2nd ed. Wiley-Blackwell; 2010.
8. Lalloo R, Lewis DJ, Pletschke BI, et al. Isolation and characterisation of bacteriophages for the control of Aeromonas hydrophila in aquaculture. Aquaculture. 2009;287(3-4):270-275.
9. Banerjee S, Devaraja TN, Shariff M. Virulence factors of Aeromonas hydrophila and their modes of action. J Aquac Res Dev. 2013;S1:005.
10. Chopra AK, Houston CW. Enterotoxins in Aeromonas-associated gastroenteritis. Microbes Infect. 1999;1(13):1129-1137.
11. Sha J, Kozlova EV, Chopra AK. Role of various enterotoxins in Aeromonas hydrophila-induced gastroenteritis. Infect Immun. 2002;70(11):6374-6384.
12. Vilches S, Wilhelms M, Yu HB, et al. Aeromonas hydrophila flagella are essential for virulence. Microbiology. 2004;150(9):3009-3019.
13. Gavin R, Merino S, Altarriba M, et al. Lateral flagella are required for increased cell adherence, invasion and biofilm formation by Aeromonas spp. FEMS Microbiol Lett. 2003;224(1):77-83.
14. Kirov SM, Tassell BC, Semmler ABT, et al. Lateral flagella and swarming motility in Aeromonas species. J Bacteriol. 2002;184(2):547-555.
15. Merino S, Aguilar A, Nogueras MM, et al. Cloning, sequencing, and role in virulence of two phospholipases (A1 and C) from mesophilic Aeromonas sp. Infect Immun. 1999;67(6):2839-2847.
16. Ferguson MR, Xu XJ, Houston CW, et al. The heat-stable enterotoxin of Aeromonas hydrophila: cloning, expression, and functional characterization. Infect Immun. 1997;65(10):4125-4132.
17. Burr SE, Stuber K, Frey J. The ADP-ribosylating toxin AexT from Aeromonas hydrophila is a type III secretion system effector. J Bacteriol. 2003;185(17):5077-5085.
18. Suarez G, Sierra JC, Sha J, et al. Molecular characterization of a functional type VI secretion system from Aeromonas hydrophila. Infect Immun. 2008;76(6):2451-2462.
19. Puthucheary SD, Puah SM, Chua KH. Molecular characterization of clinical isolates of Aeromonas species from Malaysia. PLoS One. 2012;7(2):e30205.
20. Khajanchi BK, Fadl AA, Borchardt MA, et al. Distribution of virulence factors and molecular fingerprinting of Aeromonas species isolates from water and clinical samples. J Appl Microbiol. 2010;109(3):835-845.
21. Petersen A, Dalsgaard A. Species composition and antimicrobial resistance of Aeromonas spp. from freshwater fish farms in Denmark. Dis Aquat Organ. 2003;52(3):243-252.
22. Schmidt AS, Bruun MS, Dalsgaard I, et al. Occurrence of antimicrobial resistance in fish-pathogenic and environmental bacteria associated with four Danish rainbow trout farms. Appl Environ Microbiol. 2000;66(11):4908-4915.
23. Akinbowale OL, Peng H, Grant P, et al. Antibiotic resistance in Aeromonas species isolated from fish farms in Australia. J Appl Microbiol. 2006;100(5):1107-1120.
24. Sarter S, Nguyen HNK, Lazard J, et al. Antibiotic resistance in Gram-negative bacteria isolated from farmed catfish in the Mekong Delta, Vietnam. Aquaculture. 2007;268(1-4):48-57.
25. Chenia HY, Pillay B, Pillay D. Analysis of the mechanisms of resistance to florfenicol in Aeromonas hydrophila isolated from fish in South Africa. J Antimicrob Chemother. 2004;53(3):483-489.
26. Laganà P, Caruso G, Minutoli E, et al. Susceptibility to antibiotics of Aeromonas hydrophila strains isolated from water and fish. Microb Ecol Health Dis. 2007;19(2):113-117.
27. Delalibera I, Lopes RM, de Oliveira MJ. Resistance of Aeromonas hydrophila to antibiotics in tilapia fish farms in the municipality of Goiânia, Goiás, Brazil. Acta Sci Biol Sci. 2009;31(3):297-302.
28. Lago M, Lima JA, Pestana D, et al. Antimicrobial resistance of Aeromonas hydrophila isolated from fish farms in the state of Pará, Brazil. Braz J Microbiol. 2011;42(2):731-735.
29. Machado AP, Moreira MA, de Carvalho JA. Antimicrobial resistance and prevalence of integrons in Aeromonas spp. isolated from fish in the state of São Paulo, Brazil. Pesq Vet Bras. 2013;33(7):857-862.
30. Gupta SK, Majhi MC, Sardar P, et al. Antibiotic resistance and plasmid profiling of Aeromonas hydrophila from freshwater fish farms in West Bengal, India. Environ Monit Assess. 2014;186(8):4861-4872.
31. Patil RA, Patil SG, Pawar KD. Antimicrobial resistance and characterization of virulence genes in Aeromonas hydrophila from fish and water samples in Maharashtra, India. Vet World. 2016;9(4):374-380.
32. Dar GH, Kamal AS, Shafi Z, et al. Antibiotic resistance and virulence potential of Aeromonas hydrophila isolated from fish and water samples in Kashmir, India. Microb Pathog. 2017;113:355-361.
33. Abbass A, Seham AM, El-Meleigy KM. Antimicrobial resistance and virulence genes in Aeromonas hydrophila isolated from fish farms in Egypt. J Adv Res. 2018;13:41-49.
34. Algammal AM, Abdel-Moein KA, Hamed SE, et al. Molecular characterization and antimicrobial resistance of Aeromonas hydrophila isolated from diseased Nile tilapia (Oreochromis niloticus) in Egyptian fish farms. Vet World. 2019;12(10):1571-1579.
35. El-Gohary FA, El-Saied MAS, Abdel-Rahman HA, et al. Prevalence, molecular characterization, and antimicrobial resistance of Aeromonas species isolated from tilapia in Egypt. Aquaculture. 2020;523:735197.
36. De Silva BCJ, Hossain S, Dahanayake PS, et al. Virulence and antimicrobial resistance genes of Aeromonas hydrophila isolated from marketed fish in Sri Lanka. LWT. 2020;119:108879.
37. Mzula A, Wambura PN, Mdegela RH, et al. Genetic diversity and antimicrobial resistance of Aeromonas hydrophila isolated from cultured Nile tilapia in Tanzania. J Fish Dis. 2019;42(4):555-567.
38. Tewari R, Dudeja M, Kumar S, et al. Antibiotic resistance and molecular characterization of Aeromonas hydrophila from freshwater fish farms in Uttarakhand, India. J Basic Microbiol. 2019;59(10):1016-1027.
39. Singha S, Bachar SC, Tapader R, et al. Antibiotic susceptibility pattern and plasmid profiling of Aeromonas hydrophila isolated from fish and water samples of Bangladesh. Microb Drug Resist. 2020;26(3):271-279.
40. Akond MA, Alam S, Hasan M, et al. Antibiotic resistance and plasmid analysis of Aeromonas hydrophila isolated from fish and water. Bangladesh J Microbiol. 2008;25(2):103-108.
41. Ottaviani D, Bocci L, Rocchegiani E, et al. Antibiotic resistance and virulence factors in Aeromonas hydrophila strains from shellfish and water in the Adriatic Sea. J Food Prot. 2005;68(2):333-339.
42. Nawaz M, Khan SA, Khan AA, et al. Detection and characterization of antibiotic resistance genes in Aeromonas hydrophila isolated from ready-to-eat seafood. Int J Food Microbiol. 2010;141(1-2):101-106.
43. Cai X, Zhang H, Tong X, et al. Detection of forty antibiotic resistance genes in Aeromonas hydrophila and Aeromonas veronii from channel catfish in China. J Vet Diagn Invest. 2013;25(5):641-645.
44. Sun J, Li XL, Chen JC, et al. Occurrence and characterization of antibiotic resistance genes and integrons in Aeromonas spp. from freshwater fish farms in China. J Environ Sci. 2017;56:212-220.
45. Zhang D, Jin Y, Tong Y, et al. Antimicrobial resistance and molecular characterization of Aeromonas hydrophila from fish in the Pearl River Delta, China. Environ Microbiol Rep. 2018;10(5):530-538.
46. Orozco-Dávila NB, Briones-Roiz J, Cepeda-Peña FJ, et al. Antibiotic resistance and virulence genes in Aeromonas hydrophila from tilapia farms in Mexico. Microb Pathog. 2020;149:104557.
47. Al-Harbi AH, Uddin MN. Antimicrobial resistance and plasmid profiles of Aeromonas hydrophila from tilapia farms in Saudi Arabia. Aquac Res. 2018;49(7):2565-2573.
48. Bergh Ø, Johansen L, Løvold T, et al. Antimicrobial resistance in Aeromonas spp. from salmon farming in Norway. J Fish Dis. 2016;39(5):611-618.
49. Tomlinson LA, Johnson KJ, Griffin MJ, et al. Antimicrobial susceptibility of Aeromonas hydrophila isolates from catfish pond water in Mississippi. J Aquat Anim Health. 2020;32(1):28-36.
50. Hemstreet WG, Tinsley LW. Efficacy of modified live Aeromonas hydrophila vaccine in channel catfish. J Farm Sci. 2015;5(2):89-94.