Bacterial Diseases in Farmed Fish: Recognising Clinical Patterns in Practice

A fish population develops haemorrhagic lesions, lethargy and poor appetite. Another shows exophthalmia and abdominal swelling, while a third develops ulcerative skin lesions. 

Do these signs point to the same bacterial infection? 

Not necessarily. 

Bacterial infections are among the most common infectious diseases encountered in aquaculture, and several pathogens can produce overlapping clinical findings. For the practicing veterinarian, recognising the overall clinical pattern can help narrow the differential diagnosis before disease-control measures are considered. 

Why bacterial disease can escalate quickly 

Fish maintained under culture conditions can be more vulnerable to infection than fish in natural environments. High stocking densities, retention of infected fish, inadequate nutrition, poor water quality and stressful handling can increase susceptibility to infection^[1]. 

The pathogen, host and environment interact in determining disease onset and progression. High population density, sudden temperature changes and hypoxia can accelerate the spread of pathogenic bacteria and contribute to severe outbreaks^[2]. 

This means that clinical examination should not stop with the affected fish. The production environment is part of the clinical picture. 

When septicaemia and haemorrhage dominate 

Septicaemia and skin lesions are recognised signs of bacterial infection. Among bacterial pathogens, Gram-negative bacilli are the most frequent causes of infection in fish ^[3],[4]. 

Several important pathogens can produce haemorrhagic disease, but their associated clinical patterns differ. 

Aeromonas hydrophila mainly affects freshwater fish and can be associated with haemorrhagic septicaemia, oedema, epizootic ulcerative syndrome (EUS), haemorrhagic enteritis, and red cell illness. Affected cultured species include carp, goldfish, eels, catfish, and tilapia^[5]. 

Yersinia ruckeri, associated with enteric red mouth disease, can produce septicaemia and haemorrhages. Its ability to survive under conditions of poor nutrition and on the external body of the host can facilitate transmission^[6]. 

Vibrio anguillarum is another important cause of haemorrhagic septicaemia. It can cause fatal disease in Atlantic salmon, rainbow trout and Japanese sea bass, with associated mortality and financial losses in fish production. Vibriosis also affects groupers and other commercially important fish^[7]. 

When skin and external changes provide the clue^[7] 

Skin changes can help distinguish different bacterial presentations. 

Aeromonas salmonicida, associated with furunculosis, may present with dark skin, lethargy and appetite loss. 

Flavobacterium columnare causes columnaris disease, while F. branchiophilum is associated with bacterial gill disease and F. psychrophilum with diseases involving the gills. 

Other bacterial pathogens can produce specific external findings. Tenacibaculum maritimum is associated with epidermal ulcers, fin necrosis and tail rot, while Moritella viscosa is associated with winter ulcers. 

Consider temperature and chronicity 

Not every bacterial outbreak presents in the same way or under the same environmental conditions. 

Lactococcus garvieae is associated with fatal haemorrhagic septicaemia and meningitis in fish. In rainbow trout, hot-water lactococcosis becomes a serious disease during summer when water temperatures exceed 21°C^[8]. Reported signs include anorexia, melanosis, exophthalmia and abdominal swelling. 

In contrast, mycobacteriosis is a long-term and often a deadly disease. Mycobacterium marinum is particularly significant because of its broad host range, economic implications, and zoonotic potential. Fish may develop ulcerated skin, weight loss, and granulomatous lesions, and the disease can affect freshwater, brackish-water, and marine fish^[7]. 

Clinical bottom line 

When bacterial disease is suspected, one clinical sign should not determine the differential diagnosis. Combine the affected species, patterns of lesions, systemic signs, and environmental conditions. 

Haemorrhage, exophthalmia, skin lesions, appetite loss, and lethargy may occur with different bacterial infections. A structured assessment of the complete clinical picture can therefore provide a more useful starting point for identifying the likely disease pattern and planning appropriate fish-health management.

References:

1. C. Green, A. Haukenes, The Role of Stress in Fish Disease Southern Regional Aquaculture Center, 20 https://extension.rwfm.tamu.edu/wp-content/uploads/sites/8/2019/01/SRAC_0474.pdf
2. Irshath AA, Rajan AP, Vimal S, Prabhakaran VS, Ganesan R. Bacterial pathogenesis in various fish diseases: recent advances and specific challenges in vaccine development. Vaccines. 2023 Feb 17;11(2):470. https://www.mdpi.com/2076-393X/11/2/470
3. Lafferty KD, Harvell CD, Conrad JM, Friedman CS, Kent ML, Kuris AM, Powell EN, Rondeau D, Saksida SM. Infectious diseases affect marine fisheries and aquaculture economics. Annual review of marine science. 2015 Jan 3;7(1):471-96. https://parasitology.msi.ucsb.edu/sites/default/files/docs/publications/Laffertyetal2015ARMS.pdf
4. Lieke T, Meinelt T, Hoseinifar SH, Pan B, Straus DL, Steinberg CE. Sustainable aquaculture requires environmental‐friendly treatment strategies for fish diseases. Reviews in Aquaculture. 2020 May;12(2):943-65. https://onlinelibrary.wiley.com/doi/pdf/10.1111%2Fraq.12365
5. Mzula A, Wambura PN, Mdegela RH, Shirima GM. Current state of modern biotechnological‐based Aeromonas hydrophila vaccines for aquaculture: A systematic review. BioMed research international. 2019;2019(1):3768948. https://onlinelibrary.wiley.com/doi/pdf/10.1155/2019/3768948
6. Acuña LG, Barros MJ, Montt F, Peñaloza D, Núñez P, Valdés I, Gil F, Fuentes JA, Calderón IL. Participation of two sRNA RyhB homologs from the fish pathogen Yersinia ruckeri in bacterial physiology. Microbiological research. 2021 Jan 1;242:126629. https://www.sciencedirect.com/science/article/pii/S0944501320304973
7. Senthamarai MD, Rajan MR, Bharathi PV. Current risks of microbial infections in fish and their prevention methods: A review. Microbial pathogenesis. 2023 Dec 1;185:106400. https://www.researchgate.net/profile/Rajan-Muthuswami-Ruby/publication/375006029
8. Ganesan R, Vasantha-Srinivasan P, Sadhasivam DR, Subramanian R, Vimalraj S, Suk KT. Carbon nanotubes induce metabolomic profile disturbances in zebrafish: Nmr-based metabolomics platform. Frontiers in molecular biosciences. 2021 Jul 2;8:688827. https://www.frontiersin.org/journals/molecular-biosciences/articles/10.3389/fmolb.2021.688827/pdf