Antimicrobial Resistance in Fish and Implications for One Health
World Small Animal Veterinary Association Congress Proceedings, 2017
Chris Walster, BVMS, MVPH, CertAqV, MIFM, MRCVS
The Island Veterinary Associates, Stafford, UK

Pet or ornamental fish have long been recognised as assisting in reducing stress in doctor’s and dentist’s surgeries. The trade and hobby also assists in protecting the environment through providing trade opportunities for indigenous people with Project Piaba in the Amazon rainforest being the obvious example to cite. However, concerns have been raised since the early 1970s over the use of antimicrobials in fish, and in particular aquaculture. To date the evidence appears conflicting and inconclusive although when reading the literature, each side of the argument hold strong views. Putting aside these arguments, using antibiotics can create antimicrobial resistance, and as veterinarians we have a duty to minimise that risk. This presentation looks briefly at some of the issues that arise through the trade and the treatment of ornamental fish that could cause antimicrobial resistance and their potential impact on people.

Well over 20 genera of bacteria have been isolated from fish with the most common isolates in clinical disease being aeromonads, Vibrio sp., Flavobacterium columnare, Streptococcus sp., and Mycobacterium sp.

Aeromonads are commonly isolated in cases of ulcer disease and septicaemia in fresh water fish and are ubiquitous in the environment being found in water and soil. A. hydrophila is the most commonly isolated of this group and is generally considered to be an opportunistic pathogen. Whilst A. hydrophila is recognised as a cause of gastroenteritis in people, and can cause urinary tract and skin infections in the immunocompromised, it is not reported as a zoonotic disease. Reported resistance amongst aeromonads is variable but they are considered inherently resistant to β-lactam antibiotics (i.e., penicillins and cephalosporins).

Vibrios are the marine environment equivalent of aeromonads and cause similar conditions in fish with chronic infections sometimes produce granulomas in muscle tissue. Fish pathogenic vibrios are not usually considered zoonotic although vibrios typically cause illness in people who eat contaminated seafood. The three of most concern would be: V. cholerae the cause of cholera, V. parahaemolyticus which causes non-bloody diarrhoea and V. vulnificus which causes an often fatal septicaemiae in immunocompromised individuals.

Flavobacterium columnare is the cause of columnaris disease (cotton wool disease and saddleback disease from the appearance of the lesions) which is most often seen when fish are stressed due to poor water quality, shipping, or poor nutrition. It is commonly seen as part of “new tank/pond syndrome” which occurs prior to the new system’s filter can handle the biological load. As such, it is a management issue.

Streptococcus sp. (Streptococcosis) is sometimes seen in warm water or tropical collections as infections appears to be dependent on water temperature. Some Streptococcus such as S. iniae can cause serious illness in people including toxic shock syndrome.

Mycobacteriosis is probably more common than diagnosed as the bacteria are prevalent in the environment and infection rates can be as high as 30% in fish produced in earthen ponds. Generally, it is an end-stage disease in fish suffering from chronic “poor doing”. When transmitted to people, treatment can mean prolonged courses of antibiotics and even amputation of digits or limbs.

Although the use of antibiotics by ornamental fish veterinarians is very low, probably all classes of antibiotics have been used in ornamental fish at one time or another, and there has long been usage by hobbyists from other sources, due to a lack of interest from the profession, which has led to widespread issues with resistance.

Currently the most frequently used antibiotics either separately or in combination are:

  • Fluoroquinolones - Ciprofloxacin/enrofloxacin which is used to treat aeromonad disease both in fish and people.
  • Aminoglycosides - kanamycin, amikacin, gentamicin. Streptomycin and neomycin are rarely used due to resistance. Aminoglycosides are used in people to treat multi-drug resistant tuberculosis.
  • Amphenicols - Florfenicol is generally used in veterinary medicine applications although some authors, and it is disputed, blame its use in aquaculture for the occurrence of the flo gene in Salmonella typhimurium DT104.
  • Nitroimidazole - Metronidazole used for Hole in the Head/Lateral Line erosion (HITH/LLE) particularly in Cichlids is also one of the first line treatments for Clostridium difficile infections in people.

Ornamental fish can be captive bred or wild caught, and on their way to the hobbyist are likely to pass through several transportation hubs, rather like any international traveler, disseminating infectious organisms around the world. This is exemplified by sporadic reports of hobbyists catching salmonellosis from fish. Salmonella are transitory inhabitants of the fish gut and in tracing the source of the infection it is often found to be contaminated water from one of these “hubs”. Intriguingly a recent blog from UK government scientists ( argued that the last three significant El Nino events saw new variants of waterborne pathogens emerge in South America. Not only can dissemination be manmade but also probably naturally.

Clearly from the above there is potential for the ornamental fish hobby to impact the health of people for good or for bad. Morally and ethically as aquatic animal veterinarians we need to reduce the use of antibiotics and ensure we use them effectively. Routinely taking swabs for culture and sensitivity might be an option but often getting good samples requires the sacrifice of fish which is unacceptable to hobbyists. Taking swabs from ulcers often grows A. hydrophila, however, one of the causes of ulcer disease is A. salmonicida which is overgrown by A. hydrophila. Further, if samples are sent to laboratories with little fish experience then resistance can be amplified by incorrect use of media or interpretation of resistance incorrect. As such culture and sensitivity is not really helpful in many circumstances.

The question becomes can we treat infections in ornamental fish without the use of antibiotic? Mycobacteriosis should not be treated due to the zoonotic implications and any associated fish in the tank should be kept isolated. Except for septicaemia and HITH/LLE there should be little call to use antibiotics and whenever possible they should be administered by injection. This ensures correct dosage and minimal wastage of antibiotic. Using baths or prolonged immersion should be avoided due to the amount of antibiotic used to achieve the correct dose. In-feed antibiotics can often be pointless as ill fish do not eat, although using appetite stimulants such as garlic or aniseed can help. Fish less than 5 cm in body length are difficult to inject and the injection site should always be massaged immediately afterwards or the antibiotic will leak out.

Many bacterial infections of ornamental fish can be treated and/or controlled whilst stressors such as poor water quality are improved. As example, ulcers can be healed by cleaning with a topical antiseptic such as povidone-iodine as fish skin will not seal the defect in the presence of bacteria, but can seal large deficits within 24 hours under the right conditions. In cold water fish increasing the water temperature will improve the function of the immune system and minimise scarring. Using supportive therapies such as vitamin C or levamisole (both 10 mg/l) will also improve immune function. The use of disinfectants such as Virkon Aquatic will control and decrease bacteria in the water column as will the use of salt in fresh water fish which will also decrease osmotic stress allowing more energy to fight the infection.

By learning a bit about ornamental fish and educating our clients then there will be minimal need to dispense antibiotics which is perhaps the biggest negative for One Health in ornamental fish. Then we can gain through reducing stress in our lives through watching fish in a well-maintained tank, knowing that we are helping to protect the natural environment through the economic benefits to indigenous peoples around the world.


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Speaker Information
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Chris Walster
Island Veterinary Associates
Stafford, UK

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