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Tropical Fish Highly Resistant to Tetracycline, Other Antibiotics

Oregon State University Study provides guidelines for those who consistently handle tropical fish

By | January 17, 2013

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Discus fish

The discus was one of the species of fish that the scientists reported as resistant to antibiotics. Photo by Al Castro.
A new study by Oregon State University scientists sounds the alarm of antibiotic resistance within the $15 billion global ornamental fish industry. In the study, led by Tim Miller-Morgan, scientists tested 32 species of freshwater fishes for resistance to nine types of antibiotics. They found some level of resistance to every antibiotic, with the highest level of resistance pegged at 77 percent for tetracycline, a common antibiotic used for a variety of human and fish ailments.

“The range of resistance is often quite disturbing,” the scientists wrote in the report. “It is not uncommon to see resistance to a wide range of antibiotic classes, including beta-lactams, macrolides, tetracyclines, sulphonamides, quinolones, cephalosporins and chloramphenicol.” The study found Aeromonas, Pseudomonas, Staphylococcus and other bacterial infections in the fish, some of which can infect both fish and humans.

While the report states that there have been some documented cases of disease transmission between fish and humans, the scientists say that it is not common. The study points out that those with weak or compromised immune systems, as well as those who consistently handle tropical fish may be at a higher risk.


The study offered several tips to reduce the chance of exposure to diseases while handling tropical fish:

  • Consumers should purchase only fish that are healthy
  • Avoid cleaning fish tanks when open cuts or sores are present on the hands and arms
  • Remove sick fish from tanks immediately
  • Maintain a quarantine tank and quarantine all new fish in the tank for 30 days
  • Always wash hands after handling fish
  • Never use antibiotics in a fish tank unless you are treating a fish disease that is actually caused by bacteria

“We don’t think individuals should ever use antibiotics in a random, preventive or prophylactic method,” Miller-Morgan said. “Even hobbyists can learn more about how to identify tropical fish parasites and diseases, and use antibiotics only if a bacterial disease is diagnosed.”

The global ornamental fish industry is a $15 billion business, with the United States generating $900 million every year. There are more than 6,000 species of freshwater and marine fish traded every year from more than 100 countries.

So what is the takeaway from this study? Avoid "shotgun" medicines when treating fish disease and don't over-medicate your tropical and marine fish. Follow the tips presented in the study and wash your hands.

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