The Super Bugs in Your Fish Tank

Ornamental fish, such as these discus,
 may harbor antibiotic-resistant bacteria.

As if the invasive species issue wasn’t enough, the presence of antibiotic-resistant pathogens in ornamental fish should be a significant concern to the ornamental fish industry, according to the authors of a study, “Imported ornamental fish are colonized with antibiotic-resistant bacteria,” published in the January 2013 Journal of Fish Diseases.

Not only will the industry’s ability to treat disease in fish be limited, causing both economic and emotional damage, but the antibiotic-resistant pathogens may also infect humans. The researchers noted that the U.S. The ornamental fish industry is a $900 million annual business in the U.S. and $15 billion worldwide, according to the researchers.

Dr. Tim Miller-Morgan

 “We expected to find some antibiotic resistance, but it was surprising to find such high levels, including resistance in some cases where the antibiotic is rarely used,” said Tim Miller-Morgan, DVM,  a veterinary aquatics specialist with Oregon State University. “We appear to already have set ourselves up for some pretty serious problems within the industry.”

The study sought to assess the diversity of bacteria and pathogens, and the level of antibiotic resistance, in tropical fish species imported to the U.S., according to the article's abstract. This information would be useful in treating clinical disease in fish and possible zoonotic disease in humans. Antibiotic resistance causes the antibiotic to lose all or some of its effectiveness against the pathogen.

In the study, researchers isolated and identified 64 unique bacterial colonies from kidney samples of 32 freshwater ornamental fish of various species from Columbia, Singapore and Florida. It is not immediately clear whether the Florida samples were from fish imported into Florida, produced in Florida’s tropical fish farms, or a combination of both.

The researchers detected multiple isolates of bacteria capable of causing disease in both humans and fish. The bacterial infections included Aeromonas, Pseudomonas and Staphylococcus. Researchers then examined the antibiotic resistance of each of the pathogenic isolates to nine separate antibiotics. Resistance was detected to all nine of the antibiotics, from a low for cefotaxime (16 percent resistance among isolates) to a high for the most commonly used, tetracycline (77 percent resistance).

“The range of resistance is often quite disturbing,” according to the study. “It is not uncommon to see resistance to a wide range of antibiotic classes, including beta-lactams, macrolides, tetracyclines, sulphonamides, quinolones, cephalosporins and chloramphenicol.”

Although the authors noted a human health risk, particularly for those who constantly handle ornamental fish (researchers, aquarium store employees, etc.) and individuals with compromised immune systems, the bigger concern is the diminished ability to treat fish disease effectively. Losses of fish to disease could become increasingly severe as antibiotic resistance builds, according to Dr. Miller-Morgan.

It will be interesting to determine how, if at all, the geographic origins of the samples differed, if the authors are able to capture that information from the data. That could lead to a more targeted application of some of the management practices recommended by the authors.

These practices include improvements in fish husbandry, better screening and handling, and the use of quarantines rather than antibiotics to reduce fish disease.

One of the main issues is that antibiotics are used routinely with ornamental fish, even without sign of disease, for example, when the fish face transport-related stress. But, just as for people, the judicious use of antibiotics in ornamental fish dictates that they only be used when treating a known bacterial fish disease. Otherwise, bacteria have the opportunity to develop resistance to specific antibiotics.

“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.”

To prevent spread of disease to other fish, he recommends immediately removing sick fish from tanks; buying only healthy fish; and keeping new fish in a separate quarantine tank for 30 days.

To minimize human health risk, Miller-Morgan recommends consumers buy only healthy fish, avoid cleaning tanks with open cuts or sores on their hands; use gloves; wash hands after working with fish.