Avoid Shotgun” Medicines When Treating Fish Disease

Don't over-medicate your tropical and marine fish

A recent internet news story has been circulating about a girl who contracted a rare disease from an aquarium. Some various distorted versions of it have been popping up on message boards and in less reliable sources. The simple gist of the story is this: a teenage girl scratched her hand while working in the aquarium and contracted an antibiotic resistant bacterial infection. This infection has spread into the inner tissues, and for a while, there was talks about amputating her arm.

The bacteria in question are Mycobacterium marinum, which is often called “Fish Tuberculosis” in the hobby. This is one of the very few fish diseases that humans can actually catch, so a short discussion on it is warranted. The reason that the disease is often called “Fish TB” is because the bacteria are closely related to the bacteria that causes tuberculosis, Mycobacterium tuberculosis.  It is a fairly common fish disease, though, like many other diseases, it’s usually not lethal. Heavy infections that can lead to death are often characterized by scale loss, lesions on the body, and general wasting away – however, the list of diseases that have this symptom list is exceedingly long.

This group of diseases is often simply labeled as a “bacterial infection,” and treated with broad range antibiotics. These antibiotics are the same exact ones that are used in humans for the treatment of “bacterial infections,” ranging from strept throat to systematic infections. It’s important to take a moment and note that while the active ingredient in our fish medications is the same as what a doctor may be writing you a prescription for, the surrounding materiel are not the same. Aside from the plastic that the pill is made from, a pill contains numerous ingredients to help deliver the active ingredient to the organism. These can be wildly different between fish and human medications – every so often, some genius gets the idea to self medicate himself with aquarium medications, and usually winds up in the hospital. Often the binding materiel around the pill just doesn’t agree with the person, and the medication seldom works – our internal chemistry is slightly different than that of an aquarium.

Antibiotics are a truly remarkable discovery. The discovery of Pennicilin, the first widely used antibiotic, is attributed to Alexander Fleming in 1928 (though some others had noticed the same thing: moldy bread inhibits bacteria). Once the chemical was isolated, the world changed. Without a doubt, this was one of the moments in history where historians would be able to draw a line, separating out ages. By the mid-1940s, Pennicilin was widely available, and touted as the miracle cure it truly is.  Pennicilin’s discovery so close to the outbreak of the Second World War significantly reduced the casualties of that war.

Antibiotics are derived from natural sources, meaning that they’re naturally occurring (or, at least, very similar products are). Given this, and given how toxic antibiotics are to bacteria, it should not have come as a surprise that some bacteria have a natural resistance to antibiotics. The first such resistant strains of bacteria were discovered before the Korean War.
Of course, antibiotics are widely used beyond the medical field. We use them extensively in our aquariums for the treatment of a wide range of diseases. Many of the common medications we use are blends of various antibiotics, used to treat a broad range of possibilities all at once. Often, antibiotics are even used as a preventative: ‘fish seem a little slow? Fins a little clamped? Weak appetite? Try antibiotics!’ Many retail aquarium stores, wholesalers, and trans-shippers will dose all incoming fish with antibiotics, just to be safe. This prophylactic use of antibiotics is, for the most part, the norm in the United States, though in some countries it is banned and antibiotics actually require a prescription from a licenses vet.

One of the major differences between bacteria and us is that bacterial cells are prokaryotic in nature.  While our DNA is contained in a neat little packet, known as a nucleus, the DNA of bacterial cells is just strewn throughout the cell. The majority of this DNA is contained in chromosomes, similar to ours, but some of it is contained in small rings of DNA, called plasmids. Some of these plasmids code for a special ability for the cell. I like to think of these plasmids as bacterial “upgrade” packages, as each one allows the bacteria to do something that it couldn’t previously. Many of these “upgrade” packages include the keys to antibiotic resistance.
What makes these plasmids even more interesting is that they’re replicable within the cell, meaning it can have many, many copies of the plasmid, and they can be traded. Two bacteria meet, one says to the other, “Hey, got any cool plasmids?” The second says, “Yea, check this out: Penicillin resistance.” They form a little bridge between them and exchange plasmids. This can happen between species.

 This creates a recipe for potential disaster. When you dose your aquarium with antibiotics, one of two possible things can happen. In the first case, you actually use a sufficient dose of antibiotics for a sufficient length of time to kill all the offending bacteria. Unfortunately, a few of the bacteria are naturally resistant to whatever antibiotic you used – it doesn’t matter if they’re disease causing bacteria or simply some natural bio-film on the side of your filter.  In the second case, you didn’t quite use enough, or didn’t continue the treatment for long enough. Many, many bacteria are left, including resistant individuals. These resistant individuals are in great health and quickly spread the resistant plasmids to their sickly brethren, as well as to other bacteria in the community.

 Now, keep in mind, bacteria are bacteria are bacteria. The bit of bio-film that’s on the side of the filter can transfer its resistance to bacteria on your hand. From there. . .  anything is possible.
Once the resistance is present in the community, its apt to stay there, too. So, imagine dosed with Penicillin. Next time you dose with Penicillin, nothing happens. So you try Erythromycin. Guess what? You now have a population of bacteria that’s resistant to both. So, next time, you’ll have to try Tetracycline. Superbug is coming. 

 When I was a child, I remember seeing aquariums in every single doctor and dentist office. Today, it’s not as common, particularly in patient intensive areas and sensitive areas (you still see a lot of them in waiting rooms). In 1979, an article was published in the Canadian Journal of Microbiology titled “Aquarium pets as a source of antibiotic resistant Salmonellae.”  This began the end of the age of aquariums in doctor’s offices. Coupled with increased frequency of resistant bacteria, particularly in hospitals, aquariums are slowly vanishing from medical centers.
 The bacteria that has infected the arm of the young girl appears to be resistant to most antibiotics. These resistances definitely stack.  With so many types of disease becoming resistant to antibiotics, we may someday soon find ourselves in an era before Fleming. Antibiotics are a vital natural resource that we need to work to protect.

 The first step in this is to not over-use them. Antibiotics are useful in treating bacterial infections only, and without knowing the type of bacteria involved, or at least the specific disease, treatment should be avoided. Most bacterial diseases in fish can be treated with a very complicated and hard to obtain chemical known as sodium chloride, or salt. Dissolve four cups of plain, old, uniodinised (kosher) salt in a gallon of aquarium water. Gently net out a sick fish, put him in the gallon of water and let him swim around for about minutes (watch the fish for signs of stress and remove earlier if needed). Net him out and return him to the aquarium (do NOT dump the saltwater back into the aquarium). This will take care of virtually all “bacterial” infections. For tanks of sick fish, as much as a teaspoon of salt per gallon will take care of most infections as well.

 Try to avoid the “shotgun” medications that contain several different antibiotics, all they do is expose bacteria to many different antibiotics at once, allowing them the opportunity to develop multiple resistances simultaneously.  

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Fish · Health and Care