Should you be worried about fish TB?


Well, what a stupid question. Should you be worried about an organism that can cause you pain and discomfort, and potentially ruin your hobby for you? Yes, yes you should. I guess that I should really be asking how much is it reasonable to worry about fish TB, says Nathan Hill.

What is the likelihood of picking it up? What’s it going to do to you? Is your aquarium just one festering, pus-riddled ball of illness, ready to send flesh-eating bacteria into your bones to feast on your marrow and make your teeth fall out?

If you don’t know what fish TB is, then brace yourself. It’s pretty important.

It’s important because there’s such a high degree of misdiagnosis taking place. If you go to your doctor, clutching at rancid, seeping wounds and demand an explanation and he doesn’t know you keep fish, then it’s likely you’ll get a lot of wrong medicine and painful biopsies before they get it right. Correct diagnosis is critical to getting the right help, as anyone who’s had to seek fish disease help from a retailer will no doubt testify.

Fish TB is an incredibly unpleasant zoonotic illness. By zoonotic, what we mean is that it can transfer from animals to humans, a bit like Swine flu but without the resulting media frenzy (aside what I’m doing right now, that is).

That means that if our fish can carry it, which they can, then we can get it from them, which we sometimes do.

Fish TB is caused by one of several pathogens, all belonging to the genus Mycobacterium. They can be affected by, or at least be passive carriers for, M. abscessus, M. chelonae, M. celatum, M. dienhoferi, M. fortuitum, M.flavascens, M. gordonae, M. kansasii, M. intracellulare, plus many more, and the one that really worries us — M. marinum. M. marinum is well known for its zoonotic antics, although M. fortuitum and M. chelonae can also make the jump.

To really mess things up, it’s now also suspected that fish can act as passive hosts (along with many tropical snails) for M. ulcerans, a real nasty Mycobacterium that causes the horrendous, ulcerating Buruli disease. You really don’t want that one.

Don’t worry too much about M. ulcerans, though. Unless you happen to be living in tropical Africa and eating locally caught Tilapia, it’s probably never going to appear on your radar. In fact, some researchers are still questioning the link over fish and M. ulcerans, so I’m likely to get an email from some angry scientist telling me how wrong that last paragraph is.

Back to M. marinum, and we find problematic little bacteria that’s been bothering humans for decades. It was first isolated from fish back in the 1920s, and a link between fish and human infections made in 1951 when it was isolated in a Neon tetra. Flash forward 11 years to 1962 and we have our first recorded aquarium-borne case in a human.

Up until then, it was generally accepted that people who contracted it got it from swimming pools (pre-chlorine era and all that) or from fishing-based injuries. Certain harvesters of shellfish used to be particularly at risk.

The upshot of all this is that it’s been knocking about in tanks for at least 50 years and still hardly anyone knows what it is. And people accuse fishkeepers of being slow to latch on to new ideas…

It’s now estimated that around half of all global cases of M. marinum infection come from fish and fish handling. That is to say, dealing with fish and fish tanks. Some sources think that the fish link would be higher were it not for that all troubling element in making connections — delayed diagnosis.

M. marinum is unusual as pathogenic bacteria grow. Normally germs like to hunker down in the log fire of our bodily warmth and start reproducing at a rate that would make the Daily Mail start a hate campaign against them. However, M. marinum likes things cooler. No 37°C/99°F tropical oasis for these guys, they prefer to ride it out at around 30°C/86°F or so. That’s diagnosis problem number one. Stick them in a nice body temperature culture as you would other pathogens and they don’t grow. The test reveals very little indeed about what you have in your body.

And that leads to diagnosis problem number two. Because it’s not fuelling itself on your volcanic organs, M. marinum is slower to grow than many other bugs we catch. Incubation alone can be two to four weeks before you even start to see your first lesion and several months before a full-blown outbreak. By then, you’ll have forgotten all about that run-in you had with a catfish’s spine or that cichlid that gave you what for after you got too close to its fry.

So what does this disease look like? In humans, it takes the form of a small nodule or granuloma somewhere around the site that it first entered your fertile, nourishing body (pictured above by Robert Fenner).

This nodule will get to about 3cm/1" in diameter and look pinky-purple, a bit like an angry zit but without the head. It’ll also start to hurt like billy-o. And that’s if you only get the one. The other option is that it starts to creep up the old lymph system, leading to multiple nodules as it works up the arm. Normally it starts off on the extremities, though, where the lower temperature is about right for it.

Get really unlucky and it’ll start to give you more gyp than that. Once it gets embedded in the flesh it can start to cause arthritis and tenosynovitis, where the sheath around tendons starts to swell with fluid. It hurts a lot, I’m told. And if your luck has completely failed on you, you could end up with osteitis — inflammation of the bone. Need I say more?

Immune-compromised people are much more at risk of these latter symptoms, I should add, and a healthy, fighting fit young buck should only need a course of the right antibiotics to get over a few hand nodules.

That assumes you like antibiotics because there’s no such thing as a short course. Ethambutol does the job, as does Minocycline, Rifampin, Biaxin and a Trimethoprim sulfamethoxazole combo. Expect to be on them for at least two months once diagnosis is complete. On a brighter note, M. marinum hasn’t developed immunity to any antibiotics yet, so there’s little chance of getting some superbug strain of it.

There are other treatments that have had effect on advanced cases. Some people have had to have surgery, cryotherapy (I’ll let you Google that one), electrodessication and even X-ray treatments. This is a potential illness you really need to take seriously.

And what about the poor fish harbouring this filth? Well, it screws them up, too, and more so than humans, in fact. Whereas we can recover from it after some antibiotic, the outlook for our scaly companions is bleak. To date, there’s no effective treatment in fish and the only way to get it out of the system is to get them out of the system. That is to say, infected fish need to be euthanised.

That doesn’t automatically mean that once you’ve hoiked out a handful of obviously infected fish that your worries are over. M. marinum can easily spread from fish to fish if they have any open lesions at any point. Or worse, if they eat the bodies of any infected fish, which as you’ll know, everything in your tank will do. Even the diehard herbivores will be found tucking in to a corpse’s spleen given the chance.

It’s slightly worse if you have a recirc system, too, especially one without banks of UVs and/or ozone on them. In that situation, if you’ve had a case of M. marinum in a single fish, you’ll need to assume that it’s now in the entire system; the best of luck with that.

Symptoms on fish are pretty easy to spot, although it takes a dissection by someone who knows what they’re looking for to get a successful diagnosis.

Early signs are emaciation in fish, especially a wasting of flesh and muscle tissues in the top half of the body, leading to a knife-backed appearance. Abdominal distension is another classic sign, along with spinal deformities, missing scales and open wounds and blisters on the skin. Spinal deformities often occur, with kinked backs and curious hunches. You’ll even spot bulging, exophthalmic eyes after a while.

While I’m making the point, one of the other Mycobacterium species, M. fortuitum, is heavily implicated as a cause of dropsy in fish, causing abdominal bloating and fluid accumulation, and eventual death.

As far as we know, any fish on the planet is susceptible to the disease, across both marine and freshwater domains. So far, it’s been confirmed as present in at least 160 different species, and it’s only that it isn’t being tested everywhere that this number isn’t growing.

There are some culprits who pick it up more than others, for sure. My first experiences of it were with Tiger barbs, and other barbs, goldfish, koi and danios seem to account for more than their fair share. Anabantids are buggers for it, especially some of the gouramis — as if Dwarf gouramis didn’t have enough disease issues of their own already.

It’s been found in tetras of all kinds and humble Angelfish, among other cichlids. In the marine world, it’s been the bane of Sea dragons, and Seahorse breeders the world over are all too aware of what it can do.

Don’t think it’s something you can leave to its own devices in the hope it’ll go away. In experiments, M. marinum has been shown to cause between 90 and 100% mortalities in infected danios over a 21-day period. These fish didn’t all drop at the same time, with the first going after seven days.

Fish, and you, are more at risk if you’re a bad fishkeeper, I add. Mycobacterium love low pH values, low oxygen, high soluble zinc and high fulvic and humic acids. Basically, everything you’ll have going on if you’re overstocked or can’t be arsed to maintain your aquarium properly.

Dealing with it

OK, so now you’re petrified of ever getting within 50 miles of a fish ever again, it’s worth getting some reassurance in. The likelihood of you ever getting this illness is low (personally).

Your fish are at a higher risk, but you can have some say in this. If you’re going to buy awful fish from dirty, cramped tanks, that have come from dirty, cramped farms, to stick into your own dirty, cramped tank, then you’re high on the risk chart. If you buy healthy fish from healthy tanks, from good wholesalers and put them into a clean environment at home, then you’ll be much safer. That’s not so difficult, surely?

Infection rates in people are low. In the USofA, where they seem to get it a bit more than in the UK, the rate (for all cases, not just aquaria) is around 0.27 infections per 100,000 population. In the UK, the current rate of murder is 1.22 per 100,000 population. So, you’re likelier to be killed by a loon with a machine gun than catch it. It’s pretty rare.

Avoiding it in the first place is a pretty smart idea. The easiest way to do that is to simply not go in the tank if you have any, and I mean any, cuts or lesions on your hands in the first place.

The other way to avoid it is to use gloves, obviously. Use rubber gloves like Marigolds if you have the slightest hint of suspicion about your tank, and thick, heavyset swines (the kind you’d be wearing in a nuclear disaster zone) if you’re going to be handling anything that can cut, bite, sting, graze or otherwise maul you.

It’d probably be a good move to keep a dispenser of alcoholic sterilising gel on standby, too, for the moment those hands come out of the tank.

Oh, and don’t use anything from the aquarium on anything else in the house. There was a reported case involving a child who picked up the disease after its bathing accessories were used in an aquarium.

Adults are likelier to get it than juveniles, and, as already mentioned, if you have any immune system flaws, get someone else to do those gravel cleaning chores for you.

As for the fish, it’s bad, bad news. If you get a confirmed case, the advice from all quarters is depopulation of the whole tank. If that sounds ominous, it’s because it is ominous. The best thing you can do in the event of a diagnosed outbreak of fish TB in your tank is to humanely destroy everything in there, sterilise everything associated with the tank and start again.

But seriously, don’t panic. Just be aware of it. Google images of symptoms and learn it inside and out. Know the signs and you’ll be fine.

Ignore it at your own peril.

Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.

Don't forget that PFK is now available to download on the iPad/iPhone.