The Microbiologist put down his cup of coffee, yep, coffee, it may be the hottest day of the year out there, but here in the lab it’s air-conditioned to the point of refrigeration!!!!
“Go on then” said the Microbiologist being his normal cheery self.
“This chap is in his 40s and I have been treating him for a skin infection, but it’s getting worse. He’s had courses of Flucloxacillin, Erythromycin and Doxycycline and nothing has made any real difference. I wonder what I should try next?”
The Microbiologist started to pay a bit more attention. This was a list of antibiotics usually good for treating skin infections so it sounded a bit more “interesting”… (yes ECIC, interesting!)
“What does it look like, this infection? Does he have cellulitis? Where on his body is it?” said the Microbiologist.
“It’s on his hand. It started as a slightly painful lump and then got bigger and now it looks like an infected ulcer. He also has new lumps up his arm. It looks a bit weird really,” replied the GP.
Hmmmmm, even more “interesting” thought the Microbiologist. This sounds like sporotrichosis, but I’ve already seen my one case in a lifetime so it can’t be that!
“Has there been any recent travel? What does he do for a living? Any hobbies or pastimes, pets, gardening?”
“Hold on, I’ll ask him” said the GP… the patient was clearly still in the room.
After a muffled conversation the GP came back on the line.
“No travel. He works in IT. No gardening and his only pets are a few fish… and before you ask…no, he hasn’t been bitten by them!” said the GP.
Bingo! Thought the Microbiologist… wondering if the GP really thought he thought the patient could have been bitten by a fish like Jaws. Deciding not to give the standard “detailed history is usually very relevant speech” he decided to let the GP off the hook… (pun intended!) and asked…
“What type of fish are they?”…and then to get his own back about the bite jibe, “and what are their names!?”
Another muffled conversation followed.
“He has a number of tropical fish tanks, but I forgot to ask their names” said the GP and then realised he’d fallen for a ruse. “You didn’t really need to know their names did you?!” he said accusingly.
“No” giggled the Microbiologist, “but on the bright side I think I know what’s wrong with your patient. I think he has fish tank granuloma, caused by Mycobacterium marinum. He’s going to need a biopsy and a long course of antibiotics. It might be best to let our Dermatology colleagues follow this up. To make up for the names thing I’ll have a chat with them and ask them to get him up to their clinic as soon as possible. Is that okay?”
Appeased that he wouldn’t have to try and get hold of the busy Dermatologists, the GP let the names joke slide… for now…
What is “fish tank granuloma”?
Fish tank granuloma is a skin infection caused by the bacterium Mycobacterium marinum which can be found in fresh or salt water, including the sea, swimming pools and fish tanks (any tanks… not just unclean ones!).
The bacterium enters the skin through punctures, abrasions and other wounds, usually occurring when someone is cleaning a tropical fish tank, where it sets up a chronic infection. Biopsy of a lesion shows granuloma formation like in tuberculosis (another mycobacterial disease); granulomas are a collection of macrophage white blood cells that occur in some types of chronic inflammation.
So, the name “fish tank granuloma” relates to where the bacterium is acquired from and the appearance of the lesion under the microscope… simple!
What does a fish tank granuloma look like?
Fish tank granulomas initially look like 0.5-1cm reddish purple lumps, usually on the hands (but if the exposure is from being in open water they can be anywhere). The incubation period is 2-3 weeks. The lesions usually enlarge before starting to collapse in the middle to form ulcers. The lesions often form scars unless treated early.
Occasionally the skin lesions spread along the lymphatics from the initial infection and new lumps and ulcers appear ascending up the limb. This is known as sporotrichoid spread as it is similar in appearance to the fungal infection called sporotrichosis.
How is it diagnosed?
The most important aspect to diagnosing fish tank granuloma is to take a full history. Other doctors regularly roll their eyes at Microbiologists who seem to want to know all sorts of obscure bits of information about patients, but in our defence, travel, occupation, animal exposure, and hobbies are often very important in determining what infections a patient might have been exposed to.
Once the infection is suspected the best sample to take is a biopsy both to try and grow the bacterium as well as histology to look for the granulomas.
Most laboratories in the UK use automated liquid TB culture incubators but these are not suitable for M. marinum as this bacterium requires a lower temperature to grow, 28-30oC rather than the 37oC body temperature of the automated incubator.
IF there is good clinical information on the request form such as “tropical fish tank” (yes, use that clinical info box on the form – it’s there for a reason!) the laboratory staff will put up an old-fashioned Lowenstein-Jensen media slope (often known as an LJ slope) and incubate it at 30oC for M. marinum. BUT ONLY IF THEY KNOW THE CLINICAL DETAILS!!! On Lowenstein-Jensen media the bacterium will normally grow within 7-10 days.
How is it treated?
There is no single recognised regimen for treating fish tank granuloma. However the bacterium is usually sensitive to Clarithromycin, Minocycline, Doxycycline and Co-trimoxazole, all of which are oral and can be used to treat the infection. If the infection is severe, especially if there is extensive sporotrichoid spread, then either Ethambutol or Rifampicin can be added.
I personally favour Clarithromycin as it is very familiar to doctors even if fish tank granuloma isn’t. In severe cases choosing between Ethambutol or Rifampicin would depend on whether the patient was on any other medications or had any co-morbidities that made one safer than the other.
Treatment should continue for 1-2 months after the lesions have resolved to prevent relapse. This usually means courses of 3-4 months.
It is also important to tell people who have tropical fish tanks to wear gloves when cleaning them… it’s the simplest way to avoid the infection in the first place!
So, for our patient he was seen the same day by the Dermatologists who took a biopsy and started the patient on PO Clarithromycin. The biopsy grew M. marinum confirming the diagnosis of fish tank granuloma, and after a few weeks the skin lesions had started to improve.
Oh, and if you’re wondering, the fish were called Bert, William, Tom, Doris, Mabel, Trevor… only kidding!