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Zoonotic diseases from pet fish are very rare, but rare doesn’t mean never.  That can make messaging around the disease risks from pet fish challenging, because there’s a need to balance costs vs benefits, and being proactive vs paranoid.

I have a saltwater fish aquarium, and I don’t stay up at night worrying that it’s an impending source of doom. For managing the aquarium, I use some really basic and non-disruptive common sense, though. I did some work on it a couple days ago and paid attention to two things: I kept my hand with a few cuts on it out of the tank, and after I was done I washed my hands.  Common sense and hygiene 101.

The most commonly reported zoonotic disease issue from aquariums is probably Mycobacterium marinum infection, also known as “fish tank granuloma.” A recent paper in Emerging Infectious Diseases (Dawson et al. 2021) describes a case of another type of infection acquired from a household aquarium, caused by a bug that’s gotten some press recently for other reasons: Burkholderia pseudomallei, the cause of melioidosis.

Melioidosis is a nasty and potentially life-threatening disease. In North America, melioidosis is typically associated with travel to tropical or subtropical areas where the bacterium can be found in water and soil.  People are exposed through ingestion or inhalation of the bacterium, or contamination of broken skin. However, local sources in North America have been identified, such as the recent outbreak of human melioidosis cases in the U.S. that was linked to contaminated aromatherapy products.

The case in the recent EID paper was a 56-year-old woman in the U.S. that was hospitalized because of fever, cough and chest pain. She had a somewhat complex medical history, and had been taking immunosuppressive drugs up until approximately 1 month before she got sick. Culture of her blood identified B. pseudomallei.  Fortunately, she responded well to a few months of antibiotic treatment.

The first question asked after the bacterium was isolated was likely “where have you traveled recently and when?” However, she had no history of travel.

As part of the next stage of the investigation, they found out the patient had two freshwater aquaria, from which they collected samples for testing. (It’s not clear how quickly they moved from “no travel history” to “maybe the fish tanks are the source” and how many steps were in between.)

There’s nice description of events as they relate to the aquaria. The patient had purchased two small aquaria about 3 months before she got sick. In one (tank A) she kept cherry barbs and in the other (tank B) she kept guppies. She also purchased some tiger barbs for tank B, but that was after she got sick.

Interestingly, she reported that the water in tank B had been persistently cloudier than the other tank, and it required more cleaning, which she did with bare hands.  That’s the aquarium from which B. pseudomallei was isolated.

Presumably, the woman was exposed while cleaning the aquarium, either via a break in her skin, contact of contaminated water with her nose, eyes or mouth (e.g. splashes, contact with wet hands) or maybe contact with areas that got contaminated by water from the aquarium (e.g. areas around a sink down which water was poured).

There were also dead fish in tank B when public health investigators came to visit. That raises the question of whether the fish might also have been sick. We’re not going to recommend testing all dead pet fish for melioidosis as a surveillance tool, but it once again illustrates the potential inter-relatedness of human and animal health.

The article concludes “To prevent or reduce risk of exposure, particularly among persons who have major risk factors, simple precautions can be taken when handling freshwater fish, snails, aquatic plants, aquariums, or other materials in contact with aquarium water, such as gravel, substrate, decorations, filters, and other equipment. CDC recommends thorough handwashing with soap and water before and after handling or cleaning aquariums and feeding fish, wearing gloves to cover any cuts or wounds in the hand while handling fish or aquariums or allowing wounds to fully heal first, avoiding cleaning fish aquariums if immunocompromised or in areas where immunocompromised persons might be present, and not allowing children less than 5 years of age to clean fish aquariums.”

• All pretty standard statements, although glove use is a challenge if you’re putting your hand into the water past the wrist, unless you use full arm gloves.  You have need to be aware that there will often still be some exposure of the hands through leaks in gloves.

So, don’t freak out about your fish, but use some common sense, and remember that if you’re sick, it’s always good to make sure your healthcare providers know about any animal contacts you’ve had recently (in your household or elsewhere).