Freshwater Aquarium Provides Source for Melioidosis Infection

Judy Stone, MD

January 03, 2022

A Maryland woman came down with a severe tropical infection called melioidosis from her freshwater home aquarium, says a report in the journal Emerging Infectious Diseases describing a new route of transmission. Melioidosis is caused by the bacteria Burkholderia pseudomallei in soil or water.

Until last year, almost all US cases of melioidosis were from people who lived or traveled to disease-endemic areas. It has been a rare infection in the US.

But this is not the first case of melioidosis from an unusual source. Earlier in 2021, CDC and state epidemiologists traced an outbreak of melioidosis in Georgia, Kansas, Minnesota, and Texas to B pseudomallei in a bottle of "Better Homes & Gardens Lavender & Chamomile Essential Oil Infused Aromatherapy Room Spray with Gemstones."

In the aquarium case, the patient was a 56-year-old woman with diabetes and rheumatologic disease. She had been on immunosuppressives (methotrexate, azathioprine, and prednisone) until 1 month before she became symptomatic. She was hospitalized for fever and pneumonia.

Multiple blood cultures obtained on days 1-4 grew B pseudomallei, but she had no evidence of endocarditis or intravascular seeding. Despite weeks of meropenem (Merrem), she developed evidence of a lung abscess, and trimethoprim/sulfamethoxazole (Bactrim) was added. Ultimately, the patient required a 12-week course of antibiotics.

CDC epidemiologist Patrick Dawson, PhD, first author of the report, told Medscape Medical News that although outbreak investigators always ask about pet ownership, they have not explicitly asked about fish. In this case, the patient did not volunteer exposure to the fish.

When state epidemiologists visited the patient's home, "one of the first things they saw was a few aquariums," Dawson said. Seeing the water and knowing "that most freshwater tropical fish in the US are imported from Southeast Asia" led them to culture specifically for B pseudomallei, which can be difficult for the microbiology lab to identify.

Dr Patrick Dawson

From there, Dawson explained, "The Maryland Department of Health sent a team to the local pet store" but did not find any of the bacteria there. (The patient had bought her fish 6 months earlier.) The investigators then worked with the national brand "to identify where they had actually sourced the fish from."

Two retailers supply almost all of US guppies and plants. While investigators could not find an exact matching isolate after so many months had elapsed, they found a positive PCR for B pseudomallei in a water sample from imported fish in Los Angeles.

Dawson said tropical fish are imported from southeast Asia and typically come from small family fish farms. The fish import industry has "certain products that they add to the water to hopefully kill any bacteria." He was unaware whether this included antibiotics but suggested, "we would have seen many more cases by now" of antibiotic resistance if it did.

In general advice for the public, Dawson said, "I would recommend washing hands before and after contact with the aquarium. If you have cuts or wounds on your hands, it's really important to wear gloves if you have to go clean or maintain the aquarium and you're putting your hands in the water, just for that extra layer of protection. It's probably a strong idea to just avoid that altogether if someone's immunocompromised. And not letting young children under 5 years old clean aquariums." These are the "simplest things to do to protect yourself."

Stephen A. Smith, DVM, PhD, a professor in the Aquatic Medicine Program at Virginia-Maryland College of Veterinary Medicine, Blacksburg, Virginia, also stressed the importance of careful hand hygiene when caring for aquariums. He said that the filter, filter floss, biofilm, charcoal, and gravel might have exceptionally high concentrations of bacteria. Smith also recommended gloves when cleaning aquariums and not doing this task if immunocompromised.

Smith, who was not involved in the CDC study, shared a broader perspective with Medscape Medical News, noting that "the reason why it's important to federal regulators is that [B pseudomallei] is a tier 1 select agent. And so, when that was isolated, it sent up all the red flags." The far more common Mycobacterium marinum, or fish handler's disease, is not reportable.

Mycobacterium marinum is another pathogen of concern that can be acquired from aquariums. These infections typically occur as nodular lesions on the arms and require months of therapy.

Smith stressed the importance of physicians eliciting a careful exposure history as the key to diagnosing zoonoses. For most exotic aquarium animals, he noted, "They're caught in the wild wherever they are. They're transported to a major hub to transport to the US, and a lot of times, we don't have quarantine for those animals."

Aquatic zoonoses (infections from water) are important because an estimated 11.5 million US households have pet fish, totaling about 139 million freshwater fish, Smith said.

Many infections also occur in the course of water sports — or even hiking and getting a cut or abrasion wet from a stream or lake. Aeromonas hydrophila can caue life-threatening infections. Vibrio vulnificus infections from salt-water injuries can cause sepsis and characteristic hemorrhagic bullae — large, discolored blisters filled with body fluid — during the summer. And eating contaminated shellfish has a 50%-60% death rate.

Other exposures to water-loving bacteria happen during fishing or cleaning/preparing fish. For example, Streptococcus iniae has caused cellulitis, arthritis, endocarditis, and meningitis following superficial or puncture injuries, notably from cleaning tilapia.

Other infections from contact with fish include Erysipelothrix rhusiopathiae (primarily skin infections) and gastroenteritis from Plesiomonas shigelloides, Campylobacter spp, and Salmonella spp.

Each of these zoonoses illustrates the importance of a careful exposure history when there's an atypical presentation or an infection that is not responding promptly to empiric treatment. The aquarium case broadens the differential to include melioidosis, a serious disease from southeast Asia.

Dawson and Smith have disclosed no relevant financial relationships.

Judy Stone, MD, is an infectious disease specialist and author of Resilience: One Family's Story of Hope and Triumph Over Evil and of Conducting Clinical Research , the essential guide to the topic. You can find her at or on Twitter @drjudystone .

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