In October 2019, the US Centers for Disease Control and Prevention (CDC) was notified by the Maryland Department of Health and through the CDC Laboratory Response Network (LRN) of a preliminary positive bacterial isolate of B. pseudomallei from a blood culture specimen from a Maryland resident. The clinical specimen had been forwarded to the Virginia Division of Consolidated Laboratory Services, where it was confirmed, and shipped to CDC for simultaneous confirmation and genomic analysis.

The patient, a 56-year-old woman, was hospitalized on September 20, 2019 (day 0; Figure), because of fever, cough, and chest pain with onset 2 days earlier. The patient had a history of polymyositis, rheumatoid arthritis, and diabetes mellitus, and reported to have stopped long-term immunosuppressive medications (methotrexate, azathioprine, and prednisone) 1 month before symptom onset.

A thoracic radiograph on day 0 showed a right perihilar and lower lobe infiltrate, consistent with pneumonia. A noncontrast computed tomography (CT) scan on day 3 showed air space consolidation in the right lower lobe consistent with pneumonia, additional bibasilar air space densities, and a small right pleural effusion. Other notable clinical laboratory results at presentation included an increased leukocyte count of 22,800 cells/μL (reference range 4,500‒11,000 cells/μL) and a decreased sodium level of 125 mmol/L (reference range 135‒145 mmol/L).

We obtained blood cultures on day 0 before administration of antimicrobial drugs. Gram-negative rods were identified in 4 initial blood cultures; the rods were subsequently identified as B. pseudomallei. Three additional blood cultures obtained on days 2–4 grew the same organism. The patient was given ceftriaxone and azithromycin on days 0–3, then on day 4, treatment was changed to meropenem after B. pseudomallei was identified. The patient showed gradual symptom improvement; fever resolved, and leukocyte count normalized. The intensive phase duration was extended because of persistent bacteremia. She was discharged on day 11 and continued taking intravenous meropenem as an outpatient. Antimicrobial drug susceptibility testing was not performed because the patient had been responding to treatment with meropenem before LRN confirmation of B. pseudomallei.

After 3 weeks of taking meropenem, the patient’s leukocyte count increased to 14,700 cells/μL, and fever developed (temperature 100.1°F [37.8°C]). A repeat CT scan showed a patchy opacity at the right lung base and bilateral interstitial process consistent with pneumonia, but a 45 × 24 mm mass in the right lung posteriorly could not be excluded and increased concern for pulmonary abscess. The patient was readmitted, sulfamethoxazole/trimethoprim was given, and treatment with meropenem continued. She clinically improved and was discharged after 1 week. Repeat, noncontrast, thoracic CT scans showed improvement of right basilar consolidation after 6 weeks of meropenem and sulfamethoxazole/trimethoprim and resolution after 10 weeks. The patient completed eradication therapy with a 10-week course of meropenem and 12-week course of sulfamethoxazole/trimethoprim.

This investigation of a patient who had melioidosis and no history of travel to disease-endemic areas provides strong evidence for documented transmission of B. pseudomallei from a freshwater home aquarium to a human. Although this type of transmission has not been described in the literature, contamination of aquarium transport water with B. pseudomallei used for freshwater tropical fish originating from Singapore has been reported in France (17). Furthermore, freshwater fishing practices have been identified as risk factors in countries to which melioidosis is endemic (7,18,19).

With freshwater aquariums as a newly recognized source of possible transmission of B. pseudomallei to humans, further investigations are underway to determine the extent of B. pseudomallei contamination at the pet store retailer where the patient purchased the pet fish with accompanying aquarium water and at the vendors that imported and supplied freshwater ornamental fish, aquatic plants, and associated aquarium water to this retail location. Because these vendors might distribute freshwater animals and aquatic plants to pet store retailers throughout the United States, identifying possible source(s) of introduction with B. pseudomallei in the supply chain is essential to public health.

Federal regulators classify B. pseudomallei as a Tier 1 Select Agent because of its heightened risk for deliberate misuse and major potential for mass casualties, economic disruption, critical infrastructure effects, or damaging public confidence (20). The United States is the largest importer of ornamental fish, most of which are freshwater and originate in Southeast Asia (21,22), where B. pseudomallei is widespread in the environment. An estimated 11.5 million US households have pet fish; ≈139 million freshwater fish are owned (23,24). Determining where in the supply chain introduction of the bacteria might occur can lead to development of enhanced surveillance and mitigation procedures at the critical control points, which might prevent further introductions and spread of the bacteria to retailers and homes of consumers.

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 <5 years of age to clean fish aquariums (25).

This report highlights the essential role of molecular epidemiology in public health investigations of melioidosis cases, which identified the likely geographic origin of the bacteria and prompted a public health response that characterized a novel route of exposure. There is growing evidence that US melioidosis cases are not limited to international travelers, including a 2021 multistate cluster involving an organism that is not clonal to the isolates described here, as determined by WGS (14,26–29).

We urge clinicians in the United States to consider melioidosis in patients who have clinically compatible symptoms and exposure to tropical ornamental fish and freshwater aquariums, particularly if patients are immunocompromised, even though such exposure events might be exceedingly rare and few persons show development of melioidosis after exposure to B. pseudomallei (5). This organism might be difficult for hospital laboratories to diagnose, and automated identification systems in clinical laboratories can misidentify B. pseudomallei, highlighting the need for LRN confirmation (28).

Clinicians treating melioidosis should consult established treatment guidelines, which were updated in 2020 (30,31). Likewise, public health investigators should consider inquiring about pet freshwater fish exposure in patients given a diagnosis of melioidosis who have not traveled to a disease-endemic area or have only traveled to locations inconsistent with the geographic profile of the genome of their isolate.