Mycobacteria in public aquariums: diagnose without overclaiming

Fish mycobacteriosis creates two related challenges for public aquariums. In animals, it may present as a chronic, non-specific condition. For staff, some species — particularly Mycobacterium marinum — are zoonotic hazards when contaminated water contacts damaged skin. A laboratory result sits between those concerns and needs disciplined interpretation. Identifying a mycobacterium does not establish how common infection is across a collection, nor does it prove that the organism alone caused the signs under investigation.

A 2024 study provides a useful case study. Researchers looked for non-tuberculous mycobacteria in 50 diseased aquarium fish submitted to an Iranian laboratory and recovered mycobacteria from 13. The work confirms that several relevant species can occur in aquarium fish. Its denominator, however, does not support a headline claiming 26% prevalence in aquariums generally. The practical lesson for zoological collections is methodological: animal selection, sampling, analytical method and epidemiological unit all determine what a result can legitimately answer.

Thirteen isolates representing four species

The fish were collected from several aquariums between May 2022 and January 2023. Researchers sampled internal and dermal tissues; fish weighing less than 5 g were processed whole. Following acid-fast staining and decontamination, samples were cultured on several media at 25 and 37°C for two months. Identification combined growth characteristics and biochemical tests with sequence analysis of a 441-base-pair fragment of the hsp65 gene.

Six of the 13 isolates were identified as M. marinum, three as M. fortuitum, three as M. chelonae and one as M. parascrofulaceum. Positive cultures came from five fish species: Pterophyllum scalare, Poecilia sphenops, Helostoma temminckii, Carassius auratus and Labidochromis caeruleus. Molecular identification sometimes refined or changed the phenotypic indication, especially among rapidly growing mycobacteria.

Those numbers describe isolates recovered from this sample; they do not quantify the absolute risk associated with each fish species. The animals had already been classed as diseased and referred to a laboratory, so they were not a random cross-section of healthy and unhealthy aquarium fish. The paper also lacks a detailed denominator for each source aquarium, preventing comparisons between facilities. A negative culture does not automatically rule out infection either: tissue choice, bacterial load, decontamination and incubation conditions can affect recovery of slow-growing organisms.

Identification is only one part of diagnosis

Causation has to be built from several lines of evidence. A mycobacterium isolated from a fish with wasting, internal lesions and compatible granulomatous histology carries a different weight from a stand-alone detection without matching pathology. Conversely, a group should not be declared free of infection simply because it lacks dramatic signs. A sound investigation brings together introduction and mortality history, clinical examination, post-mortem findings, histology, acid-fast staining, culture and appropriate molecular identification.

Sampling design should therefore precede sampling. Which exhibit or life-support loop is the unit being assessed? Do animals share water, equipment, staff or quarantine space? Is the aim to explain mortality, define the extent of a confirmed event or assess a group before movement? The answers determine how many animals to examine and which profiles are most informative. They also guard against opposite errors: dismissing an environmental isolate without comparing it with lesions, or assigning every death to the first mycobacterium detected.

The laboratory needs the clinical suspicion and operating context because these cultures do not follow the timetable of routine rapid bacteriology. Histology and culture samples must be handled separately and correctly. Instruments should be changed or appropriately disinfected between epidemiological units to reduce cross-contamination. In other words, an interpretable sequence result starts long before sequencing.

Genuine diversity does not translate into a simple virulence score

Interest in M. marinum diversity predates the recent survey but remains relevant. In 2012, a French-led study used MIRU-VNTR typing to compare 89 isolates: 68 from people and 21 from fish, including eight aquarium-fish isolates. The authors found genetic structuring by host and ecosystem. Fish numbers were nevertheless small and collection periods differed between hosts. The findings support distinct genetic pools, but they do not provide a simple assay that predicts the severity of an isolate recovered from an exhibit today.

In 2018, researchers sequenced 15 additional strains and compared them with four available genomes. They found substantial genomic diversity and proposed two broad groups. A 2020 correction amended the recorded origin of one zebrafish strain without changing that overall conclusion. Together, these studies show why a species name cannot capture all biological diversity. They do not support inferring strain virulence from an hsp65 fragment alone or treating every fish isolate as a future human infection.

For managed collections, this distinction has a practical consequence. Where laboratory policy allows, retaining isolates and their metadata may help compare cases over time. Exhibit, host species, date, sampled tissues, lesions and recent movements give a result epidemiological value that a bacterial name cannot provide on its own.

Protecting staff without sensationalising the hazard

CDC guidance for people in contact with aquariums emphasises straightforward controls: handwashing, gloves during aquarium care — especially when cuts, wounds, rough rocks or spiny fish are involved — and immediate cleaning of wounds with soap and water. If a lesion becomes red, painful, warm, swollen or persistent, the person should tell their healthcare provider about exposure to fish and aquarium water.

Public aquariums can translate these principles into task-based risk assessment: gloves with cuffs long enough for immersed work, secure wound covering, tools that reduce puncture and abrasion hazards, a defined post-injury procedure and occupational-health awareness. Immunocompromised personnel require an individual exposure assessment. Personal protective equipment is not a substitute for training or workflow design; a punctured glove or one filled with tank water is no longer a dependable barrier.

A coordinated response for the collection

Suspected mycobacteriosis cannot be reduced to “positive” or “negative”. It calls for coordination among aquarists, veterinarians, laboratories and occupational-health leads. Vetofish can help define cases, select fish and tissues, interpret histology alongside microbiology, and review the movement of animals, water and equipment.

The central message is deliberately measured. Mycobacteria are diverse, diagnostic methods answer different questions, and a percentage from one study cannot simply be transferred to another collection. A well-designed investigation protects fish, improves the quality of health decisions and safeguards the people who work beside the tanks.

References

• Yazdanmanesh M, Tadayon K, Bagherian Koshkghazi D, Mosavari N. “Isolation and identification of non-tuberculous mycobacteria from aquarium fish in Ilam, Iran.” Journal of Clinical Tuberculosis and Other Mycobacterial Diseases. 2024;37:100478. doi:10.1016/j.jctube.2024.100478.
• Broutin V, Bañuls A-L, Aubry A, et al. “Genetic Diversity and Population Structure of Mycobacterium marinum: New Insights into Host and Environmental Specificities.” Journal of Clinical Microbiology. 2012;50(11):3627–3634. doi:10.1128/JCM.01274-12.
• Das S, Pettersson BMF, Behra PRK, et al. “Extensive genomic diversity among Mycobacterium marinum strains revealed by whole genome sequencing.” Scientific Reports. 2018;8:12040. doi:10.1038/s41598-018-30152-y. Correction: Scientific Reports. 2020;10:5246. doi:10.1038/s41598-020-61218-5.
• Centers for Disease Control and Prevention. “Fish — Healthy Pets, Healthy People.” Updated 15 April 2024. https://www.cdc.gov/healthy-pets/about/fish.html.