Summary about Disease
Zebrafish-associated mycobacteriosis is a chronic, progressive bacterial infection in zebrafish caused primarily by species of Mycobacterium, most commonly *Mycobacterium marinum*, *Mycobacterium chelonae*, and *Mycobacterium abscessus*. It is a common disease in zebrafish research facilities and can lead to significant morbidity and mortality. The infection can manifest systemically, affecting multiple organs, and may be difficult to eradicate.
Symptoms
Symptoms can vary widely and depend on the Mycobacterium species involved, the fish's immune status, and the stage of the infection. Common signs include:
Emaciation (thinness)
Lethargy
Loss of appetite
Skin lesions/ulcers
Spinal curvature (scoliosis)
Abdominal distension (bloating)
Exophthalmia (pop-eye)
Fin rot
Granulomas (nodules) on internal organs or skin
Increased mortality
Causes
The primary cause is infection with Mycobacterium species, often *M. marinum*, *M. chelonae*, or *M. abscessus*. Introduction can occur through:
Contaminated water sources
Introduction of infected fish
Contaminated equipment (nets, tanks, etc.)
Aerosol transmission Compromised water quality, overcrowding, and stress can increase susceptibility to infection.
Medicine Used
Treatment is challenging, and complete eradication is often difficult. Common antibiotics used (often in combination) include:
Rifampicin
Isoniazid
Ethambutol
Clarithromycin
Azithromycin
Kanamycin Antibiotics are typically administered through medicated feed or immersion baths. Treatment duration is often prolonged (weeks to months). Before using medications, consult with a veterinary professional.
Is Communicable
Yes, zebrafish mycobacteriosis is highly communicable among zebrafish populations. It can spread through:
Direct contact with infected fish
Ingestion of contaminated material
Exposure to contaminated water
Aerosol transmission (less common but possible) While zoonotic transmission (from fish to humans) is rare, M. marinum can cause "fish tank granuloma" in humans, a localized skin infection, usually through open wounds exposed to contaminated water.
Precautions
Quarantine: Isolate new fish for at least 4-6 weeks to monitor for signs of disease.
Water Quality: Maintain optimal water parameters (temperature, pH, ammonia, nitrite, nitrate) to reduce stress.
Hygiene: Regularly disinfect tanks, nets, and other equipment with appropriate disinfectants (e.g., bleach solution, Virkon).
Source Control: Obtain fish from reputable sources that practice good biosecurity.
Personal Protective Equipment (PPE): Wear gloves and eye protection when handling zebrafish and their water. Wash hands thoroughly after contact. Cover any open wounds when working with tanks.
Water changes: Perform regular water changes.
Euthanasia: If fish test positive, euthanasia is common and the recommended solution to eliminate the problem.
How long does an outbreak last?
An outbreak can last for months or even years if left unaddressed. Due to the chronic nature of the infection and the difficulty of eradication, mycobacteriosis can persist in a zebrafish colony unless aggressive measures are taken, such as culling infected fish, disinfecting the system, and implementing strict biosecurity protocols.
How is it diagnosed?
Diagnosis typically involves:
Clinical Signs: Observation of characteristic symptoms.
Acid-Fast Staining: Microscopic examination of tissue samples (e.g., kidney, spleen, granulomas) for acid-fast bacteria.
Culture: Culturing the bacteria from tissue samples, although this can be slow and difficult.
PCR (Polymerase Chain Reaction): Molecular testing to detect Mycobacterium DNA in tissue or water samples.
Histopathology: Microscopic examination of tissue sections to identify granulomas and acid-fast bacteria.
Timeline of Symptoms
The timeline of symptom development is variable.
Early Stages: Subtle signs such as lethargy, reduced appetite, or slight weight loss may be present. These can be easily missed.
Progression: Over weeks to months, more obvious signs develop, such as skin lesions, spinal curvature, abdominal distension, and exophthalmia. Internal granulomas may be present even before external signs are apparent.
Late Stages: Advanced disease leads to severe emaciation, organ failure, and ultimately, death.
Important Considerations
Zoonotic Potential: While rare, be aware of the potential for M. marinum* infection in humans. Protect open wounds and practice good hygiene.
Treatment Challenges: Treatment is often prolonged, expensive, and not always successful. Drug resistance is a concern.
Euthanasia Considerations: In many cases, euthanasia of infected fish and thorough disinfection of the system is the most practical and humane approach to control the disease, particularly in research settings.
Consultation: Consult with a veterinarian experienced in fish diseases for diagnosis, treatment, and biosecurity recommendations.