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Common Fish Diseases in Chesapeake Bay
Mycobacteria

What are mycobacteria?
Mycobacteria are bacteria of the genus Mycobacterium that are widely distributed in nature. Only a few species of mycobacteria are pathogenic. For example, M. tuberculosis causes tuberculosis and M. leprae causes leprosy in humans. Other mycobacteria, such as M. marinum, M. fortuitum, M. chelonae, and the recently discovered M. chesapeakii and M. shotsii, can cause mycobacteriosis in fish.

What is mycobacteriosis?
Mycobacteriosis is a chronic wasting disease in fish caused by infection with mycobacteria. Infected fish may be appear thin and sluggish. Other external signs, such as areas of redenning, fin rot, popeye and ulcers, may or may not be present. Internally, mycobacteriosis can cause white to grayish nodules called granulomas, primarily in the spleen, kidney and liver. Mycobacteria are slow growing, so it may take months to years for the disease to become clinical and cause problems to the fish.
 
How do fish get mycobacteriosis?
The immune system of healthy fish can fight off infection from most invading bacteria. However, stress can weaken the immune system and allow bacteria, including mycobacteria, to cause an infection. Stress can be caused by poor water quality, suboptimal nutrition, excessive handling and other disease entities. Bacterial infections can also occur when the immune system is overwhelmed by the number of invading bacteria, such as could occur at the site of an open wound.
 
Are Chesapeake Bay striped bass affected?
Yes. Striped bass are known to carry or be infected with mycobacteria. There is a paucity of epidemiology data available for fish. Estimates for striped bass infection rates range as high as 73% in Maryland waters and 80% in Virginia waters. Aquacultured striped bass (California) have also been reported with very high infection rates. Below is a striped bass with a small, focal ulcer associated with Mycobacterium.

Although the external ulcer is relatively small and discrete, the extent of granulomatous nodules in the internal organs was notable. The red arrows indicate some of the granulomas on the dark and enlarged spleen. Granulomas were also noted in the anterior kidney, posterior kidney, in the liver and the heart. Although the nodules appear to be just under the surface of the spleen capsule, they are scattered throughout the tissue (see below).

This is a histological section of the spleen from the striped bass. The left image is a low magnification cross section that illustrates the severe extent of the granulomas throughout the splenic tissue. The extent of the granulomas in the anterior and posterior kidney of this fish was similar to that which was observed in the spleen. The right image is a higher magnification of the same spleen. Here you should be able to see the the individual granulomas. They are comprised of chronic inflammatory cells (macrophages and giant cells) that surround a center containing necrotic debris and bacterial rods (not evident in this photograph). These rods stain acid fast, consistent with mycobacteriosis. However, only confirmative bacterial cultures or PCR (more sensitive) can definitively diagnose mycobacteriosis and provide speciation.

Download Heckert et al.: Detection of a New Mycobacterium Species in
Wild Striped Bass in the Chesapeake Bay (884kb PDF file)

Do mycobacteria only infect striped bass?
No. Mycobacteria has been isolated from many species of wild and captive fish throughout the world. Affected Chesapeake Bay species include striped bass, yellow perch, menhaden and killifish. Mycobacteria can also infect aquarium fish. Anecdotal evidence suggests that other Chesapeake Bay fish species may be infected or carriers, but further scientific investigation is most certainly warranted.
 
Can humans be infected?
Yes. Infections are associated with persons coming in contact with infected water or fish. Persons most susceptible include those who are immuno-compromised and/or have open skin cuts or sores. Reports in the medical literature usually call these infections ‘swimming pool granuloma’ or ‘fish handler’s disease.’ Mycobacterium marinum is the most common aquatic species of mycobacteria that can be pathogenic to humans. Infection may appear as reddish bumps or nodules on extremities. Generally this infection will not spread throughout the body because the temperature of the trunk of the human body is usually too high for mycobacteria to thrive. The infection can usually be treated with a long duration antibiotic regime.
 
What should I do with a lesioned striped bass?
Anglers should be careful when handling fish, regardless of the presence of external lesions. Discard lesioned fish back into the water. Wash skin thoroughly (with soap if possible) after contact, especially at sites of cuts or scrapes. Gloves are advised for persons who handle live fish often.
 
Can I be infected by eating infected fish?
In general, you should eat only fresh, healthy looking fish, whether they are caught or purchased. Thorough cooking will destroy mycobacteria.

Check out recent articles about mycobacteria or download a mycobacteria information handout.

 

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