Unhealthy looking Striped Bass?
I've caught a striped bass whose skin has inflamed or irritated patches of red. What should I do?
Background
A striped bass (rockfish) with red or inflammed skin may be suffering from a bacterial disease called mycobacteriosis. Mycobacteria are widespread in the natural world, particularly in aquatic environments. A small fraction of mycobacterial species cause disease in animals and humans. Striped bass in Chesapeake Bay are currently suffering from a mycobacterial outbreak, with about 3 in every 4 fish infected.
A newly described species of mycobacteria, Mycobacterium shottsii, is the type most commonly associated with the current outbreak of mycobacteriosis among striped bass in Chesapeake Bay. M. shottsii was first identified by VIMS scientists in 2001, and is present in 76% of infected bass. Some infected striped bass from the Bay are also known to harbor multiple mycobacterial species. Other mycobacteria recovered from Bay bass include M. peregrinum, M. marinum, and isolates resembling M. scrofulaceum, M. szulgai, M. interjectum, and M. simiae.
The human health significance of M. shottsii is not yet known. Concern is warranted because M. shottsii is closely related to M. marinum, a species responsible for mycobacterial infections of skin and soft tissue in humans. M. marinum is also considered the primary cause of mycobacteriosis in fish in aquarium, aquaculture, and natural settings. Other more distantly related species of mycobacteria include M. tuberculosis (the cause of pulmonary tuberculosis) and M. leprae (the cause of leprosy).
Although M. shottsii is in the same genus as M. tuberculosis, mycobacteriosis in humans is not the same disease as tuberculosis. "Environmental" mycobacteria such as M. shottsii, M. marinum, and other species are collectively termed "non-tubercular" mycobacteria to distinguish them from the species that cause tuberculosis.
What are the symptoms of mycobacteriosis in striped bass?
Mycobacteriosis of Chesapeake Bay striped bass is predominantly a visceral disease, infecting organs such as the spleen and kidneys. Internal signs of the disease typically include small grayish white nodules called granulomas in these organs. A small percentage of the infected fish also exhibit unsightly shallow, rough-surfaced, reddened, or darkly pigmented skin ulcers. Loss of scales is common in these ulcers. Infected fish sometimes exhibit significant weight loss. These disease symptoms are mainly observed in the summer and fall. Fish exhibiting the unsightly skin ulcers are of greatest concern to anglers.
Can I contract mycobacteriosis by handling striped bass?
There is a slight potential for human infection from handling striped bass infected with M. shottsii and other mycobacteria. Concern is warranted because M. shottsii is closely related to M. marinum, a species known to pass from infected fish to humans via handling. However, M. shottsii prefers growth at cooler temperatures than M. marinum. It seldom grows in laboratory cultures at 30°C (86°F), suggesting that it may not produce infections in humans.
Anglers should thus:
- return any fish with skin lesions to the water
- wear gloves when handling striped bass
- take particular care if they have a cut, scrape, or abrasion on their hands or arms, and wash thoroughly with soap and water after coming into contact with fish or open water.
Individuals whose immune systems are weakened or compromised because of disease or immune suppression therapy should be especially careful to avoid wounds or abrasions.
Human infection by M. marinum following exposure to the marine environment probably requires a portal of entry and is often linked with trauma such as puncture wounds from handling marine animals such as fish, turtles, shellfish, crustaceans. M. marinum infections in humans are known by names such as "fish-handler's disease," "aquarium disease," and "swimming-pool disease."
Can I contract mycobacteriosis by eating striped bass?
There is no evidence that humans can contract mycobacteriosis by consumption of cooked fish infected by M. marinum or M. shottsii. However, because of the risk of infection via handling (see above), any striped bass that exhibit external signs of mycobacteriosis (unsightly skin ulcers) should be released or disposed of. Do not keep or eat a fish that you would not buy in a fish market.
Any fish that are consumed should be cooked thoroughly. M. shottsii, one of the bacteria responsible for causing mycobacteriosis in striped bass, prefers to grow at temperatures below about 30°C (86°F), and is killed after heating to temperatures greater than 75°C (~170°F) for 20 minutes.
For more information visit the VIMS Mycobacteria home page.