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Virginia Institute of Marine Science

Mycobacteriosis

Frequently Asked Questions

What is mycobacteriosis?

Mycobacteriosis is a generic term that describes diseases caused by a group of bacteria (simple single-celled organisms) known as mycobacteria. Mycobacteria are widespread in the natural world, particularly in aquatic environments. A small fraction of mycobacterial species cause disease in animals and humans.

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 (see below). 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. Contagious mycobacteria that cause serious disease in humans include M. tuberculosis (the cause of pulmonary tuberculosis) and M. leprae (the cause of leprosy). "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. [top]

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. [top]

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."   [top]

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.   [top]

What are the symptoms of mycobacteriosis in humans?

It is not yet known whether Mycobacterium shottsii (the dominant species isolated in the current outbreak of striped bass mycobacteriosis in Chesapeake Bay) can infect humans. Concern is warranted because M. shottsii is closely related to M. marinum, a species known to cause disease in both humans and fish.

Primary symptoms of human infection by M. marinum include infections of the skin and soft tissues. Infection most typically becomes evident as reddish raised nodules on the hands, elbows, knees, and feet. In many instances the joints may become swollen.

M. shottsii and M. marinum have very different growth rates. M. marinum grows well at 30-33°C (86-91°F), but not at the normal human body core temperature of 37°C (98.6°F). This helps explain why M. marinum tends to infect the extremities, which are cooler than the body core. M. shottsii prefers a cooler temperature (23°C or 73°F) and grows very slowly or not at all at 30¡C under laboratory conditions.   [top]

What should I do if I suspect I might have been exposed to mycobacteriosis?

Anyone who suspects they may have been exposed to mycobacteriosis from handling infected striped bass should contact their physician and inform them of the nature of the exposure.   [top]

Is fish mycobacteriosis related to Pfiesteria?

No. Although Pfiesteria has been implicated in causing skin lesions in fishes, it is a single-celled algae rather than a bacterium. For more information on Pfiesteria, visit the VIMS Pfiesteria web pages. [top]

How prevalent is mycobacteriosis among Chesapeake Bay stripers?

Studies conducted by VIMS scientists from 1999-2001 showed that mycobacteria could be cultured from the spleens of 76% of striped bass recovered from the Chesapeake Bay (Potomac River to Virginia Beach). Seventy-six percent of these infected fish are positive for M. shottsii. M. shottsii is not only the most common species of mycobacteria in striped bass, but typically occurs at much higher densities than any other mycobacterium in co-infections. This means that anglers are more likely to be exposed to M. shottsii than other mycobacterial species. Whether M. shottsii poses a threat to human health is not yet known (see above). A fall 2002 survey of striped bass health in the York, Rappahannock, Potomac, and Nanticoke rivers indicates that mycobacteriosis is present in the fish. (The survey was coordinated by the USGS and involved scientists from VIMS and Maryland.) However, the degree of its prevalence or severity will not be known until the researchers complete their analyses, a process that takes several months. Some fish, especially in the Rappahannock, had skin lesions and some were "skinny" or underweight. Stripers with similar conditions have been observed every year since 1997, starting mainly in the summer months and fall.   [top]

Does mycobacteriosis affect other Chesapeake Bay fish species?

According to available data for Chesapeake Bay, the current outbreak of fish mycobacteriosis is limited to striped bass. However, other fish species have not been studied as intensively as striped bass, and many species have not been examined at all.   [top]

How might mycobacteriosis be affecting the striped bass population in Chesapeake Bay?

There is insufficient data to determine whether mycobacteriosis will affect the stocks of striped bass in Chesapeake Bay. Anecdotal evidence from fishing tournaments shows that younger stripers (below 24 inches long) are sometimes emaciated yet show no external or internal signs of the disease, whereas older fish (greater than about 24" long) commonly exhibit external lesions and infected internal organs but are otherwise robust and healthy. Thus the relationship between "skinny" bass and mycobacteriosis is presently not clear.   [top]

What is the history of mycobacteriosis in Chesapeake Bay?

The occurrence of mycobacteriosis in striped bass (Morone saxatilis) from Chesapeake Bay was first noted by VIMS in 1997. Previous outbreaks of mycobacteriosis in wild striped bass have occurred in Pacific estuaries. Following the Chesapeake Bay outbreak, VIMS scientists isolated a new species of mycobacteria associated with skin and visceral lesions that they named Mycobacterium shottsii. This new species is closely related to M. marinum and M. ulcerans.

It is unknown how long mycobacteria may have been causing disease in Bay striped bass.   [top]

What is the focus of mycobacteriosis research at VIMS?

VIMS researchers are working to understand the extent and severity of the disease in Chesapeake Bay striped bass, the environmental conditions in the Bay that influence development of the disease, and potential impacts on striped bass stocks. The VIMS effort is part of a larger cooperative study with investigators from the USGS National Fish Health Research Laboratory in West Virginia, from the Virginia Marine Resource Commission, and from institutions and agencies in Maryland.   [top]

How is mycobacteriosis diagnosed in striped bass?

To determine whether a striped bass is infected with mycobacteriosis, researchers must first remove tissue from a fish under sterile conditions. Tissue is typically taken from the spleen, as this is an organ that helps remove bacteria from the blood. The next step is to slice the tissue thinly enough so that a section can be viewed under a light microscope. Staining and other techniques help researchers determine whether any mycobacteria and/or characteristic lesions are present in the fish tissue.

To identify the particular species of bacteria present, the researchers must isolate the bacterial cells from the fish tissue and grow them in pure culture on agar in petri dishes. Because Mycobacterium shottsii, the species predominantly isolated from striped bass with mycobacteriosis, is such a slow-growing organism, it takes at least 2 months to grow a sufficient number of these bacteria for positive identification.

To speed up the identification process, VIMS researchers are using molecular techniques that allow rapid detection of mycobacterial species from small tissue samples. Future development of genetic fingerprinting techniques may aid in rapid identification of the species present.   [top]