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Common Fish Diseases in Chesapeake Bay

Diseases can be caused by a variety or combination of agents, including bacteria, viruses, fungi, and parasites. Interestingly, many of these disease entities are commonly found in the environment. So, why don't all fish get sick? Individual fish can become predisposed based on the presence of stressors and their immune status. This webpage provides examples of common pathogens:

Bacterial Disease:

Photo credit: Maryland Sea Grant, white perch with severe hemorrhaging around the eyes and fins, caused by a bacterium called Vibrio anguillarum

• Vibrio is a gram-negative bacteria that has been found in Chesapeake Bay and its tributaries. Some common species of Vibrio found in Chesapeake Bay are: V. vulnificus, V. anguillarum, V.-Photobacterium, V. cholerae-V. mimicus, V. cincinnattiensis, V. parahemolyticus. These bacterial species are prevelant in estuarine and fresh waters and can cause disease in fish and shellfish (photo on right). The signs of vibriosis in fish are similar to those of many other bacterial diseases, usually beginning with lethargy and loss of appetite. As the disease progresses, skin discoloration and necrosis are common. Ulcers may appear on the body and bloody patches (erythema) often form around the fins and mouth. When the disease becomes systemic, it can cause exopthalmia ("pop-eye"), and the gut and rectum may be bloody and filled with fluid. It should be noted that all of these symptoms can be caused by other bacterial diseases, and are not proof of a Vibrio infection. Gross pathology alone cannot determine which bacterial pathogen is responsible for disease.

Photo credits:www.dnr.state.md.us (left) spleen tissue with white granuloma determined to be caused by mycobacterial infection; www.oregonstate.edu (right) mycobacteria granuloma stain red with acid-fast stain

• Mycobacterium is a slow-growing, acid fast, gram positive rod bacterium. Mycobacterium species isolated from fish and water samples of the Chesapeake Bay include: M. marinum, M. fortuitum, M. chelonae, M. chesapeaki. In fish, Mycobacterium can cause chronic morbidity and mortality, shallow to deep skin ulcers, pale coloration, emaciation, white nodules on viscera, and a chronic inflammatory response resulting in granulomas (above photos) in the liver, spleen (photo on left), and kidney. Mycobacteriosis is common in Chesapeake Bay striped bass populations and has also been identified in Bay menhaden and white perch. For more baic information on mycobacteria click here.

Photo credit: www.imb-ibm.nrc-cnrc.gc.ca (left) Aeromonas salmonicida, a gram-negative non-motile rod-shaped bacteria and www.disease-watch.com (right) Fish with ulcers due to Furunculosis, a disease caused by Aeromonas salmonicida infection

• Aeromonas salmonicida is an opportunistic, gram-negative rod bacterium (photo on left) that primarily affects salmonids, but also other freshwater species. These bacteria can present as Furunculosis, an ulcerative disease (photo on right), or hemorrhagic septicemia, characterized by reddening and hemmorhage of the fish's skin and fins.

Fungal Disease:

Photo credits: UMD Aquatic Pathobiology Center (left) menhaden with ulcerative tissue from which fungal hyphae of the genus Aphanomyces were isolated, and (right) isolated Aphanomyces hyphae stained black with silver stain

• Aphanomyces are slow-growing fungus. Fungal hyphae of this genus (photo on right) have been isolated in ulcerative tissue of fish in Chesapeake Bay, like the menhaden shown in the photo to the left. Aphanomyces invadans (Saprolegniaceae) is associated with the serious fish disease, epizootic ulcerative syndrome (EUS), also known as mycotic granulomatosis, characterized by necrotic ulcerative lesions typically leading to a granulomatous response.

 

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