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Diseases of Fish

V. Diseases of Fish 
A. Virus diseases

Numerous virus diseases of fish are now known to exist. By 1980, more than 30 had been reported and the number continues to grow.

Some examples of disease and their viral etiologies include:

1. Lymphocystis Disease

Iridovirus - The virus infects dermal fibroblasts: Causes formation of white to yellow cauliflower-like growths on skin and fins and sometimes other sites. The disease is self-limiting. Gains entry through dermal abrasions.

Figure 30 - Skin with multiple huge hypertophied dermal fibroblasts.

2. Other important virus diseases include IHN (infectious hematopoietic necrosis), IPN (infectious pancreatic necrosis), plus some diseases caused by herpesviruses.

B. Bacterial Diseases

Bacterial diseases cause extensive mortality in both wild and cultured fish. The actual role of these bacteria may vary from that of a primary pathogen to that of an opportunistic invader following stress.

Some pathogenic bacteria and their typical lesions include Aeromonas hydrophilia, Vibrio Anguillarum, and Aeromonas salmonicida which are characterized by hemorrhages (septicemia) and ulcers.

Flexibacter columnaris causes fraying and erosion of fins.

Mycobacterium (Fish tuberculosis) causes granuloma formations and is a zoonotic disease.

Figure 31 - Multiple granulomas in brain with presence of acid-fast bacteria typical of fish tuberculosis (10X)

Figure 32 - Higher magnification of one of the granulomas (40X)

C. Mycotic Diseases

Many fungi have been associated with disease in fish. Although some are apparently primary pathogens, many are thought to require predisposing factors in order to establish infection.

Clinically, affected fish often develop white to brown cotton-like growths on skin, fins and gills.


D. Parasitic Diseases

Many phyla in the animal kingdom have members which are parasitic on fish. Most individual fish in wild populations have parasites, often without evidence of significant harm. In cultured fish populations, however, parasites frequently cause serious outbreaks of disease. The presence of large populations of fish under similar environmental conditions often result in proliferation of certain parasites with the subsequent severe outbreaks of parasitic disease.

Many types of parasites affect fish, including protozoa, flatworms, roundworms, acanthocephala, mollusks, arthropods (crustacea and copepods), annelids and chordates (such as lampreys).

Some important examples of parasitic disease are caused by protozoa, including:

1. "Ich" or white spot disease is caused by Ichthyophthirius multifiliis. This is the largest protozoan parasite of fish. The trophozoites are up to 1 mm in diameter, ciliated and contain an oval horseshoe-shaped nucleus. Cause focal hyperplasia and tiny cysts of the epidermis with the formation of visible white spots on skin and gills. Direct life cycle.

The mature parasite leaves the cyst, descends to bottom of the aquarium or pond, produces 500-1200 tomites (small, potentially infective stage) and infects other fish. (The parasite has a direct life cycle — no intermediate host.)

Figure 33 - Roll mouse over magnifying glass for enlarged view of cyst then click for animation of life cycle.

2. Whirling Disease is caused by Myxosoma cerebralis which invade and destroy cartilagenous structures which can include those in inner ear resulting in "whirling" and incoordination (while currently classified as a protozoan, recent genetic studies suggest it may be in the metazoa group). The intermediate host is a tubifex worm. The parasite arrived in the U.S. in the 1900’s and has now killed up to 90% of rainbow trout in some major western rivers. The annual economic cost is in the millions of dollars.

Figure 34 - Copepod in gill between rows of 2° lamellae

E. Examples of Nutritional Disease

1. These include iodine deficiency, causing hyperplasia (goiter) of the thyroid tissue.

2. Vitamin E. deficiency is associated with necrosis and degeneration of skeletal and cardiac muscle, steatitis, and lipoidal liver

Figure 35 - Goiter - There is a visible mass (about 1cm diameter) ventral to the gills.
Figure 36 - Goiter - Hyperplastic follicular epithelium with irregularly sized follicles, some with vacuolated colloid.

 F. Disease due to physical properties of water

"Gas bubble" disease can result when the water is supersaturated with gas (bottom of a water falls or overactive aquarium aerator). The gas released from the supersaturated state in the blood stream of the fish can form gas bubbles which subsequently block circulation.


Figure 37 - Gas bubble disease in eel fry - spherical, clear, gas-filled dilatations in gills

G. Neoplastic Diseases

These include hepatoma (associated with aflatoxins in the feed). Can cause massive liver necrosis with bile duct proliferation.

Other neoplasms reported include melanoma, papilloma, fibrosarcoma, neurofibroma.