Lymphocystis is an iridovirus that causes a benign, generally self-limiting disease in freshwater and marine fishes. The virus infects fibroblasts causing them to undergo massive growth resulting in multiple raised nodules on the fins and skin.
Fish pox is a herpes virus (not a pox virus) that causes a superficial skin disease of cultured cyprinids (carp and minnows). Clinical signs of fish pox include focal epidermal hyperplasia of the skin presenting as raised white nodules of skin and fins.
Koi herpes virus is a serious herpes virus disease of wild and ornamental (Koi) carp with extremely high mortality rates due to secondary bacterial and parasitic infections. The pathology observed with Koi herpes virus is primarily severe hemorrhagic and necrotic gill lesions.
Most bacterial pathogens of fish are opportunistic pathogens, generally occurring secondary to poor water quality and poor husbandry. Probably the most common ubiquitous pathogen of freshwater fishes is Aeromonas hydrophila. This bacteria causes a variety of non-specific clinical signs including external hemorrhages of the skin and fins, dermal ulcerations, abdominal distention, exophthalmia and septicemia.
Aeromonas salmonicida is generally a pathogen of coldwater fish, but atypical strains of this bacteria causes a necrotic skin disease known as "goldfish ulcer disease”. Vibrio spp. are basically the equivalent marine species of the freshwater Aeromonas spp. pathogens of fish. These bacteria generally cause systemic infections of marine and estuarine fishes but can also sometimes
cause infections in freshwater fishes.
Flavobacterium (Cytophaga, Flexibacter) columnaris is primarily a pathogen of the gills and skin of pond-reared fish where it causes hemorrhage and necrosis of the epithelial tissues.
Probably the most important bacterial pathogens of both freshwater and marine fishes belong to the group Mycobacterium. Several species, such as M. marinum, M. fortuitum and M.
piscium, cause a chronic, progressive, multisystemic, granulomatous disease with clinical signs ranging from anorexia, lethargy, emaciation, exophthalmia, fin erosion, loss of normal coloration and chronic, non-healing, skin ulcerations.
The tropical and ornamental fish parasites of importance are primarily external though a few internal parasites can cause serious disease in fish. One of the more frequently observed external parasites is Ichthyophthirius multifiliis, commonly called "Ich" or "white spot disease" which infects the epithelium of the skin and gills. This freshwater ciliate parasite has an oval trophozoite stage that is embedded within the epithelium of the host causing multiple small raised lesions on skin, fins and gill of the fish.
Similar to I. multifiliis, Chilodonella spp. is an oval ciliate protozoan that infects the gills of fish, but unlike I. multifiliis does not embed itself within the epithelium of the fish. Trichodina spp. and
Trichodinella spp. are ciliated scrub brush-like protozoans with a prominent denticular ring that generally reside on the external surfaces of the skin and gills.
Another ciliate, Tetrahymena spp., can invade the skin, muscle and internal organs of the fish. Ichthyobodo spp. (Costia spp.) is a very pathogenic flagellate that attaches to the skin and gill tissue of the fish and causes hyperplasia of the tissues, while Spironucleus spp. is a small intestinal flagellate that causes inappetence, unthriftiness, emaciation and sometimes death of the fish.
Amyloodinium sp. and Oodinium sp. are parasitic dinoflagellates that attach to the skin, fins and gills of marine and freshwater fish, respectively. These parasites can cause hyperplasia of gill tissues resulting in hypoxia in the fish.
Monogeneans cause pathology to the host fish by their attachment and feeding activities resulting in increased mucus production and hyperplasia of tissues.
There are also a number of larval and adult trematodes, cestodes and nematodes that infect fish, but most cause no significant pathology. Several larger crustacean parasites, i.e. Learnea sp. and Argulus sp., can be found attached to the external surfaces and gills of fish but generally only cause irritation and a focal inflammatory response.
Fungal infections of tropical and ornamental fish are generally associated with adverse environmental conditions, poor aeration or physical trauma. Most fungal pathogens, i.e. Saprolegnia sp. and Aphanomyces sp., are usually considered secondary opportunistic invaders. Their grayish-white, non-septate hyphal mass is often associated with an underlying superficial erosion of the skin or gills.