Fish Health Center

Fish Health Program: Understanding Disease Diagnosis

Causes of Disease

Fish health can be affected by environmental problems, nutritional problems, water quality, infectious diseases, and both man-made and natural toxins. Infectious diseases can be caused by viruses, bacteria, parasites, and fungi.


Cultured fish cells being killed by the Viral Hemorrhagic Septicemia (VHS) virus. A common virus in wild Pacific salmon and herring. (Credit: J. Winton, B. Batts)	A stained kidney print from a fish infected by Renibacterium salmoninarum, the bacterium that can cause bacterial kidney Disease (BKD) in salmon and trout. The purple spots are the BKD bacteria. (Credit:David Thompson/USFWS)	Ichthyophthirius multifiliis (ich) a common single-celled parasite of wild fish that finds its way into hatcheries through their water supplies (often streams and rivers that harbor wild fish populations). (Credit: Andy Goodwin/USFWS)	Saprolegnia, a water-mold that grows on fish when their immune systems are not fully functional. It is commonly found on adult salmon that have diverted their energy reserves to migration and spawning. (Credit: Andy Goodwin/USFWS)

Diagnosing & Detecting Diseases
			Small parasites are diagnosed by looking at gills and mucus under a microscope. (At left: Gyrodactylus, Trichodina, and Epistylus parasites)

		Bacteria are often found by cultures of fish tissues on jelly-like nutrient agar (At left: Columnaris and A. salmonicida cultures)

			Viruses are often detected by putting fish samples in plastic flasks that contain a layer of living fish cells. Dying cells mean that a virus may be present. Hover over the images to the left to see the difference between healthy cells in the first image, and dying cells in the second image.

	Bacteria can also be detected by using antibodies attached to fluorescent dyes or other labels. We use an Enzyme-linked immunosorbent assay (“ELISA”) to detect Bacterial Kidney Disease (BKD) bacteria in the organs of salmon and trout. (At left: ELISA plate/USFWS)

	The labs also use modern techniques like a polymerase chain reaction (PCR), the DNA-based method used in police work and made famous by shows like “CSI.” The PCR method can be used to detect just a few viruses or bacteria in a sample of fish tissue. The PCR method makes billions of copies of the virus or bacterial DNA molecules and these copies are detected with fluorescent dyes. (At left: A PCR gel sample/USFWS)

	Our least favorite test is for whirling disease. Whirling disease is caused by a microscopic parasite that grows in the skulls and cartilage of young fish. To find it we must digest fish heads in enzymes, grind up the bones and cartilage, then concentrate the parasite spores and search for them under a microscope (shown at left). The PCR method may replace this old “grind and find” approach, but not until everybody is convinced that the new PCR tests are as sensitive as the old method. (At left: MC spores under a microscope/USFWS)

Methods for the detection of fish diseases are set by national and international agencies and organizations that include the USDA, the World Organization for Animal Health (OIE), and the American Fisheries Society Blue Book (a joint venture by the society, the FWS, NOAA, and APHIS)!

About Our Program

The Pacific Region's Fish Heath Program protects the health of salmon propagated at the region's network of 15 National Fish Hatcheries and tribal partner programs. Our veterinarians and biologists--located in three Fish Health Centers located in Olympia, Washington, Orofino, Idaho, and Willard, Washington--assist partners across the Pacific Northwest with fish health issues and conduct the National Wild Fish Health Survey. Dr. Andrew E. Goodwin manages the program.