Different viral, bacterial and parasitic diseases exist among fish populations. Many of the viral and bacterial diseases occur in salmonids. Few viral diseases such as IHN (Infectious Haematopoietic Necrosis) may also infect other species. Some diseases occur only in aquaculture (fish farming), but, for example, BKD (Bacterial Kidney Disease) occurs also in the natural waters of Finland.
More information on fish diseases from the webpage of Finnish Food Authority
All living organisms, including fish, can have parasites. Fish parasites can occur on the skin of the fish or in the gutted fish. Finnish fish have a diverse species of parasites that can live inside fish organs, such as the skin, gills, muscles, eyes, heart and liver. All parasites cannot be seen by the naked eye. The most common microscopic parasitic disease is white spot disease ( Ichyophthirius Multifilis )
Most commonly found parasites in fish are are parasitic cataracts (Diplostomum sp.), tapeworms (Diphyllobothrium dendriticum, flatworms (Triaenophorus) and Hennequya sp.
Majority of the fish parasites are harmless to man and cannot live in human body except for the wide tapeworm (Diphyllobothrium Latum) and the parasite found in common roach. (Pseudamphistomum sp., Metorchisja sp)
Crayfish plague
Crayfish plague (Aphanomyces Astaci) is a water mold, native to North America. The crayfish plague is carried and spread by Signal crayfish (Pacifastacus leniusculus), which is resistant to the disease. Crayfish plague is very harmful in the European waters. This disease has completely destroyed many populations of indigenous crayfish throughout Europe, including Finland. The disease may also be asymptomatic.
Signal crayfish is listed as a harmful alien species in the list of invasive alien species of the European Union. Signal crayfish (Pacifastacus leniusculus) restocking and farming is illegal in the EU.
In Finland prevention of Crayfish plague is taken seriously. Care must be taken to disinfect fishing and crayfish traps. Boating and fishing equipment should be dried in the sun before its use in another body of water.
Crayfish plague is an animal disease that must be reported immediately to the Finnish Food Safety Authority. Contact information here https://www.ruokavirasto.fi/en/about-us/contact