Sources and routes of spread of fish invasions

To combat fish diseases of any etiology, it is first necessary to establish the causes of their occurrence, sources and routes of penetration of pathogens. The most reliable way to prevent invasive diseases is to destroy sources or stop the routes of invasion.
Depending on the systematic position and life cycle, the mechanism of transmission of pathogens from sick fish to healthy ones can be simple when the parasite passes from one fish to another and more complicated when the parasite has a complex development cycle. SinceSince fish and their parasites are aquatic animals, the main source of invasion is water, in which parasites are well preserved and reproduce, and where the transmission of pathogens from sick fish to healthy ones occurs quite easily.
Parasites with a simple life cycle - chilodonella, trichodina, ichthyphthirius wanderers, ambiphria, dactylogyrus larvae, parasitic crustaceans are able to swim in water in a free state for a long time. They can be mechanically carried by the current to lower reservoirs and infect healthy fish there. Such cases are encountered in the practice of pond channel farms.
The bottom of the pond is also a place of preservation of pathogens of costiasis, eggs of bothriocephalus and other helminths, leech cocoons, cysts or eggs of which accumulate in bottom sediments, persist until the spring of the following year and infect fish. Trout that have recovered from salmon whirlpool disease (myxosomiasis) carry spores of the pathogen in their cartilaginous and bone tissue throughout their subsequent life, and after the death of the fish and decomposition of the corpses, the spores enter the water, bottom sediments and infect healthy fish.
One of the most common sources and methods of infection is the transmission of pathogens through direct contact of parasite-carrying fish, sick fish or corpses of fish that died from the disease to healthy fish, as happens with most parasitic protozoa, crustaceans, and helminths - monogeneans. Parasites with a complex life cycle develop with the participation of intermediate hosts, from which the fish become infected. Many fish-eating birds are definitive hosts of the pathogens of ligulosis, diplostomosis, and postodiplostomosis. Flying between reservoirs, birds bring eggs of helminths that cause fish diseases into healthy ponds. Copepods (cyclops, diaptomus) are carried into healthy water bodies with the water flow and are the first intermediate hosts of triaenophorosis and ligulosis, where they are the source of invasion of fish when they are eaten. There is also the spread of parasites (blood flagellates - trypanosomes and cryptobia) by specific carriers - leeches. By attaching themselves to the body of a healthy fish, leeches transmit parasites to it through a wound in the skin, which the leech makes with its proboscis when sucking blood.
Often the spread of invasions in fish occurs as a result of human economic activity. In the absence of veterinary control, invasive diseases can be transmitted during acclimatization and transportation of infected fish. There is a known fact of importation of about three dozen species of parasites into the European part of the Soviet Union during the introduction of Far Eastern herbivorous fish. Invasive diseases can be transmitted through feed containing pathogens of invasion. Trout become infected with costia and myxosomiasis when fed with minced fish infested with costia cysts or myxosporean spores. Prevention of invasive diseases should be based on the elimination of sources of invasion by destroying the infectious agent in intermediate hosts, in water, pond beds using disinfectants, antiparasitic treatment during transportation and quarantining fish in accordance with veterinary regulations.

The material was prepared by specialists from the Department of Veterinary Science and Risk Analysis of Food Production of the Federal State Budgetary Institution Rostov Reference Center of ROSSELKHOZNADZOR.