Animal Veterinarians finding More Diseases in Fish – And… We Eat Fish

Examples of the ornamental fish industry being accused of moving fish diseases around the world are available, but not very common. However some examples are listed below.

The British Trout Association wrote to a Parliamentary Select Committee ” It is the view of the British Trout Association that we are at great risk from the continued import of ornamental fish…” and “The threat is often belittled, but we consider it to be a potentially disastrous situation…”

water animals

The Veterinary Record June 1997 in an article entitled “Pan-Asian spread of single fungal clone results in large scale fish kills” contained the sentence “However most outbreaks of EUS can only be explained by the massive cross border movement of fish for aquaculture or ornamental fish industries.”

In an article entitled “Epizootic Ulcerative Syndrome (EUS) of Fishes” in Recent Advances in Aquaculture V the following appeared “An anomaly in the spread of the disease was its sudden occurrence in Sri Lanka. Although it reached southern India only in 1991, it had already occurred in Sri Lanka in 1987, when it particularly affected the wild snakeheads and Puntius spp. The reasons for this are not known for certain but may well have related to the transport of aquarium species between Thailand or Singapore and Sri Lanka during the period. It is thought that this route may also have accounted for the spread through the Philippines archipelago.”

In 1985 a paper in the Bulletin of European Fish Pathologists by two members of the government Fisheries Research Centre in Southern Ireland suggested that “ornamental fish could represent an important source of infection for other fish such as salmonids. It could also help explain the sudden appearance of this disease (Enteric Redmouth caused by the bacteria Yersinia ruckeri) in a number of European countries…”

May be the best documented cases of disease transfers by fish are, however, for the salmonids. Some have said this just because they have been subject to greater scrutiny than the ornamental industry. The movement of diseases such as IHN (Infectious Haemopoetic Necrosis) are closely linked the spread of the rainbow trout, which though of north American origin, is now found worldwide. Another North American animal responsible for moving disease around the world is the Louisiana Red Swamp Crayfish (Procambrus clarkii) which carried the fungus causing the crayfish plague around the world with it. The introduction of this species throughout Europe, for farming, preceded mass mortality among native crayfish.

One of the most serious cases of fish disease transfer in the salmonid industry has occurred in Scandanavia in the last 10 or so years. Massive problems have followed the movement for fish farming of stocks of salmon fron the Baltic coast of Sweden into neighboring Norway where they were destined to be farmed in cages in the sea.

Unfortunately a fluke Gyrodactylus salaris hitched a lift. This fluke caused relatively little problem to salmon from the Baltic but caused massive outbreaks of disease among Norwegian stocks. The exact reason for the difference in resistance of the two stocks is not understood. The alien invader G. salaris not only impacted farmed fish but decimated rivers it escaped to. The Norwegian authorities have had to wipe out the entire fish stocks in river systems to eradicate the organism.

The conditions in which a disease may transfer and cause a problem are that:

  • the disease causing organism (pathogen) must be present in the fish which are moved
  • the recipient water must contain a susceptible host species and allow the pathogen to survive long enough and in sufficient numbers to infect them
  • the pathogen must be exposed to the susceptible host in a manner that allows infection to occur

It is exactly these risks which will be scrutinized in the Australian Import Risk Assessment and at a meeting of the OIE (The World Health Organization for animals) early next year. Any decisions made by the OIE could affect legislation on fish imports and would probably be upheld by the World Trade Organization.