New Fish Parasite Species Described

Single-celled parasites of the genus Spironucleus are known to produce serious illness in farmed and aquarium fish. In farmed salmon, these parasites create foul-smelling, puss-filled abscesses in muscles and internal organs. After the first outbreaks of this disease were described in farmed salmon in the late 1980’s, it was assumed that the cause was Spironucleus barkhanus, which is a fairly common parasite in the intestine of wild grayling and Arctic char.

In these fish species, however, the parasite is benign. For his doctorate, Jørgensen completed genetic studies showing that the disease-causing parasite in farmed salmon is genetically quite different from the species one finds in wild salmonids, although they appear to be identical, even under high magnification in an electron microscope. Based on this observation, the parasite that causes disease in farmed salmon has now been described as a new species – Spironucleus salmonicida.

“Our work has shown that genetic methods need to be utilised for correct identification of single-celled parasites of the genus Spironucleus. Parasites that appear to be identical morphologically may in fact be significantly different genetically. An exact identification of organisms that produce disease is extremely important in the fight to find the cause of disease outbreaks and provides an important contribution to finding appropriate diagnostic methods”, says Anders Jørgensen.

Jørgensen carried out similar studies with other Spironucleus species, which he also incorporatd into his doctoral thesis. Spironucleus vortens, which causes disease in aquarium fish, is also found in wild carp in Norway. Even though these parasites appear to be identical, they are very different genetically. Jørgensen also addressed the cod parasite Spironucleus torosus, which is found in several genetic variants. Based on these new findings, Jørgensen discusses whether the genetic differences between the variants provides a basis for splitting them into separate species.

Finally, Jørgensen investigated relationships between a series of species. These investigations showed that parasites from other Spironucleus families form three primary groups, which reflect the different environments their host species live in. His thesis hints that each of these groups may constitute a separate genus.

Anders Jørgensen defended his thesis for the degree of Doctor Scientiarum at the Norwegian School of Veterinary Science on January 24, 2008, with the title “Genetic studies of diplomonad flagellates from fish”.