Amoebic Gill Disease
A growing threat for freshwater farms

Amoebic gill disease (AGD) has impacted production on several salmon production sites across Europe in recent years and now a variation of the condition has been confirmed in a number of freshwater fish farms, with a different amoeba species involved. The disease causes gill pathology; fish often lose the ability to feed, they suffer from breathing problems and if they are left untreated they die.

While the cause of the outbreak is as much of a mystery as the biology of the amoeba, it is widely believed it could have been present for some considerable time. The freshwater amoebic gill problem was first observed in Germany five years ago where it was responsible for as much as 40 percent mortality in a group of trout farms. Fish as large as 400 grams were lost.

Skretting veterinarian Frederic Esnault says the outbreak was a major problem because farmers were neither prepared nor knew how to deal with it. “Initially, it was confused with a bacterial disease and the treatments that are usually used for controlling such problems do not work on amoebic gill disease,” he says.

One German trout farmer has had some success controlling the disease with salt baths. He acquired a fish farm in 2006 knowing there had been losses that spring, and spent the next two years conducting extensive research into the cause of the mortality. In 2009, the amoebic gill pathogen was confirmed by a local animal health service.

The farmer’s amoeba-limiting measures now include reducing the feeding rate, implementing low stocking densities in warmer months and using salt baths when the pathogen is detected and quaternary ammonium baths in order to prevent secondary bacterial gill diseases. And while he has not experienced any major losses in the last two years, “breakouts” still occur in the spring and occasionally in the autumn, depending on the temperature. He also stresses that salt baths are not always a successful solution to the problem. It is also an impractical and expensive undertaking for companies with a lot of raceways, as the recommended application is 20 grams of salt per litre of water for at least one hour.

An Emerging Disease

In 2007, a similar problem to the one that had blighted the German trout farms emerged in Spain, causing high mortality rates of 30-40 percent. To date only three farms have been affected, says Skretting veterinarian Daniel Gijón Preciado, but there were two severe outbreaks last year.

“The treatment is difficult and not always successful,” warns Gijón. “We have tried several disinfectants – formalin, salt, copper sulphate – used to control other parasites and they have limited success. It is very important to detect the problem early on in order to minimise the impact and the efficacy of treatments,” he adds.

Three years ago, the amoeba was discovered in some French hatcheries. While it is not known if it was the same species, it came on rapidly and caused high mortalities. “In one case, the farmer thought it was a bacterial disease that he could not control. By the time I arrived at the farm it was too late; there were very few fish remaining in the tanks,” says Esnault.

For the time being, outbreaks have only been confirmed in Germany, Spain and France and only among trout. But Esnault calls the parasite “an emerging disease” and says it is unlikely that it has not affected farms elsewhere in Europe. “We have AGD in both marine (salmon) and now freshwater species (trout). It seems to be a growing problem. As veterinarians, we are now trained to recognise such a parasite so we might find it in many more locations. It could perhaps explain some of the past mortalities of unknown origin.”

Regular Examinations

While little is known about the factors that impact the progression of the disease, certain conditions are thought to be contributors. Esnault says outbreaks seem to be less prevalent in hard water than in very soft water. It is also suspected that too much organic matter in the ponds or raceways could exacerbate the problem.

Both Esnault and Gijón recommend changing the pond or raceway once the fish have been treated. But they say the best thing farmers can do is conduct regular examinations of their stock. “Now that we know that such a parasite exists, we are better prepared to deal with it. Amoeba is more visible at the beginning of the outbreak. When it’s too late and the gill lesions are too prominent there is a lot of swelling and mucus and it is a lot harder to find the amoeba cells on the gills,” says Esnault.

He advises that if farmers believe they have an amoebic gill problem – if, for example, they initially thought it was a bacterial disease but the usual antibacterial treatments had not worked – then they should immediately consult their local veterinarian.