The disease is well known in Tasmania, Australia, where is costs the industry up to A$230M a year, the organisms favour the warmer conditions found there. However in recent years the disease has been a growing problem in Ireland, and has also been identified in France, Norway, USA and Canada.
AGD is caused by the protozoan amoeba species Neoparamoeba perurans. The parasite causes a proliferative response in the gill epithelium (this is generically known as proliferative gill disease, or PGD, of which amoebae are just one agent). Normally oxygen would diffuse through the thin epithelium, which it can't do when the epithlium is thickened. In the parts of the gill affected, the fish effectively can't breath, explains Mr Matthews.
Mr Matthews says that the organism is probably present naturally in the water, and has become apparent in Scotland because of the unseasonally high water temperatures that have been experienced.
"Neoparamoeba perurans may have been a previously under-recognised background issue in Scotland in previous years, contributing to this seasonal pathology, and this year has become more significant - perhaps due to the warmer water temperatures favouring its biology."
Once on a farm, these amoebae divide exponentially so clinical disease can develop quickly, he says.
"Early intervention is likely to be the key to avoiding losses."
Chris Matthews, Fish Vet Group
The disease only appears to threaten farmed species, with fish in their first year at sea seeming particularly susceptible to the disease.
Mr Matthews says that in the early stages there are few obvious symptoms of the disease, as the disease develops signs of respiratory disease can be seen, including flared operculae and gasping. Fish are often also found higher in the water column.
Mucoid patches become apparent on the gills, and eventually increased mortalities will occur.
The disease is diagnosed by clinical examination by veterinarians, including a look at fresh gill preparations under a microscope. Histopathology confirms the diagnosis and is helpful in gauging the severity of the disease. RT-PCR can be used to confirm the amoeba species involved.
As of yet there is no cure as such for the disease, but if outbreaks are carefully managed, as the water temperature cools, the disease generally resolves. In extreme situations veterinarians may recommend treatment. Mr Matthews says that bathing whole cages in freshwater is the most recognised treatment in Tasmania, although recognises that this is expensive and technically difficult to achieve.
The industries in Scotland, Ireland and Tasmania are presently researching possible treatment strategies. Early intervention is likely to be the key to avoiding losses.
Mr Matthews says that careful site husbandry is required. "Avoiding stress can go a long way to mitigating the effects of gill disease."
The first cases in Scotland were seen in September 2011, says Mr Matthews. In Ireland the disease has been recognised since the 1980s, and it is thought that Tasmania has dealt with the disease for over 20 years.
Whilst the discovery of the disease in Scotland has been a particular problem in recent months, Mr Matthews says that seasonal gill disorders (of which AGD is one) can be common in the autumn and early winter, triggered by peak water temperatures at the end of summer.
The AGD agent has been identified in most of Scotland's salmon farming areas, however clinical disease has only developed in individual farms. Fish Vet Group is currently working to identify the risk factors for developing gill problems.
"Gill diseases, including AGD, are always a significant challenge for the industries in Scotland, Ireland and Norway. Because losses due to other infectious diseases are improving due to vaccination and advances in husbandry techniques, the relative importance of losses due to gill disorders like AGD has increased," he says.
Fish Vet Group is working alongside industry fish health professionals to provide advice on the management and treatment of the disease.