Introduction

Enteric Redmouth Disease (ERM) is caused by the bacterial pathogen Yersinia ruckeri. The first instances of the disease were reported in Rainbow trout (Oncorhynchus mykiss) in the USA in the 1950s. Subsequent clinical outbreaks occurred in the UK in 1982 and in most other parts of Europe over the following few years. It is now endemic in all trout producing countries where it can cause severe economic losses. It is also becoming a more significant pathogen of farmed salmon, primarily in freshwater but it has been reported to cause losses in the sea as well.

According to estimates from within the British Trout Association in 1998, the cost of ERM in the UK trout industry alone was approximately 1.3-1.5 million per year. This figure is based on the costs of mortality, growth penalties and subsequent reduction in feed conversion rate, grading problems, withdrawal periods due to antibiotic treatments and subsequent harvesting delays as a result of the disease. This can be calculated as 10% of the production cost of the industry. Extrapolating these figures to the European trout industry producing 200,000 tons per year indicates ERM causes a potential economic loss of 20 million each year.

Aetiology

ERM is characteristically a disease of rising and falling water temperatures. The highest risk of disease occurs between 8^o and 16^oC although mortality can occur at temperatures as low as 4^oC. At the higher temperatures mortality can rise quickly. Levels of 15 - 20 % have been reported even with antibiotic treatments. Fish of all sizes are affected, however, fish of 50 - 200g are the most susceptible. Stressful conditions such as high stocking densities and poor water quality will increase the level of susceptibility and mortality.

Symptoms

The most distinctive external and internal signs and symptoms of the disease are:

• Black, lethargic fish "hanging" in areas of low flow at the edges of ponds and raceways or against outlet screens.
• Bilateral exophthalmia
• Abdominal distension as a result of fluid accumulation
• Haemorrhages of the mouth and gills. The so-called "Redmouth" is rarely seen in acute infections but may be present in chronic infections.
• General septicaemia with inflammation of the gut
• Petechial haemorrhage of the pyloric caecae
• Diffuse haemorrhages within the swim bladder. These are the most diagnostic symptoms of ERM
• The spleen is often enlarged and can be almost black in colour

The external symptoms mentioned above are also seen with other bacterial and parasitic infections so some further diagnosis must be carried out. Yersinia ruckeri is easily isolated from diseased fish on TSA plates or preferably upon a semi-selective media designed for the purpose.

Following an incubation period of 24-48 hours shiny off-white colonies develop. Identity can be rapidly confirmed using specific antisera such as Mono YR test kits

Epidemiology

Known to be present over large areas of the USA and Europe (Denmark, France, Germany, Italy, Norway and UK). It is also present in Australia, South Africa and Chile, all of which received imported salmonid eggs and fry from these areas. However, there is some variation in the strains and serotypes within the genus associated with geographical areas and it cannot be ruled out that it is of a more widespread occurrence in a natural manner than was at first thought.

Treatment

Good husbandry can play an important part in limiting the level of mortality caused by ERM. Reducing stocking densities at times of high risk and avoiding stressful procedures such as grading will help.  

• Salmon
• Trout

References

© CAB INTERNATIONAL 1999. Fish Diseases and Disorders, Volume 3:
Viral, Bacterial and Fungal Infections (eds P.T.K. Woo and D.W. Bruno)