Enteric Redmouth Disease

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Yersinia ruckeri
Kingdom	Bacteria
Phylum	Proteobacteria
Class	Gammaproteobacteria
Order	Enterobacteriales
Family	Enterobacteriaceae
Genus	Yersinia
Species	Y. ruckeri

Also Known As: ERM — Redmouth — Redthroat — Yersiniosis

Caused By: Yersinia ruckeri

Introduction

Enteric redmouth disease is a disease of fish caused by the gram negative, rod shaped, slightly curved bacterium, Yersinia ruckeri. Most of the bacteria are motile due to having seven or eight flagellae, although they only function at temperatures above 9⁰C and below 35⁰C .

There are six known serovars of Y. ruckeri. Many fish species involve a carrier state and are only detected when stressed, becoming a significant source of disease.

ERM can be acute or chronic. Economic losses due to the disease can be significant.

The disease is not zoonotic and no isolates can initiate growth at 37⁰C.

Distribution

Europe, North and South America, Africa, Asia and Australia.

ERM can be transmitted via water and it is poorly understood whether this is due to saprophytic function or merely prolonged survival in the presence of sediment and organic matter facilitating transmission and infection. The bacterium is shed in the faeces of infected fish and the quantity increases exponentially with stress. Once infected, a population can maintain the disease indefinitely.

Signalment

ERM is particularly associated with intensive fish culture and poor water quality. Fish appear able to withstand exposure to large numbers of bacteria without developing disease in the absence of stress. Poor feeding routine and nutrition can contribute, but poor water quality is the primary concern, usually relating to high ammonia, poor flow, high temperatures or lots of organic matter/sediment. Handling fish can also exacerbate/trigger disease.

The disease is primarily of cultured salmonid fish but a number of other fish families have been found to be infected leading to the belief that its host range is quite diverse.

Young fish 1-5 months old are more susceptible to ERM than adults and brook trout appear to be most vulnerable.

Infections have not been reported below 10⁰C.

Clinical Signs

The reddening of the throat and mouth that give the disease its name are caused by subcutaneous haemorrhage and are common but not always present. This can progress to erosion of the jaw and palate if untreated.

Affected fish also have generally darkened skin and fins with haemorrhagic lesions around the gill tips, vent and fins and are lethargic and inappetant. They may have a distended abdomen leading to a “pot-bellied” appearance. Fish are often pale around the gills and may demonstrate exopthalmos with periorbital oedema. They are anorexic and may swim close to the surface.

Mortalities are frequent and variable. They may occur in small fry with virtually no preceding signs. Losses of 15% over a cycle are not uncommon and can reach 90% on small scale units.

Diagnosis

Diagnosis is often presumed from history at a site and clinical signs but these are not specific and represent many other gram negative septicaemic diseases.

Confirmation is most easily made by culture from tissue samples of the spleen, heart and kidney. Media, such as tryptone soya, are widely available and very effective. They can be supplemented with 5% blood to increase success. The bacterium can then be serologically diagnosed by traditional methods, monoclonal or polyclonal antibody testing and antigen ELISA.

Antibodies to ERM can be detected using latex agglutination and can also detect subclinical infections.

The most recent method is PCR and is capable of detecting very low levels of Y. ruckeri but is insensitive and not commercially available. It is recommended that identification of pure cultures be based on amplification and sequencing of the 16S rRNA gene using universal bacterial primers 27F and 1492R.

At necropsy, there is congestion of blood vessels throughout the peritoneum and white-grey patches of necrosis and petechial haemorrhages are found in the liver, swim bladder, pancreas, lateral muscles and adipose tissue. Serosanguinous fluid often accumulates in the body cavities and muscle which become yellow and mucoid within the intestinal tract. There may also be severe damage of the haematopoietic renal tissues.

Acute anaemia with a haematocrit as low as 23% has also been noted and is thought to be due to endotoxic effects.

Treatment

ERM responds well to antibiotic treatment with oxolinic acid, oxytetracycline, sulphamerazine and tribrissen. Amoxicillin is now licenced for use in the UK.

Vaccines can also be used as treatments for infected fish.

Control

Good husbandry and water quality are key to minimising the stress that precipitates disease.

Two very effective vaccines are also available but may not provide comprehensive cross-protection in all situations.

Enteric Redmouth Disease Learning Resources
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References

Barnes, A.C. 2011. Enteric Redmouth Disease (ERM) (Yersinia ruckeri). In: Fish Diseases and Disorders Volume 3: Viral, Bacterial and Fungal Infections (eds. P.T.K. Woo and D.W. Bruno), CABI, Walingford, UK, pp. 484-511.

This article was originally sourced from The Animal Health & Production Compendium (AHPC) published online by CABI during the OVAL Project. The datasheet was accessed on 10 July 2011.

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