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Tackling enteric redmouth disease (ERM)

22 February 2018, at 9:00am

Virginia Iglesias, a veterinarian based at the Fish Vet Group’s Inverness office, explains the causes, diagnosis and control of the disease - which is a major problem in global salmonid aquaculture.

Enteric redmouth disease (ERM) is usually thought of as a freshwater disease of salmonids, though seawater cases are occasionally reported following transfer of Atlantic salmon to sea. Anecdotal evidence suggests the number of such cases may have risen in recent years in regions where the pathogen is present.

Also commonly known as Yersiniosis, it has become a major constraint to the expansion of salmonid culture worldwide, affecting both Atlantic salmon (Salmo salar) and rainbow trout (Onchorhynchus mykiss). The disease is caused by the pathogen Yersinia ruckeri, a gram-negative bacterium with tropism for the gastrointestinal tract. Bacteria from the genus Yersinia are agents of invasive human and animal diseases (for example, Yersinia pestis, the causative agent of the bubonic plague; Yersinia pseudotuberculosis or Yersinia enterocolitica, causative of important animal-origin food poisoning outbreaks in humans).

Accumulation of waste feed and organic debris in a fry tank (top) and in a fish grader (below). Some bacterial fish pathogens form biofilms on aquaculture substrates, which resist the effect of antibiotics and water disinfectants and are potential source of infections to the fish population. Periodical removal is essential to minimise the bacterial load.
Accumulation of waste feed and organic debris in a fry tank (top) and in a fish grader (below). Some bacterial fish pathogens form biofilms on aquaculture substrates, which resist the effect of antibiotics and water disinfectants and are potential source of infections to the fish population. Periodical removal is essential to minimise the bacterial load.

Although infections with Y. ruckeri have been reported in many freshwater fish species, including European eels, sturgeon and carps, salmonids seem to be more sensitive to this bacterium and prone to suffer disease outbreaks, especially in young life stages, leading in the worse cases to significant mortalities and economic losses for the industry.

Accumulation of waste feed and organic debris in a fish grader. Some bacterial fish pathogens form biofilms on aquaculture substrates, which resist the effect of antibiotics and water disinfectants and are potential source of infections to the fish population. Periodical removal is essential to minimise the bacterial load.
Accumulation of waste feed and organic debris in a fish grader. Some bacterial fish pathogens form biofilms on aquaculture substrates, which resist the effect of antibiotics and water disinfectants and are potential source of infections to the fish population. Periodical removal is essential to minimise the bacterial load.

ERM transmission

Transmission of the disease is mainly by direct contact with infected fish. The possibility of a carrier state has also been identified, where asymptomatic fish carry the bacteria in the lower intestine and whenever any stressful situation occurs, the disease outbreak takes place. Infected fish can shed the bacteria to the surrounding water through their faeces, and these can survive up to several months on surfaces and sediments in form of biofilms, which are source of recurrent infections. Therefore, optimisation of hygiene in the facilities is essential to reduce the bacterial load in the environment.

 

The Health and Welfare of Atlantic Salmon.

It is vital that fish farm operatives who are responsible for farmed fish are trained in their health and welfare. This will help to ensure that fish are free from disease and suffering whilst at the same time promote good productivity and comply with legislation.

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Identifying ERM

Yersinia ruckeri is associated in fish with acute septicaemia, where the bacteria invade the bloodstream along with their toxins. Signs are therefore unspecific and these are initially seen as an increase in mortalities above the normal rate, fish swim near the surface or in the edges of the unit, showing darkening and poor feeding response.

Exophthalmos with intraocular haemorrhage is a classic sign of enteric redmouth disease
Exophthalmos with intraocular haemorrhage is a classic sign of enteric redmouth disease

Common features include exophthalmos (“pop-eye”) with blood spots in the eye, haemorrhagic congestion of fins, pale gills and vent distension. The presence of vascular lesions (hyperemia, ecchymotic haemorrhage) around the mouth are classic signs of this disease, although these are found infrequently.

Congestive fins can be a tell-tale sign of enteric redmouth disease
Congestive fins can be a tell-tale sign of enteric redmouth disease

Clinical signs of ERM

Many bacteremias and viremias encounter oral congestion and haemorrhages, in addition to poor water quality conditions, therefore the diagnosis of the disease cannot be concluded exclusively based on the clinical signs. These include the isolation of the bacterial pathogen from samples of spleen or kidney in generic freshwater agar based media like tryptone soy agar (TSA), and the species confirmation can be made by phenotypic profiling or molecular assays as PCR. Further histological examination can provide evidence of the level of infection in the fish organs and tissues and the degree of circulatory collapse.

Colonies of Yersinia ruckeri in standard TSA medium: round entire colonies, creamy in colour and translucent in appearance
Colonies of Yersinia ruckeri in standard TSA medium: round entire colonies, creamy in colour and translucent in appearance

Preventative measures

Prevention measures are essential to control ERM. Vaccination has helped to control the significant mortalities due to ERM, especially those that act against several biotypes of Y. ruckerii (or polyvalent vaccines). Monovalent vaccines have been developed to control the disease in areas where specific bacterial strains are more prevalent than others, based on epidemiological studies of bacterial populations in fish farms.

Treatment relies mainly on in-feed oral use of antibiotics, including routinely used amoxicillin, oxolonic acid and florfenicol. Since in vitro testing has shown rapid resistance development against several antibiotics, considering the limited availability of antibiotics in fish medicine it is highly recommended to carry out antibiotic sensitivity testing to check the ability of the specific bacterium strain to survive in the presence of the antibiotic in question and alleviate the growing drug-resistance problem.

Veterinarian at Fish Vet Group
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