Rainbow trout fry syndrome (RTFS) explained

Flavobacterium psychrophilum is the causative agent of rainbow trout fry syndrome (RTFS), a disease with an important economic impact in salmonid farms and restocking facilities worldwide, especially in rainbow trout (Oncorhynchus mykiss). Although disease outbreaks have been recorded in Europe since the late 80s, the lack of knowledge regarding the epidemiology of this bacterium have limited the development of disease management tools, and it still remains an important issue on British fish farms, where rainbow trout are the second most produced finfish by harvest volume.

The implementation of strategic biosecurity plans and optimization of disinfection and husbandry practices have reduced the overall impact of this disease. However, the impact of these control measures are limited by the widespread nature of this bacterium in the UK’s freshwater bodies. Flavobacterium psychrophilum has been previously isolated from rearing water and pond sediments, and since trout farming currently depends mainly on natural water sources, the water intake to the farms is a potential vector for its spread. To reduce this risk, farming companies are increasingly looking towards sourcing water from boreholes and springs, especially during the early stages of fish growth.

Both living and dead fish can be a F. psychrophilum source, but dead fish show a higher rate of bacterial release into the surrounding water, therefore regular removal of mortalities is a highly recommended practice. Although the transmission of this pathogen from native fish populations like eels (Anguilla anguilla) and brown trout (Salmo trutta) to farmed stocks remains to be demonstrated, latent carriers of virulent strains without clinical signs are possible.

The epidemiology of RTFS is similar regardless of the geographical location. Disease outbreaks commonly occur at water temperatures between 4 - 10ºC, but mortalities have been reported in recent years at temperatures up to 16ºC. Although juvenile fish are particularly susceptible to the disease, diverse sizes of fish have been involved in F. psychrophilum infections, including table size yearlings (300-500 g) and broodstock. A range of clinical signs of RTFS in rainbow trout have been described but the more common ones include reduced appetite, dark skin pigmentation, necrosis of adipose fin and caudal peduncle, with exposure of the underlying musculature and even the vertebral column in chronic progression of the disease. Dermal ulcerations can sometimes be deep and usually appear in one flank of the fish. Exophthalmia (“pop-eye”) is also commonly exhibited. In cases of important bacterial infestation and acute manifestation, fish generally die in high numbers due to septicaemia. Microscopically, pathologic changes indicative of systemic bacterial infection (including inflammation, haemorrhage, congestion and oedema) and the presence of typical filamentous bacterial rods confirmed by Gram stain are commonly found.

Where preventative strategies have failed, the principle infection control measure remains the oral (in-feed) administration of antibiotics, most commonly florfenicol (Florocol). The use of bacterial culture and sensitivity testing is a critically important practice in selecting the correct antibiotic for RTFS and monitoring for potential antibiotic resistance.

RTFS can be a secondary infection, following on from an initial viral challenge, but its widespread presence can be misleading for fish pathologists. In addition to this, the isolation of the pathogen does not necessarily correlate with the disease, since several studies have confirmed that not all strains of F. psychrophilum are equally virulent, and virulent and non-virulent strains can be found and isolated together in the same fish.

Although many aspects of the bacterial disease have been widely studied, control measures are limited and currently there is no successful commercial vaccine available. Some advances have been made at a local scale with the use of autogenous vaccines to prevent disease outbreaks and the use of bacteriophages as prophylaxis has demonstrated potential protection against bacterial infection in fry under experimental conditions in Chile. In addition, a number of breeding companies offer rainbow trout eggs which are more resistant to flavobacteriosis.

The use of genotyping assays has brought together new data on the epidemiology of F. psychrophilum in rainbow trout farms in countries including Scotland, Chile, Norway, USA, Denmark and France. The results confirm a high genetic diversity in bacterial populations in the facilities, with the prevalence of one or two types. However, the relative virulence of each type remains to be fully evaluated. As with other bacterial species F. psychrophilum strains demonstrate a range of virulence factors including serological, genetic, stress response mediators, elastin-degrading and adhesive properties. A better knowledge of this variability could have important implications for studies of epidemic population structure. This could be used to further investigate the distribution of bacterial strains and their link with the movement of eggs and live fish, as well as to establish improved disease management strategies and vaccine development.