Pseudomonas anguilliseptica infection as a threat to wild and farmed fish in the Baltic Sea

The transport of live fishes related to the growth of the fish farming industry worldwide may increase the transfer of previously known bacterial pathogens into new geographic areas and new host species, but also facilitate the introduction of completely new bacterial pathogens. Species belonging to the genera Vibrio and Aeromonas are well known in many countries, infecting a large number of fish species. Other bacterial fish pathogens like Pseudomonas anguilliseptica species, up to now considered less harmful, may constitute a potential threat to a developing fish farming industry, especially of European whitefish.

Pseudomonas anguilliseptica is a fish pathogenic bacterium infecting mainly farmed fish in brackish and marine environments. The pathogen was initially reported from diseased farmed Japanese eel (Anguilla japonica) in Japan in 1971, and named ‘sekiten-byo’ or red spot disease¹. Since 1981, P. anguilliseptica has also been isolated from cultured European eel (Anguilla anguilla) in different European countries^2–5. Although P. anguilliseptica was initially considered a pathogen closely associated with eel culture it appeared that this pathogen infects a number of different fish species in different water areas. It has been isolated from farmed fish species, like ayu (Plecoglossus altivelis)⁶, cod (Gadus morhua)⁷, gilthead seabream (Sparus aurata)⁸, sea bass (Dicentrarchus labrax), turbot (Scophthalmus maximus)⁹, striped beakperch (Oplegnathus fasciatus)¹⁰ and recently from lumpsucker (Cyclopterus lumpus)¹¹.

The disease signs associated with P. anguilliseptica infections in fish are characterised by petechial haemorrhages in the peritoneum and in the skin on the ventral side of the fish (Figure 1). Sometimes haemorrhages are also present in the liver and adipose tissue of visceral organs. Occasionally spleen and kidney are soft in consistency and enlarged^1,12,13. P. anguilliseptica has in some occasions caused significant mortalities in farmed eel^3,14 and Atlantic salmon¹³ suggesting this pathogen has the potential to be a serious problem for farmed fish especially if left untreated.

Figure 1. Farmed European whitefish (Coregonus lavaretus) with Pseudomonas anguilliseptica infection showing haemorrhages in the skin and loss of scales.

P. anguilliseptica is an aerobic, motile, Gram-negative rod, producing slow-growing colonies on agar plates. The bacterium is cytochrome oxidase-positive, catalase positive and does not produce acid from glucose and has a low metabolic reactivity for many different carbohydrates. The inclusion of this pathogen into the genus Pseudomonas has been questioned, and it has been suggested that it could be classified into Alcaligenes or Deleya or even to a newly described genus¹⁵.

In the Baltic Sea area in northern Europe, in the middle of the 1980s P. anguilliseptica has been almost simultaneously isolated from farmed European eel in Denmark¹⁶, in Sweden (Eva Jansson, pers. comm.) and from different farmed salmonids in Finland (Atlantic salmon (Salmo salar), sea trout (Salmo trutta m. trutta), European whitefish (Coregonus lavaretus) and rainbow trout (Oncorhynchus mykiss))¹³. Subsequently, occasional disease outbreaks associated with P. anguilliseptica have been reported from farmed eel in Sweden (Eva Jansson, pers. comm.), although the number of eel farms in the Baltic Sea area were rather low during the past 20–30 years. Although in the northern Baltic Sea, initially P. anguilliseptica was isolated from several farmed salmonid species, it is today mainly associated with disease in European whitefish and to lesser extent with diseased rainbow trout. Both species are farmed in net pens in brackish water (salinity = 4–6‰). During 1986–1991, 2 to 17 disease outbreaks associated with P. anguilliseptica were recorded per year¹⁷. Lately, 2–5 disease outbreaks both in European whitefish and in rainbow trout have been recorded per year (T. Wiklund, unpubl. data)¹⁸. Initially P. anguilliseptica was often co-isolated with other bacterial fish pathogens like Vibrio anguillarum, and Aeromonas salmonicida subsp. salmonicida¹⁷, suggesting a compromised immune system of the fish facilitating the infection with several bacterial species. Now the majority of farmed European whitefish and rainbow trout in Finland are vaccinated against vibriosis and furunculosis, but P. anguilliseptica is still causing disease outbreaks, mainly in European whitefish.

So far, Finland seems to be one of the few countries where P. anguilliseptica is causing disease outbreaks in farmed salmonids from marine and brackish water. In Sweden P. anguilliseptica was recently reported from diseased rainbow trout in freshwater (Eva Jansson, pers. comm.). Disease outbreaks in fish from freshwater associated with P. anguilliseptica are uncommon. P. anguilliseptica has previously been isolated from tilapia farmed in fresh water¹⁹ and the bacterium seems to be present also in fresh water environment as reported from a river in India in rather high concentrations²⁰.

Reports of P. anguilliseptica in wild fish are rather rare in literature. The pathogen has been isolated from wild European eel¹⁷ and from wild Atlantic salmon, sea trout and Baltic herring (Clupea harengus membras) with eye lesions in the Baltic Sea²¹. The Baltic herring suffered from haemorrhages in the eyes and in some specimens the cornea was punctured. Additionally haemorrhages in the fins and head and blood containing ascites were present. The isolates from Baltic herring were, however, of low pathogenicity for rainbow trout. The role of P. anguilliseptica as the etiological agent of the observed eye lesions in Baltic herring remained unsolved, and the authors concluded that the bacterium might have been a secondary invader²¹.

Although P. anguilliseptica has been associated with disease outbreaks in different fish species in the Baltic Sea, the most significant impact today is on European whitefish. Infections with P. anguilliseptica have been treated with trimethoprim/sulfamethoxazole or florfenicol. Both antibiotics are efficient if the treatment is applied immediately in the onset of a disease outbreak. In contrast, oxytetracycline has been reported to be of limited effect against this pathogen¹³. In Japan and Scotland P. anguilliseptica infections in eels have been controlled by raising the water temperature temporary to above 27°C^3,22. However, this procedure to treat the infection is not possible for salmonids.

It can be concluded that P. anguilliseptica seems to be present in different areas of the Baltic Sea. The pathogen has the potential to negatively impact future large scale farming of European whitefish and European eel in brackish water.