Lactococcus garvieae, an emerging pathogen for the Portuguese trout culture

We report here the first description in North Portugal of epizootic episodes of lactococcosis causing important mortalities in cultured rainbow trout associated with high water temperatures. The outbreaks that occurred during the warm seasons of 2002 and 2003 affected all size fish, reaching up to 90% mortality in some farms. The taxonomic studies of the isolates obtained indicated that they belonged to Lactococcus garvieae. This identification was confirmed by PCR amplification using specific primers. The isolates proved to be highly pathogenic for rainbow trout (LD50<10 cells), and belonged to the same genetic group as revealed by RAPD analysis. All these findings indicate that lactococcosis by L. garvieae is an emerging disease for the Portuguese trout culture and that appropriate measures must be taken in order to avoid the spread of this disease to other areas of the country. Lactococcus garvieae, one of the major Gram positive cocci pathogenic for fish, is considered a serious problem in cultured marine and freshwater fish species such as yellowtail (Seriola quinqueradiata) in Japan and rainbow trout (Oncorhynchus mykiss) in Europe and Australia (Diler et al., 2002; Eldar & Guittino, 1999; Eldar et al., 1999; Kusuda et al., 1991). In Spain, also within the Iberic peninsula, this disease has appeared in rainbow trout farms from 1991 onward (Doménech et al., 1993; Ravelo et al., 2001) and, at present, is considered one of the most important risk factors in the trout industry during the summer months. To our knowledge, and regardless of the inclusion in some works of Portuguese isolates of L. garvieae (Doménech et al., 1993; Ravelo et al., 2003), there are no reports of lactococcosis outbreaks in Portugal. One reason for this lack of descriptions in Portugal can be the difficulty in the identification of L. garvieae on the basis of phenotypical tests, which can barely differentiate this bacterium from other Gram positive cocci such as the human pathogen L. lactis subsp. lactis (Elliot et al., 1991; Fefer et al., 1998) or the Enterococcus – like strains isolated from diseased fish (Eldar et al., 1999; Toranzo et al., 1994). Besides fish, L. garvieae has been isolated from cows and buffalos (Teixeira et al., 1996) and it has also been recovered from human sources (Elliot et al., 1991; Fefer et al., 1998). In addition, this bacterium has been isolated recently in Taiwan from diseased freshwater prawn Macrobrachium rosembergii (Chen et al., 2001). All these facts indicate the expanding importance of L. garvieae. Bull. Eur. Ass. Fish Pathol., 24(6) 2004, 275 Between June and October of 2002, several disease outbreaks causing high mortalities occurred in four rainbow trout farms located in the north of Portugal. Epizootics were associated with a rise in the water temperatures, which in some cases were higher than 21°C for more than two months. An initial treatment with oxytetracycline was applied in two of the farms, but although at first it seemed to control the outbreaks, after three weeks the mortalities started again with higher virulence. Therapy was subsequently changed to enrofloxacin and erythromycin with similar results. Total fish losses at the end of September amounted from 20 to 90% depending on the farm. It is interesting that fish of all sizes were affected, from 5 g to market-size (>200 g), although mortalities were heavier in bigger fish (>100 g). A similar situation occurred during 2003, where cumulative mortalities between 25 and 30% were observed in the farms at the end of the warm season. The use of vaccination as preventive measure during 2004 avoided the appearance of epizootic outbreaks in most of the affected farms. In all cases, diseased fish showed a pronounced unior bilateral exophthalmia with periocular haemorrhages and, in some cases, loss of the eye. Internally, clinical signs consisted of an accumulation of ascitic fluid in the peritoneal cavity and haemorrhages in the liver and muscle. These signs are in concordance with those previously described for lactococcosis outbreaks (Austin & Austin, 1999).