The Disease

Synonyms
WDS

Introduction
Over the last few years a disease referred to as Winter Disease Syndrome (WDS) has been affecting (mainly) sea bream in the Mediterranean area. The diseased fish show typical behaviour for this disease, i.e., they swim belly-up and, as the name indicates, they are affected mainly during winter when water temperatures are low.

WDS was first seen in the Mediterranean area in cultured gilthead seabream in 1991; sea bream are apparently affected most but infections in turbot and sea bass have also been reported.

Etiology
The precise etiology is still a point of discussion amongst researchers. Some consider the disease to be related to a dysfunctional immune system at low water temperatures while others think that a nutritional aspect can not be excluded. We have found a single bacterial species, Pseudomonas anguilliseptica, in all the cases that were analysed in our laboratory. This observation is in accordance with the findings of most other research groups.

However, the disease condition is most probably multifactorial in etiology. This viewpoint is further highlighted by the difficulties different researchers have when trying to establish experimentally-induced infections. Most reports on artificial challenge show that the disease can be reproduced only in conjunction with a severe external stressor. Thus, the disease probably arises from the combined action of low temperature, nutritional imbalance, immunosuppression and the presence of P. anguilliseptica.

Incidentally, P. anguilliseptica was first described as the causative organism for 'red-spot disease' in eels in 1972 in Japan. Since the original papers, infections in European eel, turbot, Atlantic salmon, rainbow trout, sea bass, herring and others have been described, so freshwater fish as well as marine fish can be affected.