Vibriosis in fish: a clinical, pathological and bacteriological study of the disease in Norwegian fishfarms

Abstract

The work divides naturally into three sections, the first section dealing with the literature apposite to the study. The literature review starts with the historical discovery of the disease, followed by description of morphology, metabolism, antibiotic sensitivity, serology and toxin production of Vibrio anguillarum. Literature on the normal and experimental pathogenesis of the disease is cited with descriptions of attempts to type Vibrio anguillarum into biotype groups. This section also includes a description of the host range and economic significance of the disease. The second section of the study gives a short description of Norwegian fish farms in general, followed by a description of the methods of collection of pathological material from outbreaks of vibriosis in Norwegian fish farms. This section also gives a description of the isolation procedures, bacteriological tests and histological techniques used in the study, followed by a description of the pathology and treatment of the disease. The fourth chapter of the second section gives the results of the bacteriological examinations carried out on 163 strains of Vibrio anguillarum obtained from outbreaks of vibriosis in Norway. This chapter includes a description of morphology, viability, antibiotic sensitivity, biochemical properties and production of specific proteinases identified by means of the caseinate precipitation inhibition test (CPI-test). The third section of the work consists of a computer analysis study of the bacteriological data obtained in the second section of the work. A review of the literature concerning numerical taxonomy is given, followed by a description of different methods for computation of the material. This includes a description of principal components analysis (PCA) and of numerical taxonomy by means of Single Link Listing (SLL). Finally a description of the results obtained with these two methods is given. The PCA method gave two distinct groupings of the strains and allowed all strains to be included in the two groups. There seemed to be an even geographical distribution of both group*, and none of the groups could account for specific pathological findings. Two strains of Vibrio anguillarum obtained from the American Type Culture Collection fell into one group each and Vibrio metchnlkovi (a human vibrion strain which had been included in the study), was demarcated well outside group I. There seemed to be no specific group distribution of fish species, except in the case of isolates from saithe (Gadus virens) which almost all fell into one computer defined group. Examination of the material by means of SLL gave five acceptable groups at 88.7%, leaving 23 strains ungrouped at this level. Comparative study of the groups defined by PCA and by SLL showed that the SLL defined group I and XI fitted into PCA group II, while SLL groups III, IV and V corresponded to PCA group I. It is therefore concluded that the PCA method provided the most suitable way to classify the material and that the taxonomic determinants allowing best definition of strains in the two PCA groups were citrate utilization together with arabinose, lactose, cellobiose and trehalose fermentation. The two groups defined by the computer for Norwegian isolates of Vibrio anguillarum thus did not correspond to the groups proposed by Nybelin (1935) and Smith (1961) who based their grouping on indole production and mannitol and sucrose fermentation.