The peripheral blood of O.mossambicus was examined using light and electron microscopy and was found to contain four forms of leucocytes: lymphocytes, thrombocytes, monocytes and three types of granulocytes. The monocyte and two types of granulocyte were found to be phagocytic and ingest colloidal carbon and bacteria. The alimentary tract was found to contain a number of leucocytes, some showing a morphological similarity to those in the peripheral blood, while others were unique to the gut tissue. These intestinal leucocytes were found mainly as a diffuse cell population in the epithelium and lamina propria, and only occasionally as discrete lymphoid accumulations within the gut tissue. Ontogenic studies showed that a limited number of leucocytes were found in the gut tissue after hatching, however, there was a gradual increase in these numbers once exogenous feeding began. The intestinal enterocytes of both the anterior and posterior intestine were found to take up intubated macromolecules. An electron microscopical investigation revealed that these macromolecules were absorbed by pinocytosis and were found within large intraepithelial macrophages. These macromolecules were also absorbed and transported into the systemic circulation. In juvenile fish macromolecules were detected in the plasma following both oral and anal intubation, however, in adult fish they were detected in the plasma only after anal intubation, and in smaller quantities. Macromolecular absorption in O.mossambicus was compared to that in two other fish species, Cyprinus carpio and Sa1mo gairdneri, and it was found that higher levels of absorbed macromolecules were found in the plasma of O.mossambicus. Bovine serum albumin absorption by the gut of the three species revealed that both the 'intact' macromolecule and smaller antigenic fragments, probably resulting from enzymatic modification, were ansorbed and transported into the plasma.

Diplostomum metacercariae were collected from three sources (a) the lenses of sticklebacks from Culter Compensation Dam. (b) the lenses of rainbow trout from Mill of Cantray trout farm. (c) the humours of rainbow trout from Mill of Cantray trout farm. The metacercariae from the lenses of Culter Dam sticklebacks produced infections lasting 12-15 weeks with an egg production of 6,000-10,000 eggs per day, in gulls. The metacercariae from Mill of Cantray trout however, produced short Infections of only 3-4 days with an egg production of less than 1,000 eggs per day, in the same bird hosts. All metacercariae from all sources appeared morphologically identical at the light microscope level. However, the body length and width dimensions were significantly different between all 3 types of metacercariae. Using the available taxonomic keys, all three sources of metacercariae were identified as D. spathaceum. The life cycle of D. spathaceum was established in the laboratory using metacercariae from Culter Dam sticklebacks and maintained throughout the study. Parasite egg production was recorded from herring gulls four days after infection with metacercariae from Culter Dam sticklebacks. Egg output rose to a maximum of 10,200 eggs per day after 4 weeks and then oscillated between 6,000 and 10,000 eggs per day for the. following 9 weeks, after which egg production rapidly declined. 7. Mlracidia hatched from eggs incubated at 29°C after 8-11 days. 8. At 14°C, 29% of the Initial number o£ cercarlae successfully established In the lenses of exposed rainbow trout. 9. An epidemiological survey of D. spathaceum was carried out on rainbow trout from Mill of Cantray trout farm and 3-spined sticklebacks from Culter Dam over a 30-month period. 10. The Infection period at both sites was normally between May and September each year. Transmission from snail to fish did not occur when temperatures were below 10°C. 11. A low snail prevalence of patent infections in L. pereger (0-8%) was recorded at both sites throughout the survey. 12. During the summer of 1982 rainbow trout in Raceway 3 at Mill of Cantray became infected with 139 (humour) and 70 (lens) metacercariae. In April, 1983 the raceway was cleaned and the entire length treated with copper sulphate. This resulted in a 60% reduction in the numbers of metacercariae infecting trout during the following summer. 13. The prevalence of D. spathaceum metacercariae declined from 100% to less than 20% and the abundance of metacercariae per fish declined from 12 to less than 2 during the survey period. Despite this decline a pattern of seasonal variation in prevalence and abundance was observed in both 1982 and 1983. 14. Significant correlation coefficients between abundance of metacercariae per fish and length and weight of sticklebacks Indicated that abundance Increases with size. 15. Within the confines of a flume In which temperature, flow rate, and cercarlal concentration could be manipulated Independently, It was shown that (a) It is possible to control the Infection rate of fingerllng rainbow trout by the manipulation of flow rate. (b) the relationship between mean abundance of metacercariae per fish and cercarlal concentration is linear, with a regression coefficient of 0.99 (c) that no metacercariae are found in rainbow trout infected and maintained below 10°C. (d) that it is migration of cercariae which is inhibited at low temperature and not penetration or attachment. 16. Rainbow and brown trout do not produce circulating antibody at detectable titres in response to infection with D. spathaceum cercariae. 17. A significant difference occurred in the rate of infection of rainbow trout given weekly infections of D. spathaceum cercariae in winter and summer. 18. Rainbow trout injected with a suspension of dead cercariae acquired significantly fewer metacercariae when exposed to a challenge infection.

The properties of the cell surface of Aeromonas salmonicida were studied, with particular emphasis on the additional surface layer (A-layer), found on virulent strains. This was identified by electron microscopy, as having a tetragonal subunit morphology; and by + ejectrophoresis of membrane components as a 51 kdal protein on A strains. A and A- strains, (the latter isolated by growth at elevated temperature), were compared biochemically and their interactions with various cell types investigated. Strains of A. salmonicida possessing the A-layer were shown to be more hydrophobic than those devoid of this protein. The influence of culture age, medium composition and subculture on hydrophobicity were investigated and hydrophobicity related to culture characteristics. No difference in enzyme susceptibility between A+ and A A. salmonicida was found and both phenotypes showed similar tolerance to other environmental conditions. The interactions between A. salmonicida and cells in vitro were studied using adhesion and association assays involving rädiolabelled bacteria, viable count determinations, haemagglutination and chemiluminescence. A+ A. salmonicida were found to adhere to a greater extent than A bacteria to tissue culture cell lines and to isolated fish tissue by non-specific hydrophobic interactions. Adhesion was maximal to a fish epithelial cell line and the effects of various environmental conditions on adhesion were determined. A- bacteria more commonly exhibited haemagglutination which was inhibited by specific sugars. A+ bacteria associated more than A- organisms with mouse peritoneal macrophages and rainbow trout phagocytes when in salts solutions. The effects of opsonization on A. salmonicida were strain dependent. Incubation in a variety of sera resulted in a decrease in A+ surface hydrophobicity, often accompanied by abolition of characteristic autoagglutinability, whereas opsonized A cells became more hydrophobic. These properties directly influenced the chemiluminescence response of trout macrophages.

The in vitro and in situ immune responses of carp Cyprinus carpio to Ichthyophthirius multifiliis were investigated in order to characterise the immune mechanisms involved in protection. 'O' group and adult carp were immunised against I.multifiliis by controlled infection procedures. Sterile immunity was not achieved; theronts were observed to penetrate the skin of immunised fish, although in the majority of cases this did not lead to successful trophozoite establishment. It was concluded that most parasites prematurely exited the epidermis of immunised fish within two hours of penetration as an active survival strategy. Trophozoites remaining in immunised fish beyond two hours post infection were able to complete normal development. Serum from carp immunised against I.multifiliis displayed specific in vitro theront immobilising activity, and antibody was detected against parasite ciliary membranes and mucocyst organellae; similar activity was not detected in cutaneous mucus. Significant amounts of antibody could not be located at the immediate host/parasite interface of trophozoites in situ in immune skin; prevention of antibody binding may be achieved by means of a mucocyst-derived "sheath" around the parasite and the formation of a layer of necrotic host tissue debris. Parasite development in immunised fish initiated a localised cellular infiltration, predominated by type III granulocytes ("basophils'') and mast cell-like cells, the activity of which may augment further cellular and humoral infiltration. Sites of premature parasite exit from the epidermis of immunised fish were infiltrated by actively phagocytic cells, predominantly macrophages, probably in response to localised antibody/antigen complex deposition. Pronephric leucocytes of immunised fish displayed greater in vitro non-specific phagocytic activity than cells of carp naive to I.multifiliis; the relevance of this to enhanced antigen uptake in immunised fish is discussed. Based on results of the present study, a model for the Mode of protection in fish against I.multifiliis is proposed.

No description available.