• Yersinia ruckeri infection
• Onchorhynchus mykiss
• human-gamma-globulins
• Freund’s Complete Adjuvant

For the experimental design, 203 adult rainbow trout were randomly assessed to 6 groups of about 30 fish each and treated by intraperitoneal injection (i.p.) with various combinations of HGG,Freund's Complete adiuvant (FCA), Phosphate Buffered Saline (PBS), and formalin-inactivated Yersinia ruckeri (Yr) with and without the adjuvant. Pre-treated fish were challenged 40 days later with Yr using the LD50 approach. The occurrence of clinical signs and mortality were monitored up to 14 days and pathological findings were recorded. Fish were submitted to bleeding procedure before and after bacterial challenge to evaluate serum immunological indicators. The study has been developed trough the following experimental approaches:
• characterization of specific and aspecific serum immunological parameters before and after the experimental challenge with Yr;
• investigation on the prevalence of pathology and lesion distribution in challenged rainbow trout.
The goal of this work was to highlight any differences among different treatment groups in susceptibility to the bacterium that may correlate with a preliminary stimulation of either non-specific or specific immune factors.

Furthermore, we perform a morphometric study to evaluate the morphometric modifications of kidney macrophage aggregates (MAs) in the treatment groups of Yr-infected rainbow trout. Morphometrical findings from infected fish have been compared with data from control fish. We assumed that kidney would be a suitable organ to perform the morphometric analyses and we hypothyzed that the quantification of the area occupied by MAs pigments, would be an accurate means of evaluating MAs in trout organs. For this purpose we evaluated the area occupied by MAs pigments and MAs number using a computerizing image analysis.
Summarising the aims of this part of the study were: (1) to develop an objective and quantitatively rigorous method for evaluating total MAs area and MAs number in rainbow trout; (2) to determine if any difference exists in MAs area and number that may correlate with a preliminary specific and aspecific stimulation of the immune system and with the health status in different groups of Yr-infected trout.