Production of gill-associated and serum antibody byrainbow trout (Oncorhynchus mykiss) following immersion immunization with acetone-killed Flavobacterium branchiophilum and the relationship to protection from experimental challenge

Rainbow trout (Oncorhynchus mykiss) were either injected twice intraperitoneallywith 0·1 ml of a 1:10 dilution from broth culture of acetone-killed Flavobacterium branchiophilum, immersed twice for 1 h in a 1:10, 1:100 or 1:1000 dilution from a broth culture of acetone-killed F. branchiophilum, or utilized as unexposed controls. After bath challenge with live F. branchiophilum the percent cumulative mortality of each group was as follows; controls, 45·3%, i.p.-injected, 32·1%; immersed in a 1:1000 dilution, 38·0%; immersion in a 1:100, 40·9%; and immersion in a 1:10 dilution, 11·7%. There was no relationship between the group treatment and the levels of F. branchiophilum antigen detected by enzyme immunoassay following challenge. The amount of gill-associated and serum antibody to F. branchiophilum detected in the immersion immunized groups before challenge increased with increasing concentration of antigen in the bath. The i.p.-injected group had the highest serum antibody of any group while gill-associated antibody was comparable to those groups which were bath exposed with the lower concentrations of F. branchiophilum antigen. A similar pattern of antibody response was seen with five groups of rainbow trout treated exactly as the five groups above, in which antibody was monitored over time, but which were not challenged. Gill-associated and serum antibody were detectable following primary antigen exposure regardless of route. The serum and gill-associated antibody responses were present for longer following a second antigen exposure by the same route, and the gill-associated but not the serum antibody was increased over those following primary antigen exposure. The greatest increase in gill-associated antibody response was seen in the group which received the highest concentration of antigen via bath.