Comparison of passive and active immunisation of fish against microbial diseases with consideration of the mechanisms involved

Passive immunisation of fish was carried out to determine whether anti-Vibrio anguillarum antibodies (AVA), anti-Streptococcus sp. antibodies (ASA), and antiParamoeba sp. antibodies (APA) raised in sheep, rabbits and rainbow trout (Oncorhynchus mykiss) were persistent and biologically active in rainbow trout. Results of passive immunisation were compared with results of active immunisation in which fish were immunised by immersion in or injection with formalin-killed cells. Assessments of passive and active immunisation were undertaken concurrently for up to three months in order to demonstrate their relative efficacies and, especially, to evaluate the practical potential of passive immunisation. Sheep and rabbit antibodies were detected in fish sera by enzyme-linked immunosorbent assay (ELISA) for up to 70 days after intraperitoneal (i.p.) injection. The relative percent survival (RPS) of fish challenged with virulent V. anguillarum after an i.p. injection (0.1 ml 100 g-1 fish body weight) of sheep, rabbit or fish AVA was 93.3, 86.6, 40% at one month; 25, 18.7, 12.5% at two months and 13.3, 4.1, 9.0% at three months post-immunisation (p.i.) respectively. In order to obtain equivalent protection to undiluted fish serum, rabbit and sheep sera had to be diluted 1: 8 and 1: 50 respectively. Protection conferred by immune sera was shown to be due to the specific antibodies alone. Also inactivation of complement from sera had no effect on the potency of either immune or non-immune sera. Affinity purified sheep and rabbit AVA sera were demonstrated to have equivalent protective potential as whole antisera in rainbow trout. Fish actively immunised by immersion showed RPS of 86.6, 93, and 81.8% after one, two and three months p.i. respectively in the same trials. Fish immune responses to sheep and rabbit antisera were determined (using specific ELISA) with the greater response being to the rabbit serum. Rainbow trout given sheep AVA conjugated to LTB (the GM-1-binding subunit of Escherichia coli heat-labile toxin) orally had a RPS of 37.5% at 15 days and a RPS of 27% after one month post-immunisation. In contrast, sheep AVA conjugated to TraT (an internal membrane of E. coli) and Quil-A had RPSs of 18.7 and 6.2% after fifteen days and 13.3 and 0% after one month p.i. respectively. The relative percent survival of rainbow trout challenged with virulent Streptococcus sp. after an i.p. injection (0.1 ml 100 g-1 fish body weight) of sheep, rabbit or fish anti-Streptococcus sp. antibodies was 88.8, 50, 0.0% after one month ; 33.3, 6.8, 6.8% after two months and 13.3, 0, 6.6% after three months p.i. respectively. Fish immunised actively had RPS of 88.8 and 11.1% after one month, 38.1 and 4.7% after two months 36 and 0.0% after three months p.i. for the injection and immersion routes respectively. Thus, passive immunisation shows potential as a therapeutic and prophylactic against streptococcosis as it gives similar results to active intraperitoneal vaccination and would be expected to provide immediate protection. In one trial Atlantic salmon immunised i.p. with Paramoeba vaccines showed marked humoral responses. In another trial, Atlantic salmon were immunised passively with an i.p. injection (0.1 ml 100 g-1 fish body weight) of sheep APA. Immunised fish (both actively and passively) were exposed to a natural infection (by cohabitation with infected Atlantic salmon) one month post-immunisation. Transmission of the disease was successful. No unequivocal protection was demonstrated in any of the immunised fish in this experimental challenge trial. Moreover, local antibody against Paramoeba sp. in gill mucus of experimentally infected Atlantic salmon was not detected (by ELISA). It is shown in this study that passive immunisation has significant potential in disease prevention when given to fish at strategic times, especially in the face of an outbreak when there is not enough time for an active immune response. Thus, there should be a future for the use of mammalian hyperimmune sera against fish diseases, in particular, those for which useful immunity by active immunisation is not available.