Immunization of Atlantic salmon against the salmon louse: identification of antigens and effects on louse fecundity

Injection of extracts derived from adult caligid copepods induced a partial immunity to Lepeophtheirus salmonis in Atlantic salmon. Antigens were derived from the supernatant of adult lice extracts and were partially purified by Con A affinity chromatography. The antigens were also present in the pellet derived from adult lice extracts, Immunohistochemistry showed that antibodies present in the serum of rats immunized with the Con A purified extract bound predominantly to the gut of L. salmonis. Components of apparent sizes > 205 000, 165 000, 133 000, 130 000, 125 000, 114 000, 110 000, 96 000, 82 000, 78 000, 65 000, 46 000, 35 000, 31 000 and < 29 000 were present in the extract. Although the numbers of attached lice were initially higher on those fish in the immunized group, throughout the course of the challenge experiment the total numbers of lice did not vary greatly between the immunized or control groups of fish. Initially, an overall average of about 17 attached stages were recorded from each fish sampled and by the end of the experiment this figure had fallen to about five adult lice per fish. However, compared with control fish fewer gravid female lice were present on immunized fish and furthermore these lice possessed fewer eggs (P<0.01). No major differences in egg hatching success were recorded. Further exploitation of this would require the isolation and purification of the antigens responsible for the observed effects. The possibilities for the development of a salmon louse vaccine are promising.