Oral immunization of rainbow trout, Salmo gairdneri Richardson, against vibriosis with vaccines protected against digestive degradation

. In order to reduce digestive degradation of vaccines against vibriosis when administered orally to fish, two methods of protecting the antigens were tested. Lyophilized vaccine was either incorporated into a slow‐release pellet or coated by an acid‐resistant film. These vaccines were compared to both oral vaccination by an unprotected vaccine as well as to standard vaccination methods of immersion and injection. However, mortalities after an artificial challenge showed that the efficacy of the protected vaccines was lower than that of the unprotected vaccine given orally and the standard vaccination procedures. The reason for this is probably that the important antigens of vibriosis vaccines are lipopolysaccharides which are little affected by gastric digestion, and the coating of the vaccine or incorporation into slow‐release pellets resulted only in reduced absorption of the antigens.