Can passive immunization prevent disease outcome in gilthead sea bream exposed to Enteromyxum leei?

Abstract

Passive immunization is used in humans for treating or preventing some infectious diseases, but it also constitutes an emerging field of interest in aquaculture, particularly with the restrictions for antibiotic use. Intracoelomically-injected antibodies can be detected in fish sera within the first 8 h and their half-life ranges from 7 to 22 days post-injection, depending on the species. The fish models studied so far used fast-acting pathogens such as virus, bacteria or ciliate parasites. The current work aimed to determine if passive immunization could help to prevent enteromyxosis in gilthead sea bream (GSB, Sparus aurata). Enteromyxum leei is a myxozoan intestinal parasite that invades the paracellular space of the intestinal epithelium, producing a slow-progressing disease, leading to anorexia, cachexia and mortalities. We have previously demonstrated that GSB that survive E. leei infection become resistant upon re-exposure, and this resistance is directly related to the presence of high levels of specific serum antibodies. In the current study, we evaluated whether injection with sera from resistant animals would protect naïve fish when challenged by effluent exposure to the parasite. Serum from a pool of resistant (R) and naïve (N) animals (intact or heat inactivated, 10 ¿l/g BW) was intracoelomically injected 24 h prior to the E. leei-effluent challenge and at 9 days post-challenge (dpc). At 23 dpc, the different groups were allocated in separate tanks and the effluent exposure was terminated. A non-lethal parasite diagnosis was performed at 56 dpc. At the final sampling (100 dpc), blood, serum and tissues were collected for hematology, circulating antibodies, histological and molecular diagnosis and gene expression. Groups injected with R sera had lower prevalence and intensity of infection than those with N sera, both in the intermediate and final samplings. At 100 dpc, the prevalence of infection in the PBS and N groups was 70%, whereas in R group it only reached 55%. Condition factor (CF) and specific growth rate (SGR), key parameters affected by enteromyxosis, were higher in R group. There was a significant correlation between prevalence of infection and SGR and CF. Immunohistochemistry and gene expression studies will reveal whether this partial protection was due to higher presence of specific antibodies or specific cell populations. These results show that, even with this long term disease, passive immunization can confer some degree of protection. The administration of specific antibodies during exposure, probably provided fish with time to activate the specific defenses before the parasite proliferated.