Immune response of Salmo salar (exotic fish) and Eleginops maclovinus (native fish) during Francisella noatunensis horizontal transference

Bacterial transmission between cultured and native fish is potentially of great epidemiological significance. We evaluated horizontal transmission of Francisella noatunensis subsp. noatunensis from Salmo salar (Ss) to Eleginops maclovinus (Em) and vice versa by conducting cohabitation experiments. We detected F. noatunensis DNA in infected fish "Trojans" (fish injected with F. noatunensis) and cohabitants (non-injected fish in the same tank as trojans) of both species from day 1. Additionally, we detected bacterial replication from day 1 post injection (dpi) in trojan E. maclovinus and day 14 in the other experimental conditions. We also found that hematocrit values in trojan fish were significantly decreased; however, we did not detect any corresponding decrease in cohabitating fish. Furthermore, we found that transcription of genes involved in the innate and acquired immune response in trojan fish and cohabitants, such as TRL1, TRL5, TRL8, MyD88, IL-1 beta, IL, MHCI and MHCII, especially at 1-3 dpi, and at 7, 14, 21 and 28 dpi in several tissues, with a tissue-dependent response. Finally, we observed an increase in serum IgM Anti-F. noatunensis antibody levels from 14 dpi in trojan fish but not in cohabitating fish. Our results demonstrate that F. noatunensis can be transmitted horizontally from S. salar to E. maclovinus and vice versa.