Successive Oral Immunizations Against Piscirickettsia Salmonis and Infectious Salmon Anemia Virus are Required to Maintain a Long-Term Protection in Farmed Salmonids

Abstract

Currently, there is a growing demand to determine the protective status of vaccinated fish in order to prevent diseases outbreaks. A set of different parameters that include the infectious and immunological status of vaccinated salmonids from 622 Chilean farms were analyzed during 2011–2014. The aim of this study was to optimize the vaccination program of these centers through the determination of the protective state of vaccinated fish using oral immunizations. This state was determined from the association of the concentration of the immunoglobulin M (IgM) in the serum and the mortality rate of vaccinated fish. Salmonids were vaccinated with different commercial mono- or polyvalent vaccines against salmonid rickettsial septicemia (SRS) and infectious salmon anemia (ISA), first by the intraperitoneal injection of oil-adjuvanted antigens and then by the stimulation of mucosal immunity using oral vaccines as a booster vaccination. The results showed that high levels of specific IgM antibodies were observed after injectable vaccination, reaching a maximum concentration at 600–800 degree-days. Similar levels of antibodies were observed when oral immunizations were administrated. The high concentration of antibodies [above 2750 ng/mL for ISA virus (ISAv) and 3500 ng/mL for SRS] was maintained for a period of 800 degree-days after each vaccination procedure. In this regard, oral immunizations maintained a long-term high concentration of anti-SRS and anti-ISAv specific IgM antibodies. When the concentration of antibodies decreased below 2000 pg/mL, a window of susceptibility to SRS infection was observed in the farm, suggesting a close association between antibody levels and fish protective status. These results demonstrated that, in the field, several oral immunizations are essential to uphold a high level of specific anti-pathogens antibodies and, therefore, the protective status during the whole productive cycle.

Immunization Strategies against Piscirickettsia salmonis Infections: Review of Vaccination Approaches and Modalities and Their Associated Immune Response Profiles

“…The underlying mechanisms were not studied and are not known in detail but could be due to pathogen exposure and antibody consumption, as has been seen in IPN-vaccinated salmon (57). In line with such a thinking, the authors showed that declining levels of anti- P. salmonis antibodies coincide with increase in SRS-related mortality (34) indicating that pathogen exposure at least partly explained antibody consumption. Further, oral revaccination increased the circulating antibodies to levels equivalent to or higher than what was achieved from the primary injection vaccine (34), and even more pronounced when a second oral boost was administered (Figure 5).…”

“…However, reduced sensitivity and increased resistance to penicillin, streptomycin, oxolinic acid, and oxytetracycline have been reported (12). There are commercial vaccines for intraperitoneal injection available, but these have shown variable results (34). Vaccinated fish come down with SRS toward the end of the production cycle which causes severe economic losses, i.e., death of fish close to harvest.…”

“…Vaccinated fish come down with SRS toward the end of the production cycle which causes severe economic losses, i.e., death of fish close to harvest. In Chile, oral vaccines are used as a boost immunization the primary injection vaccination when the antibody titers decline (34). The different types of vaccines tested and those available commercially and their efficacy profiles are discussed below.…”

“…The fact that protection against disease is seen at early stages after sea transfer/early stage post-vaccination could favor an interpretation of spread through the extracellular space. Further to this, there are good indications of antibody consumption over time, i.e., decline in circulating levels of antibodies as a result of pathogen exposure/infection (34, 47). The approach has been to boost the primary response by oral antigen delivery with the purpose to raise the level of circulating antibodies.…”

“…Current vaccination strategies based on non-replicating vaccines will elicit a humoral immune response and to a lesser extent cell-mediated immunity. The kinetics of the antibody response is sparsely studied, but recently, Tobar et al (34, 56) showed that immunized fish had increased antibody levels up to 800 degree days post-vaccination after which they started to decline (Figure 5). There is a successive decline in antibody levels after they have peaked and they reach pre-vaccination levels by 1800–1900 degree days post-vaccination.…”