Immunization of Nile Tilapia (Oreochromis niloticus) Broodstock with Tilapia Lake Virus (TiLV) Inactivated Vaccines Elicits Protective Antibody and Passive Maternal Antibody Transfer

Abstract

Tilapia lake virus (TiLV), a major pathogen of farmed tilapia, is known to be vertically transmitted. Here, we hypothesize that Nile tilapia (Oreochromis niloticus) broodstock immunized with a TiLV inactivated vaccine can mount a protective antibody response and passively transfer maternal antibodies to their fertilized eggs and larvae. To test this hypothesis, three groups of tilapia broodstock, each containing four males and eight females, were immunized with either a heat-killed TiLV vaccine (HKV), a formalin-killed TiLV vaccine (FKV) (both administered at 3.6 × 10⁶ TCID₅₀ per fish), or with L15 medium. Booster vaccination with the same vaccines was given 3 weeks later, and mating took place 1 week thereafter. Broodstock blood sera, fertilized eggs and larvae were collected from 6-14 weeks post-primary vaccination for measurement of TiLV-specific antibody (anti-TiLV IgM) levels. In parallel, passive immunization using sera from the immunized female broodstock was administered to naïve tilapia juveniles to assess if antibodies induced in immunized broodstock were protective. The results showed that anti-TiLV IgM was produced in the majority of both male and female broodstock vaccinated with either the HKV or FKV and that these antibodies could be detected in the fertilized eggs and larvae from vaccinated broodstock. Higher levels of maternal antibody were observed in fertilized eggs from broodstock vaccinated with HKV than those vaccinated with FKV. Low levels of TiLV-IgM were detected in some of the 1-3 day old larvae but were undetectable in 7-14 day old larvae from the vaccinated broodstock, indicating a short persistence of TiLV-IgM in larvae. Moreover, passive immunization proved that antibodies elicited by TiLV vaccination were able to confer 85% to 90% protection against TiLV challenge in naïve juvenile tilapia. In conclusion, immunization of tilapia broodstock with TiLV vaccines could be a potential strategy for the prevention of TiLV in tilapia fertilized eggs and larvae, with HKV appearing to be more promising than FKV for maternal vaccination.

Keywords: antibody; inactivated vaccines; maternal passive immunity; tilapia broodstock.

Figure 1

Diagram illustrating the experimental design for broodstock TiLV vaccination, mating and sampling.

Figure 2

TiLV-specific IgM levels (OD₄₅₀) from 6 to 14 weeks post primary vaccination in (a) vaccinated male broodstock (diluted 1:1024, n = 1 per treatment weekly), (b) vaccinated female broodstock (diluted 1:1024, n = 1–2 per treatment weekly), (c) egg supernatant (diluted 1:8, n = 1–2 per treatment weekly) and (d) larval supernatant (diluted 1:2, n = 1 per treatment at different sampling time points). The OD₄₅₀ values were compared with significantly statistical cut-off values. HKV, FKV, and Control mean that the broodstock, eggs or larvae originate from the heat-killed vaccine group, the formalin-killed vaccine group and the control group, respectively.

Figure 3

Average percent survival of Nile tilapia juveniles passively immunized with pooled sera from female broodstock by intramuscular injection (IM) and then challenged with TiLV TH-2018 at 9 × 10⁵ TCID₅₀ per fish. The differences were statically significant between group 1, 2, 4 and group 3 (n = 20 per group, Log Rank test: p < 0.0001). HKV, FKV, and control mean broodstock fish were immunized with heat-killed vaccine, formalin-killed vaccine and L15 medium, respectively. The L15 group is negative control group treated with L15 medium without virus (n = 20).