Immunisation of tilapia broodstock as a strategy to prevent vertical transmission of tilapia lake virus (TiLV)
Aquaculture is the fastest growing food-production sector globally, with over 1 billion people relying on fish as their major protein source. Within low and middle income countries (LMIC), aquaculture has an important role in improving food security, as well as the economic and social development of the country. Tilapia (particularly Oreochromis spp.) is the world’s second most predominant aquaculture species after carp. Reaching harvest size in just 6 months, it provides an important source of revenue for many low income families. Disease in tilapia culture is associated with intensification of the farming system, and both bacterial and viral diseases are severely impacting on expansion of tilapia farming. Tilapia fry are actively traded for aquaculture purposes, globally. There is a huge risk of widespread disease resulting from the movement of live fish without appropriate control measures in place e.g. tilapia lake virus (TiLV) in tilapia, white spot virus in shrimp, etc. Thus, production of specific pathogen-free, immunised fry is considered an ideal approach for disease control and preventing the spread of disease worldwide. Maternal antibody transfer from female broodstock to offspring has been confirmed to occur in several finfish species, and has been shown to protect early embryos and fry from infection. However, little is known about maternal antibody transfer in tilapia. TiLV has recently emerged as a new viral disease problem in tilapia culture, and its rapid spread has raised global concern for the tilapia aquaculture industry. Vaccinating broodstock maybe a novel disease control strategy in tilapia, but it requires vaccinated tilapia broodstock to be able to pass specific antibodies to their embryos, and for these antibodies to prevent vertical transmission. We intend to test these hypotheses in this project using TiLV as our model pathogen.
In this project, we have developed two injectable vaccines (heat-killed (HKV) and formalin-killed (FKV)) for the prevention of disease caused by TiLV. The efficacy of these vaccines was initially tested in juvenile tilapia. The results showed that both HKV and FKV vaccines induced systemic and mucosal IgM against TiLV, and conferred significant protection, with relative percentage survival of 71.3% and 79.6%, respectively. Up-regulation in the expression of IgM, IgD and IgT and CD4 (genes encoding proteins involved in humoral immunity) and CD8 (cell-mediated immunity) following immunization with HKV and FKV suggests that the vaccines are able to activate both arms of the specific immune response in Nile tilapia. Protection from these vaccines is, therefore, likely to result from a synergistic effect of humoral (B-cell) and cellular immune (T-cell) responses. We then tested these vaccines in tilapia broodstock using the same vaccination protocol. Three groups of broodstock, each containing 4 males and 8 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, sera from the immunized female broodstock were administered to naïve tilapia juveniles to assess if antibodies induced in immunized broodstock were protective (passive immunity). 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 anti-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 short persistence of 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, this study showed that both HKV and FKV are promising injectable vaccines for the prevention of disease caused by TiLV in Nile tilapia. Immunization of tilapia broodstock with these 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. However, protective antibodies had a short persistence in the larvae leaving a gap between maternal immunity and immunocompetence. Further vaccination is therefore likely to be needed to protect fish from TiLV infection during this gap as well as later stages of development. A number of monoclonal antibodies have been prepared against tilapia IgT, the efficacy of which are still being evaluated.