Seed preparation and field efficacy of palm oil adjuvanted feed-based Streptococcosis vaccine [EMBARGOED]

Abstract

Streptococcosis poses a considerable threat to tilapia farming, necessitating the urgent development of an effective vaccine. Previously, the potential usage of palm oil as a vaccine adjuvant in the feed-based streptococcosis vaccine was investigated with promising result following laboratory trials. The present study further investigated the vaccine’s efficacy using a similar formulation under field conditions. Prior to the vaccine preparation, the S. agalactiae growth curve was developed to understand its growth dynamic and obtain the optimal numbers of bacterial isolates. After that, the proximate composition of the feed-based vaccine was determined to ensure the nutrients in the prepared vaccine is comparable with the commercial feed. The findings indicated that twelve hours incubation of S. agalactiae at 30? reaches the peak of the exponential phase, and the nutrient composition (moisture, ash, crude fiber, crude fat, and crude protein) of the vaccinated pellet was within the recommended nutritional value. Meanwhile, six thousand healthy red hybrid tilapias with an average bodyweight of 40 ± 20g were equitably distributed into six cages at Kenyir Lake, Terengganu. The selected red hybrid tilapias were screened for S. agalactiae and ensured to be free from those infections before being transferred to the field trial site. Fish from Group 1 in cages 1 and 2 were not vaccinated, serving as a negative control group. Group 2 in cages 3 and 4 were vaccinated twice on week 0 and week 2 (single booster), while Group 3 in cages 5 and 6 were vaccinated thrice on week 0, week 2, and week 6 (double booster). The palm oil adjuvanted bacterin vaccine was administered orally at an average of 5% body weight. Blood serum and organ samples of the eye, brain, and kidney were collected at two weeks intervals until the conclusion of the 16-week trial period. The collected samples were then subjected to bacterial isolation and immunoglobulin M (IgM), lysozyme, and complement-3 quantification. Following vaccination and the first booster, there was a significant (p<0.05) increase in IgM levels in all vaccinated groups from week 2 that peaked at week 4 before gradually declining. However, an additional booster given at week 6 was significantly (p<0.05) increased the IgM levels and remained highest until the end of the field trial. Similarly, an increasing lysozyme and complement-3 activity were observed following each administration, especially in the double booster group. The average isolation rate of S. agalactiae was 19.6 ± 20.2%, 11.5 ± 11.3%, and 8.2 ± 9.4% from unvaccinated, single, and double booster groups, respectively. At the end of the study period, the mortality rate was 23.6 ± 1.8% for unvaccinated, 14.5 ± 5.9% for single booster, and 6.4 ± 0.4% for double booster groups. These results indicate that oral administration of palm oil adjuvanted vaccine stimulates both specific and non-specific immune responses, reduces the incidence of streptococcosis to 11.4%, and improves the survival rate to 93.6%.