Effect of dietary essential oils of clove basil and ginger on Nile tilapia (Oreochromis niloticus) following challenge with Streptococcus agalactiae

Nile tilapia is the most farmed freshwater fish, however disease outbreaks are the main cause of losses in the production. Active compounds from plants are an alternative to synthetic chemicals to enhance growth and disease resistance. This study aimed to verify the effects of dietary supplementation with essential oils of clove basil (Ocimum gratissimum) and ginger (Zingiber officinale) on tilapia, regarding growth, hemato-immunological parameters and survival to challenge with S. agalactiae. Juveniles fed for 55 days with supplemented diets (0.5%, 1.0% and 1.5%) were challenged by gavage with S. agalactiae and monitored for 10 days. At 35 and 55 days of supplementation and 10 days after infection, blood was collected for hemato-immunological assays. For growth parameters, fish were weighed every 10 days. Clove basil at 0.5% significantly improved (p < 0.05) feed conversion ratio (1.34 +/- 0.13 in control, with final weight of 19.25 +/- 238). At 55 days, neutrophils were higher in fish fed ginger 1.0% (p < 0.05) compared to ginger 0.5% and 1.5%. Ten days after infection, there were significant differences in thrombocytes, total leukocytes, lymphocytes and neutrophils. Except for 1.5% ginger, all supplemented diets showed increased phagocytic activity (p < 0.05). After challenge, the only group with 100% relative survival was 0.5% ginger, while control had 2.86% mortality. Dietary supplementation with essential oils was useful for improving growth, immune responses and disease resistance. Statement of relevance: This study reports the use of natural compounds for prophylactic purposes as an alternative to chemical antibiotics, whose hazardous consequences regarding environmental and health questions have been reported. Streptococcus agalactiae is a highly pathogenic and potentially zoonotic bacteria affecting farmed tilapia. The tested compounds proved to enhance the growth and immune response of fish against S. agalactiae. (C) 2016 Elsevier B.V. All rights reserved.