Effects of dietary thyme essential oil on hemato-immunological indices, intestinal morphology, and microbiota of Nile tilapia

Abstract

Thyme (Thymus vulgaris) essential oil (TVEO) is a herbal medicine with one of the highest levels of antimicrobial activity. Although TVEO has been broadly used in poultry and swine production due to its immunostimulatory and growth-promoting characteristics, the effects of TVEO on fish are poorly characterized. In this study, Nile tilapia (Oreochromis niloticus L.) were fed 0, 0.1, 0.5, and 1% TVEO for 15 days. Subsequently, blood parameters, intestinal morphology, and the population of Bacillus bacteria in the intestine were evaluated. The numbers of lymphocytes (p < 0.05) and leukocytes (p < 0.05) significantly increased in the blood of the fish fed the highest dose of TVEO. Based on the normal behavior of the fish and the alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels, which were not altered (p > 0.05), this study concluded that the diets were safe and showed no negative or toxic effects. Even at doses as high as 1%, TVEO did not alter the population of beneficial Bacillus bacteria in the gut. In conclusion, supplementation with TVEO stimulated the cellular components of the non-specific immune response of Nile tilapia without causing deleterious effects or altering the population of important intestinal bacteria.