# Dietary Combined Thyme Meal and Bacillus subtilis to Promote Growth Performance, Immune Function, Gene Expression, Antioxidant Defense, and Cecal Microbiota in Growing Rabbits Under Heat Stress Conditions

## TL;DR

A mix of thyme meal and B. subtilis improves growth and health in heat-stressed rabbits by boosting immunity, digestion, and gut health.

## Contribution

The novel combination of thyme meal and B. subtilis is shown to mitigate heat stress effects in rabbits through multiple physiological and genetic mechanisms.

## Key findings

- The CBT mixture increased body weight gain, carcass weight, and improved feed conversion in heat-stressed rabbits.
- The supplement enhanced immune response and antioxidant activity by increasing IgA, IgG, GPx, and SOD levels.
- The CBT mixture improved gut health by reducing pathogens and increasing gene expression related to nutrient absorption.

## Abstract

Heat stress is a major obstacle to the sustainability of the rabbit industry. Heat stress causes significant economic losses through its negative impacts on growth performance, intestinal microbiota, gut tissue, immune response, and meat quality, as well as its exposure to oxidative stress. There has been increased interest in nutraceutical supplements to protect rabbits from the harmful effects of heat stress. These nutraceutical supplements include probiotics, organic acids, antioxidants, enzymes, and medicinal plants, due to their diverse properties, which include disease prevention, anti-inflammatory, immunomodulatory, and biotic balance maintenance, and antioxidant activity, thus supporting productive performance. The objective of the current study is to investigate the effect of adding a B. subtilis and thyme meal mixture on reducing the impacts of heat stress on the physiological performance and growth of growing rabbits.

This study investigates the nutritional effects of a thyme meal and B. subtilis mixture on growth performance, immune function, antioxidant capacity, gene expression, and gut microbiota in heat-stressed rabbits. One hundred and twenty male New Zealand White rabbits were divided into four dietary treatments (five replicates/group). The rabbits in the first group were fed a basal diet, while the other three groups were fed a basal diet containing B. subtilis, thyme meal, and their mixture, respectively. The B. subtilis and thyme meal mixture increases the heat-stressed rabbits’ body weight gain and carcass weight, and enhances nutrient digestibility and the feed conversion ratio. Supplementing the CBT mixture improved the lipid profile and liver and kidney function via decreasing plasma triglycerides, cholesterol, LDL, creatinine, urea, and AST levels, while increasing total protein and albumin levels. Furthermore, the CBT mixture enhanced the immune response and oxidative stability by increasing IgA and IgG levels, GPx enzyme activity, and SOD, while decreasing plasma MDA content. Adding the CBT mixture enhanced gut health by reducing pathogens and inflammation, as well as increasing volatile fatty acid levels and the expression of CAT-1, MUC-2, and SGLT-1 genes. The combination of a thyme meal and B. subtilis enhanced growth, immune function, antioxidant capacity, gut microbiota modification, and the expression of gut health nutrient absorption-related genes in heat-stressed rabbits.