**Background:** The growing concern over antibiotic resistance in livestock production has driven the search for natural alternatives to antibiotic growth promoters. Frankincense (Boswellia serrata) resin oil (FO) contains numerous biologically active constituents including monoterpenes, sesquiterpenes, and diterpenes with reported antimicrobial, anti-inflammatory, and immunostimulatory properties. However, studies exploring FO supplementation on broiler performance, particularly its effects on intestinal histomorphology, fatty acid composition of meat, and immune status, were limited. This study aimed to determine the effect of dietary FO supplementation on growth performance, intestinal histomorphology, fatty acid profile of breast muscle, blood biochemical parameters, and immunoexpression of CD3 and CD20 in the spleen of broiler chickens.
**Methods:** Four hundred three-day-old male Ross 308 broiler chicks were randomly assigned into four treatment groups (100 chicks each; 10 replicates/group; 10 chicks/replicate) after a three-day adaptation period. The groups received a basal diet supplemented with FO at 0 (control, FO0), 200 (FO200), 400 (FO400), or 600 (FO600) mg/kg diet for 35 days. The feeding period was divided into starter (4th–10th day), grower (11th–23rd day), and finisher (24th–35th day) phases. Growth performance parameters (body weight, body weight gain, feed intake, feed conversion ratio) were measured at 10, 23, and 35 days. At the end of the experiment, samples were collected for carcass traits, fatty acid analysis of breast muscle (n=5/group), intestinal histomorphology (n=10/group), serum biochemical and immunological parameters (n=30/group), and immunohistochemical examination of CD3 and CD20 in spleen tissues (n=10/group). The bioactive compounds in FO were identified using gas chromatography-mass spectrometry (GC-MS). Data were analyzed using one-way ANOVA with polynomial orthogonal contrasts for linear and quadratic effects, and significance was set at p < 0.05.
**Key Results:** GC-MS analysis identified farnesol (12.42%), ç-elemene (12.42%), α-farnesene (12.42%), and phenolic compounds including phenol, bis(1,1-dimethylethyl) (7.15%) as major bioactive compounds in FO. FO supplementation significantly improved growth performance: during the starter period, BW and BWG increased quadratically (p=0.04) in the FO200 group; during the grower period, BW increased linearly (p=0.001) and quadratically (p=0.02), while BWG (p=0.001) and FCR (p=0.002) improved linearly; during the finisher and overall periods, FO supplementation decreased FCR (p<0.01) with no effect on total feed intake. The final body weight was highest in the FO200 group and lowest in the FO0 group (p<0.05). No significant effects were observed on dressing percentage or internal/immune organ weights. FO supplementation linearly increased (p<0.01) the percentages of α-linolenic acid (18:3 n-3), eicosapentaenoic acid (20:5 n-3), docosapentaenoic acid (22:5 n-3), docosahexaenoic acid (22:6 n-3), and arachidonic acid in breast muscle, with increased total n-3 (linear p<0.01, quadratic p=0.03) and n-6 (linear p<0.01, quadratic p=0.01) fatty acids and an increased n-3/n-6 ratio (linear p<0.01, quadratic p=0.04), highest at 600 mg/kg. Intestinal histomorphology showed improvements including increased duodenal and jejunal villus height, crypt depth, and muscular coat thickness. Serum biochemical analysis revealed linear increases (p<0.01) in total protein, growth hormone, T3, and T4 levels, with albumin increased in the FO600 group (p<0.01) and total globulins increased linearly (p=0.001) in FO400 and FO600 groups. Glucose and leptin levels were not altered (p>0.05). Immune parameters showed linear and quadratic increases (p≤0.01) in lysozyme, complement 3, and interleukin 10 levels across all FO treatments, with best results at FO600. Immunohistochemical analysis of spleen sections revealed increasing percentages of CD3-positive T-cells (1.39%, 5.34%, 15.9%, and 26.3% for FO0, FO200, FO400, and FO600, respectively) and CD20-positive B-cells (17.4%, 28.84%, 32.61%, and 43.25% for FO0, FO200, FO400, and FO600, respectively).
**Clinical Implications:** This study demonstrates that dietary supplementation of frankincense (Boswellia serrata) oil at up to 600 mg/kg can serve as an effective natural growth promoter and immune-stimulating feed additive in broiler chicken production. The improvements in growth performance are attributed to enhanced secretion of growth and thyroid hormones, improved intestinal histomorphology with increased absorptive surface area, and better nutrient utilization. The enrichment of breast muscle with n-3 and n-6 polyunsaturated fatty acids offers potential benefits for meat quality and consumer health. The immunomodulatory effects, evidenced by increased lysozyme, complement 3, interleukin 10, and upregulation of CD3 and CD20 expression in the spleen, suggest FO can enhance both innate and adaptive immunity in broilers. These findings are particularly relevant given the global challenge of antimicrobial resistance in animal production, as FO provides a natural alternative to antibiotic growth promoters. The study validates the prophylactic benefits of FO in commercial broiler production systems, though further research may be needed to optimize dosage and evaluate effects under field conditions with potential disease challenges.