Effects of commercial herbal oil mixture on some hematological, biochemical and immunological parameters of rainbow trout (Oncorhynchus mykiss) and its preventive efficacy against Yersinia ruckeri infection

This study was conducted to evaluate the effects of different levels of a commercial herbal oil mixture (Mix-oil (R)) on the hematological, biochemical and immunological responses of rainbow trout as well as the resistance against infection with Yersinia ruckeri. The Mix-oil (R) includes, Thymus vulgaris, Origanum vulgare and Eucalyptus sp. essential oils. Experimental diets were supplemented with Mix-oil (R) at 0 (control group), 50, 200 and 400 ppm. Fish with an initial weight of 31.0 +/- 0.1 g were divided into four groups and reared for 56 days. The anti-bactericidal activity of Mix-oil (R) revealed that the minimum inhibitory concentration (MIC) value for Streptococcus iniae, Aeromonas hydrophila and Lactococcus garviae was 6.25 mu l ml(-1) while it was 12.5 mu l ml(-1) for Y. ruckeri. Significant improvements in red blood cell count, hematocrit, immunoglobulin, lysozyme, total protein, complement component C3 and C4, and alternative complement pathway (ACH(50)) in fish fed with 400 ppm Mix-oil (R) were observed compared with the control group (p<0.05). No significant difference was observed between different treatments in terms of MCV, MCH, MCHC levels and hemoglobin percentage aspartate aminotransferase (AST) and alanine aminotransferase (ALT) levels (p>0.05). The results suggested that dietary administration of Mix-oil (R) at the level of 400 ppm could enhance hematological parameters and immune function of rainbow trout as well as the resistance against Y. rukeri. Thus, using Mix-oil (R) as an immunostimulant is recommended for farmed rainbow trout.