The increasing global demand for medicinal leeches has emphasized the need for reliable production sources. Providing safe, high-quality blood, essential for growth and cocoon production, is a major challenge in leech rearing. In Iran, over 90% of breeders use slaughterhouse blood for feeding leeches. This study aimed to compare the effects of feeding slaughterhouse blood versus sterile (hygienic) blood from Holstein cattle on the growth and survival of the Iranian medicinal leech (Hirudo orientalis), rather than to compare the two blood types with each other. A total of 140 leeches were quarantined for four months, divided into two groups (slaughterhouse and sterile blood, 10 replicates each), and fed twice over a 30-day period. Blood was analyzed for biochemical and microbial parameters using an automated hematology analyzer at an accredited veterinary laboratory. Red blood cell count, hemoglobin, hematocrit, and total protein were higher in slaughterhouse blood, while platelet count, glucose, albumin, sodium, and potassium were higher in sterile blood (P<0.05). Slaughterhouse blood was heavily contaminated with extensive bacterial and fungal growth, whereas sterile blood remained uncontaminated after 72 hours. Leeches fed sterile blood showed higher final body weight (1.87 vs. 1.41 g), growth rate (117.44% vs. 58.42%), and survival (84.28% vs. 68.57%) than those fed slaughterhouse blood (P<0.05). Conclusion: Although fresh slaughterhouse blood may reduce costs, it raises the risk of microbial contamination and negatively affects growth and survival; continuous use can lower productivity and increase mortality on leech farms.