Background: Cryptosporidium parvum is a protozoan parasite causing severe diarrhea in neonatal calves, impacting dairy farm productivity. This study investigates the prevalence and management of this infection in dairy herds in Mashhad, Khorasan Razavi, Iran. Methods: From October 2023 to July 2025, fecal samples were collected from 70 diarrheic Holstein-Friesian calves (aged 5–21 days) across seven dairy farms in Mashhad. Calves were housed individually post-colostrum feeding. Samples, obtained rectally, were analyzed using modified Ziehl-Neelsen staining to detect Cryptosporidium oocysts under microscopy (×400–×1,000 magnification). Results: Cryptosporidium spp. were detected in 56 of 70 (80%) diarrheic calves, confirming its role as a major pathogen. Clinical signs included watery diarrhea, dehydration, lethargy, weight loss, and abdominal pain. All farms reported oocyst shedding, with higher prevalence in moist, crowded conditions. Environmental persistence of oocysts and inadequate sanitation were key risk factors. Control measures, including 3% hydrogen peroxide disinfection and isolation of sick calves, were effective in reducing transmission. No curative treatments exist, but supportive fluid therapy improved outcomes. Conclusion: Cryptosporidium is a significant cause of neonatal calf diarrhea in Mashhad, with an 80% prevalence. Robust sanitation, isolation, and water management are critical for control. The zoonotic potential underscores the need for worker protection. Further research on seasonal trends and prophylactic treatments is warranted to enhance dairy herd health in Iran.

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