Cryptosporidium parvum is a parasitic pathogen that causes gastroenteritis in cattle and humans. The host innate immune response to C. parvum infection is associated with expression of enteric antimicrobial peptides that may represent a useful prophylactic to prevent C. parvum infection. The aim of this study is to evaluate a model of C. parvum infection and to evaluate the prophylactic efficacy of a C. parvum-specific monoclonal antibody, 4H9, fused to the human cathelicidin LL-37 (4H9-G1-LL37) in C. parvum challenged calves. Holstein bull calves (n = 18) were randomly assigned to receive either 0 mg (control), 20 mg, or 100 mg of fusion protein 4H9-G1-LL37 orally upon birth. Approximately 36–44 h later, calves were challenged with 1 × 10⁷C. parvum oocysts. Fecal consistency, daily total fecal volumes, daily oocyst concentration (oocysts/mL), and total daily oocysts shed were assessed over a 10-d post-challenge period. Prior to experimental C. parvum challenge all calves tested negative for C. parvum and by d 3 post-challenge all calves were shedding C. parvum oocysts in feces and had developed diarrhea. The control calves exhibited features of a natural infection, with diarrhea occurring between 4 and 7d post-challenge. The daily oocyst concentration was determined by immunofluorescence microscopy. Enumeration of total daily oocysts shed, was calculated as the product of the oocyst concentration and the total fecal volume collected for the corresponding day. The daily oocyst concentration was highly correlated with total daily oocysts shed for d 2–10 in the control calves, validating both quantification methods as robust measures of shedding even during peak oocyst production. Prophylactic treatment (20 mg) of calves before challenge tended to reduce total oocyte shedding on d 6 compared with control calves (P = 0.085). These results validated the oocyst quantification technique and demonstrate the potential efficacy of the prophylactic fusion protein 4H9-G1-LL37 in reducing C. parvum shedding. These results also suggest that additional studies with a larger sample size are warranted.