Background/Objectives: Cryptosporidium parvum is a zoonotic enteroparasite causing severe diarrhea in newborn calves, leading to significant economic losses in dairy and beef farming. This study aimed to evaluate whether C. parvum p23-specific IgY antibodies could control neonatal calf diarrhea caused by C. parvum. Methods: A recombinant immunogen comprising the p23 protein fused to the antigen-presenting cell homing (APCH) molecule was expressed using the baculovirus system. Hens were immunized with the APCH-p23 immunogen, and the resulting IgY was spray-dried for treatment use. Eight newborn calves were included in the study and received commercial colostrum within the first 12 h of life. Four calves were treated with 20 g of powdered egg containing IgY (p23-specific IgY titer of 256 in milk) twice daily for 7 days. The remaining four calves received regular non-supplemented milk. All calves were orally infected with 6 million oocysts and monitored for 21 days. Results: Calves treated with p23-specific IgY exhibited significantly reduced diarrhea duration (3.5 vs. 7.5 days; p = 0.0397) and oocyst shedding duration (6.50 vs. 12 days; p = 0.0089). In addition, the total number of excreted oocysts, as measured by the change of the area under the curve (AUC), was significantly reduced in the treated group (14.25 vs. 33.45; p = 0.0117). Although the onset of diarrhea was delayed (3.5 to 6.5 days post-infection; p = 0.1840), and diarrhea severity was reduced (24.25 to 17 AUC; p = 0.1236), both parameters were not statistically significant. Conclusions: P23-specific IgY antibodies effectively reduced the C. parvum-induced duration of diarrhea in experimentally infected calves. These findings highlight the potential of this passive treatment as a promising strategy for controlling neonatal calf diarrhea.

Sarcocystis aucheniae is a coccidian parasite that produces macroscopic sarcocysts in South American camelid (SAC) muscles and causes a disease known as SAC sarcocystosis. This parasitosis hampers the commercialization of llama and alpaca meat, a vital economic activity in the Andean regions. No control or prevention methods are available, and diagnosis is based on postmortem visual inspection of carcasses. The aim of this study was to identify S. aucheniae B-cell epitopes suitable for the development of diagnostic methods for SAC sarcocystosis. To this end, sarcocyst immunoreactive protein bands were analyzed via mass spectrometry, and proteins in each band were identified in silico by searching in the parasite transcriptome. Five highly antigenic, hydrophilic B-cell epitopes, predicted not to cross-react with antibodies against other coccidia, were selected for future development of peptide-based serological tests. In addition, conserved domains present in the identified proteins allowed us to unravel metabolic pathways and mechanisms active in the parasitic stages present in sarcocysts, including aerobic respiration, antioxidant activity, signal transduction, protein synthesis and processing, and host–pathogen interactions. This study provides novel information on the biology of S. aucheniae, as well as new protein sequences that can be used for the development of diagnostic tests and chemotherapeutic approaches for SAC sarcocystosis.