** AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** HISTOMONOSIS IS A DEVASTATING DISEASE CAUSED BY THE PROTOZOAN,HISTOMONAS MELEAGRIDIS.MORTALITY MAY EXCEED 80% IN AFFECTED COMMERCIAL POULTRY FLOCKS. CURRENTLY, THERE ARE NO APPROVED INTERVENTIONAL OR CONTROL STRATEGIES TO MITIGATEH. MELEAGRIDISINFECTIONS IN POULTRY. THE OVERALL GOAL OF THIS PROJECT IS TO INVESTIGATE TARGETINGH. MELEAGRIDISVIRULENCE-ASSOCIATED PEPTIDASES AND RELEVANT INHIBITORS AS NOVEL METHODS TO CONTROL HISTOMONOSIS IN POULTRY. IN OBJECTIVE 1, WE WILL ASSESS THE EXPRESSION PATTERN AND FUNCTIONS OF PUTATIVE VIRULENCE-ASSOCIATED CYSTEINE PROTEASES IN VIRULENT AND HIGHLY PASSAGED, OR ATTENUATEDH. MELEAGRIDISSTRAINS. IN OBJECTIVE 2, THE ENZYMATIC PARAMETERS OF THESE CYSTEINE PROTEASES WILL BE CHARACTERIZED, AND POTENT INHIBITORS WILL BE IDENTIFIED THROUGH HIGH THROUGHPUT SCREENING. IN OBJECTIVE 3, WE WILL EVALUATE THE PROTECTIVE EFFICACY OF VACCINATION WITH RECOMBINANT CYSTEINE PROTEASES GENERATED IN OBJECTIVE 1 AND THE THERAPEUTIC POTENTIAL OF TOP CANDIDATE INHIBITORS SELECTED IN OBJECTIVE 2 AGAINSTH. MELEAGRIDISCHALLENGE IN TURKEY POULTS. THE RESULTS OF THIS PROJECT WILL PROVIDE WORKFLOW AND INSIGHTS RELATED TO THE EXPRESSION OF VIRULENCE-RELATED PROTEASES IN PARASITE INFECTION AS WELL AS THE EFFICACY OF RECOMBINANT VACCINES OR INHIBITORS TARGETING THESE PROTEASES IN PARASITE VIRULENCE. THIS PROPOSED PROJECT COULD LEAD TO THE DEVELOPMENT OF NOVEL COMMERCIALLY APPLICABLE METHODS TO CONTROL HISTOMONOSIS IN COMMERCIAL POULTRY FLOCKS AND PROVIDE A BETTER UNDERSTANDING OF HISTOMONAD BIOLOGY.