A method, system, and kit are provided for diagnosing L. intracellularis infection or exposure in a subject. The method includes purifying whole L. intracellularis from host cells and host debris produced in or on a suitable medium and adhering the purified whole L. intracellularis on a suitable material to form an antigen substrate for determining whether a subject produces L. intracellularis-specific antibodies against the antigen to thereby indicate L. intracellularis exposure or infection in the subject. The kit includes purified whole L. intracellularis produced from host cells and host debris adhered to a suitable material to form an antigen substrate adapted for screening serum from a subject. A method for diagnosing L. intracellularis exposure or infection in a subject includes acquiring a serum sample from a subject, introducing the serum sample to an antigen substrate followed by detecting a presence of L. intracellularis-specific antibodies in the serum.