An isolated or synthetic nucleic acid from having a sequence selected from the group consisting of SEQ ID NOs: 9, 13 and 1-8, or a homologues thereof, an isolated or synthetic nucleic acid complementary to a sequence selected from the group consisting of SEQ ID NOs: 1-14, or a homologues thereof, an isolated polypeptide encoded by the nucleic acid, or an antibody that binds thereto, may be used as an immunogenic composition or diagnostic, or in a method of treatment or prevention, in respect of a microsporidian infection, in particular a Tetramicra brevifilum infection in animals, particularly fish. Additionally, methods of testing an agent for the prevention and/or treatment of a microsporidian infection, preferably a Tetramicra brevifilum infection, and kits for screening for a microsporidian infection, are provided.
