A composition comprising one or more purified polypeptides consisting of: (a) an amino acid sequence set forth as SEQ ID NO: 8, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (b) an amino acid sequence set forth as SEQ ID NO: 1, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (c) an amino acid sequence set forth as SEQ ID NO: 4, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (d) an amino acid sequence set forth as SEQ ID NO: 5, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (e) an amino acid sequence set forth as SEQ ID NO: 6, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (f) an amino acid sequence set forth as SEQ ID NO: 7, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (g) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NO: 8, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (h) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NO: 19, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (i) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NOs: 1, 4, 5, 6, 7, 9, 12, 15, 16, 17, 18, or 20, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (j) a polypeptide having at least about 94% identity to SEQ ID NOs: 1, 4, 5, 6, 7, 9, 15, 16, 17, 18, or 20, which comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; or (k) a polypeptide having at least about 94% identity to SEQ ID NOs: 8 or 19, which comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys. (a) an amino acid sequence set forth as SEQ ID NO: 8, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (b) an amino acid sequence set forth as SEQ ID NO: 1, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (c) an amino acid sequence set forth as SEQ ID NO: 4, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (d) an amino acid sequence set forth as SEQ ID NO: 5, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (e) an amino acid sequence set forth as SEQ ID NO: 6, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (f) an amino acid sequence set forth as SEQ ID NO: 7, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (g) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NO: 8, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (h) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NO: 19, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys; (i) at least 17 contiguous amino acids of an amino acid sequence set forth as SEQ ID NOs: 1, 4, 5, 6, 7, 9, 12, 15, 16, 17, 18, or 20, wherein the polypeptides comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; (j) a polypeptide having at least about 94% identity to SEQ ID NOs: 1, 4, 5, 6, 7, 9, 15, 16, 17, 18, or 20, which comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma phagocytophilum; or (k) a polypeptide having at least about 94% identity to SEQ ID NOs: 8 or 19, which comprise an antigenic determinant that is recognized by antibodies specific for Anaplasma platys.