Anaplasma phagocytophilum surface protein AipA and/or fragments thereof which comprise an invasin domain are used in compositions suitable to elicit an immune response to treat or prevent infections caused by tick-born bacteria of the Anaplasmatacaea family. AipA proteins and protein fragments or antibodies directed to AipA proteins and protein fragments are also used in diagnostic assays to detect exposure to and/or infection with Anaplasmatacaea. AipA and/or fragments thereof are also used for these purposes in combination with one or both of Asp14 and OmpA proteins and/or fragments thereof which comprise an invasin domain. Homologs of these proteins are also used in the compositions and assays.
