Previous studies revealed that the heart suffers significant injury during experimental Lyme and relapsing fever borreliosis when the immune response is impaired (D. Cadavid, Y. Bai, E. Hodzic, K. Narayan, S. W. Barthold, and A. R. Pachner, Lab. Investig. 84:1439-1450, 2004; D. Cadavid, T. O'Neill, H. Schaefer, and A. R. Pachner, Lab. Investig. 80:1043-1054, 2000; and D. Cadavid, D. D. Thomas, R. Crawley, and A. G. Barbour, J. Exp. Med. 179:631-642, 1994). To investigate cardiac injury in borrelia carditis, we used antibody-deficient mice persistently infected with isogenic serotypes of the relapsing fever agent Borrelia turicatae. We studied infection in hearts 1 to 2 months after inoculation by TaqMan reverse transcription-PCR and immunohistochemistry (IHC) and inflammation by hematoxylin and eosin and trichrome staining, IHC, and in situ hybridization (ISH). We studied apoptosis by terminal transferase-mediated DNA nick end labeling assay and measured expression of apoptotic molecules by RNase protection assay, immunofluorescence, and immunoblot. All antibody-deficient mice, but none of the immunocompetent controls, developed persistent infection of the heart. Antibody-deficient mice infected with serotype 2 had more severe cardiac infection and injury than serotype 1-infected mice. The injury was more severe around the base of the heart and pericardium, corresponding to sites of marked infiltration by activated macrophages and upregulation of interleukin-6 (IL-6). Infected hearts showed evidence of apoptosis of macrophages and cardiomyocytes as well as significant upregulation of caspases, most notably caspase-1. We conclude that persistent infection with relapsing fever borrelias causes significant loss of cardiomyocytes associated with prominent infiltration by activated macrophages, upregulation of IL-6, induction of caspase-1, and apoptosis.