Antibody responses to MAP 1B and other Cowdria ruminantium antigens are down regulated in cattle challenged with tick-transmitted heartwater

Serological diagnosis of heartwater or Cowdria ruminantium infection has be en hampered by severe cross reactions with antibody responses to related eh rlichial agents. A MAP 1B indirect enzyme-linked immunosor bent assay that has an improved specificity and sensitivity for detection of immunoglobulin G (IgG) antibodies has been developed to overcome this constraint (A. H. M . van Vliet, B. A, M. Van der Zeijst, E. Camus, S. M. Mahan, D. Martinez, a nd F. Jongejan, J, Clin. Microbiol. 33:2405-2410, 1995). When sera were tes ted from cattle in areas of endemic heartwater infection in Zimbabwe, only 33% of the samples tested positive in this assay despite a high infection p ressure (S. M;I. Mahan, S. M. Samu, T. F. Peter, and F. Jongejan, Ann. N.Y. Acad, Sci 849:85-87, 1998). To determine underlying causes for this observ ation, the kinetics of MAP 1B-specific IgG antibodies in cattle after tick- transmitted C. ruminantium infection and following recovery were investigat ed, Sera collected weekly over a period of 52 weeks from 37 cattle, which w ere naturally or experimentally infected with C. ruminantium via Amblyomma hebraeum ticks, were analyzed. MAP 1B-specific IgG antibody responses devel oped with similar kinetics in both field- and laboratory-infected cattle. I gG levels peaked at 4 to 9 weeks after tick infestation and declined to bas eline levels between 14 and 33 weeks, despite repeated exposure to infected ticks and the establishment of a carrier state as demonstrated by PCR and xenodiagnosis, Some of the serum samples from laboratory, and field-infecte d cattle were also analyzed by immunoblotting and an indirect fluorescent-a ntibody test (IFAT) to determine whether this observed seroreversion was sp ecific to the MAP 1B antigen. Reciprocal IFAT and immunoblot MAP 1-specific antibody titres peaked at 5 to 9 weeks after tick infestation but also dec lined between 30 and 45 weeks. This suggests that MAP 1B-specific IgG antib ody responses and antibody responses to other C. ruminantium antigens are d own regulated in cattle despite repeated exposure to C. ruminantium via tic ks. Significantly, serological responses to the MAP 1B antigen may not be a reliable indicator of C. ruminantium exposure in cattle in areas of endemi c heartwater infection.