Serological diagnosis of heartwater orThe rickettsia Cowdria ruminantium is the causative agent of heartwater, an acute, fatal infectious disease of domestic and wild ruminants (5, 40) which is transmitted by ticks of the genus Amblyomma (45). Efforts to study the epidemiology of heartwater and to implement disease control have been hampered by the lack of reliable serodiagnostic tests. Available tests are based on cultured organisms or antigen extracts (7,9,13,15,23,26,34,36) and on the major antigenic protein of C. ruminantium, MAP 1 (2), which has been targeted as a diagnostic antigen because of its immunodominance (2,11,14). These tests are, however, limited by the extensive antigenic similarity between C. ruminantium and closely related agents of the genus Ehrlichia (1,8,12,14,17,35,41), some of which also infect ruminants. Recently, a partial fragment of MAP 1, MAP 1B, which spans amino acids 47 to 92 of the mature protein (42, 43), has been shown to have high specificity for C. ruminantium in an indirect enzyme-linked immunosorbent assay (ELISA) (24,25,43). The assay does not detect antibodies to ehrlichial agents infecting domestic ruminants, such as E. bovis, E. ovina, and E. phagocytophila. It does detect antibodies to E. canis (which infects dogs) and antibodies to E. chaffeensis (a pathogen of humans). In the United States, E. chaffeensis infects white-tailed deer (6, 16), a species that is highly susceptible to heartwater. In areas of Zimbabwe (22,43) and the Caribbean (24, 25) that are designated heartwater free by the absence of Amblyomma ticks and clinical heartwater, the MAP 1B indirect ELISA demonstrated a high specificity with cattle, sheep and goat sera. This assay is also reliable for the detection of experimental infections in small ruminants, and it detects antibodies to geographically diverse C. ruminantium isolates from different countries (24, 43). Hence, its use has been proposed for diagnosis and surveillance of heartwater.In a preliminary serological survey of heartwater in Zimbabwe using the MAP 1B indirect ELISA, only 33% of cattle sera from areas with endemic heartwater infection tested positive (22). The low seroprevalence was unexpected, given the high infection pressure in these regions and the consequent likely high prevalence of infection (27). Epidemiological studies conducted on some of these farms over several years demonstrated a tick infection rate of 10% and a vector attachment rate of between one and four ticks every 2 days (31). At this tick attack rate, it was estimated that cattle were exposed to