Most estimates of the prevalence of
anaplasmosis have been based on serologic data using the
complement-fixation (CF) and/or card agglutination tests. Since these tests are considered to be only about 50 percent reliable for detecting carrier cattle in enzootically stable herds, the need for more sensitive diagnostic tests is widely recognized. The objective in the present study was to compare the sensitivity of the CF test with that of the indirect immunofluorescence (IIF) test and a recently developed
DNA probe in determining the prevalence of Anaplasma marginale
infection in cattle from an enzootic area. The study herd consisted of 52 8-month-old steers and 13 3-year-old cows of mixed beef breed. All cattle were initially tested for this comparative purpose. All but one animal (one that was a positive reactor as assessed by all three tests, and served as a positive control), were treated with long-acting
oxytetracycline in an attempt to clear any carrier
infections. Each animal was then retested at 1 month and 2 months post-treatment (PT), in an effort to determine if the
DNA probe could be used to evaluate the effectiveness of the
drug. Six of the 65 (9.2%) initial serum samples were CF positive. In contrast, 60 (92.3%) and 64 (98.5%) of the samples were positive as assessed with the IIF test and the
DNA probe, respectively. The
DNA hybridization reactions varied in intensity within the sample population indicating different individual levels of
infection. The
DNA probe hybridized with two samples taken at 1 month PT, and with two different samples taken at 2 months PT. The mean IIF titers were reduced at both the 1 month and 2 month sampling times. These results suggest that the
drug did not eliminate
infections in all cattle. Some may have been cleared, but, in any event, the
drug did reduce the level of
infections below the sensitivity of the
DNA probe and interrupted continuity of stimulation of antibody. Therefore, the
DNA probe and the IIF test appear to be considerably more sensitive in detecting carrier
infections than the CF test, and should be considered in future epidemiologic studies.