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Objective—To compare sensitivity of a complement fixation (CF) test and competitive ELISA (cELISA) for detection of Anaplasma marginale in experimentally infected steers.

Animals—40 crossbred (Angus-Simmental) steers.

Procedures—Steers were inoculated with 2.6 × 109 A marginale–infected erythrocytes (day 0). Blood samples were collected on days 9, 13, 20, 28, 34, 41, 61, 96, 126, and 156 days after inoculation. The percentage of parasitized erythrocytes (PPE) was determined by microscopic examination of stained blood films, and sera were evaluated with the CF test and cELISA by use of USDA-approved methods. Sensitivity and agreement (κ statistic) between the 2 methods were determined. Persistent infections were confirmed by inoculation of blood obtained from infected steers into susceptible, splenectomized calves.

Results—9 days after inoculation, sensitivity of the cELISA was 47.5%, whereas the CF test failed to identify seropositive steers. After day 13, sensitivity of the cELISA and CF test was 100% and 20%, respectively. During peak parasitemia (day 20), sensitivity of the cELISA and CF test was 100%. Thereafter, sensitivity of the CF test fluctuated between 7.5% and 37.5%, whereas sensitivity of the cELISA remained at 100%. Overall sensitivity of the cELISA and CF test was 94.8% and 26.5%, respectively (κ statistic, 0.039).

Conclusions and Clinical Relevance—The cELISA had superior sensitivity for serologic detection of A marginale.The CF test and cELISA each had a high percentage of false-negative results during the prepatent period. These findings are relevant for export certification and anaplasmosis prevention or eradication programs.

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in American Journal of Veterinary Research