Evidence for a high rate of false-positive results with the indirect fluorescent antibody test for Ehrlichia risticii antibody in horses

John E. Madigan From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616 (Madigan, DeRock); the Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1092 (Rikihisa, Mott); and the Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348 (Palmer).

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Yasuko Rikihisa From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616 (Madigan, DeRock); the Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1092 (Rikihisa, Mott); and the Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348 (Palmer).

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Jonathan E. Palmer From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616 (Madigan, DeRock); the Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1092 (Rikihisa, Mott); and the Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348 (Palmer).

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Elfriede DeRock From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616 (Madigan, DeRock); the Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1092 (Rikihisa, Mott); and the Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348 (Palmer).

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Jason Mott From the Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616 (Madigan, DeRock); the Department of Veterinary Biosciences, College of Veterinary Medicine, The Ohio State University, Columbus, OH 43210-1092 (Rikihisa, Mott); and the Department of Clinical Studies, New Bolton Center, School of Veterinary Medicine, University of Pennsylvania, Kennett Square, PA 19348 (Palmer).

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Objective—

The original objective was to determine seroprevalence of Ehrlichia risticii antibody among horses in California. On the basis of the unexpected results of the survey, an investigation into the accuracy and reproducibility of results of the indirect fluorescent antibody ( ifa) test for E risticii was carried out.

Design—

Prospective, seroprevalence study.

Animals—

Healthy horses (n = 655) and horses with clinical signs of equine monocytic ehrlichiosis ( eme; n = 514) from various regions of California.

Procedure—

The ifa test was performed. Results were compared with results of an elisa and with results of western immunoblot analysis.

Results—

Overall, 104 of 655 (15.9%) healthy horses had evidence of an antibody response. However, 84 of 514 (16.3%) horses with clinical signs of eme also had positive test results, and of the 8 seropositive diseased horses for which paired (acute and convalescent) samples had been submitted, only 1 had a rise in antibody titers between the acute and convalescent samples. Comparison of results for the ifa test, elisa, and western immunoblot analysis revealed a high rate of false-positive results for the ifa test. Subsequent studies suggested that routine vaccination of horses with non-E risticii vaccines may have contributed to the false-positive reactions.

Clinical Implications—

The data failed to provide conclusive evidence of E risticii infection among California horses. Owing to the high percentage of false-positive test results, caution is advised when using the ifa test to diagnose eme in horses or to determine the necessity for E risticii vaccination.

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Cover Journal of the American Veterinary Medical Association
Journal of the American Veterinary Medical Association
Issue:
01 Dec 1995
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