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Use of antibody titers measured via serum synergistic hemolysis inhibition testing to predict internal Corynebacterium pseudotuberculosis infection in horses

Jennifer M. Jeske DVM, MPVM1, Sharon J. Spier DVM, PhD, DACVIM2, Mary Beth Whitcomb DVM3, Nicola Pusterla DVM, PhD, DACVIM4, and Ian A. Gardner bvsc, MPVM, PhD5
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  • 1 Departments of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 2 Departments of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 3 Surgical and Radiological Sciences, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 4 Departments of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.
  • | 5 Departments of Medicine and Epidemiology, School of Veterinary Medicine, University of California-Davis, Davis, CA 95616.

Abstract

Objective—To estimate likelihood ratios (LRs) of correctly identifying internal Corynebacterium pseudotuberculosis infection in horses by measurement of antibody titers via serum synergistic hemolysis inhibition (SHI) testing.

Design—Retrospective case-control study.

Animals—170 horses (171 records; 92 cases of C pseudotuberculosis infection and 79 controls).

Procedures—Medical records were reviewed, and horses were grouped on the basis of evidence of internal or external C pseudotuberculosis infection. The LRs and 95% confidence intervals for identification of internal C pseudotuberculosis infection by use of SHI test results were estimated.

Results—LRs for C pseudotuberculosis infection increased as antibody titers increased when all horses were included in analyses; LRs for detecting internal infection were significantly > 1 (null value) for reciprocal antibody titers ≥ 1,280 overall and > 160 when horses with external abscesses were excluded. Likelihood ratios for detecting internal infection did not differ from 1 (indicating no change in pretest-to-posttest odds of internal infection) when only horses with external C pseudotuberculosis infection (horses with external and internal abscesses vs those with external abscesses only) were included. The LR for detecting internal infection was 2.98 (95% confidence interval, 2.19 to 4.05) for horses with titers ≥ 512.

Conclusions and Clinical Relevance—In the study population, higher titers were typically more indicative of active external or internal C pseudotuberculosis infection than of internal disease specifically. The SHI test was not a useful predictor of internal C pseudotuberculosis infection in horses with external abscesses but was useful in the absence of external disease.

Abstract

Objective—To estimate likelihood ratios (LRs) of correctly identifying internal Corynebacterium pseudotuberculosis infection in horses by measurement of antibody titers via serum synergistic hemolysis inhibition (SHI) testing.

Design—Retrospective case-control study.

Animals—170 horses (171 records; 92 cases of C pseudotuberculosis infection and 79 controls).

Procedures—Medical records were reviewed, and horses were grouped on the basis of evidence of internal or external C pseudotuberculosis infection. The LRs and 95% confidence intervals for identification of internal C pseudotuberculosis infection by use of SHI test results were estimated.

Results—LRs for C pseudotuberculosis infection increased as antibody titers increased when all horses were included in analyses; LRs for detecting internal infection were significantly > 1 (null value) for reciprocal antibody titers ≥ 1,280 overall and > 160 when horses with external abscesses were excluded. Likelihood ratios for detecting internal infection did not differ from 1 (indicating no change in pretest-to-posttest odds of internal infection) when only horses with external C pseudotuberculosis infection (horses with external and internal abscesses vs those with external abscesses only) were included. The LR for detecting internal infection was 2.98 (95% confidence interval, 2.19 to 4.05) for horses with titers ≥ 512.

Conclusions and Clinical Relevance—In the study population, higher titers were typically more indicative of active external or internal C pseudotuberculosis infection than of internal disease specifically. The SHI test was not a useful predictor of internal C pseudotuberculosis infection in horses with external abscesses but was useful in the absence of external disease.

Contributor Notes

Dr. Gardner's present address is Department of Health Management, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, C1A 4P3 PE, Canada.

The authors thank Dr. Sharon Hietala for technical assistance.

Address correspondence to Dr. Jeske (JenJeskeDVM@gmail.com).