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Serologic titers to Leptospira in vaccinated pigs and interpretation for surveillance
Susan K Schommer1, Nicholas Harrison2, Michael Linville2
1National Swine Resource and Research Center, Division of Animal Sciences, University of Missouri, Columbia, Missouri, United States of America.
Abstract:
Diagnosis and surveillance of pathogenic Leptospira is difficult as organisms may be intermittently shed and in small numbers. Therefore, serologic testing by the microscopic agglutination test (MAT) is the primary screening method for leptospirosis. While a MAT titer ≥1:100 is considered to be a positive result, interpretation is complicated by the use of commercial vaccines in pigs. Most guidelines for interpretation of MAT titers in pigs were published in the 1970's and 1980's, prior to the development of the current multivalent vaccines. We evaluated MAT titers in routinely vaccinated healthy research pigs compared to their unvaccinated cohorts. Our study confirmed previous reports that the Pomona serovar elicits minimal antibody response even after a second booster 6 months after initial vaccination. However, MAT titers of ≥1:3,200 were detected as early as 4 weeks post initial vaccination for serovars Bratislava and Icterohaemorrhagiae and remained as high as ≥1:1,600 prior to booster at 24 weeks post vaccination. Our study determined that high levels of MAT titers can occur from vaccination alone and high titers are not necessarily indicative of infection. Therefore, the interpretation of MAT titers as indicators of Leptospira infection should be readdressed.
Insights
High microscopic agglutination test (MAT) titers for Leptospira in pigs can result from vaccination alone. This challenges the interpretation of MAT titers as indicators of infection, necessitating updated guidelines for accurate diagnosis.
Area of Science:
- Veterinary Immunology
- Bacteriology
- Diagnostic Pathology
Background:
- Leptospira diagnosis is challenging due to intermittent shedding and low organism numbers.
- Microscopic agglutination test (MAT) is the primary serologic screening method for leptospirosis.
- Current MAT interpretation guidelines for pigs predate modern multivalent vaccines.
Purpose of the Study:
- To evaluate microscopic agglutination test (MAT) titers in vaccinated versus unvaccinated pigs.
- To determine if high MAT titers can be solely attributed to vaccination.
- To reassess the interpretation of MAT titers for Leptospira infection in pigs.
Main Methods:
- Comparison of MAT titers between routinely vaccinated and unvaccinated healthy research pigs.
- Analysis of antibody responses to specific Leptospira serovars (Pomona, Bratislava, Icterohaemorrhagiae).
- Longitudinal monitoring of MAT titers post-vaccination and pre-booster.
Main Results:
- Vaccination alone can induce high MAT titers (≥1:3,200) for Bratislava and Icterohaemorrhagiae serovars.
- Pomona serovar elicited a minimal antibody response even after booster vaccination.
- High MAT titers persisted for months post-vaccination, indicating vaccination-induced immunity.
Conclusions:
- High MAT titers in pigs are not always indicative of active Leptospira infection.
- Existing guidelines for MAT titer interpretation require revision due to modern vaccine impacts.
- Updated diagnostic strategies are needed to differentiate vaccination responses from infection in pigs.
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