Although the microscopic agglutination test (MAT) is the most common test used to diagnose acute leptospirosis, it is poorly sensitive early in the disease process because serum antibodies are not present and can have poor specificity due to inability to differentiate antibodies induced by vaccine versus infection.

This prospective study evaluated real-time PCR results in 20 healthy dogs recently vaccinated for Leptospira. Real-time PCR is a promising modality with good sensitivity and specificity in detecting Leptospira DNA in blood and urine. Study dogs were divided into 2 vaccine groups, with each group receiving 1 of 2 commercial 4-serovar Leptospira vaccines. Blood was drawn for real-time PCR and MAT titers on days 0 (prevaccination), 3, 4, 7, 14, 21, 28, 35, 42, 49, and 56. Results showed that real-time PCR was negative for Leptospira DNA in all dogs at all time points. By contrast, 18 dogs mounted a positive MAT response to multiple Leptospira serovars by day 7. The authors concluded that real-time PCR testing for Leptospira DNA is not affected by recent vaccination for canine leptospirosis. Further study is warranted to determine how this test would perform in dogs naturally infected with leptospirosis and whether concurrent antibiotic treatment may yield false-negative results.

Commentary
The influence of leptospirosis vaccination on diagnostic tests is a pragmatic concern. Although killed vaccines do not replicate, killed viral vaccines can potentially cause positive PCR test results, thereby prompting investigation of whether commercial 4-serovar Leptospira bacterins would induce a positive real-time PCR reaction in whole blood of vaccinated dogs. This small study provides reasonable assurance that positive real-time PCR tests for leptospirosis indicate infection (not vaccination) in dogs. False-negative PCR tests, however, may occur in infected dogs with very low quantities of circulating bacterial DNA.—George E. Moore, DVM, PhD, DACVPM, DACVIM

Source
Effects of recent Leptospira vaccination on whole blood real-time PCR testing in healthy client-owned dogs. Midence JN, Leutenegger CM, Chandler AM, Goldstein RE. J VET INTERN MED 26:149-152, 2012.

This capsule is part of the One Health Initiative