Vaccination with monovalent Leptospira interrogans serovar Copenhageni and its implication for the diagnosis of natural infection in dogs.

Using microscopic agglutination test (MAT), a single titre ≥ 1/800 in dogs unvaccinated against leptospirosis is considered suggestive of infection. The emergence of canine leptospirosis in Sydney, Australia, led to considerable vaccination uptake with monovalent Leptospira interrogans serovar Copenhageni, yet the humoral response post-vaccination has not been described. Aims were to characterise magnitude and duration of humoral response post-vaccination with serovar Copenhageni (Protech®C2i, Boehringer Ingelheim, Australia) in healthy dogs to assess potential effects on diagnosis. Serial MAT testing (24 serovars) was performed before and after annual booster vaccination (group 1, n = 13) and primary vaccination (group 2, n = 14). Serum was collected before and 1, 2, 4, 8, 12, 26, 39 and 52 weeks post-vaccination. Antibodies against serovar Copenhageni developed in 62 % (8/13) in group 1 (highest titre 1/100) and 71 % (10/14) in group 2 (highest titre 1/400). Antibodies developed against seven other serovars both from the same and from multiple different serogroups and exceeded titres to Copenhageni in 33 %. In group 2, titres reached 1/800 in weeks 1 and 2 post-vaccination (serovar Bratislava). One dog (group 1) developed antibodies to Bratislava but not Copenhageni. Four in group 1 (31 %) and group 2 (29 %) had no antibody response detected. There was no association between vaccination history and seroconversion, however response was more pronounced after primary vaccination. Most (group 1, 77 % (10/13); group 2, 71 % (10/14)) had no antibodies by week 12. Vaccination elicited a weak, short humoral response, unlikely to interfere with diagnosis, except within two weeks of completing a primary vaccination course.