Hot Literature: Who's got leptospirosis?

We've heard a lot recently about the resurgence of leptospirosis in dogs. Despite the availability of canine vaccines against various serovars, this zoonotic disease seems to be making a comeback. Researchers from Purdue's School of Veterinary Medicine conducted a serosurvey to analyze the microscopic agglutination test (MAT) results for more than 33,000 canine serum samples submitted to one commercial laboratory for Leptospira species antibody testing. The time period of the survey extended from 2000 to 2007 and consisted of submissions made by veterinarians suspicious of Leptospira species infection.

While bacterial culture of urine, blood, or organs is the most reliable diagnostic test for leptospirosis in dogs, the difficulty in obtaining rapid results makes the MAT more appealing to most practitioners. For this study, to negate the effects of vaccination on the evaluation of the MAT results, a cutoff titer of 1:1,600 was used, beyond which samples were considered negative. While MAT is a serogroup-specific assay, cross-reaction commonly occurs among serovars. Five common Leptospira species serovars were tested for in each case-Canicola, Icterohaemorrhagiae, Pomona, Bratislava, and Grippotyphosa. For the years when the tests were available, Autumnalis and Hardjo were also tested for. The age, sex, and breed of each dog were recorded, along with the postal code from the submitting hospitals to gauge geographic distribution.

Samples were received from each state in the continental United States but were unevenly distributed, predominantly coming from the Pacific coastal states and northeast regions of the country. During the eight-year period, the number of samples submitted increased annually, from 1,528 in 2000 to 8,189 in 2007. The MAT results were positive for one or more Leptospira serovars in 8.1% of these samples. Positive results were highest in 2007, but 2004 was a close second, suggesting that there has not been a linear or steady increase in the prevalence of Leptospira species. Additionally, while there were more positive MAT results in the months of November and December of several years, there was no evidence of a seasonal focus for the positive results. Positive MAT results occurred throughout the year and may have been correlated to periods of increased rainfall, the start of hunting seasons when some dogs may be more likely to be exposed, or periods when the reservoir host (raccoons) may be more active.

Of the positive antibody test results, 78.7% had the highest titer against just one serovar. Most of the positive samples were seropositive for antibodies against Grippotyphosa and Autumnalis. While Autumnalis has not been definitively detected in dogs in 50 years and is not likely to have been the infective serovar in these cases, it is thought to cross-react with other Leptospira species serovars. This cross reactivity along with the seroprevalence correlations between various serovars suggest an ability of Leptospira species serovars to imitate each other, such that a host’s antibodies (IgM or IgG) agglutinate leptospires of multiple serovars. Furthermore, this suggests that some cross-protection can be achieved with vaccination.

Working and hunting breeds are likely overrepresented for Leptospira species testing. However, in this survey, mixed breeds as well as small and large identifiable breeds had similar distribution of their results. Age did play a small role. Dogs more than 10 years of age were least likely to have positive results, and the highest percentage of positive antibody titers was found in dogs between the ages of 2 and 6 years. This may simply be a reflection of the peak years of exposure.

This study suggests that Leptospira species can affect any type of dog and at most any age. Potential exposure due to weather conditions, outdoor activity, or the behavior of reservoir hosts is more critical to the risk for infection than season or breed. Because of MAT result limitations, bacterial culture is necessary to definitively determine the infective Leptospira species in dogs. Additional study integrating the vaccination status of participants and the inclusion of a randomly selected population of dogs to further evaluate the seroprevalence and geographic distribution of Leptospira species would be beneficial.

Gautam R, Wu CC, Guptill LF, et al. Detection of antibodies against Leptospira serovars via microscopic agglutination tests in dogs in the United States, 2000-2007. J Am Vet Med Assoc 2010;237(3):293-298.

For a link to the abstract, click here.