To help confirm a diagnosis of canine leptospirosis, veterinarians today must rely on serologic testing with the microscopic
agglutination test (MAT). The highest dilution of serum that agglutinates 50% of the Leptospira organism is the titer. Interpreting the test can be problematic, though, especially early on in the disease or in a recently
vaccinated dog.
AN EARLY NEGATIVE TITER SHOULDN'T DISSUADE YOU
A low MAT titer could indicate previous vaccination against Leptospira species or possibly infection with another type of spirochete because cross-reactivity occurs. However, a titer ≥ 800 for
a nonvaccinal serovar along with appropriate clinical signs is suggestive of active disease. Also, a fourfold increase in
titer in a convalescent serum sample indicates a recent Leptospira species infection.1
For example, you're treating a sick dog (dogs with leptospirosis may have acute renal failure, vasculitis, disseminated intravascular
coagulation, or hepatic disease) that has a titer ≥ 800 for Leptospira
pomona, but the dog was vaccinated two months earlier against L. pomona. You submit another serum sample seven to 10 days later, and the titer is now ≥ 3,200. This fourfold increase in titer indicates
infection, most likely due to L. pomona during the illness.
Keep in mind that because titers may decline quickly after antibiotic therapy, you may miss rapidly rising titers if a convalescent
titer is drawn after the typical three-week interval. So if you suspect leptospirosis, don't let an early negative titer result
dissuade you; start treating and submit another sample for an MAT within seven to 10 days.
HOW LONG DO VACCINAL TITERS LAST?
Leptospira species vaccinal titers may last six months to a year or longer. However, antibody testing may not be a good way to assess
protection because at least one study has shown that dogs were well-protected from a challenge with Leptospira organisms a full year after vaccination despite low serum MAT titers.2
A TEST ON THE HORIZON
Veterinarians may someday have a better diagnostic test that will allow earlier diagnosis of canine leptospirosis. In people,
ELISA tests are used as screening tools for leptospirosis, and similar types of tests are currently being evaluated in dogs.
Richard E. Goldstein, DVM, DACVIM (small animal internal medicine), DECVIM-CA, Department of Clinical Sciences, College of
Veterinary Medicine, Cornell University, Ithaca, NY 14853.
REFERENCES
1. Greene CE, Sykes JE, Brown CA, et al. Leptospirosis. In: Greene CE, ed. Infectious diseases of the dog and cat. 3rd ed. Philadelphia, Pa: WB Saunders Co, 2006;402-417.
2. André-Fontaine G, Branger C, Gray AW, et al. Comparison of the efficacy of three commercial bacterins in preventing canine
leptospirosis. Vet Rec 2003;153(6):165-169.
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