Laboratory Diagnosis Update of Canine Leptospirosis
By: Kenneth Harkin, Veterinary Health Center KSUCVM
The two tests routinely utilized for the diagnosis of canine leptospirosis are serology by the microscopic agglutination test (MAT) and polymerase chain reaction (PCR) testing of urine as these tests offer the best combination of sensitivity, specificity and ease of submission. The optimum confirmation of a diagnosis of leptospirosis with the MAT is to document a 4-fold rise or higher in the reciprocal titer over a 2-4 week period (e.g., 800 initially and then 3200 two weeks later). If the 2-week convalescent titer fails to document the expected 4-fold rise, it is recommended to check at 4-weeks. For various reasons, however, it may be inconvenient or impossible to do the convalescent titer. In those scenarios, a single reciprocal titer of 6400 or higher would yield a high confidence in the diagnosis. The sensitivity of the test declines as a single reciprocal titer cut-point for a diagnosis increases, but the specificity improves, eliminating the effect of prior vaccinations. Lower titers in the unvaccinated dog may also be considered supportive of the diagnosis when clinical signs are consistent with the disease. The MAT is intended to identify the serogroup of the infecting serovar and not the specific serovar, but even the infecting serogroup is misidentified up to 50% of the time by the MAT. This does not diminish the value of the MAT in establishing a diagnosis, however.
Some dogs, especially those < 1 year of age, may never seroconvert, but a diagnosis can still be obtained by use of the PCR. The PCR identifies the presence of the leptospiral organism in urine and is unaffected by previous vaccinations. The test is reported by the laboratory as either positive or negative for pathogenic leptospires. In the presence of supportive clinical signs, a positive PCR confirms the diagnosis of leptospirosis. The PCR can also be used to detect the carrier/shedding status of animals, something that the MAT cannot predict. A urine sample for PCR testing is best obtained prior to the initiation of antibiotic therapy, although some dogs will remain positive for several days after starting antibiotics. Urine can be collected and stored in the refrigerator for up to 5 days with no significant reduction in sensitivity of the PCR test, allowing the veterinarian time to decide whether or not to submit the test while not delaying therapy.
The PCR may be negative if the patient is not shedding leptospires in the urine or there are too few organisms to be detected, either because the patient is not in the leptospiruric phase (shedding leptospires in the urine) of the disease or due to antibiotic therapy initiated prior to diagnostics. Although it is rare that a dog with clinical disease from leptospirosis would not be in the leptospiruric phase, the PCR can be performed on other bodily fluids (blood, CSF, semen) or tissues (liver, kidney, lung, and brain), so contact the laboratory if you would like alternative samples tested.
The PCR and the MAT can be performed on any species of animal, not just dogs.
Given that false negative test results may occur with either the MAT or PCR, performing both tests on a patient in which leptospirosis is suspected is recommended and the combination provides the greatest diagnostic accuracy.
Pricing: 6 serovar MAT: $15.50 ; PCR: $33 ; Combination: $43.50