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In this VETgirl blog, we review the clinicopathologic testing for leptospirosis. The diagnosis of canine leptospirosis is based on clinical suspicion, clinical signs, and clinicopathologic results consistent with leptospirosis. Clinicopathologic findings consistent with leptospirosis include the presence of: neutrophilia, a left shift, lymphopenia, a mild to moderate non-regenerative anemia, hemoconcentration (seen with dehydration), hemolysis (seen with cattle), thrombocytopenia (seen in up to 58% of dogs), azotemia (seen in > 80-90% of dogs), increased liver enzymes (including increases in ALT, AST, ALP, and total bilirubin; these changes are almost always seen with concurrent azotemia with leptospirosis), electrolyte abnormalities (e.g., hypokalemia, hyponatremia, hypochloridemia, hyperphosphatemia), and increased creatinine kinase. Additional findings consistent with leptospirosis include isosthenuria, bilirubinuria, hematuria, glucosuria, proteinuria, and evidence of coagulopathy (e.g., increased fibrinogen, FDP, FSPs). Prolonged PT or PTT may be seen in 6-50% of dogs with leptospirosis.
Other advanced diagnostics may include radiology (to look for evidence of pulmonary lesions secondary to leptospirosis, which may appear as a nodular interstitial or alveolar pattern) and abdominal ultrasound (to rule out other underlying disease processes such as neoplasia, etc.). Ultrasound findings may reveal non-descript findings including renomegaly, pylectasia, perirenal fluid accumulation, a medullary band of increased echogenicity, increased cortical echogenicity, and rare other findings (e.g., splenomegaly, mild abdominal lymphadenopathy, etc.).
The most “definitive” diagnosis of leptospirosis is typically based on serology by the microscopic agglutination test (MAT), which tests for antibodies to leptospires. The MAT tests for the highest serum dilution causing agglutination of 50% of the leptospires. MAT testing typically includes L. canicola, L. icterohaemorrhagiae, L. pomona, L. grippotyphosa, L. hardjo, and L. bratislava. Unfortunately, there are several limitations of the MAT, including the hazardous need to maintain live cultures of pathogenic serovars, difficulty in standardizing the test, expense, cross-contamination of serovar cultures, and false negatives (e.g., due to acute disease) or false positives (due to previous vaccination, exposure, etc.). Another limitation of the MAT is that some cross-reactivity may occur between different serogroups. Keep in mind that one of the key limitations of MAT is that during the first week of acute disease, dogs may test negative. For this reason, convalescent titers are generally recommended 2-4 weeks later (at the same laboratory) to look for the presence of seroconversion. Typically, a 4-fold increase in titer is suggestive of infection; however, recent vaccination or antimicrobial therapy may affect the results (e.g., antimicrobial therapy may blunt the expected response). Interpreting MAT results must be done with care, as a result. Titers post-exposure can persist for at least one year, but are thought to declined by 4 months secondary to vaccination.
There are other diagnostic tests that can be used to screen for leptospirosis including dark field microscopy, silver staining of biopsy specimens (e.g., renal), immunohistochemistry, PCR, in situ hybridization, culture, and Idexx’s leptospirosis PCR & antibody ELISA in-clinic test. Note that each has their limitations. Dark field microscopy is technically difficult and has low specificity; this has fallen out of favor and is rarely used now. Silver staining of renal biopsy tissue can be performed, but lacks sensitivity and can result in false negatives. Fluorescent antibody testing and PCR can be performed on urine or tissue. Note that culture, PCR, and even antibody ELISA tests (Idexx) can all be affected by recent microbial therapy. For this reason, pre-treatment blood work should always be utilized for submission in the patient suspected to have leptospirosis.
For more information, check out the ACVIM Consensus Statement on Leptospirosis here.
References available upon request. Copyright, VETgirl, LLC 2014.