Abstract
Leptospirosis is a common bacterial disease in California sea lions (Zalophus californianus) however there remain gaps in our understanding of maintenance hosts relative to animals demonstrating clinical disease. To effectively study the epidemiology of leptospirosis in any species, a solid understanding of the performance of diagnostic tests for different stages of infection is required. This study compares renal histopathology to immunohistochemistry (IHC) and polymerase chain reaction (PCR) for detection of Leptospira interrogans in California sea lions with and without evidence of clinical renal disease.
The study included stranded California sea lions that presented to The Marine Mammal Center in Sausalito, CA between July 1, 2008 and March 30, 2009 that either died or were euthanized due to the presence of severe disease. Animals were examined post mortem and both fresh (for PCR) and fixed kidney (for histopathology and IHC) tissue was available for diagnostic testing.
Kidney samples were collected from 134 animals; 42 with clinical and serum chemistry changes consistent with renal disease, 25 without, and 67 with no ante-mortem blood work. Of 134 kidneys, 35 were positive for Leptospira interrogans on PCR; all of those kidneys were also positive on IHC as were an additional 17 kidneys that were PCR negative. The kappa value (agreement) between the two tests was 0.71 (95% CI 0.55–0.88). Discordant results between the two tests had no unique histologic or host characteristics and both the histologic lesions and IHC staining patterns were morphologically consistent with leptospirosis.
Histologic lesions were present in the kidneys of 109 (81%) animals. The most common histologic change was interstitial lymphoplasmacytic inflammation that was either patchy or more regionally extensive and associated with tubular changes. This latter pattern along with severity of inflammation was statistically associated with Leptospira infection detected by IHC and PCR. Tubular lesions (most commonly suppurative tubulitis with tubular degeneration) and glomerular lesions (glomerulonephritis) were observed in 57 (43%) animals and also associated with infection. Animals with clinical renal disease were more likely to be positive for Leptospira than those with no ante-mortem chemistry changes; however, two animals were positive on IHC in the absence of serum chemistry changes; nephritis in these two cases was only mild.
Results of this study suggest that IHC may be a more sensitive tool relative to PCR for diagnosis of Leptospira infection in California sea lions when using kidney tissue. Severity and pattern of histologic changes in the kidney are associated with leptospirosis, but non-clinical animals can also be positive suggesting that asymptomatic infection may play a role in maintenance of infection within sea lion populations.