Identifying predictors for detecting Leptospira DNA in the urine of California sea lions (Zalophus californianus)

The California sea lion (Zalophus californianus) population is affected by Leptospira interrogans serovar Pomona annually, causing low levels of infection year-round and recurrent, cyclical outbreaks of disease. Efforts to... [ view full abstract ]

The California sea lion (Zalophus californianus) population is affected by Leptospira interrogans serovar Pomona annually, causing low levels of infection year-round and recurrent, cyclical outbreaks of disease. Efforts to mathematically model leptospirosis transmission dynamics in sea lions depend on our ability to estimate the extent of pathogen shedding in the wild population. The microscopic agglutination test (MAT) is the standard diagnostic tool to detect serum anti-Leptospira antibodies (an indication of prior exposure), while polymerase chain reaction (PCR) can be used to detect Leptospira DNA in urine (leptospiruria; an indication of current infection and active shedding). While serum is relatively easily obtained from sea lions, urine can only be collected aseptically from anesthetized or deceased animals; therefore we sought a tool to estimate leptospiruria from more easily acquired data. Using aseptically collected urine samples and other associated data from stranded sea lions at The Marine Mammal Center, we applied logistic stepwise regression, CART and random forest analyses to determine if more readily available clinical (e.g. serum chemistry) or demographic data (e.g. age, sex) can be used to predict leptospiruria. We found that Leptospira interrogans serovar Pomona antibody titers had the strongest relationship with leptospiruria, and a Spearman's correlation coefficient between these two variables was statistically significant (rho=0.65, n=728, p<0.001). Using this correlation and predictive logistic regression models, we calculated the probability of urine being PCR positive (i.e. infected and shedding leptospires) based on MAT results (95% CI). By applying these probabilities to cross-sectional data on antibody titers over the last 20 years, we are able reconstruct the prevalence of urinary shedding in the California sea lion population over time. Estimating levels of sea lion leptospire shedding is key to understanding the disease dynamics of this system, including the risk of transmission to humans and other mammalian species along the California coast.