Understanding Leptospirosis Epidemics in Zalophus californianus: Effects of Total & Self-Reactive Immunoglobulin Titers on Health & Survival

Abstract

Since 1970, marine researchers have documented mass strandings in California sea lions (Zalophus californianus; CSLs) attributed to leptospirosis (caused by Leptospira interrogans serovar Pomona) roughly every three to five years. Past research into the cause of these cyclical outbreaks has focused on hypotheses about demographic and environmental influences. The present study looks at what role the inherent immune responsiveness could play in CSL epidemics. We measured self-reactive and total immunoglobulin (Ig) titers (IgM and IgG) in yearlings and juveniles and compared these titer distributions between epidemic and non-epidemic years and between seasons during epidemic years to see if there are differences in these titers on a population level. In juveniles, we found higher self-reactive Ig titers and lower total Ig titers in epidemic years compared to non-epidemic years, suggesting damage repair (e.g. clearing apoptotic cells), poor body condition, and/or increased investment in cross-reactive Ig in response to infection. On the individual level, self-reactive IgM is positively associated with blood urea nitrogen (BUN) levels in L. interrogans-infected juveniles. Self-reactive and total IgM positively predict creatinine levels. Neither self reactive nor total Ig titers were predictive of juvenile survival or L. interrogans Pomona seroconversion in minimal logistic regression models. Seroconversion to L. interrogans Pomona was positively predicted by the interaction between self-reactive IgG and total IgM. These results suggest a possible role for self-reactive and total Ig (possibly cross-reactive) in CSL response to L. interrogans Pomona infection. We recommend expanding this study to include females and more age classes to see if these findings vary and potentially explain why juvenile males are most affected during outbreaks. We also recommend including nutritional assays to try to distinguish between poor body condition and investment in cross-reactive Ig. Future studies should include wild-caught sea lions to eliminate potential bias from our sample population of all stranded individuals.