Analysis of the Enigma of Leptospirosis Epidemics in Zalophus californianus: Seroprevalence as a Biomarker of Exposure and Persistence

Abstract

For almost 50 years, researchers have documented the zoonotic disease leptospirosis, which infects mammalian hosts every three to five years. Their findings in attempting to describe the persistent and cyclical nature of this disease in California Sea Lions (Zalophus californianus; CSLs) are primarily centered around environmental factors. It is still unknown whether Leptospira spread and accumulate in the sea lion population or if it is a recurrent infection resulting from external reservoirs. The present study focuses on immune responsiveness and its role in CSL leptospirosis epidemics intertwined with environmental drivers of disease. I analyzed antibodies IgM, IgG, and avidity IgG (IgGU) in lepto-positive and non-lepto-positive yearlings, juveniles, sub-adults, and adults. I also analyzed these antibodies with respect to the fall, winter, spring and summer seasons as well as in outbreak years and non-outbreak years (which were also years of El Niño occurrences). I found that adult CSLs produced the lowest amount of IgM and IgG of all age classes suggesting their high likelihood of re-exposure. In general, between lepto-positive and non-lepto-positive individuals of all age classes, antibody production was lowest in individuals that were non-lepto-positive. Comparing antibody titers to one another depicted a positive correlation amongst all titers besides one anomaly in the juvenile data where IgM and IgGU had a negative relationship. This suggests that age class has a significant role in leptospirosis epidemics. Outbreak and El Niño years which are most conducive to Leptospira survival had greater amounts of IgG and IgGU in CSL samples than non-outbreak/El Niño years. However, this same trend did not exist with IgM titers in the yearling and juvenile age classes. This suggests that environmental sensitivity to leptospirosis infection in terms of IgM production is impacted by age more so than IgG and IgGU. Seasonal analysis yielded that IgM and IgG titers were highest in the winter and lowest in the spring and fall. IgGU on the other hand, was produced in greater amounts in the fall and summer seasons, agreeing with past research. Furthermore, environmental anomalies may be changing our knowledge of leptospirosis as the winter season may play more of a role in epidemics than it was previously found to. Longitudinal analysis depicted a general declining trend of IgM, IgG, and IgGU in all CSL individuals throughout leptospirosis persistence. Furthermore, longitudinal IgM and IgGU titers were found to have statistically similar patterns as time persists, both differing from longitudinal IgG titers patterns. This confirms past literature and its conclusion that IgM and IgGU function similarly in acting as a marker for distinguishing between primary and subsequent infections. Future research should consider examining a greater number of CSLs equally distributed between the four seasons to further explain seasonal deviation effecting leptospirosis epidemics. Additionally, future researchers should incorporate more than one region of CSL residence into analysis to eliminate varying environmental effects on the nature of leptospirosis persistence.