The seasonal influx of susceptible animals in a population (through births) is a key driver of seasonal disease outbreaks in many wildlife host species. Environmental conditions can have additional effects on outbreak size and seasonality by acting on host sensitivity to infection, Leptospira transmission rates, or susceptible recruitment. A key challenge in disease ecology has been to assess the relative importance of such intrinsic and extrinsic drivers of transmission.
California sea lions (Zalophus californianus) along the US West Coast experience seasonal outbreaks of leptospirosis (caused by Leptospira interrogans bacteria) of different magnitudes. As a first attempt at revealing the drivers behind this variation in outbreak size, we analyzed the relative contributions of environmental oceanic conditions and host demography.
Data consist of annual time series (1984-2012) of case counts, size estimates of population age classes, and local sea surface temperature. Taking full advantage of the availability of age- and year-specific population size estimates, we developed a method that allows the detailed reconstruction of the proportion of susceptible animals in the population. To quantify the relative contributions of extrinsic vs. intrinsic drivers, we fitted generalized additive models (to allow for nonlinearity).
Results/Conclusions
Results indicate that outbreak size and seasonality are driven by the availability of susceptible animals, but with an important additional effect of oceanographic conditions. There are a number of candidate underlying biological mechanisms that could link environmental conditions to transmission dynamics. Oceanographic processes such as upwelling of cold nutrient-rich ocean currents and ENSO events (El Niño and La Niña) are known to influence short- and long-term food availability, with bottom-up effects on sea lion movement and intraspecific contact patterns, food intake, body condition and possibly susceptibility to infection. The next steps in understanding how environment and demography interact to drive Leptospira transmission will be to determine how these mechanistic processes might affect transmission.