title: Detecting signals of chronic shedding to explain pathogen persistence: Leptospira interrogans in California sea lions.
abstract: Identifying mechanisms driving pathogen persistence is a vital component of wildlife disease ecology and control. Asymptomatic, chronically infected individuals are an oft-cited potential reservoir of infection, but demonstrations of the importance of chronic shedding to pathogen persistence at the population-level remain scarce. Studying chronic shedding using commonly collected disease data is hampered by numerous challenges, including short-term surveillance that focuses on single epidemics and acutely ill individuals, the subtle dynamical influence of chronic shedding relative to more obvious epidemic drivers, and poor ability to differentiate between the effects of population prevalence of chronic shedding vs. intensity and duration of chronic shedding in individuals. We use chronic shedding of Leptospira interrogans serovar Pomona in California sea lions (Zalophus californianus) as a case study to illustrate how these challenges can be addressed. Using leptospirosis-induced strands as a measure of disease incidence, we fit models with and without chronic shedding, and with different seasonal drivers, to determine the time-scale over which chronic shedding is detectable and the interactions between chronic shedding and seasonal drivers needed to explain persistence and outbreak patterns. Chronic shedding can enable persistence of L. interrogans within the sea lion population. However, the importance of chronic shedding was only apparent when surveillance data included at least two outbreaks and the intervening inter-epidemic trough during which fadeout of transmission was most likely. Seasonal transmission, as opposed to seasonal recruitment of susceptibles, was the dominant driver of seasonality in this system, and both seasonal factors had limited impact on long-term pathogen persistence. We show that the temporal extent of surveillance data can have a dramatic impact on inferences about population processes, where the failure to identify both short- and long-term ecological drivers can have cascading impacts on understanding higher order ecological phenomena, such as pathogen persistence.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California - Los Angeles, Los Angeles, CA, USA.|Fogarty International Center, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Michael G
authors.identifier: 0000-0002-5996-8927
authors.initials: MG
authors.lastname: Buhnerkempe
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California - Los Angeles, Los Angeles, CA, USA.|Fogarty International Center, National Institutes of Health, Bethesda, MD, USA.
authors.forename: Katherine C
authors.initials: KC
authors.lastname: Prager
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California - Los Angeles, Los Angeles, CA, USA.
authors.forename: Christopher C
authors.initials: CC
authors.lastname: Strelioff
authors.affiliation: The Marine Mammal Center, Sausalito, CA, USA.
authors.forename: Denise J
authors.initials: DJ
authors.lastname: Greig
authors.affiliation: National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, Seattle, WA, USA.
authors.forename: Jeff L
authors.initials: JL
authors.lastname: Laake
authors.affiliation: National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, Seattle, WA, USA.
authors.forename: Sharon R
authors.initials: SR
authors.lastname: Melin
authors.affiliation: National Marine Mammal Laboratory, Alaska Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, Seattle, WA, USA.
authors.forename: Robert L
authors.initials: RL
authors.lastname: DeLong
authors.affiliation: The Marine Mammal Center, Sausalito, CA, USA.
authors.forename: Frances M D
authors.initials: FM
authors.lastname: Gulland
authors.affiliation: Department of Ecology and Evolutionary Biology, University of California - Los Angeles, Los Angeles, CA, USA.|Fogarty International Center, National Institutes of Health, Bethesda, MD, USA.
authors.forename: James O
authors.initials: JO
authors.lastname: Lloyd-Smith
mesh_terms: D000818:Animals; D002140:California / Q000453:epidemiology; D004196:Disease Outbreaks / Q000662:veterinary*; D005260:Female; D015994:Incidence; D007921:Leptospira interrogans / Q000502:physiology*; D007922:Leptospirosis / Q000453:epidemiology / Q000382:microbiology / Q000635:transmission / Q000662:veterinary*; D008297:Male; D008962:Models, Theoretical; D015995:Prevalence; D046070:Sea Lions; D012621:Seasons; D017201:Virus Shedding
keywords: asymptomatic infection; birth pulse; critical community size; epidemic drivers; maintenance host; marine mammal stranding; partially observed Markov process; pathogen reservoir; seasonal transmission; subclinical shedding