Abstract
Infectious diseases can cause population declines and even extinctions. The amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), has caused population declines and extinctions in amphibians on most continents. In the tropics, research on the dynamics of this disease has focused on amphibian populations in mountainous areas. In most of these areas, high and low elevation sites are connected by an assemblage of streams that may transport the infectious stage of the pathogen from high to low elevations, and, also, this pathogen, which grows well at cool temperatures, may persist better in cooler water flowing from high elevations. Thus, the dynamics of disease at low elevation sites without aquatic connections to higher elevation sites, i.e., non-contiguous low elevation sites, may differ from dynamics at contiguous low elevation sites. We sampled adult common mistfrogs (Litoria rheocola) at six sites of three types: two at high (> 400m) elevations, two at low elevations contiguous with high elevation streams, and two at low elevations non-contiguous with any high elevation site. Adults were swabbed for Bd diagnosis from June 2010 to June 2011 in each season, over a total of five sampling periods. The prevalence of Bd fluctuated seasonally and was highest in winter across all site types. Site type significantly affected seasonal patterns of prevalence of Bd. Prevalence remained well above zero throughout the year at the high elevation sites. Prevalence declined to lower levels in contiguous low sites, and reached near-zero at non-contiguous low sites. Patterns of air temperature fluctuation were very similar at both the low elevation site types, suggesting that differences in water connectivity to high sites may have affected the seasonal dynamics of Bd prevalence between contiguous and non-contiguous low elevation site types. Our results also suggest that reservoir hosts may be important in the persistence of disease at low elevations.
Highlights
Emerging infectious diseases (EIDs) can pose major threats to wildlife species [1]
For the first time to our knowledge, that connectivity between high and low elevation sites may influence the seasonal patterns of prevalence of Batrachochytrium dendrobatidis (Bd) in frog populations, such that sites connected to high elevation sites by water flow had higher prevalence of disease in adult frogs
We demonstrated that the dynamics of Bd infections on L. rheocola were influenced by temperature and season, effects that have been reported in previous studies on several species in Australia and elsewhere [15,18,19,20,35,36,37]
Summary
Emerging infectious diseases (EIDs) can pose major threats to wildlife species [1]. At the individual level, disease can cause illness and may cause death, and at the population level, disease can cause declines and extinctions [2,3,4,5,6]. Rates of transmission to susceptible hosts, and pathogen load within hosts are the driving forces behind the dynamics of infectious diseases [7,8]. Extinctions are possible, when a pathogen has a resting stage, or when a pathogen persists in the environment by other means, or has a reservoir host or hosts [1]. Reservoir hosts are typically less susceptible species that can sustain the pathogen, allowing it to persist in the environment even when susceptible species decline and disappear from the system [4,9,10,11]. Determining the dynamics of diseases is important in understanding their effects on host populations, and in providing possible ways of mitigating host declines (e.g., by reducing transmission rates)
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