Infection dynamics in frog populations with different histories of decline caused by a deadly disease

Abstract

Pathogens can drive host population dynamics. Chytridiomycosis is a fungal disease of amphibians that is caused by the fungus Batrachochytrium dendrobatidis (Bd). This pathogen has caused declines and extinctions in some host species whereas other host species coexist with Bd without suffering declines. In the early 1990s, Bd extirpated populations of the endangered common mistfrog, Litoria rheocola, at high-elevation sites, while populations of the species persisted at low-elevation sites. Today, populations have reappeared at many high-elevation sites where they presently co-exist with the fungus. We conducted a capture-mark-recapture (CMR) study of six populations of L. rheocola over 1 year, at high and low elevations. We used multistate CMR models to determine which factors (Bd infection status, site type, and season) influenced rates of frog survival, recapture, infection, and recovery from infection. We observed Bd-induced mortality of individual frogs, but did not find any significant effect of Bd infection on the survival rate of L. rheocola at the population level. Survival and recapture rates depended on site type and season. Infection rate was highest in winter when temperatures were favourable for pathogen growth, and differed among site types. The recovery rate was high (75.7-85.8%) across seasons, and did not differ among site types. The coexistence of L. rheocola with Bd suggests that (1) frog populations are becoming resistant to the fungus, (2) Bd may have evolved lower virulence, or (3) current environmental conditions may be inhibiting outbreaks of the fatal disease.

Using a Bayesian network to clarify areas requiring research in a host–pathogen system

“…The latter case is consistent with a hypothesis for persistence of populations of common mist frogs ( L. rheocola) with B. dendrobatidis , whereby overdispersed infection loads result in many individuals with very low infection levels, changing infection states, and a few individuals with high infection loads (Abell ; Roznik ; Sapsford et al. ).…”

“…When all frogs became infected and intensity of infection increased, probability of decline was 100%, but when probability of a frog becoming infected was 100% but the intensity of infection decreased, the probability of decline was 40%. The latter case is consistent with a hypothesis for persistence of populations of common mist frogs (L. rheocola) with B. dendrobatidis, whereby overdispersed infection loads result in many individuals with very low infection levels, changing infection states, and a few individuals with high infection loads (Abell 2002;Roznik 2013;Sapsford et al 2015).…”

“…The strongest change in population trend was driven by infection prevalence (frog becomes infected), reflecting the importance of transmission (Dobson 2004). When frog becomes infected was set to a 100% and intensity of infection was decreasing, the probability of population decline was 40%, which is consistent with findings that susceptible species persist with pathogens endemic to the host population because infections are cleared, and reestablish on individuals without necessarily causing mortality or population declines (Phillott et al 2013;Roznik 2013;Sapsford et al 2015).…”

Disease and climate effects on individuals drive post‐reintroduction population dynamics of an endangered amphibian

“…This flexibility is critical for understanding disease impacts in wild populations. Multi-state mark-recapture methods have been applied with great success to understand the population-level impacts of Bd, and differences in demographic rates between infected and uninfected classes (Murray et al 2009, Pilliod et al 2010, Sapsford et al 2015, Hudson et al 2016.…”

Amphibian species traits, evolutionary history and environment predict Batrachochytrium dendrobatidis infection patterns, but not extinction risk

“…Second, there is evidence that pathogenicity of virulent B. dendrobatidis strains can potentially decline over time (Refsnider, Poorten, Langhammer, Burrowes, & Rosenblum, ), suggesting that both hosts and pathogens may eventually attain some level of equilibrium. Finally, these two points are strengthened by cases where amphibian species previously devastated by chytridiomycosis are recolonizing sites where B. dendrobatidis is still endemic (Knapp et al., ; Sapsford, Voordouw, Alford, & Schwarzkopf, ; Scheele et al., ). Together, these lines of evidence suggest that while B. dendrobatidis susceptibility is important, other mechanisms may be just as important for ultimately determining species fates in environments where B. dendrobatidis is introduced.…”