Decline in the prevalence of chytridiomycosis in frog populations in North Queensland, Australia

Abstract

In the early 1990s stream-associated amphibian populations in tropical upland North Queensland experienced severe declines resulting in extinction of three species, local elimination of four species, marked reductions in one species and apparently no declines in other species, Chytridiomycosis, a disease due to the amphibian chytrid fungus, Batrachochylrium dendrobatidis, was the likely cause of this epidemic. We conducted a monitoring study for chytridiomycosis in four species of frogs in North Queensland from October 1998 to October 2002 by collecting specimens in the field and using histology of removed digits to diagnose chytridiomycosis. Chytridiomycosis was diagnosed in 112 (7,1%) of the 1 578 specimens and prevalence was significantly associated with season and altitude, with higher prevalences in winter and above 300 metres altitude. A multivariate model adjusting for potential confounding effects arising from the sampling process demonstrated a significant decline in the time trend of prevalence of chytridiomycosis. The study supports the hypothesis that B. dendrobatidis becomes endemic after the initial epidemic wave. Since the surviving species of stream-associated frog, Litoria genimaculata, has increased to pre-decline numbers, the decline in prevalence of chytridiomycosis is evidence of a changed pathogen-host relationship. The reasons for this change are speculative but could be due to an increase in innate host resistance in response to selection pressure by B. dendrobatidis or to lower rainfall associated with an EI Nino effect. These findings justify management strategies that assist susceptible amphibian species to survive an initial epidemic wave of chytridiomycosis.

Widespread occurrence of the amphibian chytrid fungus in Kenya

“…Furthermore, an enzootic state of Bd may be a consequence of populations being in a post-decline phase and that declines were merely not observed as they happened. There are now numerous examples of enzootic Bd infection in amphibian populations globally (Retallick, McCallum & Speare, 2004;McDonald et al, 2005;Longcore et al, 2007;Brem & Lips, 2008;Woodhams et al, 2008;Padgett-Flohr & Hopkins, 2009), and recent compelling evidence suggests that Bd can have a marked impact even decades after becoming enzootic (Murray et al, 2009). Hence, the apparently enzootic state of Bd found in the present study should be regarded as a conservation concern and requires further investigation.…”

Context-dependent symbioses and their potential roles in wildlife diseases

“…We suggested earlier that context-dependent changes in the activity of mutualistic bacteria against Bd could explain the relatively high virulence sometimes exhibited by chytridiomycosis at temperatures well below those optimal for Bd growth. Additionally, frogs have reappeared at some upland sites from which they were initially extirpated [96]. It is not presently known whether this reflects growth from relictual in situ populations or recolonization.…”

Condition-dependent reproductive effort in frogs infected by a widespread pathogen

“…By the mid-1990s, chytridiomycosis had extirpated this IUCNEndangered species [33] at higher elevations (more than 400 m) throughout its geographical range [31]. However, many populations have subsequently recovered or recolonized these areas [34] and now coexist with the pathogen [25]. Several aspects of the ecology of L. rheocola make it ideal for examining how disease influences calling effort across seasons and with body condition.…”