Prior Infection Does Not Improve Survival against the Amphibian Disease Chytridiomycosis

Abstract

Many amphibians have declined globally due to introduction of the pathogenic fungus Batrachochytrium dendrobatidis (Bd). Hundreds of species, many in well-protected habitats, remain as small populations at risk of extinction. Currently the only proven conservation strategy is to maintain species in captivity to be reintroduced at a later date. However, methods to abate the disease in the wild are urgently needed so that reintroduced and wild animals can survive in the presence of Bd. Vaccination has been widely suggested as a potential strategy to improve survival. We used captive-bred offspring of critically endangered booroolong frogs (Litoria booroolongensis) to test if vaccination in the form of prior infection improves survival following re exposure. We infected frogs with a local Bd isolate, cleared infection after 30 days (d) using itraconazole just prior to the onset of clinical signs, and then re-exposed animals to Bd at 110 d. We found prior exposure had no effect on survival or infection intensities, clearly showing that real infections do not stimulate a protective adaptive immune response in this species. This result supports recent studies suggesting Bd may evade or suppress host immune functions. Our results suggest vaccination is unlikely to be useful in mitigating chytridiomycosis. However, survival of some individuals from all experimental groups indicates existence of protective innate immunity. Understanding and promoting this innate resistance holds potential for enabling species recovery.

Amphibians acquire resistance to live and dead fungus overcoming fungal immunosuppression

“…The only previous study to repeatedly infect and clear amphibians of Bd seemed to conflict with our results 20 . In this study 20 , the authors concluded that there was no evidence of acquired resistance in the critically endangered booroolong frog ( Litoria booroolongensis ) because frogs previously cleared of Bd with the fungicide itraconazole did not have significantly lower prevalence or mortality upon re-infection than frogs that were not previously exposed to Bd or the fungicide (20 of 32 versus 14 of 28 infected, respectively; χ 2 1 = 0.9, P = 0.33).…”

“…In this study 20 , the authors concluded that there was no evidence of acquired resistance in the critically endangered booroolong frog ( Litoria booroolongensis ) because frogs previously cleared of Bd with the fungicide itraconazole did not have significantly lower prevalence or mortality upon re-infection than frogs that were not previously exposed to Bd or the fungicide (20 of 32 versus 14 of 28 infected, respectively; χ 2 1 = 0.9, P = 0.33). However, the fungicide alone increased Bd prevalence in Bd -naive frogs (no previous fungicide: 14 of 28 infected, previous fungicide: 10 of 11 infected, χ 2 1 = 5.5, P = 0.01), consistent with other studies suggesting that itraconazole is immunosuppressive 27 .…”

Assessing host extinction risk following exposure to Batrachochytrium dendrobatidis

“…This corresponds to findings suggesting that infested waters and sand might be the primary source of infection [21,28,32]. Infections might also change due to within-host disease dynamics, modelled through transition probability rates between DOIs.…”

“…For example, the yearly host survivorship is halved at a certain DOI tolerance level, T s and the adult breeding rate is halved at a certain DOI, T br . Individuals die immediately if their DOI reaches a lethal threshold [28,32,34]. We also explicitly considered the case where tadpoles completely tolerate their infections, e.g.…”

Confronting inconsistencies in the amphibian‐chytridiomycosis system: implications for disease management

“…Recently, Cashins et al (2013) used itraconazole (an antifungal) to clear frogs of their Bd infection and found that frogs infected with Bd, treated with itraconazole, and then re-exposed to Bd had a higher prevalence of infection compared to frogs only exposed to Bd once. Recently, Cashins et al (2013) used itraconazole (an antifungal) to clear frogs of their Bd infection and found that frogs infected with Bd, treated with itraconazole, and then re-exposed to Bd had a higher prevalence of infection compared to frogs only exposed to Bd once.…”