Retreat sites of rain forest stream frogs are not a reservoir for Batrachochytrium dendrobatidis in northern Queensland, Australia

Abstract

Chytridiomycosis is a potentially fatal disease of amphibians caused by Batrachochytrium dendrobatidis, and is implicated in declines and extinctions of amphibian populations and species around the world. To cause local host extinction, a disease organism must persist at low host densities. One mechanism that could facilitate this is the ability to persist in the environment. In the laboratory, B. dendrobatidis spreads by both frog-to-frog and environment-to-frog transmission, and can persist on a number of biotic substrates. In the field, B. dendrobatidis has been detected on environmental samples taken during an epidemic, but it is not known if it persists in the environment when endemic. Retreat sites of 2 species of Australian rain forest stream frogs Litoria lesueuri and L. nannotis were sampled 0 to 3 d after occupation during the wet and dry seasons in northern Queensland, Australia, where chytridiomycosis has been endemic for at least 10 yr. The intensity and prevalence of infection in frogs during sampling were comparatively low compared with epidemics. Diagnostic quantitative polymerase chain reaction did not detect B. dendrobatidis in any retreat site samples. It thus appears that retreat sites are not a major environmental source of infection when B. dendrobatidis occurs at low prevalence and intensity on frogs. This suggests that control efforts may not need to eliminate the organism from the environment, at least when prevalence and intensity of infection are low in frogs. Simply treating hosts may be effective at controlling the disease in the wild. KEY WORDS: Batrachochytrium dendrobatidis · Amphibian chytrid fungus · Chytridiomycosis · Environmental reservoir · Disease transmission · FrogsResale or republication not permitted without written consent of the publisher Dis Aquat Org 74: 7-12, 2007 B. dendrobatidis is the only chytrid known to cause disease in vertebrate hosts (Berger et al. 1998), presumably breaking down keratin that occurs in the epidermis of adult amphibians and the mouthparts of larval anurans. Chytridiomycosis can cause rapid mortality (Nichols et al. 2001), with infected frogs of susceptible species dying within 3 wk of infection in the laboratory (Berger et al. 1998(Berger et al. , 2004. In the laboratory, the disease is highly contagious (Nichols et al. 2001), spreading within and among individuals via motile, waterborne zoospores .One of the unusual aspects of chytridiomycosis is that it drives many host species to local extinction during outbreaks (Berger et al. 1998, Lips et al. 2006. For a pathogen to cause local host extinction it must be capable of persisting and infecting new hosts even at very low host population densities (Anderson & May 1986, Dobson & May 1986. One potential mechanism for this is the persistence and growth of Batrachochytrium dendrobatidis in tadpoles and adults that do not die from infection (Berger et al. 1998, 1999. The broad host range of B. dendrobatidis also provides a reservoir of infection, enabling the species most susceptib...

Elevation, Temperature, and Aquatic Connectivity All Influence the Infection Dynamics of the Amphibian Chytrid Fungus in Adult Frogs

“…Adults of other frog species (e.g. Litoria wilcoxii ; [15,41]), other higher taxa [42,43], or the environment (water, soil, etc., [6,16,44,45]) may act as reservoirs at non-contiguous sites. We have evidence that terrestrial stages of other frog species in non-contiguous sites were also uninfected in summer ( Nyctimystes dayi, Litoria serrata, Litoria infrafrenata, Litoria leseuri (complex); A. McNab, unpublished data).…”

“…At the individual level, disease can cause illness and may cause death, and at the population level, disease can cause declines and extinctions [2-6]. Infection prevalence, rates of transmission to susceptible hosts, and pathogen load within hosts are the driving forces behind the dynamics of infectious diseases [7,8].…”

Lack of Evidence for the Drought-linked Chytridiomycosis Hypothesis

“…One explanation, the climate-linked epidemic hypothesis, has received increased attention in recent years (Pounds and Crump, 1994;Pounds et al, 1999;Kiesecker et al, 2001;Pounds, 2001;Harvell et al, 2002;Ron et al, 2003;Pounds and Puschendorf, 2004;Lampo et al, 2006;Pounds et al, 2006;Santiago-Paredes and La Marca, 2006;Alford et al, 2007;Bosch et al, 2007;Di Rosa et al, 2007;Pounds et al, 2007;Laurance, 2008;Lips et al, 2008). Although it is increasingly clear that various climate anomalies can alter the dynamics of host-pathogen systems (Harvell et al, 2002;Pounds et al, 2007), I suggest caution before accepting the hypothesis that prolonged or intensified dry seasons trigger or exacerbate epidemics of chytridiomycosis, a scenario hypothesized by several authors (Pounds et al, 1999;Ron et al, 2003;Burrowes et al, 2004;Pounds and Puschendorf, 2004;Lampo et al, 2006;Santiago-Paredes and La Marca, 2006), but for which no empirical data exist.…”

“…Batrachochytrium dendrobatidis has yet to be detected in amphibian retreat sites (Rowley et al, 2007). However, search effort has been low and it is certainly conceivable that amphibians may aggregate at hibernacula during dry periods and thus increase disease transmission.…”

Distribution Limits of Batrachochytrium dendrobatidis: A Case Study in the Rocky Mountains, USA

“…We would have preferred to sample lower-elevation wetlands that were warmer and more productive, but it is unlikely they are never used by amphibians. In work with a similar objective as ours, Rowley et al (2007) sampled terrestrial retreat sites used by Bd-positive stream frogs (Litoria spp.) and did not detect Bd.…”