DISEASES OF AQUATIC ORGANISMS Dis Aquat Org Vol. 73: 257–260, 2007 Published January 18 Smith (2007, this issue) raises important issues re- garding the interpretation of the results of chytrid diagnostic assays, and highlights the blurred distinc- tions between the presence of Batrachochytrium den- drobatidis on a sample, infection by B. dendrobatidis, and the disease chytridiomycosis. In this reply, we (1) clarify the distinction between a chytrid infection and the disease chytridiomycosis; (2) suggest that the detection of chytridiomycosis (as opposed to the detection of B. dendrobatidis) has only been possible for dead or dying frogs, and is therefore of limited use to researchers; (3) demonstrate the biological rel- evance of a single B. dendrobatidis zoospore, and thus the importance of maintaining a conservative approach to the interpretation of diagnostic results; (4) confirm the usefulness of the quantifications pro- duced by the quantitative (real-time) polymerase chain reaction (qPCR) assay; and (5) re-iterate our view that qPCR outperforms histology in virtually all respects, that the benefits of histology are far out- weighed by its shortcomings, and thus qPCR should indeed be adopted as the primary means of detecting B. dendrobatidis on live amphibians. Infection as opposed to disease. While an infection is merely the ‘the establishment of a pathogen in its host after invasion’, disease is ‘a condition of the living animal or plant body or of one of its parts that impairs normal functioning and is typically manifested by dis- tinguishing signs and symptoms’ (http://merriam- webster.com). We will rely on these definitions for the remainder of this reply. When and how can we detect chytridiomycosis? It has long been clear that the visual assessment of dis- ease status in infected frogs is unreliable. While frogs often do not die until approximately 1 mo after being exposed to Batrachochytrium dendrobatidis (Longcore et al. 1999, Nichols et al. 2001, Daszak et al. 2004), dis- tinguishing signs and symptoms of chytridiomycosis (i.e. lethargy, anorexia, excessive sloughing of skin) are often restricted to infected animals in the final stages of disease progression and are thus not evident until a few days prior to death (Berger et al. 1999, 2004, Nichols et al. 2001). Sometimes they go completely undetected: in a die-off of 28 captive poison dart frogs (Dendrobates azureus and D. auratus), only 2 frogs showed premonitory clinical signs (anorexia and lethargy), and these not until 1 d prior to death (Pessier et al. 1999). Thus the need to develop more sensitive diagnostic techniques, such as histology and PCR, to detect the potential for disease in individuals and pop- ulations before disease outbreaks actually occur. Smith (2007) states that since a positive histological result is dependent on the presence of clusters of Batrachochytrium dendrobatidis zoospores, histology provides a robust confirmation of chytridiomycosis. We disagree. Using histology, Hanselmann et al. (2004) found high prevalence (96%, n = 48) of chytrid infec- tion in bullfrogs Rana catesbeiana in Venezuela. Many frogs had B. dendrobatidis infecting up to 40% of their epithelial cells, yet no mortality or clinical signs of dis- ease were observed. Further, the bullfrog population appeared to be rapidly increasing in numbers, rather © Inter-Research 2007 · www.int-res.com *Email: kerry@savethefrogs.com REPLY COMMENT On the biological relevance of a single Batrachochytrium dendrobatidis zoospore: a reply to Smith (2007) Kerry M. Kriger 1, * , Kevin J. Ashton 2 , Harry B. Hines 3 , Jean-Marc Hero 1 1 Centre for Innovative Conservation Strategies and 2 Heart Foundation Research Centre, Griffith University, PMB 50 Gold Coast Mail Centre, Queensland 9726, Australia 3 Queensland Parks and Wildlife Service, PO Box 64, Bellbowrie, Queensland 4070, Australia