Lee
Berger and Rick Speare
School of Public
Health and Tropical Medicine
James Cook University
Townsville 4811
Australia.
Amphibian
chytridiomycosis is the most formidable disease of amphibians globally (Speare
et al 2001) and is the most serious fungal disease. It is discussed elsewhere
at this site and summarised in Formidable
Infectious Diseases of Amphibians.
The other fungus that is a significant primary pathogen, Mucor amphibiorum,
is also discussed elsewhere at this site.
Basidiobolus ranarum
Basidiobolus ranarum is
a zygomycete that can be frequently
isolated from the intestines of healthy amphibians and lizards
(Reichenbach-Klinke and Elkan 1965). B.
ranarum was reported to cause an epizootic of cutaneous mycosis in captive Hymenochirus curtipes in America (Groff
et al 1991), but the identification of the fungus involved appears doubtful and
B. ranarum may have been cultured as
a contaminant. The morphology of the organism in the skin was different to the
appearance of cultured B. ranarum and
more typical of B. dendrobatidis
since it occurred as a spherical form in the skin with no hyphae typical of B. ranarum. Experimental transmission
could not be achieved using cultured B.
ranarum, but the disease was transmitted when healthy frogs were exposed to
sick frogs (Groff et al 1991).
Saprolegniasis
Saprolegniasis in captive amphibians is similar to the disease in fish with pale tufts of fungus growing on the skin. The disease mainly effects aquatic species and life-stages and usually occurs secondary to epidermal damage. Saprolegnia ferax and S. parasitica are the most common species isolated (Anver and Pond 1984). S. ferax was found to be responsible for high mortality rates in eggs of B. boreas in the wild in northwest USA (Blaustein et al 1994).
Chromomycosis
Chromomycosis refers to infection with a range of pigmented, septate fungi from the phylum Ascomycota. Many reports mention difficulties in identifying fungal species due to a lack of sporulation in tissues and in culture. Pigmented fungi including Fonsecaea pedrosi, F. dermatitidis, Cladosporium sp. Scolecobasidium sp. and Phialophora sp. have been isolated from lesions in a range of captive amphibians including B. marinus, R. pipiens, R. catesbiana, Hyla caerulea, Phyllobates trinitatis, Ceratophrys ornata, Rhacophorus sp., H. septentrionalis (Beneke 1978; Cicmanec et al 1973; Elkan and Philpot 1973; Rush et al 1974; Miller et al 1992) and wild B. melanostrictus (Dhaliwal and Griffiths 1963). These organisms have also been isolated from tanks housing captive frogs. Clinical signs are of chronic debilitating disease, and papules and ulceration may occur. Frogs died 1‑6 months after first showing signs of infection. Multiple grey nodules occurred in liver, kidney, heart, lung, skeletal muscle, meninges, bone marrow and other organs. These were fibrous granulomas with mononuclear cells, epithelioid cells and multinucleate giant cells around pigmented, septate fungi or spherical chlamydospores. The granulomas coalesced and replaced much of the parenchyma. Central caseation occurred in very large granulomas. A haematogenous spread was suspected due to the multi-organ infections. Transmission experiments had variable results. Rush et al (1974) transmitted disease in healthy, unstressed frogs whereas Elkan and Philpot (1973) could not infect healthy frogs by intraperitoneal inoculation. Cicmanec et al (1973) transmitted the disease by intracoeloemic injection only if toads were stressed by refrigeration, monthly feeding, or limited water.
Dermocystidium and Dermosporidium
Dermocystidium spp. and Dermosporidium spp. grow as large spore-filled cysts in subcutaneous tissue or the dermis and can cause inflammation and ulcerations (Broz and Privora 1951; Jay and Pohley 1981). Infections have been found in Europe and America, and can occur at high prevalence in a population (Reichenbach-Klinke and Elkan 1965). Sequencing of small-subunit rRNA genes from Dermocystidium spp. showed this genus to be part of a clade of protistan parasites near the animal-fungal divergence (Ragan et al 1996).
Fungal disease of
tadpoles
Two outbreaks of fungal disease were described in tadpoles of B. marinus near Townsville (Berger et al 2001). The disease had an unusual appearance with tufts of fungi growing primarily on the head of tadpoles. When the disease was encountered a second time, more detailed investigations were undertaken. Although a range of fungi were cultured, all were considered to be secondary infections or contaminants except for an Aphanomyces sp. that is likely to be the primary agent. Mortality rate could not be determined, but the high prevalence (37%) and debilitating lesions show the disease would have impacted on the tadpole population. However, this would be unlikely to affect the adult population size as toads are phenomenally fecund. Also in normal circumstances tadpole survival has been reported to be very low (5%) due to density dependent predation (Calef 1973).
Updated 16 February, 2003
Rick Speare