Published: 17 March 2015
E.H. Graham Centre (NSW Department of Primary Industries and Charles Sturt University), PO Box 588 Wagga Wagga, NSW 2650, Australia
School of Animal and Veterinary Sciences, Charles Sturt University, Locked Bag 588, Wagga Wagga, NSW 2678, Australia
Tel: +61 2 6933 2218
Mucormycosis in the platypus and the anuran (frogs and toads) is a serious fungal disease affecting these aquatic taxa. Mucor amphibiorum infection causes significant morbidity and mortality in free-living platypuses in Tasmania. Infection has also been reported in free-ranging cane toads and frogs from mainland Australia, but not confirmed in platypuses from the mainland. This paper reviews mucormycosis in the platypus and anuran, including consideration of the clinical, epidemiological, pathological and diagnostic features.
Mucor amphibiorum is a dimorphic fungus in the Mucorales order of the Zygomycetes class of fungi. Its sporangiospores, when found in infected tissues, occur as the yeast form (spherule-like structures, containing 2–11 daughter spherules) or develop into the more usual non-septate hyphal form on culture media or in the environment1,2. Infections (Table 1) have been reported in a range of anurans (frogs, toads), and the platypus1,3,4,6,8–10. Transmission between captive anurans and salamanders has been documented; while experimentally infected reptiles remained clinically healthy with only small lesions at necropsy, and no lesions were reported in experimental infections of laboratory animals1,6. Mucor amphibiorum appears to be endemic in Australia, infecting free-living frogs and toads in Queensland, New South Wales and Northern Territory, with accidental introductions into captive frogs in Melbourne and Perth5,7 and platypuses in Tasmania. It seems unlikely that it was introduced into Australia with cane toads in 1935, as Speare et al (1994)3 was unable to isolate it from 41 cane toads sampled in Hawaii or Costa Rica.
Mucor amphibiorum was first reported from a German Zoo in 1972, where it resulted in disseminated disease in a common green tree frog (Litoria caerulea) imported from Australia, and subsequently in frogs, toads and salamanders in neighbouring exhibits1,6. In amphibians, mucormycosis caused a disseminated disease with multiple white nodules in liver, kidney, bladder and lung, the emaciated animal dying within 2–4 weeks. Skin involvement via lymphatic spread was observed in 42% of infected toads, but skin ulcers were rare11. Histologically, nodules consisted of granulomas and pyogranulomas containing thick-walled spherules (5–37 μm diameter, containing 0 to 10 daughter spherules). Of nine M. amphibiorum isolates from cane toads (Rhinella marina), five were positive mating strains and four were negative mating strains11. The route of entry of M. amphibiorum in the anuran is likely ingestion of soil contaminated by faeces excreted by infected animals. M. amphibiorum was isolated from 2/20 soil specimens from an endemic site in Townsville where resident cane toads had mucormycosis. Furthermore, the organism has been shown to grow and sporulate in soil11.
Munday and Peel (1983)8 first described four cases of ulcerative dermatitis in dead and debilitated Tasmanian platypuses from the Elizabeth River in Campbell Town, but the causative agent was not identified as M. amphibiorum until 19939. M. amphibiorum causes a severe granulomatous and often ulcerative dermatitis in the platypus, which may progress to involve underlying muscle and occasionally disseminate to internal organs, particularly the lungs9, leading to death. In the absence of the systemic spread of the organism, death can also result from secondary bacterial infections or impaired thermoregulation and mobility.
All 17 platypuses with mucormycosis captured during a 12 month Tasmanian study10 were alert and displayed normal responses to capture and handling. Gross appearance of skin lesions varied from non-ulcerated, hairless nodules and abscesses, to ulcers with under-run or thickened margins, sinuses exuding pus, or exuberant granulation tissue (Figure 1). Some lesions appeared as discrete entities. Others coalesced to form plaques. Lesions were found on haired regions including the hind limbs (38%), forelimbs (6%), tail (19%), trunk (6%) and head (6%), and unhaired regions such as the webbing of the forelimbs (13%) or bill (6%). Some affected animals had lesions at more than one site. One platypus had a tail ulcer which reduced in size over a three month period. Of 13 isolates of M. amphibiorum from 17 diseased platypuses, all were of the positive mating strain10. One platypus M. amphibiorum isolate was tested showed susceptibility to amphotericin B, but resistance to both itraconazole and fluconazole10. In a pathogenicity study using cane toads, Stewart and Munday (2004)12 found that the positive mating strain and platypus-derived isolates of M. amphibiorum were more pathogenic than negative mating strains or anuran-derived isolates. In a disinfectant trial, a positive mating strain of M. amphibiorum from a platypus was more resistant to disinfectants (Phytoclean®, Path-X®, F10sc®) than a negative strain from a frog13. M. amphibiorum was not isolated from 40 faecal or 8 healthy skin samples from platypuses or 14 environmental samples including soil, water, frog faeces, and Ixodes ornithorhynchi ticks. Mucor circinelloides was isolated from samples of soil, platypus and frog faeces; Mucor hiemalis was cultured from platypus faeces and Mucor saturninus from soil samples from the study site. Mucor circinelloides was reported from one platypus ulcer14, but was later thought to be a contaminant as it was incapable of infecting cane toads12.
The sudden emergence of mucormycosis in Tasmanian platypuses in 1982 may have resulted from accidental introduction of this pathogenic fungus with ‘banana box frogs’ from Queensland12 to a naïve Tasmanian platypus population (similar to the recent introduction of the chytrid fungus into Tasmania15). Alternately, an endemic Tasmanian strain of M. amphibiorum may have mutated to become pathogenic for platypuses16,18. Since the index cases of mucormycosis in the platypus in 19828, the distribution of the disease has slowly expanded but remained endemic to the catchments draining into the Tamar River. Spread of the agent could be via movement of platypuses and other aquatic hosts or fomites such as contaminated fishing gear and tyre treads. In 1994, mucormycosis prevalence in the platypus at Brumbys Creek was 33%10. By 2009, the prevalence of platypus mucormycosis across Tasmania appeared to be declining17,18.
Diagnosis of mucormycosis is based on culturing M. amphibiorum from characteristic lesions. Aseptically collected representative specimens (including fine needle aspirates, swabs and punch biopsies) should be inoculated onto Sabouraud’s dextrose agar with and without gentamicin (50 IU/mL) and incubated at 28°C. Single colonies can then subcultured onto plates containing Sabouraud’s dextrose agar without antibiotics or potato dextrose agar for more detailed morphological studies and mating experiments8. Two mating strains, CBS 763.74 (positive type strain) and CBS 185.77 (negative reference strain) were used to assess zygospore production in aerial hyphae2. By definition, positive strains produce zygospores only in test matings with negative strains.
Clinical signs (Figure 1), the presence of spherules in cytology preparations or histological sections from lesions (Figure 2) further support a diagnosis of mucormycosis in anurans or platypus, but are less specific than culture. Corynebacterium ulcerans and an unidentified fungus were isolated from cutaneous lesions resembling mucormycosis in two platypuses18. Several environmental Mucor species other than M. amphibiorum display dimorphism including M. circinelloides, M. hiemalis and M. saturninus, and could potentially result in similar-appearing spherules in lesions. In the platypus, M. amphibiorum-specific serum immunoglobulin may be detected by ELISA19. To date, no PCR has been used to identify M. amphibiorum DNA from clinical (platypus and amphibian) or environmental samples, although panfungal PCR assays have been used to detect a range of fungi in fresh and paraffin-embedded tissues20,21 and the fungal microbiome of canine duodenal samples22.
A collection of 21 Mucor isolates representing isolates from platypus, frogs, toads and environmental samples were obtained for genotypic analysis (Figure 3)23. Internal transcribed spacer (ITS) region sequencing and GenBank comparison confirmed the identity of most isolates. Platypus isolates formed a clade containing the reference isolates of M. amphibiorum from the CBS repository. The M. amphibiorum isolates showed close sequence identity with Mucor indicus and consisted of two haplotypes, differentiated by single nucleotide polymorphisms within ITS1 and ITS2 regions. Except for one, all isolates from platypuses were in one haplotype. Multi-locus fingerprinting via the use of intersimple sequence repeats (ISSR) PCR identified 19 genotypes. Two major clusters were evident: (1) M. amphibiorum and Mucor racemosus; and (2) Mucor circinelloides, Mucor ramosissimus, and Mucor fragilis. Seven M. amphibiorum isolates from platypuses were present in two subclusters, with one isolate appearing genetically distinct from all other isolates. Isolates classified as M. circinelloides by sequence analysis formed a separate subcluster, distinct from other Mucor spp. The combination of sequencing and multilocus fingerprinting has the potential to provide the tools for rapid identification of M. amphibiorum.
Future work should include the development and refinement of molecular tools to detect free-living forms of M. amphibiorum in the environment as well as infective forms in tissue lesions. The potential for other aquatic vectors for M. amphibiorum needs to be assessed. Such developments will likely lead to an improved understanding of the environmental niche of the fungus and how it is spread in Tasmania. This could lead to control measures to prevent further spread of this disease.
Joanne Connolly teaches Veterinary Microbiology and is the Course Coordinator of the Captive Vertebrate Management Program at Charles Sturt University in Wagga Wagga. The major themes of Dr Connolly’s research are veterinary microbiology, public health, as well as wildlife biology and disease. Research topics of interest include Mucor amphibiorum, Cryptococcus neoformans, Salmonella, Campylobacter jejuni, Escherichia coli and Chlamydophila in animals and host-agent-environmental relationships.
The tale of a tiny worm, the bacteria that live inside her, and a tree being munched on by a grub.