Assoc Prof Rick Speare
James Cook University
Townsville 4811
Australia
richard.speare@jcu.edu.au
- What is mucormycosis?
- Causative agent
- Where does M. amphibiorum occur?
- What amphibians have been found to be infected?
- How does M. amphibiorum transmit?
- Preventing spread
in captivity
- Literature cited
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enlarged image Figure 1: Mucormycosis due to Mucor amphibiorum in a cane toad, Bufo marinus, Townsville, Australia. |
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enlarged image Figure 2. Section of a liver nodule from a Bufo marinus showing histiocytes and lymphocytes in a chronic inflammatory response with several foreign body giant cells and sphaerules of Mucor amphibiorum. |
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enlarged image Figure 3: Mucor amphibiorum growing on Saubaurad's agar as a typical filamentous fungus. |
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enlarged image Figure 4. Experimental infection of Bufo marinus by sporangiospores with initial infection established in the intestine. |
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enlarged image Figure 5. Mucor amphibiorum causing skin ulceration in a wild B. marinus. |
Amphibian
mucormycosis is a systemic disease caused by the fungus,
Mucor amphibiorum. Infected frogs and toads have fungi
disseminated through their internal organs and skin. The fungi incite formation
of granulomas that consist of inflammatory cells and fibrous tissue. At
postmortem, the liver contains small pale nodules, up to about 5 mm in
diameter, and usually in massive numbers
(Fig. 1). These nodules can also be seen in
other organs such as the kidney, lung, mesentery, urinary bladder, subcutaneous
sinuses and skin. The microscopic fungi are found inside these nodules.
Green tree frog with mucormycosis
M. amphibiorum is a primary pathogen and can infect normal
amphibians, but in the wild it appears to cause only sporadic infections.
Possibly the usual inoculating dose in the wild is not high enough to cause
epidemic disease. In captivity it can cause fatal outbreaks in collections.
The causative or aetiological agent, Mucor amphibiorum, is a zygomycete fungus not related to
Batrachochytrium dendrobatidis,, the amphibian chytrid. M. amphibiorum is
a dimorphic fungus. In the amphibian host it is a a yeast-like spherical structure, called a sphaerule. M. amphibiorum has the unusual characteristic of forming daughter spherules inside the mother spherule, and these cane be seen in histological sections of organs or on direct microscopic examination of infected tissue (Fig. 2). The
spherical shape and the daughter spherules is a key diagnostic feature in
tissue sections. Sphaerules range in size from 37 to 4.9 microns. When M. amphibiorum is growing outside the amphibian host, it becomes thread like, forming hyphae that form a mat, or mycelium (Fig. 3). Two mating types of M. amphibiorum exist, and when these meet they form resistant structures called zygospores (Fig. 4). Spores are eventually formed, and these are infectious to amphibians when ingested. M. amphibiorum grows on soil and will sporulate on the soil. Hence,
we assume that amphibians can become infected when they catch prey along with
soil containing spores.
M. amphibiorum appears to be an endemic Australian fungus.
Cases of mucormycosis have been found in amphibians in Queensland, Northern
Territory, Western Australia, and New
South Wales (see table below for details). M. amphibiorum also has been
found causing disease in platypuses in Tasmania. However, it has not been found
in amphibians in that state.
The quirky thing is that
mucormycosis due to M. amphibiorum first became known to science after
it was discovered in a captive collection in Germany (
Frank et al 1974). In
this collection it caused death in the Australian green tree frog (Litoria
caerulea) and in Dendrobates from South America. The source of the fungus
was not identified, but it may have been imported from Australia with the L.
caerulea. Another notable Australian export!
M. amphibiorum can be found in the soil, and grows well on soil
when infected tissues from frogs are placed on soil. In one epidemiological
study M. amphibiorum was isolated from soil in a greenhouse where a
series of cane toads (Bufo marinus) had died from mucormycosis over 2
years (Speare et al 1994). The available evidence suggests that M.
amphibiorum is an environmental fungus that occasionally infects
amphibians.
What amphibians have
been found to be infected? Mucormycosis has been
found in cane toads (B. marinus), green tree frogs (L. caerulea),
white lipped frogs (Litoria infrafrenata) and striped marsh frog (Limnodynastes peronii)
in the wild in Australia and in slender tree frogs (Litoria adelensis), green tree
frogs, white lipped tree frogs and dendrobatid frogs in captivity in Australia
and Germany (see table). 0.7% of cane toads in a survey we did in Queensland,
NSW and Northern Territory were infected (Speare et al 1994).
Species infected Wild / captive Deaths Location Reference captive yes Germany
Frank et al 1974; Frank 1976; Dendrobates captive yes Germany
Frank et al 1974; Frank 1976 Bufo marinus wild yes Australia (Qld, NT,
NSW)
Speare et al 1994; Speare et al 1997;
Berger 2001 Limnodynastes peronii wild yes
no Australia
(Rockhampton, Qld) L. caerulea wild yes
no Australia
(Biloela, Qld) Litoria
infrafrenata wild yes Australia (Cairns) Speare & Mendez (pers obs) Litoria adelensis captive yes Australia (Perth) captive yes Australia (Perth)
Experimentally we were
able to infect cane toads by feeding them sporangiospores from cultures
(Fig 4). The
fungus appeared to establish first in the intestinal wall and then to enter the
bloodstream and become widely disseminated. After this the fungus appeared to
spread further via the subcutaneous lymph sinuses and in the peritoneal cavity.
Infections in the skin appeared to be initiated mainly from the subcutaneous
lymph sinuses and to enter the epidermis from below in the dermis
(Fig. 5). The initial intestinal route of infection was also
indicated by severe infections with
fungus in the intestines of captive frogs in the Perth Zoo (Creeper et al
1998). Slender tree frogs had started dying within 7 days of entering
quarantine. In this outbreak transmission to a quarantined group of white lipped tree frogs may have occurred via contaminated feed bowls (Creeper et al 1998).
In a study on wild cane
toads M. amphibiorum was isolated from the faeces of two toads. Ulcerative
lesions in the skin may also allow Mucor to escape into the environment.
In the wild the fungus
has yet to be shown to have a significant impact on amphibian populations.
However, in captivity mortality rates in outbreaks have been high. No control
strategies have been trialed. From what we know of the biology and transmission
of M. amphibiorum, we can devise theoretical control strategies for
captive colonies.
Stopping transmission
Infective event: Sporangiospore is ingested with food.
Strategies:
M. amphibiorum in the captive environment: Strategies: DDAC is a highly active
fungicide. It kills Batrachochytrium at a concentration of 0.1%
(Johnson et al 2002). DDAC
has not been evaluated against M. amphibiorum, but it appears to be active
against a wide range of fungi, including many found in the natural environment.
Use at 0.1% concentration for soaking equipment and tanks and at 1% for
spraying in complex environments. DDAC is available in
Australia as Path-X from Nutri Tech Solutions, Eumundi, Qld. Visit -
http://nutri-tech.com.au/articles/path-x.htm
For additional details on quarantine procedures for amphibians in captivity see Lynch 2001.
Literature Cited
Berger L. Diseases in Australian frogs. PhD Thesis, James Cook University, Townsville. 2001:pp330.
Berger L, Speare R, Humphrey J. Mucormycosis in a free-ranging green tree frog from Australia. Journal of Wildlife Diseases 1997;33(4):903-907.
Creeper JH, Main DC, Berger L, Huntress S, Boardman W.
An outbreak of mucormycosis in slender tree
frogs (Litoria adelensis) and white-lipped tree frogs (Litoria
infrafrenata). Australian Veterinary Journal 1998;76(11):761-762.
Frank W. 1976. Mycotic infections in
amphibians and reptiles. Pp 73-88. In: Proceedings of the Third International
Wildlife Disease Conference. Ed L. A. Page. Plenum Press, New York.
Frank W, Roester U, Scholer HJ.
Sphaerule formation by a Mucor species in the internal organs of amphibia. Zentrablatt für Bakteriologie und Parasitkunde 1974;226:405-417.
Lynch M. Amphibian quarantine protocols: Melbourne Zoo. Attachment 6. In: Speare and Steering Committee of Getting the Jump on Amphibian Disease. Developing management strategies to control amphibian diseases: Decreasing the risks due to communicable diseases. School of Public Health and Tropical Medicine, James Cook University: Townsville. 2001:179-184.
Schipper MAA. On certain species of Mucor with a key to all accepted species. Studies in Mycology 1978;17:1-52.
Slocombe R, McCracken H, Booth R, Slocombe J, Birch C. Infectious skin diseases of captive frogs. In: Proceedings of the Australian Society for Veterinary Pathology May 1995:
Melbourne. Page 14.
Speare R, Berger L, O’Shea P, Ladds PW,
Thomas AD. Pathology of mucormycosis of cane toads in Australia.
Journal of Wildlife Diseases 1997;33:105-113.
Speare R, Thomas AD, O'Shea P, Shipton WA. Mucor amphibiorum in the cane toad, Bufo marinus, in Australia. Journal of Wildlife Diseases 1994;30:399-407.
Litoria caerulea
Australia (Adelaide)
Berger 2001
Australia (Brassall & Rockhampton, Qld)
L. infrafrenata
Controlling the
source
Sporangiospores are generated from the mycelium after the fungus has been
growing for at least a week. In captive husbandry, the fungus would have to
become established on a substrate that is suitable for growth. Soil is such a
substrate. We do not know if the fungus grows on plant substrate. By preventing
growth of the fungus sources within collections should be eliminated.
Dimethyl didecyl ammonium chloride (DDAC)
Updated 25 October, 2002
Rick Speare