Great Barred Frog (Mixophyes fasciolatus) with Chytridiomycosis and Mycobacteriosis

An adult, male, wild caught frog from the North Coast (Watagans National Park, Cooranbong NSW) 01/01/15, was housed in a group of 13 other M. fasciolatus at a research facility. The frog was found on the morning of 16/06/15 lying on its back, minimally responsive. Euthanasia with MS-222 was elected, and tissues were sent to the Registry for assessment.

GROSS FINDINGS (Prosected by submitter)

Exterior findings: Weight 25g. Body condition is good. Skin appears healthy with no apparent lesions. Oral cavity appears normal. Eyes look fine.

Coelomic cavity: Liver and lungs were very dark in appearance, almost black. This is possibly normal for this species. 2 partially digested crickets in stomach and intestine. All other organs appear normal.

SAMPLES TAKEN: Liver, gall bladder, heart, lung, kidney, gonad and femur were sampled into formalin or frozen. Swabs were taken from the oral cavity and from the skin. A skin scrape was performed and the material deposited into saline.

HISTOPATHOLOGY

Tissues were submitted fixed in formalin.

The tissues are well preserved.

Lesions are not evident in the following tissues: kidney, lung, air sac.

The following observations are notable:

Skin:  The epidermis is moderately to markedly thick, contains transmigrating leucocytes, and is coated with a variably thick layer of compact keratin.  The stratum corneum and keratin layers contain moderate to marked numbers of zoosporangia, many of which have clear evacuation tubes. Mats of fine bacteria multifocally coat the surface of the keratin layer. The dermis appears normal.

Testes:  Spermatogenesis is prevalent throughout the spermatic cords.

Liver:  Hepatocytes have abundant cytoplasm.  The tissue contains numerous melanomacrophage centres.  Some of these centres contain non-pigmented histiocytes and central eosinophilic hyalin material. Focally, one of these centres also contains concentric layers of fibroblasts forming a more distinct granuloma.

Kidney: Distal renal tubules contain moderate quantities of golden cytoplasm. A small number of proximal renal tubular epithelial cells are large and multinucleate.

ZN stain (A):  Numerous acid fast, slightly beaded bacilli are evident within macrophages in the centre of the discrete granuloma described above.  A smaller number of organisms are present within the wall of the granuloma, and within the less well defined foci of granulomatous inflammation.  Rare organisms are evident within hepatic melanomacrophage centres.

 

DIAGNOSIS

Euthanasia

Epidermitis with acanthosis and hyperkeratosis – extensive, moderate to marked, subacute – chytridiomycosis

Hepatitis – granulomatous, focal, subacute to chronic – Mycobacteriosis

COMMENT

The frog was euthanased after being found moribund after being in captivity for approximately 6 months. The tissues were in excellent condition and well preserved.  The frog had marked, extensive chytridiomycosis.  Numerous bacterial colonies were also evident on the thickened and irregular epidermis, but these were relegated to the surface of the keratin layer, and are most likely opportunistic colonisers.

The frog also has mild granulomas (disguised within melanomacrophage centres) in the liver.  The granulomas were small and focal, and most likely incidental to the cause of death in this animal, but they contained acid fast organisms consistent in morphology with Mycobacteria sp.  Various species of saprophytic Mycobacteria can reproduce and cause disease in captive populations of amphibians. Ensuring that water filtration systems include a microbial filter (ozone, UV, etc) can help to control burdens of Mycobacteria in captive amphibians.