AVIAN/FUR/EXOTIC SPECIES

Yellow fungus disease in bearded dragons

Marina Brash, Durda Slavic, Hugh Cai, Pat Bell-Rogers, Megan MacAlpine

In the last quarter of 2014, the AHL received numerous submissions of primarily juvenile bearded dragons with histories of lethargy and death often following the development of skin lesions. Gross skin lesions ranged from yellow to grey/brown ulcerated to thickened, roughened and scaly patches involving the face, head, neck, abdomen, legs and tail (Fig. 1) with splitting of the skin reported in some chronic cases.

Histologically, variable numbers of fungal hyphae were on the surface and within a variably thickened hyperkeratotic layer of epidermis, and the underlying epithelium was eroded/ulcerated or hyperplastic. Often the dermis was expanded, hypercellular, fibrotic, and infiltrated by nodules or sheets of macrophages and multinucleated giant cells, often containing fungal hyphae. Occasionally the nodules had central cores of eosinophilic necrotic debris. Granulomas were also within the underlying skeletal muscle, connective tissue between muscle bundles, and adjacent to resorbing bone. Organs including lung and liver were less frequently affected (Fig. 2).

Fresh skin samples were submitted for fungal culture, and wet mount microscopic examination, which revealed a large number of hyphae confirming histological observations. Fungal cultures yielded Chrysosporium anamorph of Nannizziopsis vriesii (CANV) which was confirmed by the mycology laboratory at the Public Health Agency of Canada and also by 18S rRNA gene sequencing at the AHL.

In the past, it was widely accepted that CANV and a variety of Chrysosporium spp. cause yellow fungus disease in reptiles. Advanced molecular characterization of over 40 isolates from reptiles showed that they had been phylogenetically misclassified. Isolates from reptile dermatitis cases belong to 3 different fungal genera, namely Nannizziopsis, Paranannizziopsis, and Ophidiomyces. These isolates may have species predilections, with O. ophiodiicola causing disease in snakes only, whereas N. guarroi primarily affects bearded dragons.

There are slight morphological differences among the isolates belonging to these genera. Since, at present, molecular characterization is time consuming, expensive, and likely does not affect the clinical outcome, AHL reports fungal isolates from reptile dermatitis cases as CANV complex which currently encompasses all 3 fungal genera noted above.   

Orange bearded dragon with grey/brown irregularly thickened and crusted skin of the caudal back, R leg, and proximal tail. The R leg and tail are swollen, and the tail is shortened with a dark crust on the tip. b. Ventrum of head of a bearded dragon; generalized yellow discoloration of the skin and yellow crusts of exudate at the neck. c. Skin of the ventral abdomen is slightly thickened and yellow with scattered small brown foci.

Surface keratin contains large numbers of fungal hyphae (*). The underlying epidermis is hyperplastic (200X, H & E). b. Multiple nodules and clusters of macrophages and giant cells (M), often containing fungal hyphae (see inset (400X, H & E) are within the expanded, fibrotic dermis and infiltrating into the underlying skeletal muscle (S) (100X, H & E). c, d. Granulomas occasionally containing fungal hyphae (*) within liver (c) and lung (d). Exudate (E) is accumulating in faveolar lumens

Figure 1 a. Orange bearded dragon with grey/brown irregularly thickened and crusted skin of the caudal back, R leg, and proximal tail. The R leg and tail are swollen, and the tail is shortened with a dark crust on the tip. b. Ventrum of head of a bearded dragon; generalized yellow discoloration of the skin and yellow crusts of exudate at the neck. c. Skin of the ventral abdomen is slightly thickened and yellow with scattered small brown foci. (Photo courtesy of Dr. Rick Axelson, The Links Road Animal & Bird Clinic). 

Figure 2 a. Surface keratin contains large numbers of fungal hyphae (*). The underlying epidermis is hyperplastic (200X, H & E). b. Multiple nodules and clusters of macrophages and giant cells (M), often containing fungal hyphae (see inset (400X, H & E) are within the expanded, fibrotic dermis and infiltrating into the underlying skeletal muscle (S) (100X, H & E). c, d. Granulomas occasionally containing fungal hyphae (*) within liver (c) and lung (d). Exudate (E) is accumulating in faveolar lumens