ISVD‐2 Chytridiomycosis – an emerging and fatal infectious skin disease of amphibians

Abstract

Chytridiomycosis, a mycotic cutaneous infection of amphibians, was first described in frog populations in Australia and Central America and was consequently found in wild and captive anurans and urodeles worldwide. It is now considered an emerging infectious disease and a key factor in the dramatic global decline of amphibians. The aetiological agent is Batrachochytrium dendrobatidis , a member of the fungal phylum Chytridiomycota in the order Chytridiales. Batrachochytrium dendrobatidis is the first and only chytrid known so far to be pathogenic for vertebrates. The keratinophilic fungus causes significant mortality in postmetamorphic amphibians, while tadpoles are spared. Clinical signs vary depending on species and include abnormal posture, lethargy, behavioural changes, loss of righting reflex, bilateral mydriasis, reddening as well as greyish‐white discolouration of the skin, abnormal sloughing, and occasional erosions and ulcers. Death usually occurs within a few days after the onset of clinical signs. At necropsy, gross lesions are confined to the skin and are characterized by greyish‐white discolouration and thickening and sloughing of the skin with occasional erosions and ulcers, particularly on the extremities and the abdominal and pelvic regions. Histologically, the skin lesions are characterized by marked hyperkeratosis with numerous spherical fungal thalli (zoosporangia) within the thickened keratin layer, irregular epidermal hyperplasia, intercellular oedema, and occasional necrosis of epidermal cells, as well as exocytosis of low to moderate numbers of mainly lymphocytes. A mild inflammatory infiltrate (heterophils, lymphocytes, and macrophages) may be present in the dermis. The pathogenesis of chytridiomycosis is largely unknown. However, it is thought that the epidermal hyperplasia and hyperkeratosis seriously compromise the function of the skin which is essential for respiration, water absorption, regulation of body temperature, and homeostasis of electrolytes and serves as a shield against pathogenic organisms. Diagnosis of chytridiomycosis can be achieved by routine histology. The fungal structures can be readily detected on H&E sections. Microscopic examination of skin scrapings stained with lactophenol is a quick method for detection of B. dendrobatidis on dead and living animals. Electron microscopy, immunohistochemistry, PCR, and fungal culture are useful tools to confirm the diagnosis. Administration of azoles (e.g. fluconazole, itraconazole), baths with formalin/malachite green solution, and elevation of the environmental temperature have been described for treatment. Funding: Self‐funded.