Cutaneous chytridiomycosis in poison dart frogs (Dendrobates spp.) and White's tree frogs (Litoria caerulea)

Summary

This paper reports the occurrence of chytridiomycosis in 3 species of captive amphibians at the National Zoological Park, Washington DC, wild cricket frogs (Acris crepitans) in Illinois, and captive arroyo toads (Bufo microscaphus californicus). Cases at the National Zoological Park occurred between 1996 and October 1997 in 24 juvenile blue poison dart frogs (Dendrobates azureus), 4 juvenile green-and-black poison dart frogs (Dendrobates auratus) and 3 aged White's tree frogs (Litoria caerulea).

Key Points

• Lesions most prominent on ventral abdomen, pelvis and legs. Poison dart frogs also had lesions adjacent to oral mucocutaneous junctions.
• Pathological lesion was epidermal hyperplasia with orthokeratic hyperkeratosis.
• Chytrid zoosporangia with diameters of 7-15 micrometre present inside keratinocytes of the stratum corneum.
• Bacteria on surface of epithelium.
• Inflammation uncommon.
• Four forms of chytrid visible in H&E sections:
  • uninucleate form containing homogenous basophilic cytoplasm
  • multiple nuclei separated by cytoplasm
  • cyst-like form containing multiple discrete 2-3 micrometre round to oval basophilic spores.
  • Empty thalli, delinated by a residual cyst wall.
• Discharge papillae occasionally seen arising from zoosporangia.
• Staining characteristics: Walls of thalli positive by PAS, Gomori's methanamine silver stain, and Gridley's fungal stain. Not acid fast by Kinyon's stain. Zoospores Gram positive and stained weekly with Giemsa.
• Chytrid can be observed in smears of superficial epidermis examined using wet mounts, Wright's stain or Diff-Quik stain.

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