Amphibian Chytridiomycosis

Overview

• Important emerging disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd). Pathogenic, virulent and highly transmissible.
• Infection has been recognised in a wide range of bot anuran (frogs and toads) and urodele (salamanders and newts) amphibians.
• A problem in both captive and free-ranging animals, chytridiomycosis is associated with mass mortality events and population declines in the United States, Europe, Latin America, and Australia.
• Amphibian population declines attributed to chytridiomycosis are well known in stream-dwelling species at high elevations in the rain forests of Central America and eastern Australia.
• Links to the spread of Bd have been made with the international movement of amphibians for the pet trade, laboratory research, and food. The African clawed frog (Xenopus laevis) and the bullfrog (Rana catesbeiana) are good potential reservoir hosts for Bd since they carry infection without clinical signs, and have also been widely transported on a global scale.
• Caused the decline or extinction of up to around 200 species of frog.
• Is an international notifiable disease (World Organisation for Animal Health, 2008).

Characteristics

• "Chytrids" are true fungi that produce characteristic, motile, posteriorly uniflagellate zoospores within discrete fungal bodies called thalli.
• Ubiquitous in aquatic enrivonments.

Pathogenesis

• Lesions limited to keratinizing epithelium in the skin of postmetamorphic animals and the mouthparts (tooth rows and jaw sheaths) of tadpoles.
• Varying degrees of epidermal hyperplasia and hyperkeratosis, with intralesional thalli characterisitic of Bd.
• Normal skin function is disrupted (important in amphibians with regard to water absorption and electrolyte balance), and secretion of a mycotoxin are probable causes of death by Bd.
• Transmission of Bd infection is via zoospores during direct contact between animals or contact with water or substrates.

Clinical signs

• Variable, ranging from unexpected death without previous signs, to evidence of skin disease.
• Skin (typically microscopic and not detectable): excessive sloughing; granular changes in skin texture; hyperemia skin discolouration. These are frequently observed on the ventral body and feet of terrestrail species, but may be diffuse in completely aquatic animals.
• Behaviour: abnormal sitting position; increased or decreased preference for water; anorexia; lethargy.
• Neurologic: Loss of righting reflex; tetanic spasms; paralysis.

Diagnosis

• Light Microscopy
  • Examination of skin slough with or without staining.
  • Examination of histological sections stained with haemotoxylin and eosin, with silver stain, or with an immunoperoxidase stain using a polyclonal antibody against Bd.
• Real-time Taqman PCR which quantifies DNA in the sample.

Prevention and Control

• In captive populations, enclosures should be disinfected and husbandry protocols implemented to prevent cross-contamination of water or substrate. Quarantine of newly aquired animals is essential.
• Temperature: Cultures of Bd die above 32°C (optimum growth is 17°-25°C) and so this has potential for use as a primary or adjunctive treatment in species that tolerate higher environmental temperatures.
• Adult animals may be bathed in 0.01% itraconazole suspension for 5 minutes daily, up to 11 days.
• Surveillance should be performed by complete necropsies, including histopathology, on all dead animals. Screening for Bd by PCR of skin scrapings or skin swabs may be implemented, in addition to prophylactic antifungal treatment of entire groups originating from high-risk areas.