The fungus Batrachochytridium dendrobatidis has been implicated in catastrophic amphibian population declines on several continents.  This fungus feeds on keratin found in amphibian skin and tadpole beaks.  Known as chytridiomycosis or chytrid infection, this skin infection often results in deaths of untreated amphibians.

Infected frogs often develop a brownish cast to their skin and being to shed more than normal.  Some may have reddening of the drink patch, the belly skin and feet, and may have discolored toes.  Some amphibians die without obvious signs of illness.  Infected frogs may crouch with a curved spine; some may spend more time in water than normal.  Some species of amphibians, such as bullfrogs (Rana catesbeiana) are asymptomatic carriers.  ‘Carriers’ are often a source of re-infection in a captive collection and in the wild may have devastating impacts if inhabiting wetlands that support susceptible species.  Tadpoles lack keratinized skin except for their beaks which become deformed with infection.  Froglets often quickly succumb to chytridiomycosis soon after completing metamorphosis.

Chytrid fungi have an infective life stage (zoospore) that swims using flagella. Once it has attached to the keratin in the skin of an amphibian, it becomes a more typical fungus, growing asymmetric tubes of tissue as the infection spreads.  At certain junctures, vial shaped structures develop called sporangium.  These sporangia are the “incubators” for future zoospores.  When the zoospores have matured, the surface of the sporangium in contact with the environment ruptures, spewing out swimming zoospores which proceed to look for a suitable colonization site, either on the same host of infecting a new host.

Diagnosis relies on identifying the agent in skin scrapings or other tissue samples either through PCR analysis or histology.  PCR analysis much more effective than histology.  PCR samples are collected by scraping a sterile wood applicator across the skin of the amphibian.  Several areas of the skin should be scraped, including the ventral abdomen, obvious lesions, and the tips of the digits.  The sample should be preserved in 1.0 ml of 70% ethanol and submitted for analysis.  It may take a month or longer for results to be available, so treatment often is started without waiting for confirmation of infection. 

We recommend regular screening of amphibian collections for this pathogen as well as it being an essential part of the quarantine screening process.

There are many different treatments that are effective but each have their advantages and disadvantages so a treatment plan needs to be designed specifically for your amphibian.  Some of the drugs that are used include itraconazole, chloramphenicol, and florfenicol, while heat treatment is effective in species that can tolerate temperatures over 85°F.  Make sure that there is adequate humidity and open water available during heat treatment so the amphibians do not desiccate.

Many infected amphibians benefit from soaks in of 0.6% saline to help keep their electrolytes stable while their skin is healing.  This can be made by add 6 g of noniodized table salt to 1 liter of distilled water.  Rapid treatment is essential to cure infected amphibians.  Entire collections may need to be treated to eliminate silent carriers of the infection.

Unless you have prior experience with a species, we recommend treating a few individuals first before exposing the entire population to a treament to make sure it is safe.

Cages and equipment may be disinfected quite easily.  Household bleach, diluted to 1% sodium hypochlorite, is a readily available effective disinfectant.  Other alternative exist sush as 1 mg/ml benzalkonium chloride or 70% ethanol.

There is some evidence that low levels of vitamin A may predispose amphibians to this infection.  Please make sure that your amphibians are getting a multivitamin supplement that contains pure vitamin A at least one a week.

Copyright 2009
Drs. Kevin Wright and Jay Johnson
Arizona Exotic Animal Hospital, LLC
744 N Center Street
Mesa, AZ 85201
info@azeah.com