Chytridiomycosis in Commercially Purchased African Dwarf Frogs at Flippersandfins.net

Diagnosis of Chytridiomycosis in Commercially Purchased African Dwarf Frogs

by Dr. Barb and JBOT

The newly identified chytrid fungus of the genus Batrachochytrium is infecting amphibian populations in Australia, Central and South America, the United States, and Canada at an alarming rate. Chytridiomycosis causes death and has led to the decline of many amphibian species. As moderators of an internet-based aquarium fish and frog board, we observed numerous cases where African Dwarf Frogs inhabiting the same tank died from similiar illnesses. We were suspicious that a chytrid infection may have been involved in the deaths. To our knowledge, this is the first case report identifying the deadly chytrid fungus in commercially purchased African Dwarf Frogs in the United States.

The African Dwarf Frog is a popular aquarium inhabitant of the family Pipidae and genus Hymenochirus. This fully-aquatic, tongueless frog with four webbed limbs reaches a length of 1.5-2 inches, snout to vent length, when fully grown. With proper care, an African Dwarf Frog will live four to seven years. African Dwarf Frogs make their way for commercial availability via both local and large commercial breeders, and capture in the wild. Another species of frog that is popular in the aquarium trade, the African Clawed Frog (Xenopus laevis), can be a carrier of the chytrid fungus but the species does not get sick or die from the fungus. Fortunately, fish do not contract this infection.

An author of this report purchased an apparently healthy adult female African Dwarf Frog, which we will refer to as the “source frog”, from a large chain pet store. After three weeks of quarantine, this still apparently healthy frog was added to the author's home 20 gallon aquarium containing ten healthy African Dwarf Frogs that had already been established in the tank for seven months. One week later, the source frog became ill with lack of appetite, lethargy, and rough, flaking skin. The water parameters were excellent (ammonia = 0, nitrites = 0 and nitrates = 7 ppm). The source frog was quarantined and treated with a commercially-purchased anti-fungal medication and antibiotic, but died.

Within the next two weeks, one frog after another in the tank became ill with the same constellation of signs and symptoms including thrashing at the water’s surface and attempting to climb out of the water. Five more frogs then perished. Four of the deceased frogs were preserved in 10% formalin within minutes of death in anticipation of histological examination (February 2006).

Healthy African Dwarf Frog amplecting with a chytrid infected frog (foreground).

The chytrid fungus only invades the stratum corneum and stratum granulosum. Therefore, the skin is the only organ required for diagnosis. The fungus is most often found in the stratum corneum on the belly of the frog (rather than the back of the frog). Accordingly, microscopic slides were prepared using vertical strips of skin from the belly region of three of the preserved frogs. The specimen taken from the fourth frog was prepared by placing the frog in Bowen's solution at room temperature for about 24 hours to fully remove all of its bones. The frog was then cross-sectioned. All of the specimens were stained with hematoxylin and eosin.

Under high dry magnification, zoospores within zoosporangia (a finding which indicates the existence of a chytrid infection) were present in all specimens. Zoospores are small round or oval basophilic (blue staining) bodies. Zoosporangia are eosinophilic (red staining) to slightly basophilic, spherical structures that wall off the zoospores. They possess discharge openings, not readily visible upon microscopy, through which zoospores escape. Empty zoosporangia are often present on histology; however our examination found a great number of zoosporangia that were full of zoospores. (Chytridiomycosis can also be diagnosed by PCR (polymerase chain reaction) testing but such testing can be costly to the aquarist.)

Chytrid infections can be treated in the early stages with benzalkonium chloride (an easily obtainable disinfectant) or itraconazole aka Sporonox (which requires a prescription). Some sources indicate that chytridiomycosis cannot be successfully treated with benzalkonium chloride. However, according to Groff et. al., benzalkonium chloride is an effective treatment for the Hymenochirus frog (African Dwarf Frog) that reduces mortality to 3% versus 74% for the control (untreated) group.

The remaining surviving frogs in the author’s tank were treated, two weeks after exposure, with a 2 mg/liter benzalkonium chloride bath once every three days for a total of six treatments. None of the treated frogs succumbed to chytrid infection and twelve months later they all remain alive and healthy.

A frog can be infected with the chytrid fungus and yet not show any visible symptoms until it is too late for treatment to work. Therefore, when treating with benzalkonium chloride, it is essential to treat all frogs in the tank, and not just the ones showing symptoms. It is important that the tank, equipment, and water all be cleaned with bleach both to kill any spores that remain and to avoid introducing the chytrid fungus into the water supply or the wild. Tank water should be treated with 200 mg/liter sodium hypochlorite (i.e. one tablespoon 5.25% bleach per gallon) for fifteen minutes before discharge into the sewer system or the natural environment. Gravel and plants should be bagged and thrown away. The infected tank and its equipment should then be cleaned using a 10% bleach solution (one part bleach to nine parts tap water). Acrylic tanks should be cleaned with one part bleach to nineteen parts tap water. Finally, rinse with copious amounts of water containing ten times the usual amount of dechlorinator in order to fully neutralize the bleach. Allow all items to air dry. To avoid reinfection, the frogs must not be returned to the tank until their treatment is over and the tank has been completely cleaned. For further details on how to perform the benzalkonium chloride treatment, please contact the authors at flippersandfins.net.

Chytridiomycosis is a dangerous and deadly infection. Yet, proper quarantine procedures on the part of breeders and aquarists could limit or even stop the spread of this infection within the aquarium hobby. The chytrid fungus has an incubation period of two months, necessitating a period of quarantine for at least this long for the African Dwarf Frog. Also, the chytrid fungus can survive three to four weeks in tap water, so it is imperative that pet trade companies and hobbyists approach this infection seriously to stop further introduction of the fungus into the wild. We wish to acknowledge and thank Dr. Sylvan Cohen for his preparation of the specimens and assistance with the histological readings.

References:

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2. Berger, L, Speare, R, Daszak, P, Green, D E, Cunningham, A A, Goggin, C L, Slocombe, R, Ragan, M A, Hyatt, A D, McDonald, K R Hines, H B, Lips, K R, Marantelli, G and Parkes, H. (1998). Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America. Proceedings of the National Academy of Science, USA 95: 9031-9036.

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6. Groff, JM et al, An Epizootic of Cutaneous Zygomycosis in Cultured Dwarf African Clawed Frogs (Hymenochirus curtipes) due to Basidiobolus ranarum. World Wide Web file, http://www.jcu.edu.au/school/phtm/PHTM/frogs/papers/groff-1991.pdf, 22 February, 1991.

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8. Vance, J. (2006) Benzalkonium Chloride Procedure for Treating Chytridiomycosis in African Dwarf Frogs, World Wide Web file, http://www.flippersandfins.net/chytridBCtreatment.htm.

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