Aquarium Science: Diagnosis of Chytridiomycosis in Pet African Dwarf Frogs
Author: Barbara Scott, MD and Jay Vance
A groundbreaking diagnosis of chytridiomycosis, a dangerous and deadly amphibian infection, in the pet aquatic frog hobby, with tips on eliminating it through quarantine and treatment.
A New Threat to Amphibians
The newly identified chytrid fungus in 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 online aquarium fish and frog board, we have seen several cases where African dwarf frogs in the same tank died from a similar illness, and we were suspicious that a chytrid infection was involved. 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 1½ to 2 inches, snout to vent length, when fully grown. With proper care, an African dwarf frog will live 4 to 7 years. African dwarf frogs make their way for commercial availability via both local and large commercial breeders, and from capture in the wild.
The First Victim
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 a home 20-gallon aquarium of 10 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 7ppm). The source frog was quarantined and treated with a commercially purchased anti-fungal medication and antibiotic, but it still died.
Within the next two weeks, one frog after another 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 died. Four of these deceased frogs were preserved in 10 percent formalin within minutes after death for histological examination (February 2006).
The chytrid fungus only invades the stratum corneum and stratum granulosum, and 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. The microscopic slides were prepared by using vertical strips of skin from the belly region of three of the frogs. The specimen 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 and then the frog was cross-sectioned. All specimens were stained with hematoxylin and eosin.
Under high dry magnification, zoospores within zoosporangia were present in all specimens—a finding which indicated a chytrid infection. 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 visibly upon microscopy, through which the zoospores escape. Most often empty zoosporangia are present on histology; however we found a great number that were full of zoospores.
Chytrid infections can be treated in the early stages with benzalkonium chloride or itraconazole. Some sources indicate that chytridiomycosis cannot be successfully treated with benzalkonium chloride, but according to Groff et al., this is an effective treatment for the Hymenochirus frog (African dwarf frog) that reduces mortality to 3 percent, versus 74 percent for the control (untreated) group. The remaining 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 six treatments. None of the treated frogs succumbed to chytrid infection, and 11 months later they remain alive and healthy.
Chytridiomycosis is a dangerous and deadly infection, but with proper quarantine procedures on the part of breeders and aquarists the spread of this infection could be limited or even stopped 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. The chytrid fungus can stay alive for three to four weeks in tap water, so it is imperative that the pet trade companies and hobbyists approach this infection seriously, so as to stop the reintroduction of the fungus back into the wild. African Clawed Frogs Xenopus laevis can be carriers of the chytrid fungus but do not get sick or die from this fungus and fortunately fish do not contract this infection.
We wish to acknowledge and thank Dr. Sylvan Cohen, who prepared the specimens and assisted with the histological readings.