A team of scientists led by the Wildlife Conservation Society (WCS) and the National University of Singapore (NUS), revealed in a new study, for the first time, the presence of the pathogenic chytrid fungus (Batrachochytrium dendrobatidis) in amphibians sampled in Singapore. And the American bullfrog may be a central player in the spread of the disease.
The study appears in the current issue of the journal EcoHealth, and is the first to consider the role that Southeast Asia's commercial trade plays in the spread of amphibian pathogens.
Demand for amphibians through local and international trade is high and fueled by use of frogs as pets, food, bait, and as a source of traditional 'medicine.' More than 40 percent of amphibian species are in decline globally due, not only to chytrid fungus, but also overharvesting, competition from invasive species, habitat loss, pollution, and climate change.
In the study, scientists collected samples from 2,389 individual animals in Lao PDR, Cambodia, Vietnam, and Singapore at 51 different sites including farms, locally supplied markets, pet stores, and from the wild.
The molecular testing of samples was led by Dr. Tracie Seimon at WCS's Molecular Diagnostic Laboratory at the Bronx Zoo. Results showed that frogs from Lao PDR and Vietnam tested negative for chytrid. In Cambodia, one frog intended for food tested positive. In addition, 74 animals in Cambodia and Vietnam were screened for ranavirus and tested negative, suggesting that these specific pathogens are not yet a conservation threat in species tested from these countries.
In Singapore, however, 13 samples tested positive for chytrid and represent the first report of chytrid in the territory. Eleven of those samples were collected from four pet stores and the remaining two were taken from amphibians in the wild.
The scientists noted that the chytrid detections were most prevalent in the American bullfrog (Lithobates aka Rana catesbeiana), a common species in the trade and one that is tolerant of chytrid infections.
"Finding chytrid in four of the seven Singaporean pet stores we sampled is cause for concern," said lead author and WCS Scientist Martin Gilbert. "Since the American bullfrog is able to tolerate this pathogen, it may act as a carrier for spreading chytrid to the region when it is imported through commercial trade."
In another alarming discovery, the scientists found that all 497 frogs sampled from 23 frog farms in Vietnam had skin lesions ranging from swelling and inflammation to ulcers and deformed or missing digits in the most severe cases. Disease examination revealed four of the animals had bacteria associated with the lesions that in two cases appeared to have spread to other organs.
While the bacteria and its role as primary or secondary pathogen could not be positively identified, the scientists noted that frog farms could serve as a source of infection for the wider environment.
The study noted that lesions among frogs raised at commercial facilities in Vietnam are of particular concern, in light of the low level of bio-security that exists. All of the farms in the study disposed of untreated wastewater directly into natural watercourses, which becomes an avenue to spread infection to other places and other species.
According to the authors, the World Organization for Animal Health (OIE) requires its 174 member countries, which include the four countries in this study, to conduct surveillance for chytrid fungus, report confirmed cases, and implement measures to control their spread.
Co-author of the study, Assistant Professor David Bickford from the Department of Biological Sciences at the NUS Faculty of Science, said, "In light of the fact that this emerging infectious disease is now known to be spread by commercial trade, it is in everyone's best interest to eliminate it from the trade in live animals before both the native amphibian populations of Southeast Asia are affected and before it completely decimates the commercial trade and people are unable to make a living. This is not just about the frogs."
The paper concludes, "There is an urgent need to conduct wider surveys of wild amphibians in Southeast Asia to determine the extent and severity of chytrid fungus and other infectious diseases among a range of species, and whether and how these change over time. Studies should focus on differentiating Batrachochytrium dendrobatidis strains that may be endemic to the region from exotic strains that may be introduced through routes including international trade."