Schloegel, Lisa Marie (2012) Investigations on the dispersal of 'Batrachochytrium dendrobatidis' and Ranaviral disease through the international live animal trade in the Americas and Asia. (PhD thesis), Kingston University, .
Abstract
Enigmatic amphibian declines are strongly associated with infectious diseases, including chytridiomycosis, caused by the fungus 'Batrachochytrium dendrobatidis' (Bd), and ranaviral disease induced by ranaviruses. A series of data analyses, field sampling, laboratory experimentation and molecular techniques were utilized to test the hypothesis that the international live animal trade is contributing to the spread of these amphibian pathogens. Regions sampled included specific locations in North America, South America and Asia. The magnitude of the live importation of amphibians into the United States (U.S.) was calculated in the millions of animals per year. Batrachochytrium dendrobatidis and ranavirus infections were identified in live food animals in: (a) U.S. wet markets (in 'Lithobates catesbeianus', the North American bullfrog); (b) frog farms in Taiwan (in 'L. catesbeianus'); and (c) wet markets in Taiwan (in 'Rana tigrina', the Chinese bullfrog). 'Batrachochytrium dendrobatidis' infections were also identified in 'L. catesbeianus' farmed in Brazil and in 8 species of amphibians in the U.S. pet trade (in captive bred and directly imported animals). Detection of Bd in individuals upon initial entry into the U.S. demonstrated definitively the trans-continental movement of this potentially lethal amphibian pathogen. Laboratory experimentation illustrated transmission of Bd from a highly traded carrier host ('L. catesbeianus') to frogs of the same species, and to a known susceptible frog species ('Litoria caerulea', White's tree-frog). Molecular data linked U.S. market frogs and their associated ranaviral infections to origins in Asia and South America. Genotyping of pure, cultured isolates of Bd from farmed 'L. catesbeianus' in Brazil showed that they were most similar to isolates from native amphibians in Latin America when compared to a global dataset, providing evidence for transmission between wild and farmed amphibians. Analysis of isolates from market 'L. catesbeianus' led to the discovery of a novel genotype of Bd in Ypsilanti, Michigan that appeared to have a disparate lineage compared with the previously identified panzootic genotypes. DNA sequence analysis from the Ypsilanti frog indicated its origin in Brazil, where the novel genotype was also discovered in native Brazilian frog populations. The demonstration of amphibian pathogens on frog farms, in live wet markets and at border crossings, the evidence of inter- and intra-species transmission, and the existence of a novel genotype in traded anurans, all indicate that international transport of amphibians and their pathogens could produce not only mixing of pathogen genotypes, but the inadvertent spread of strains of unknown virulence that could negatively impact amphibian health. In 2008, data from research contained in this thesis were instrumental in the decision to list both Bd and ranaviral disease in the World Organization for Animal Health Aquatic Animal Health Code, providing guidelines (such as quarantine procedures) to limit the spread of these pathogens through the trade. The evidence provided in this work re-enforces the need for urgent action to minimize the potential spread of amphibian diseases through international trade routes, for the benefit of amphibian populations throughout the world.
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