Batrachochytrium dendrobatidis is a chytrid fungus that causes the lethal skin disease chytridiomycosis in
amphibians. It is regarded as an emerging infectious disease affecting diverse amphibian populations in many
parts of the world. Because there are few model amphibian species for immunological studies, little is known
about immune defenses against B. dendrobatidis. We show here that the South African clawed frog, Xenopus
laevis, is a suitable model for investigating immunity to this pathogen. After an experimental exposure, a mild
infection developed over 20 to 30 days and declined by 45 days postexposure. Either purified antimicrobial
peptides or mixtures of peptides in the skin mucus inhibited B. dendrobatidis growth in vitro. Skin peptide
secretion was maximally induced by injection of norepinephrine, and this treatment resulted in sustained skin
peptide depletion and increased susceptibility to infection. Sublethal X-irradiation of frogs decreased leukocyte
numbers in the spleen and resulted in greater susceptibility to infection. Immunization against B. dendrobatidis
induced elevated pathogen-specific IgM and IgY serum antibodies. Mucus secretions from X. laevis previously
exposed to B. dendrobatidis contained significant amounts of IgM, IgY, and IgX antibodies that bind to B.
dendrobatidis. These data strongly suggest that both innate and adaptive immune defenses are involved in the
resistance of X. laevis to lethal B. dendrobatidis infections.