Neurotropic mesomycetozoean-like infection in larvae of the southern toad Anaxyrus terrestris in Florida, USA.

As part of a state-wide multispecies survey of amphibian diseases, sampling was conducted at Archbold Biological Station, Venus, Florida, USA, on 15 April 2011. Gross examination of southern toad (Anaxyrus terrestris) larvae was unremarkable, but infections by a mesomycetozoean-like organism were observed in longitudinally sectioned routine haematoxylin and eosin-stained histologic slides. In 100% of the sectioned specimens examined (n = 5), a high density of the organism, representing several developmental stages, was found in the central nervous system, mainly in the spinal cord, brain, retina and optic nerve. No host inflammatory responses were found to be associated with the parasitic infection. Free, mature schizonts were occasionally found in the gill chamber and, more commonly, in the dorsal roof area. No organisms were found in other organs examined histologically, i.e. liver, kidney, heart, alimentary tract, exocrine pancreas and skeletal muscles. Presumptive mesomycetozoean ichthyophonids in anurans are usually reported to be pathogenic, especially affecting skeletal muscle tissue and causing death. To our knowledge, this is the first report of a similar organism infecting primarily the central nervous system in an amphibian.