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

Abstract

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. KEY WORDS: Mesomycetozoean-like · Southern toad · Amphibian · Central nervous system · LarvaResale or republication not permitted without written consent of the publisher Dis Aquat Org 113: 157-162, 2015 and emaciation causing mortality and morbidity (Green et al. 2002, Densmore & Green 2007.Lacking molecular data for confirmation, we tentatively assigned the parasites infecting southern toads to the Mesomycetozoea, possibly in the Order Ich thyo phonida, based on comparable morphology of other infections by these organisms in amphibians and fish. Herein we report a case study of an ap parently unusual, nonpathogenic organotropism for these organisms using descriptive terms for piscine Ichthyo phonus stages as proposed by Kocan (2013), and attempt to classify the cell stage types based upon size and morphology. MATERIALS AND METHODS SamplingWhen sampling in ponds at the Archbold Biological Station (Venus, FL), we used 4.8 mm mesh (3/16 inch) dip nets (Model HDD-2, Memphis Net and Twine) to collect amphibian larvae (n = 5) for histopathology. At each pond, live larval specimens were immediately transferred into a Ziploc ® bag or a plastic bucket of pond water, sorted by species and processed onsite. Dip nets, buckets, boots and other field gear were sprayed with 3% chlorine bleach following recommended disinfection procedures (Phillott et al. 2010). Necropsy and histopathologyLive larvae were euthanized with 200 mg l −1 tricane methanesulfonate (Tricane S, Western Chemical) buffered with 400 mg l −1 sodium bicarbonate (NaHCO 3 , Fisher Scientific) in pond water. Following anaesthesia, digital callipers were used to measure specimens (snout-to-vent length [SVL]) to the nearest 0.1 mm. Then, a midventral longitudinal incision was made to allow the fixative to penetrate i...