Amphibians are in decline as a result of habitat destruction, climate change and infectious diseases. Tadpoles are thought susceptible to infections because they are dependent on only an innate immune system (e.g. macrophages). This is because the frog adaptive immune system does not function until later stages of their life cycle. In 1920, Nöller described a putative infectious agent of tadpoles named Nematopsis temporariae, which he putatively assigned to gregarine protists (Apicomplexa). Here, we identify a gregarine infection of tadpoles using both microscopy and ribosomal DNA sequencing of three different frog species (Rana temporaria, R. dalmatina, and Hyla arborea). We show that this protist lineage belongs to the subclass Gregarinasina Dufour 1828 and is regularly present in macrophages located in liver sinusoids of tadpoles, confirming the only known case of a gregarine infection of a vertebrate.

Biology Centre Czech Academy of Sciences Institute of Parasitology Branišovská 31 České Budějovice 370 05 Czech Republic

Biosciences University of Exeter Geoffrey Pope Building Exeter EX4 4QD UK

Department of Botany and Zoology Faculty of Science Masaryk University Kotlářská 2 Brno 611 37 Czech Republic

Integrated Microbial Biodiversity Program Canadian Institute for Advanced Research Toronto Canada

National Council for Technological and Scientific Development Brazil

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Diverse alveolate infections of tadpoles, a new threat to frogs?