"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

May 12, 2016

Cystodiscus axonis

Myxozoans are a group of very unusual parasites. Despite their simplified structure, latest research has shown that their closest living relatives are actually jellyfish (which means they are technically animals). They are found in a variety of tissues and organs in their hosts which are usually fish, and in some cases, amphibians. Some causes diseases such as the salmon whirling disease, and in the case of certain fish-infected myxozoans, after the host dies, the parasite causes the flesh to melt - much to the frustration of fishermen. While the majority of known myxozoans infect fish, in the last decade, there has been increasing interest in studying myxozoans that infect amphibians, and the parasite featured today is one such species.
Photo of Cystodiscus axonis spore from this paper

Cystodiscus axonis is a myxozoan species which lives in either the brain or the gall bladder of their frog hosts. This parasite and its close relative C. australis is found in a range of native Australian frogs. This parasite was previously classified in another genus called Myxidium and its discovery (and reclassification) featured a number of twist and turns.

The parasite was first recorded in cane toads which have been introduced to Australia, so it was originally thought to be a parasite that the cane toads had brought with them into Australia and had since taken to infecting Australia's native amphibians. However, examinations of older frog specimens from museum collections, including those that were collected before the introduction of cane toads to Australia, revealed that the parasite had been in native Australian frog all along - the cane toad simply picked it up when they arrived and they turned out to be a really hospitable home for this parasite.

Given that C. axonis is not too discriminating when it comes to whether it infects native Australian frogs or introduced cane toads, there is potential for this parasite to infect other amphibians as well. And that's what the scientists behind today's featured study decided to find out. This time, they once again look to museum specimens, in this case from the Natural History Museum in London, and specifically they examined preserved specimens of caecilians for myxozoan parasites

Caecilians are very strange looking amphibians - they are legless, look kind of like giant earthworms, and they are very different to either toads and frogs. For this study, the scientists examined 148 caecilian specimens spanning across twelve species which are found in a variety kind of habitats, ranging from terrestrial, burrowing forms to aquatic species. Out of those, they found seven specimens which had myxozoan spores floating in their gall bladder. All the infected caecilians belong to one of two species - Typhlonectes natans and Typhlonectes compessicauda - and both of them are aquatic caecilians.

Based on the shape of those spores and sequences of their DNA, the parasite they found was almost identical to C. axonis from Australian frogs. So somehow, C. axonis has managed to successfully make a jump to caecilians too - but how? The native frogs of Australian and caecilians are separated not just by a vast ocean, but also 300 million years of divergent evolution - so how did these legless amphibian parasites end up with a parasite which is originally found in Australian frogs? A vital clue might be the fact that the infected specimens were originally captive animals.

Myxozoans use different host in their lifecycle - they usually alternate between a vertebrate and invertebrate host, so the infected caecilians might have become infected when they were fed invertebrates, such as tubifex worms, which were parasitised by C. axonis. Alternatively, they might have been housed or shared a water supply with other captive amphibians that were infected.

Given its ability to jump to a dissimilar host like caecilians, this explains why they were so receptive to cane toads when they were brought to Australian. Compared with the evolutionary gulf that separate frogs from caecilians, the native frogs of Australia and the introduced cane toads are practically kissing cousins. Given the presence of an Australian frog parasite in South American caecilians, just how widespread have C. axonis and similar parasite have become?

The lethal amphibian chytrid fungus Batrachochytrium is an amphibian pathogen which has now been spread all over the world due to the global trade in amphibians. So what other parasites might be lurking in the loads of frogs, salamanders, and caecilians which are currently being shipped all over the globe?

Hartigan, A., Wilkinson, M., Gower, D. J., Streicher, J. W., Holzer, A. S., & Okamura, B. (2016). Myxozoan infections of caecilians demonstrate broad host specificity and indicate a link with human activity. International Journal for Parasitology 46: 375-381.

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