"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

January 15, 2023

Leucochloridium passeri

Leucochloridium paradoxum is one of those parasites which is immediately recognisable on sight. Commonly known as the "zombie snail parasite", its habit of turning the eyestalks of snails into pulsating candy canes has also earned it the name "green-banded broodsac", and it has appeared in various forms of media, including the opening of the Chainsaw Man anime. But far from just being a bizarre one-of-a-kind oddity, L. paradoxum is just one out of ten known species in the Leucochloridium genus which infect amber snails and produce these "broodsacs" structures. And these colourful, pulsating sacs are the key for distinguishing different species of Leucochloridium.

Left: Snail infected with Leucochloridium passeri collected from Hemei Township (Changhua County) by Jui-An Lin, photo from Fig. 1 of the paper. Top right: Labelled L. passeri broodsac from Fig. 1 of the paper. Bottom right: A trio of L. passeri broodsacs with metacercariae removed from an infected snail, from Supplementary video 3 of the paper

The adult stage of Leucochloridium are found in birds where they dwell in the cloaca or a special organ called Bursa of Fabricius. While parasite identification is usually based upon the various anatomical features of the adult parasite, in the case of Leucochloridium, the adult flukes of different species all look rather similar to each other. In contrast, the broodsac stages come in a wide variety of colours and patterns that are extremely noticeable and unique to each species. So short of comparing their DNA sequences, the colours and stripes of the larval broodsacs are the most reliable way to tell apart the different species.

This blog post features a study on Leucochloridium passeri, a species that was first described as adult flukes from Eurasian tree sparrows in Guangdong, and has subsequently been found across the Indomalayan realm. It is one of five different Leucochloridium species found in Taiwan, but it is the only one for which their broodsac stage has been documented. While not as well known as L. paradoxum, its broodsacs nevertheless present an attention-grabbing sight. You might recognise it from this video, which has gone viral and been posted all over the internet, usually without credit or attribution of the original source.

It can be easily distinguished from L. paradoxum and other Leucochloridium species by a distinctive wide band of red-brown patches or longitudinal stripes in the mid-section of each mature broodsac. Many people who have some familiarity with this parasite would know about the pulsating sacs forcing their way into the snail's eye tentacles, but what they might not know is that those are only part of the entire parasite mass residing within the snails.

Those pulsating "broodsacs" are actually the parasite's asexual larvae. In addition to the very flamboyant mature broodsacs, there are also translucent immature broodsacs which are tucked away deeper in the snail's body. Digenean flukes have an asexual stage in their life cycle, and in most flukes they produce hundreds to thousands of sausage-shaped asexual larvae in the snail's body. Those wriggly sausages would then give birth to free-swimming larvae called cercariae that are release into the environment where they infect the next host in the life cycle. In the case of Leucochloridium, the cercariae stay in those wriggly sausages and develop into round, jelly-coated cysts within the snail. Each mature broodsac can contain up to two hundred cysts, so when a bird swallows one of these colourful wriggling sausages, they are inviting hundreds of flukes to take up residency in their cloaca.

The L. passeri broodsacs described in this study were found in Yilan County in Taiwan, and they look very similar to some Leucochloridium broodsacs which have been found in Okinawa, Japan. They both have the distinctive wide band of red-brown stripes and splotches, and when researchers compared their DNA sequences, they found that they both belong to the same species - Leucochloridium passeri.

Relatively little is known about the birds that can serve as the final hosts for L. passeri, but researchers have noticed that the distribution of various Leucochloridium species in different zoogeographical regions seems to be related to the distribution of birds and amber snails which are native to those particular regions. Since some of those birds are migratory, this provides Leucochloridium with the means to cross oceans while seated snugly in the butt of their feathery host, ready to settle down wherever there are amber snails to infect. 

Chiu, M. C., Lin, Z. H., Hsu, P. W., & Chen, H. W. (2022). Molecular identification of the broodsacs from Leucochloridium passeri (Digenea: Leucochloridiidae) with a review of Leucochloridium species records in Taiwan. Parasitology International 102644.

P.S. Leucochloridium is a very distinctive parasite and has been subjected to numerous artistic depictions, here's my own artistic depiction of Leucochloridium in the form of a Parasite Monster Girl.

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