"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 7, 2024

Prosthogonimus cuneatus

Prosthogonimus is a genus of flukes that live in a special part of a bird's anatomy. It is usually found either in the Bursa of Fabricius, an organ that only birds have, or in the oviduct, and it's this latter location which lend this parasite its common name, the oviduct fluke. This fluke is found all over the world in many different species of birds, and while it doesn't seem to cause much issues for wild birds, it presents a major problem for the poultry industry.

Left: Dragonflies Sympetrum vulgatum (top) and Sympetrum depressiusculum (bottom), Right: A metacercaria cyst of Prosthogonimus cuneatus. Photos from Fig. 2 and Fig. 3 of the paper

Since this fluke lives by clinging to and feeding on the surface of mucosal membranes, its activities can leave lesions and cause inflammations, and heavy infection of Prosthogonimus can lead to all kinds of oviduct disorders in chickens. This includes leaking milky discharges from the cloaca, laying soft-shelled or malformed eggs, or even egg peritonitis, where egg yolk material gets displaced into the hen's body cavity, leading to secondary infections and death. In some cases, the fluke can even end up getting bundled into the egg itself, which seems pretty mild compared with what I have mentioned above, but it would nevertheless be a nasty surprise for anyone looking to make an omelette. To make matters worse, there are currently no effective treatments available for getting rid of this fluke once a bird is infected.

So how do birds end up with this peculiar parasite? Prosthogonimus has a multi-host life cycle that takes it across three very different animals - freshwater snails, dragonflies, and birds. In the dragonfly, the larval Prosthogonimus lies in wait as a dormant cyst called a metacercaria, waiting for its host to get eaten by a bird. That is why this parasite is usually associated with free-range chickens, as they have more opportunity to feed on a variety of things. Most studies on Prosthogonimus have focused on the effects it has on the bird hosts, but surprisingly fewer studies have investigated the source of the infection - parasitised dragonflies.

To rectify this oversight, a group of researchers undertook a truly herculean effort to investigate the presence of Prosthogonimus in dragonflies from the Heilongjiang province, China. The researchers collected over TEN THOUSAND dragonflies, composed of 12 different species from 41 locations. They identified each of the dragonflies before dissecting them for Prosthogonimus metacercariae, which are usually located in the abdominal muscles. The researchers noticed that infected dragonflies tend to have softer abdominal muscles, possibly due to injuries caused by the presence of the Prosthogonimus cysts.

They found three different species of Prosthogonimus in those dragonflies, of which Prosthogonimus cuneatus was the most common. Overall, about 20% of the dragonflies they examined were infected by Prosthogonimus, but it was more common in some species than others. The spotted darter (Sympetrum depressiusculum) was most frequently infected (28.53% prevalence), followed closely by the vagrant darter (Sympetrum vulgatum) (27.86% prevalence) and the autumn darter (Sympetrum frequens) (20.99% prevalence). The highest number of fluke larvae in a single dragonfly goes to an unlucky Sympetrum kunckeli which was packed with 157 Prosthogonimus metacercariae in its abdomen.

But dragonflies are aerial predators - how do they end up being infected with fluke larvae which are shed from freshwater snails? Well, before becoming acrobatic flying hunters, dragonflies spend their early life as underwater predators. But this aquatic life also expose them to Prosthogonimus' waterborne larvae, which are drawn into the dragonfly nymph's body through its respiratory current - in other words, they get sucked through the dragonfly nymph's butt whenever it takes a breath. Even as the dragonflies metamorphose into airborne adults, they carry the legacy from their youth in the form of Prosthogonimus cysts

Overall, the study found that Prosthogonimus was most common in Heihe, which might be due to the presence of large wetlands in the area. Those wetlands are home to high levels of biodiversity which help support the life cycle of this parasite - they provide habitats for numerous snails that can host the asexual stage of Prosthogonimus, along with wild birds that would usually act as the final host for this parasite. Just add dragonflies, which are always common around water bodies, and the circle of life is complete for Prosthogonimus.

Studying and elucidating the life cycles and ecological role of parasites in their natural context is an important part of disease ecology research. Understanding what these parasites actually do in nature can help us prevent them from causing problems in the animals that we raise.

Li, B., Lan, Z., Guo, X. R., Zhang, A. H., Wei, W., Li, Y., Jin, Z. H., Gao, Z. Y., Zhang, X. G., Li, B., Gao, J. F., & Wang, C. R. (2023). Survey of the Prosthogonimus spp. metacercariae infection in the second intermediate host dragonfly in Heilongjiang Province, China. Parasitology Research 122: 2859-2870.