"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

July 18, 2022

Dolichoperoides macalpini

Australia has some of the most venomous snakes in the world, but the mouths of those reptiles are filled with more than just venomous fangs. In some cases, they are filled with tiny digenean flukes, specifically Dolichoperoides macalpini. This species of fluke was first reported from the lowland copperhead snakes in the 1890s, but it wasn't until 1918 that it was formally identified and described, and in 1940 it was placed in its own genus when it was recognised that it was specifically associated with elapid snakes. Since then, there hasn't been much further studies on this fluke, and the research team behind the paper in this post seeks to fill in that knowledge gap.

Left: Dolichoperoides macalpini in a snake's mouth, Right: Dolichoperoides macalpini in a snake's lungs
Photos from Fig. 1 of the paper

For this study, the researchers collected snakes from parts of Tasmania and Western Australia.
In Tasmania, they collected roadkills composed of Tiger Snake (Notechis scutatus) and Lowland Copperhead (Austrelaps superbus). While in Western Australia, they obtained freshly caught and euthanised Western Tiger Snake (Notechis scutatus occidentalis) which were collected as a part of another, larger project examining tiger snakes from wetlands in and around Perth. Dolichoperoides macalpini were mostly found in the snake's mouth, oesophagus, and stomach. And when the snake's mouth is open, the flukes are clearly visible as tiny black specks that clung to the roof of the snake's mouth (see accompanying photo). However, the snakes from Tasmania had D. macalpini in their lungs and intestine as well. So what's going on there? 

This could be because the snake specimens examined in Tasmania were roadkills. In some cases, after the host dies, its parasites may move from their usual location to different parts of the host's body, possibly due to some last ditch survival instincts. This phenomenon is well-known in anisakid nematodes, which is a major seafood-borne zoonotic parasite. After their fish host is caught, these worms often migrate from their host's viscera to its flesh. In the case of D. macalpini, once they sense that their host had died, perhaps they evacuated away from the mouth and throat to other, deeper parts of the body such as the lungs and intestine in a desperate bid for survival.

This may also explain some of the other differences the researchers found in the infection patterns of different snake populations. The Tassie snakes generally had fewer flukes than those which were caught around Perth. Since the Tasmanian snakes were found as roadkill, it is possible that the flukes which didn't crawl to the lungs or intestine had just ended up abandoning the snake altogether.

But this difference in fluke abundance may have also been influenced by other more innate factors of the snakes' ecologies. The encysted larval stage of D. macalpini are found in frogs, which the Perth snakes were particularly fond of, with frogs accounting for almost 90% of their diet. This provided them with ample opportunities to encounter the infective larval stages of D. macalpini through their food. In contrast, the Tassie snakes had a more varied diet consisting of rodents, birds, and lizards - but no frogs.

Additionally, there were also other differences among the flukes themselves. For example, while the snakes from Perth were more heavily infected, their flukes were only about half the size of those found in the Tasmanian snakes. While such size differences might have indicated that the flukes in those separate snake populations may in fact be different species, genetic analyses showed otherwise. The 18S rRNA gene and ITS gene sequences - which are key genetic markers for delineating different species among these parasites - were identical for the flukes from both Tasmanian and Perth snakes.

So there must be other reasons for such marked differences in their sizes. Perhaps in more heavily infected hosts, the crowded environment may have limited the flukes' growth? Studies on other species of flukes have found that those from more heavily infected hosts tend to be smaller on average than their counterparts from less parasitised hosts. This diminished growth may be the result of competition over limited resources, be it host nutrient, or simply available space for growth. Or perhaps there are slight variations between the biology of different snake species that can influence the fluke's growth?

The result of this study offers a brief glimpse into the distribution and infection patterns of D. macalpini in Australian snakes, and it raises some tantalising questions about the parasite's ecology. But there are many other reptile parasites in Australia for which little is known about them outside of a taxonomic description. Despite having one of the world's richest reptile fauna, the parasites fauna of Australian reptiles are relatively understudied. Not only are they an integral part of Australia's biodiversity, understanding these parasites can also tell us about how their reptile hosts are connect with the rest of the ecosystem.