Benign pinworms, intestinal vampires, and fluffy bug suckers

We've reached the end of yet another year and as usual there were many interesting new papers published this year, but so little time to write about them on this blog. Speaking of which, one of my paper was recently accepted in Journal of Animal Ecology - this is a study I conducted with Dr Janet Koprivnikar comparing parasitic worms found in lizards and asking the question: Why are some lizards more wormy than others?

Parasite collection on display at the Meguro Parasitological Museum (photo credit: Dr Tommy Leung)

But back to the blog, so what were featured in 2018? Well, some highlights from this year includes: pinworms from animals that most people would not have associated with pinworms, such as a species that live in tadpoles (but they're gone once the tadpoles become frogs), and a pinworm that makes its home in the gut of cockroaches.

But while pinworms are relatively benign as far as parasitic roundworms go, there are some that are nastier - like gut-dwelling, blood-feeding hookworms - and of the hookworms, those that infect seals and other pinnipeds are especially nasty - and this has something to with their host's life style. Speaking of nasty things that burrow in the belly of aquatic animals, this year we also featured a parasitic isopod that live in the belly of an armoured catfish.

And the, there are hosts that can't seem to keep their parasites all to themselves, for example, the introduction of the Burmese Python to Florida as resulted in a series of parasite exchanges between it and Florida's native snakes. So even if you end up living in a new location, you can never truly escape from your parasites - even for salamander living out their quiet lives in a cave, they can also become host to some hungry leeches.

But it's not just the parasites of vertebrate animals which get the spotlight here. This year the blog also featured posts on two parasites that give cicadas a bad time. One is a fluffy caterpillar that simply cling on to cicadas and suck their blood, while the other is a fungus that cause the infected cicada's butt to disintegrate. That fungus also seize control of the cicada's behaviour, and it is not alone in doing so. This year I also wrote a post about how the lancet fluke puts itself in the pilot seat of an ant.

Meanwhile, we continue to feature more student guest posts about topics such as a parasitoid wasp's bodyguard caterpillar, whale lice, how parasitoids are affected by what their hosts eat, the cuckoo's thicker egg shells, and a maggot that eat baby birds.

And for those who thought I was done with drawing Parasite Monster Girls, I have some bad news - I'm at again; meet the medically proficient Dr Delilah, and the elegantly composed Sayuri. They've even made their way out of the digital into the the physical realm, with prints of the Parasite Monster Girls being featured at the University of New England Library. I also recently got the opportunity to visit the Meguro Parasitology Museum which should be on the top of the bucket-list for any fans of parasites or parasitology.

That does it for this year on this blog, but until next year, you can continue to follow my parasite-related and other antics on my Twitter @The_Episiarch if you wish to do so. See you all in 2019!

Grillotia sp.

Most people probably think of tapeworms as being parasites that infect their pets, livestock, or even themselves - so mostly as parasites of land mammals. But the vast majority of tapeworms are actually found in the sea, completing their life cycles by being transferred from one marine animal to another through the food chain. The tapeworm species featured in this blog post came from a monkfish which was caught in the Tyrrhenian Sea off the coast of Civitavecchia. The fish was sent to Istituto Zooprofilattico Sperimentale del Mezzogiorno for further examination when it was found that its flesh was thoroughly dotted with numerous tiny white ovoids.

Top left: tapeworm larvae in the caudal fin of the fish, Top right: tapeworm larvae embedded in fish muscle
Bottom left: the front of Grillotia, showing the four unextended tentacles, Bottom right: a partially extended tentacles
Photos from Fig. 1, 2, and 3 of this paper

Also known as anglerfish or goosefish, monkfish are large, sea bottom-dwelling predatory fish that can grow to two metres long. They are commonly sold on fish markets but usually as pieces of pre-cut fillets since a whole monkfish would be rather unwieldy to handle for most people, and its appearance is probably off-putting sight for many would-be customers. While reducing a monkfish down to fillets would have made it presentable at a fish market, that would not have worked for the monkfish featured in this paper, which was infected with 1327 tapeworm larvae which were later identified as belonging to the genus Grillotia.

Grillotia belongs to a group of tapeworms called Trypanorhyncha. While most tapeworms have suckers and hooks for clinging to the intestinal wall of their final host, trypanorhynchan tapeworms have a different and rather unique tool in its arsenal. Concealed within its front end are four forward-facing tentacles lined with recurved hooks. Upon reaching their final host, those tentacles shoot out like harpoons and embed themselves into the intestinal wall.

But before they get there, they need to pass through multiple different host animals. The life cycle of a trypanorhynchan tapeworm goes something like this: Upon hatching from an egg, the first host they infect are tiny crustaceans called copepods, this is followed by larger crustaceans, fish or squid that feed on the said copepod, and the life cycle is complete when those infected animals are eaten by the right final host. While monkfish eats practically anything that it can swallow (even puffins), they are unlikely to be feeding (at least intentionally) on tiny copepods. So it must have been infected through eating larger fish and squid. Being a voracious predator, the monkfish act like a parasite sink as it accumulate tapeworm larvae from its prey.

Once inside the monkfish, the tapeworm larvae embed themselves into the chunky tail muscles, the subcutaneous tissue, and the fins. Histology sections showed that the larvae left behind trails of necrotic tissue as they migrated through the fish's flesh. Despite how heavily-infected it was, the monkfish was just a stopover and not the final destination for those parasites. In order to reach sexual maturity and begin the life cycle anew, they need to enter the gut of its final host - sharks. The adult stage of Grillotia have been previously reported from the guts of variety of sharks. Of those that are known to prey on monkfish, the sixgill sharks and nursehound sharks seems to be the most likely candidates as the final hosts for those tapeworms.

While it may seem that a big scary monkfish should have few predators, the sixgill shark is known for feeding on marine mammals, so a monkfish is certainly fair game, and the nursehound can feed on juveniles or scavenge on dead monkfish. If a shark had come along and eaten that monkfish, it would have swallowed a few hundred tapeworm with every bite. In that way, the monkfish acts as an effective staging ground for the tapeworm larvae so they can  infect the final host en masse.

While it may seem that infecting the final host in such numbers all in one go would increase competition for the limited space available in a shark's gut, for trypanorhynchan tapeworms, the shark also serves as a place for sexual reproduction, and for that, the more potential mating partners the better. Of the 977 known species of tapeworms that infect sharks, the full life cycle is only fully known for four of them. Such is the case for most parasitic worms with complex life cycles, but especially those that infect marine animals.

The secrets of the ocean aren't just found in difficult to access location like the deep sea, but are often within the animals that people take for granted. While the sight of a freshly caught fish riddled with parasites might be a horrifying sight for most people, it is also a snapshot into a cycle of life which has gone on in the ocean for millions of years - and we are barely beginning to understand any of it.

Reference:
Santoro, M., et al. (2018). Grillotia (Cestoda: Trypanorhyncha) plerocerci in an anglerfish (Lophius piscatorius) from the Tyrrhenian Sea. Parasitology Research 117: 3653-3658.