"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

April 11, 2024

Anoplocephala gorillae

Tapeworms are found in all kinds of vertebrate animals, and while their life cycles and transmission usually rely upon parasitised prey being eaten by predatory final hosts, some tapeworms have evolved ways to infect herbivorous animals as well. Anoplocephala is a genus of tapeworms that parasitise a wide range of herbivorous mammals including elephants, rhinos, hyrax, zebras, and more. The most well-studied species is Anoplocephala perfoliata because it happens to be a parasite of horses, and heavy infection with that tapeworm can cause gastrointestinal diseases. But the species featured in this post are found in a close relative of humans, specifically the Mountain Gorilla (Gorilla beringei beringei), and its name is Anoplocephala gorillae.

Left: Anterior of four Anoplocephala gorillae with their scolices (attachment organ) visible. Right: Proglottids (reproductive segments) of Anoplocephala gorillae collected from faecal samples.
Photos of the parasite from Figure 2 and Figure 4 of the paper

This post is about a study which took place at the Volcanoes National Park (VoNP), in the Rwandan part of the Virunga Massif - a complex of protected areas spanning the borders of Rwanda, Uganda and the Democratic Republic of the Congo (DRC). The aim of the study was to examine the epidemiology of tapeworms in mountain gorillas, and to improve the diagnostic tools for detecting such parasites. To do so, researchers examined faecal samples which were collected by park personnel and Gorilla Doctors veterinarians from groups of habituated gorillas in the VoNP. Whenever possible, each of those samples were identified to specific gorilla individuals, allowing veterinarians to keep track of each gorilla's health and parasite status.

Researchers estimated the abundance of tapeworms in the gorillas by counting the number of tapeworm eggs in each gram of faeces. Generally speaking, more eggs means more worms, but egg production varies between individual worms at different times, so multiple samples needed to be taken to ensure a more accurate count. Out of the 1500 samples they examined, about seven percent had egg counts of over a thousand eggs per gram of faeces, though the average was much lower at 384 eggs per gram. While A. gorillae seems to dominate the tapeworm fauna of these gorillas, the faeces of one gorilla, an infant male named Inkingi, also had another tapeworm species in the genus Bertiella. It is relatively easy to distinguish the eggs from those two different tapeworms - Anoplocephala has quadrangular or triangular-shaped eggs with flat sides and thick shells, whereas Bertiella has spherical eggs with thin shells

In addition to those faecal samples, any gorillas that had died were retrieved from the wild and necropsied as a part of the local veterinary surveillance program. For the purpose of this study, five deceased gorillas that were recovered between 2015–2018 were necropsied and examined for tapeworms. In total, 53 A. gorillae tapeworms were collected, and they varied in size from 1.5 to 13 cm long. Most of them were found in the small intestine, but there were also some in the caecum and colon.

So how do the gorillas end up with all those tapeworms in the first place? While the eggs are released into the environment packaged in the gorilla's faeces, they cannot infect the gorillas directly. Like other tapeworms, they have to go through an intermediate host, which as mentioned earlier, is usually a prey animal. But since gorillas are herbivores, how can tapeworms gain entry into their guts? 

Based on what is known for other Anoplocephalidae tapeworms, gorillas become infected by swallowing mites that are parasitised by the tapeworm's larvae. These mites are tiny, barely pinhead-size, thus can be easily swallowed among a mouthful of foliage. While the prevalence of Anoplocephala among mites might be extremely low, like other herbivorous mammals, gorillas go through a lot of plant matter, eating 18-45 kilograms of vegetation a day. So just a few infected mite sprinkled in would be enough to ensure that the gorillas get infected,

While the deceased gorillas that were necropsied in this study had large numbers of tapeworms dwelling in their gut, they were all in good condition, and had died from other sources of trauma rather than disease. So in contrast to A. perfoliata which can cause major pathologies in horses, A. gorillae is content with a more peaceful existence, just living quietly as a part of the gorilla's regular gut symbiont fauna.


March 9, 2024

Veneriserva pygoclava

There are many ways to become a parasite, and there are parasites with vastly different ancestries that end up joining the same path on the road of parasitism. In some cases, sharing the same path can also mean adopting a certain shape. This post is about Veneriserva pygoclava, a worm that lives inside a worm, more specifically it is a polychaete worm that has evolved to parasitise another type of polychaete worm which are commonly called "sea mice".

Top left: Ventral view of an infected Aphrodita longipalpa with a Veneriserva pygoclava parasite inside. Bottom Left: MicroCT scan image of an Aphrodita longipalpa with Veneriserva pygoclava female highlighted in yellow and juvenile highlighted in blue. Right: A female Veneriserva pygoclava (top) and a male (bottom).
Photos from Fig. 1 and Fig. 3 of the paper

The genus name of this parasitic polychaete translates into "Venus' servant" though this worm is certainly a servant for nobody but itself. You'd think that living inside the body of another animal would restrict how big it can get, but the female Veneriserva grows to about seven centimetres long, which is twice as long as its host. Surprisingly enough, being longer than the host is not unusual among these kinds of parasitic polychaete worms. Despite its size and the amount of space it occupies within the host, it does not seem to cause any injuries or damage to the host's internal organs.

Living this endoparasitic lifestyle requires some specialised adaptations, and over the course of its evolution, Veneriserva has ended up with a body plan which is very similar to that of tapeworms. Despite both being called "worms", tapeworm and polychaete worms are from entirely separate animal phyla and their path to this "tapeworm body plan" (for the lack of a better term) were very different.

Tapeworms evolved from free-living flatworms, which are fairly simple animals, at least in terms of their body plan. A flatworm has no body cavity, its gut is more or less a blind-end sac (with some branches in larger flatworms), and it doesn't even have a circulatory system. If anything, in order to adapt to a parasitic lifestyle, tapeworms have evolved to become more complex than their free-living ancestors. Over the course of the tapeworm's evolution, they have gained a new attachment organ - the scolex - which is a heavily modified head, while the rest of the body has become an efficient conveyor belt of reproductive organs. These parasitic flatworms have even evolved a brand new type of "skin" called a tegument which allows it to absorb nutrients as well as protect itself against the host's enzymes, and some tapeworms even have the most complex central nervous system among all the flatworms, enabling them to navigate and maneuver in the dark, fleshy tunnels that are their host's intestinal tract.

In contrast, polychaetes are segmented worms, and are actually more similar to us in their body plan, equipped with a full body cavity, muscular gastrointestinal tract, and a closed circulatory system with blood vessels. But Veneriserva has abandoned much of that, because when you're living inside another animal, being built like a tapeworm seems to be the way to go.

Veneriserva does have a mouth of sorts, but it is not connected to any digestive tract to speak of. In fact, the digestive tract has been reduced down to a throat with a blind-end. Instead, the mouth of Veneriserva serves as a grabber to hold the parasite in place, functioning much like a tapeworm's scolex. Additionally, Veneriserva has also evolved its own version of the tapeworm's tegument, which is covered in fine microscopic finger-like projects (rather like the lining of your small intestine, just inside out) allowing it to absorb nutrients through its skin. There are also patches of cilia on the skin which may serve to stir the host's bodily fluids in order to bring more nutrients into contact with the parasite's skin.

However, when it comes to sex, there is one key difference between Veneriserva and tapeworms. Tapeworms are hermaphroditic - any tapeworm can mate with any other individual of the same species, or even with itself if it is desperate and alone. In contrast Veneriserva have separate female and male sexes which are clearly distinguishable - male worms are tiny compared to their much larger partners (see accompanying photo).

This "attachment organ + loads of gonads" type of body plan that tapeworms and Veneriserva have both independently evolved is also found in other internal parasites. For example, acanthocephalans - thorny-headed worms - are parasitic worms which live in the gastrointestinal tract of vertebrate animals, and are somewhat related to rotifers. Despite being in a different phylum, they share some key anatomical similarities to tapeworms, with their own version of the tegument, a body dominated by gonads, and a prickly anchor at its "head" to stay attached to the host's intestinal wall. Another example is Thyonicola, the parasitic snail which uses a thin stalk to attach itself to the intestines of its sea cucumber host, while the rest of the body is simply a long tube of reproductive organs and developing eggs. There are even some parasitic dinoflagellates that have evolved to resemble tapeworms. 

Judging from how common this "tapeworm-style" anatomy is across different parasite groups, it seems that when you are an internal parasite, you have to get into shape - and that shape happens to be that of a tapeworm.


February 12, 2024

Ascarophis globuligera

To land-dwelling humans, deep sea hydrothermal vents would seem like a vision of hell, amidst the crushing darkness you have plumes of superheated water, mixed with noxious sulfides, erupting from fissures on the seafloor. But for many deep sea animals, this "hell" is in fact a vibrant oasis in the middle of the abyss. This lively habitat is made possible thanks to bacteria that are able to extract energy from the sulphurous waters billowing from those vents. In the absence of sunlight, these chemoautotrophs form the foundation of the food chain. Some tube worms have been able to co-opt the power of these bacteria by housing the microbes in their gills, enabling them to grow to enormous sizes. Their tubes form dense, forest-like habitats for many other animals including other polychaete worms, fishes, crustaceans, and molluscs. This sets the stage for all kinds of complex ecological interactions, and that includes parasitism.

Left: Anterior of Ascarophis globuligera from Fig. 6 of the paper, Right: Photo of Thermarces cerberus (pink vent fish) by Dr Lauren Dykman, used with permission 

This post is about a paper reporting on three newly described species of Ascarophis nematodes that have been found in the guts of some deep sea hydrothermal vent fishes. Some of those worms were collected as a part of a larger study which focused on looking for parasites from hydrothermal vent animals, and along with freshly caught specimens, the researchers also examined preserved fishes collected by past expeditions. While they only managed to recover a few specimens of Ascarophis nematodes, some of which were fragmentary, those were enough to provide a scientific description for three different species - A. justinei, A. globuligera, and A. monofilamentosa.

The three species differed slightly in which fish species they infect - A. justinei is found in both the pink vent fish and a species of viviparous brotula, whereas A. globuligera has only been found in the pink vent fish, and A. monofilamentosa lives in a species of zoarchid fish named Pyrolycus manusanus. While it is not possible to conduct experimental infections to work out exactly how these nematodes transmit between hosts, their life cycles can be inferred based on what is known about other Ascarophis species which are found in shallower waters. This usually involves a crustacean, often amphipods, serving as the intermediate host for the parasite's larvae. Amphipods are plentiful around hydrothermal vents, and these crustaceans are eaten by a range of animals including deep sea fishes such as the pink vent fish, making them the ideal vehicle for Ascarophis to complete its life cycle.

The need for Ascarophis to reach an amphipod host may explain why each of those Ascarophis species has differently shaped eggs. For example, A. justinei has eggs which are regular, ovoid shape rather similar to other known species of Ascarophis, but the eggs of A. globuligera have a bulge on their side (which gave the species its name), and A. monofilamentosa eggs have a long filament dangling from them which is about fifteen times the length of the egg itself. These differently shaped eggs could mean slightly different transmission strategies. The extra ornament on the eggs of A. globuligera might serve to entice a hungry amphipod by resembling something edible (as with some tapeworm eggs that infect crustaceans by mimicking diatoms), or in the case of A. monofilamentosa, its long filament may prevent the eggs from drifting away into the empty abyss by wrapping them around a structure, or entangle them around something which might get eaten by an amphipod.

Some Ascarophis species are actually known to take a shortcut with their life-cycles -  instead of waiting for a fish host to come along, they become sexually mature and start laying eggs inside the amphipod, bypassing the need to enter a fish host to complete their life cycles. It is unknown whether any of the newly described deep sea species are capable of doing this, but in an ephemeral habitat like hydrothermal vents, it would be useful to have such an option as insurance.

There are many biomes on this planet which are completely inhospitable to humans. But that does not stop them from being as rich and vibrant as those that we are more familiar with, and wherever there is a thriving ecosystem, you will find parasites taking part in its web of interactions.

Moravec, F., Dykman, L. N., & Davis, D. B. (2024). Three new species of Ascarophis van Beneden, 1871 (Nematoda: Cystidicolidae) from deep-sea hydrothermal vent fishes of the Pacific Ocean. Systematic Parasitology 101: 2.

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.