"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

May 12, 2016

Cystodiscus axonis

Myxozoans are a group of very unusual parasites. Despite their simplified structure, latest research has shown that their closest living relatives are actually jellyfish (which means they are technically animals). They are found in a variety of tissues and organs in their hosts which are usually fish, and in some cases, amphibians. Some causes diseases such as the salmon whirling disease, and in the case of certain fish-infected myxozoans, after the host dies, the parasite causes the flesh to melt - much to the frustration of fishermen. While the majority of known myxozoans infect fish, in the last decade, there has been increasing interest in studying myxozoans that infect amphibians, and the parasite featured today is one such species.
Photo of Cystodiscus axonis spore from this paper

Cystodiscus axonis is a myxozoan species which lives in either the brain or the gall bladder of their frog hosts. This parasite and its close relative C. australis is found in a range of native Australian frogs. This parasite was previously classified in another genus called Myxidium and its discovery (and reclassification) featured a number of twist and turns.

The parasite was first recorded in cane toads which have been introduced to Australia, so it was originally thought to be a parasite that the cane toads had brought with them into Australia and had since taken to infecting Australia's native amphibians. However, examinations of older frog specimens from museum collections, including those that were collected before the introduction of cane toads to Australia, revealed that the parasite had been in native Australian frog all along - the cane toad simply picked it up when they arrived and they turned out to be a really hospitable home for this parasite.

Given that C. axonis is not too discriminating when it comes to whether it infects native Australian frogs or introduced cane toads, there is potential for this parasite to infect other amphibians as well. And that's what the scientists behind today's featured study decided to find out. This time, they once again look to museum specimens, in this case from the Natural History Museum in London, and specifically they examined preserved specimens of caecilians for myxozoan parasites

Caecilians are very strange looking amphibians - they are legless, look kind of like giant earthworms, and they are very different to either toads and frogs. For this study, the scientists examined 148 caecilian specimens spanning across twelve species which are found in a variety kind of habitats, ranging from terrestrial, burrowing forms to aquatic species. Out of those, they found seven specimens which had myxozoan spores floating in their gall bladder. All the infected caecilians belong to one of two species - Typhlonectes natans and Typhlonectes compessicauda - and both of them are aquatic caecilians.

Based on the shape of those spores and sequences of their DNA, the parasite they found was almost identical to C. axonis from Australian frogs. So somehow, C. axonis has managed to successfully make a jump to caecilians too - but how? The native frogs of Australian and caecilians are separated not just by a vast ocean, but also 300 million years of divergent evolution - so how did these legless amphibian parasites end up with a parasite which is originally found in Australian frogs? A vital clue might be the fact that the infected specimens were originally captive animals.

Myxozoans use different host in their lifecycle - they usually alternate between a vertebrate and invertebrate host, so the infected caecilians might have become infected when they were fed invertebrates, such as tubifex worms, which were parasitised by C. axonis. Alternatively, they might have been housed or shared a water supply with other captive amphibians that were infected.

Given its ability to jump to a dissimilar host like caecilians, this explains why they were so receptive to cane toads when they were brought to Australian. Compared with the evolutionary gulf that separate frogs from caecilians, the native frogs of Australia and the introduced cane toads are practically kissing cousins. Given the presence of an Australian frog parasite in South American caecilians, just how widespread have C. axonis and similar parasite have become?

The lethal amphibian chytrid fungus Batrachochytrium is an amphibian pathogen which has now been spread all over the world due to the global trade in amphibians. So what other parasites might be lurking in the loads of frogs, salamanders, and caecilians which are currently being shipped all over the globe?

Hartigan, A., Wilkinson, M., Gower, D. J., Streicher, J. W., Holzer, A. S., & Okamura, B. (2016). Myxozoan infections of caecilians demonstrate broad host specificity and indicate a link with human activity. International Journal for Parasitology 46: 375-381.

April 25, 2016

Trophomera marionensis

This planet is full of parasites, and no matter what you are or where you live, there seems to be no escape from getting parasitised. A few years ago, I wrote a post about some microsporidian parasites which live in deep sea nematodes (roundworms) - well this time it is a deep sea nematode which is the parasite. Trophomera marionensis is a nematode which is found in one of the deepest part of the ocean, in the inky depths of the Kermadec Trench about 7000 to 10000 metres below sea level. This is a part of the ocean known as the Hadal Zone - a realm of perpetual darkness and immense water pressure, named after the underworld of Greek mythology.

Sample of the deep sea amphipods (top left), parasitised H. dubia (bottom right), an immature female T. marionensis (right)
Image from Fig. 1 and Fig. 4 of the paper
Trophomera marionensis belongs to a family of roundworms call Benthimermithidae which are mostly found in the deep sea. They share a similar lifecycle to the Mermithidae and Marimermithidae families which are found in the sunlit realm - some of which have previously been featured on this blog here, here, and here. Much like those families of nematodes, the benthimermithids are also body-snatchers that infects their host, take over the insides, and make a xenomorph-style exit at the end of their stay. But whereas those shallow water roundworms infect mostly insects and crustaceans, these deep sea nematodes are found in a more diverse range of hosts.

While T. marionensis infects the deep sea amphipod Hirondellea dubia which makes it comparable to some of its shallow water marine mermithid cousins, the hosts of the other 40 or so known species of Trophomera covers a wide variety of deep sea invertebrate animals. Given how sparsely distributed potential hosts are in the deep sea, you tend to take what you can get. The ecology of the hadal zone had placed enormous evolutionary selection pressure on the benthimermithids to diversify and infect invertebrates other than just arthropods. In addition to infecting deep sea crustaceans, species from that genus have been recorded from priapuplid worms (also known as the penis worm), mussels, and even other nematodes.

Much like other deep sea creatures, the population of T. marionensis is very sparsely distributed. Out of the several thousand amphipods that the researchers examined, they only came across 32 infected ones, containing a total of 40 worms. Most amphipods were infected with a single worm, though there was one rather unfortunate individual that was host to four worms. Furthermore, all the worms they found were female worms - so at this point we don't know how the male worms look like!

The most likely way that those deep sea amphipods become infected by T. marionensis is through accidentally ingesting the larval parasite during early stages of their development, while feeding on scraps of "marine snow" which had settled on sea floor. Currently, it is unclear what effects T. marionensis has on its crustacean host, but given the size of this nematode in comparison with the amphipod, they must have at least some effects on their growth and reproduction.

Amphipods are common in deep sea habitats, and benthimermithid nematodes have also been recorded in deep sea environments from all over the world. So there is no doubt there are many more parasite-host combinations lurking in the dark abyss of deep sea habitats which are yet to be discovered.

Leduc, D., & Wilson, J. (2016). Benthimermithid nematode parasites of the amphipod Hirondellea dubia in the Kermadec Trench. Parasitology Research 115: 1675-1682

April 11, 2016

Pseudolynchia canariensis (revisited)

Ever since birds and mammals have evolved to have feathers and fur respectively, many different orders of insects have also evolved to take advantage of the opportunities that they provide. Fleas, lice and some families of flies have become ectoparasites that dwell in the cosy environments offered by animals covered in feathers or fur.

Top: P. canarienesis with hitch-hiking lice
Bottom: Pigeon lice; (A) Columbicola columbae,
(B) Campanulotes compar, (C) Hohorstiella lata,
and (D) Menacanthus stramineus. Image from the paper
While there are many biting flies that feed on the blood of feather- and fur-covered animals, few are as specialised as the hippoboscid flies - also known as Louse Flies. Louse flies are flies that have evolved to be obligate ectoparasite - some of them can fly, but they prefer spending most of their time crawling around the feathers of birds or the fur of mammals. The species featured in the study we are covering today is Pseudolynchia canariensis, which parasitises the common rock pigeon (Columba livia). The flattened body and long legs of the louse fly allows it to go scrambling amidst the feathers of its host, as it finds a sweet spot to chow down on some pigeon blood. The prime spot to do so is on the pigeon's belly amongst all the soft downy feathers

But P. canariensis is not the only ectoparasite on pigeons - as would be expected, pigeons are also home to many other parasites include a variety of actual lice. There are four species of lice that regularly hang out around the pigeon's belly, Columbicola columbae (which has been featured on this blog before), Campanulotes compar, Hohorstiella lata, and Menacanthus stramineus. So it can get pretty crowded on a pigeon's belly and there are plenty of opportunities for these parasites to mingle

Unlike the louse flies, lice don't have wings - so to travel from one host to another, they have to either crawl the whole way themselves, or borrow someone else's wings. By that I mean some lice hitch a ride on their fly-based namesake. In this study, scientists measured the mobility of the above mentioned four lice species commonly found on the rock pigeon, and their ability to hitch a ride on those P. canariensis.

They first tested how well each of those lice move about on their own by placing them on a piece of filter paper, and watch how far they managed to move in two minutes. Because lice tend to dislike light, the scientist shone a small light on them to coax them to move. Next, they tested the lice's ability to attach to louse flies by placing louse individually in a clear tube with a louse fly, and see how quickly they climb onboard - if at all. Finally, they test how well these lice managed to stay on the louse fly by repeating the attachment experiment, but this time they let P. canariensis does its thing and fly to the other side of a small room with a closed window, then recapture it to see whether the hitch-hiking louse had manage to hang on.

They found that not all lice are equally adept when it comes either moving on their own, or the finer art of leaving on a louse fly - and it seems aptitude in those two skills are inversely related. The most athletic lice like M. stramineus are also the worst at attaching themselves to P. canariensis, whereas those that can't move all that well off-host, such as C. columbae, are louse fly riders par excellence. Rock pigeons are pretty gregarious, so for the more mobile lice, they can easily cover the distance under their own steam. At the speed which the scientists recorded, M. stramineus is capable of covering one metre in the period of six minutes, which makes it quite the marathon runner in the louse world. In contrast, Co. columbae and Ca. compar are downright helpless anywhere away from a bed of pigeon feathers, but they are very skillful when it comes to piggybacking on a louse fly.

For some lice, leaving on a louse fly is not such a lousy way to travel.

Bartlow, A. W., Villa, S. M., Thompson, M. W., & Bush, S. E. (2016). Walk or ride? Phoretic behaviour of amblyceran and ischnoceran lice. International Journal for Parasitology 46: 221-227.

March 27, 2016

Confluaria podicipina

Most of the time, being infected with parasites is costly to the host in some way. But sometimes there might be circumstance when the presence of parasites might be a good thing. For brine shrimps (known to most as "sea monkeys"), it seems like tapeworm larvae might be a worthwhile accessory - admittedly one that turns you bright red and make you more likely to be eaten by a bird.

Photo of infected (red) and uninfected (transparent) brine shrimps
From Fig 1 of the paper
The study being featured today were based on a population of brine shrimps living at salt marshes in southwestern Spain which are infected by nine different species of tapeworm larvae. The most common species are Flamingolepis liguloides (which have previously been featured on this blog here) and Confluaria podicipina. At the site where the scientists conducted this study, about two-thirds of the brine shrimps were infected with either F. liguloides or C. podicipina, and about a third of them are unlucky enough to be simultaneous infected by both species (alongside a bunch of other less common species).

All these parasites are using the shrimps as a temporary vehicle for getting into final host where they can mature into adult worms, and for that to happen, the shrimp needs to be eaten by a bird. However, in the environment that these shrimps dwell in, tapeworms like C. podicipina can convey some unexpected benefits. It seems that shrimps infected with tapeworms are more resistant towards arsenic.

Previously, we have featured a study on how tapeworms can act as a sink for heavy metal in seabirds soaking up the toxin before they get absorbed into the host's tissue. But that study was on adult tapeworms living in the gut of a bird host. Though they are also tapeworms, the physiological interaction between an adult tapeworm in the gut of a vertebrate host is very different to that of a larval tapeworm residing inside a small arthropod.
Flamigolepis liguloides cysticerocoid (larger one on the left) and Confluaria podicipina cysticercoid (indicated by arrows)
From Fig 2 of the paper
In this case, the tapeworm larvae increased the level of various fatty substances - C. podicipina increases triglyceride level, while F. liguloides increase the amount of lipid in the host. Together, these fatty droplets help soak up any arsenic in the brine shrimp. Additionally, the tapeworms also help the shrimp sequester carotenoid which enhances the shrimp's capacity to produce antioxidant enzymes which mops up harmful free radicals, and help the shrimp deal with the presence of arsenic in their bodies.

Whereas F. liguloides seems to be present in high numbers all the time, C. podicipina only appear in April. This might be related to the seasonal movement of their final host - which are flamingos in the case of F. liguloides, but for C. podicipina, the final hosts are grebes, which only visit the lake during certain time of year. Indeed, that was the finding of a previous study which has been featured on this blog.

Additionally, it seems that the brine shrimps are better at handling arsenic in May when they are mostly only infected with F. liguloides. So why is that the case? Well, it could be that (1) C. podicipina is not as good at helping their host deal with arsenic, (2) it is harmful to the host in other ways that offset their detoxification effects, and (3) it only appears during the warmer months when the brine shrimp's overall resistance to arsenic is lower anyway, so it simply coincided with their appearance.

Of course, neither F. liguloides and C. podicipina are doing this as some kind of favour to the host - C. podicipina and its fellow tapeworm larvae are doing this for their own benefit. They are manipulating host physiology to make the host a more suitable shelter and vehicle for reaching the final host - increasing the fat content of the host makes it a cosier site for development, and increasing the carotenoid level makes the shrimp bright red and stand out more to the bird host. But it just so happens that all these changes also have a side effect of benefiting the shrimp, even if temporarily, before they end up between the beaks of a bird

Sánchez, M. I., Pons, I., Martínez-Haro, M., Taggart, M. A., Lenormand, T., & Green, A. J. (2016). When Parasites are Good for Health: Cestode Parasitism Increases Resistance to Arsenic in Brine Shrimps. PLOS Pathogen 12(3): e1005459.

March 15, 2016

Trichobilharzia szidati

If you have ever gone for a swim in a lake and later found your arms and legs covered in red itchy welts resembling mosquito bites, it is quite likely that you have encounter parasites related to the one being featured today. Trichobilharzia szidati is an avian blood fluke, and it has relatives living all over the world in both freshwater and marine environments. While they usually infect waterbirds like duck, they are not very good at telling birds apart from humans. To them, any warm-blooded terrestrial vertebrate animal is fair game, which is rather unfortunate for both humans and flukes alike - more so for them than us. As a result of this encounter, we end up covered in intensely itchy spots, but getting under the skin of a human means immediate death for such flukes.
Cercaria of Trichobilharzia regenti, a species related to T. szidati
Scale bar = 200 μm. Photo from this paper

So why is that the case? Blood flukes are masterful molecular mimics - they are able to disguise themselves with proteins that resembles the host's own molecules, allowing them to stealthily sneak pass the host's immune system. But Trichobilharzia szidati and similar avian blood flukes have evolved to bypass the immune system of birds, and when it encounters a mammalian immune systems like ours - all bets are off. Our immune system takes immediate action against this intruder with extreme prejudice, which results in an inflammatory reaction that manifest itself as "duck itch" or "swimmer's itch".

But aside from getting inside the circulatory systems of ducks or giving us a nasty itch, it seems that trematode larvae like those of T.szidati are also making a contribution to the environment which usually get overlooked.

As a part of their lifecycle, parasitic flukes turn snails into parasite factories - churning out a continuous stream of free-swimming parasite larvae called cercariae, which in the case of T. szidati is the stage that infects birds and cause us temporary grief. But most of these cercariae don't actually end up infecting a bird or getting (and dying) under the skin of an unsuspecting human swimmer. The majority of them end up entering the food web as food for a range of other animals. To aquatic insects and fish, the swimming parasite larvae is simply another tasty morsel. Alternatively, the cercariae simply use up their limited energy reserves and expire, becoming food for all manner of scavengers and detritivores. So how much food is being provided by these tiny parasite larvae?

In the study being featured today, scientists collected some T. szidati-infected snails from a fish pond in Czech Republic and made daily observations on the amount of cercariae they were pumping into the environment. The noticed that most cercariae came streaming out upon first light in the morning, in order to coincide with the daily routine of the bird host, then dwindled as the day went by. But throughout the day, it adds to to hundreds and thousands of larvae.

When they conducted the first set of observations in April, they found that on average infected snails were releasing about 1000 cercariae per day, with a maximum of over 4500. However, when they made another series of observation again in September, the average daily output was ten times that of the snails they studied in April, with a maximum output of almost 30000 cercariae per snail per day. It is worth noting that while they made four sets of observations for the April sample, only one set was conducted during September, which means the sample could have be skewed by an unusual sample. Additionally, the snails in the September sample were larger than those from April, and larger hosts are usually able to produce more parasite larvae. But these are the kind of seasonal and individual variations which would have exist in the natural environment anyway.

Since each infect snails are releasing thousands of cercariae per day, though they are microscopic, those contributions really adds up. Based on the numbers they obtained from the study, the research estimated that over its lifetime, an infected snail produce as much as its own body mass (or more) in the form of T. szidati larvae. Therefore, in a large fish pond with a relatively low infection prevalence such as 5%, the infected snails would be contributing about a 500 kilograms of biomass per year in the form of T. szidati cercariae. But in some location where almost half the snails are infected at any given period, the yearly output of all these snails can add to to 4.65 tons of parasite larvae, which weighs as much as an Asian elephant.

Trichobilharzia szidati and other avian blood flukes do not exist in isolation - the snails they infect can also host an entire communities of other flukes species, some of which have been recorded to churn out even more cercariae than T. szidati. When you put them together, they provide quite a substantial food source for all the aquatic organisms that they share the environment with. These parasitic flukes are the unseen elephant(s) in the pond.

Soldánová, M., Selbach, C., & Sures, B. (2016). The Early Worm Catches the Bird? Productivity and Patterns of Trichobilharzia szidati Cercarial Emission from Lymnaea stagnalis. PloS One 11: e0149678.

February 23, 2016

Anisakis pegreffii

Raw fish are eaten all over the world. However, when preparing fish fillet for a meal, one might come across some parasitic worms, much to some people's shock and revulsion. Most of these parasitic worms are anisakid nematodes, and these parasites had other plans for that fish before they ended up on someone's chopping board.
Photo of hagfish (left) by Linda Snook, photo of anisakid worms in hagfish (right) from Fig. 1 of the paper
Anisakids is a family of parasitic nematodes which really gets around. It is found in oceans all around the world and they have complex lifecycles that carry them across most part of the oceanic food web, infecting all kinds of marine animals from krill to fish to whales. The larval stages of the parasite are found in crustaceans, squid, and fish, while sexually-mature adult worms live in the gut marine mammals such as whales and seals where they mate, and produce prodigious number of eggs which are released into the ocean to start the lifecycle again. Sometime humans can interrupt this cycle, resulting in a disease call anisakiasis.

While the larval worms are usually found in the gut of their fish host, after their host dies, the worm tend to migrate into the muscle, where they are sometimes found by people preparing fish fillets for a meal. Since its lifecycle takes place in the open sea, anisakids have incorporated many marine animals into acting as their hosts at various stages of their development.

One seemingly unlikely animal that they have incorporated is the hagfish. Hagfish themselves are often mistakenly consider as a parasite when they are in fact mostly scavengers. This misconception has probably arisen from their habit of burrowing into the body cavity of dead and dying fish while feeding, however unlike the other living lineage of jawless fish - the lamprey - hagfish do not exhibit any parasitic habits.

Hagfish are also well-known as living slime machines, able to produce buckets worth of slime that act as a deterrent to many would-be predators. But despite their slime defence, this does not stop them from being eaten by a variety of large marine animals as well - some of which happens to be the final host for anisakid nematodes. Of course hagfish have also been incorporated into culinary dishes from around the world, which means they can also be a potential source of anisakiasis.

In the study being featured today, scientists examined 265 hagfish purchased from a fishing port in northeastern Taiwan, composing of four different species. From that sample they were able to find eight species of anisakids. By far the most common species was one call Anisakis pegreffii, which made up about 80% of the worms that were found. And not all the hagfish were equally parasitised - the host with the most was Eptatretus burgeri, otherwise known as the Pacific hagfish. Anisakis pegreffii has also been found in many other marine fish including anchovies, cod, and mackerel, and their final hosts are dolphins. So what's the advantage of using hagfish as a part of their lifecycle?

Because hagfish often scavenge on dead fish, they might actually be a slimy saviour for many larval anisakid worms found in those fish. Since the final host of these worms (marine mammals), don't usually go around picking up dead rotting fish from the seafloor, anisakid nematodes in dead and dying host would have usually been consigned to the same fate as their fish host.

However, the intervention of the hagfish can keep these worms in circulation, giving them another chance of reaching their final host where they can reproduce. Thus, the scavenging hagfish act as a slimy saviour for these parasites on their life's journey.

Luo, H. Y., Chen, H. Y., Chen, H. G., & Shih, H. H. (2016). Scavenging hagfish as a transport host of Anisakid nematodes. Veterinary Parasitology 218: 15-21

February 12, 2016

Briarosaccus regalis

If you come across a crab which has some kind of kidney-shaped blob sticking out of its abdomen and an extensive network of root-like filaments throughout its body - do not be alarmed  - it is merely infected with some kind of body-snatching parasitic barnacle. So let say you then find another crab, of a different species, which seems to have the same affliction. You might think that it is also infected with the same species of barnacle as that first crab. But looks can be deceiving.

Photo from Figure 3 of this paper
Parasites vary in the range of hosts that they can infect. Some are generalists that can infect a wide range of hosts, but the majority are specialists that can only live on a few or even a single host species. With the advent of molecular biology, some of those versatile "generalists" parasites have actually turned out to be a bunch of specialists that each infected their own particular host, but they just happened to look very similar to each other. Such is the case with the parasite we are featuring today - Briarosaccus regalis.

Briarosaccus is a type of rhizocephalan - a group of highly-modified parasitic barnacles - the most well-known example is Sacculina carcini. As you can see in the photo above, rhizocephalans look about as similar to a seashore barnacle as a haggis. The kidney-shaped orange part is the externa - the parasite's reproductive organs. It might not look like much, but it is capable of undergoing at least 33 breeding cycles, producing up to 500000 larvae each time. The rest of this parasite, call the interna, are actually those luxurious green threads which are wrapped around the crab's internal organs.

Not surprisingly, we generally have trouble telling apart what looks like a kidney-shaped blob sprouting a bundle of delicate green roots from other similarly adorned kidney-shaped blobs. This is where DNA can be useful. The new study analysed sections of the mitochondrial DNA of some Briarosaccus specimens from 52 king crabs collected in the fjords of Southeastern Alaska. Previously, the Briarosaccus genus is only known to contain two species, one of which is Briarosaccus callosus which was described in 1882 and has been documented to infect many different species of king crabs, three of which are commercially fished.

Since it infects such a wide range of king crabs, it was assumed to be found across all the world's oceans. But the new study that we're featuring today shows that some specimens which have previously been identified as B. callosus actually consist of two other different species - B. regalis which infects the red king crab and the blue king crab, and B. auratum which is only found on the golden king crab.

It turns out we've been lumping two previously undescribed species together and treating them as if they belong to another species which we are more familiar with. What this study revealed is that instead of just one species (B. callosus) infecting all kinds of king crabs, there's actually a bunch of specialised parasites which happens to look the same us. While both B. regalis and B. auratum are found in the same region and their respective hosts occur in close proximity to each other, these parasites are faithful their own hosts. Since there are other plenty of other king crabs nearby, why have neither of them made a switch?

Given the extremely intimate relationship that rhizocephalan parasites have with their host - sending delicate roots throughout the crab's body and manipulating their physiology, all without setting off the immune system - they are finely tuned towards their particular host species. So even when there are alternative potential hosts available, neither species can make a switch. From the parasite's perspective, there's no need to do so when your host is so abundant.

During their evolution, many parasites have lost physical characteristics which would otherwise allow us to visually distinguish them from their close relatives. Because of that, their differences may not be immediately obvious to us. The use of molecular biology techniques has enabled us to start seeing the true diversity of parasites - most of which are hidden in plain sight.

Noever, C., Olson, A., & Glenner, H. (2016). Two new cryptic and sympatric species of the king crab parasite Briarosaccus (Cirripedia: Rhizocephala) in the North Pacific. Zoological Journal of the Linnean Society, 176: 3-14.

January 24, 2016

Artystone trysibia

The tongue-biter Cymothoa exigua is arguable one of the most (in)famous fish parasite in the world. It was famous enough to get a mention on the Colbert Report, and while the world recoil in collective horror at the sight of a fish which had its tongue replaced by a parasite, among its fellow parasitic crustaceans, tongue-biter's modus operandi is actually rather quaint. It can easily be upstaged by other parasitic isopods in the horror department, and today's post is about one those species.

The parasite we are featuring today is Artystone trysibia - it is in the same taxonomic group as the tongue-biter (Cymothoidae), and it parasitises a number of freshwater fish in the Amazonian basin. But unlike its more famous cousin which is content with merely living in the host's mouth, A. trysibia cranks the nightmare fuel up to eleven and lives inside a fleshy capsule in the host's body cavity.
Photo from Figure 2 of this paper
This parasite and others like it are actually relatively common. This parasite has been documented from a range of freshwater fish from South America, and has also been reported from aquarium ornamental fish. They're so common that A. trysibia has gained a common name - "ghili" - among the Kichwa people.

Female A. trysibia (right) and Female A. trysibia with larvae
Photo from Figure 3 of this paper
This study documented its presence in the Bristlemouth Armoured Catfish (Chaetostoma dermorhynchum). Despite its armouring, this fish has no protection against A. trysibia. Usually, the only sign of the parasite's presence is a small, gaping hole on their belly or their flank. But that hole serves as a window for the parasite within. For this study, these catfish were sampled from three pristine sites at the Tena River in the Amazonian region of Ecuador.

These catfish are fairly small fish, and most of them are about 15 - 20 cm long (6-8 inches), but A. trysibia can grow to 1.5 - 3 cm (0.6-1.2 inches) long and takes up quite a lot of space within the catfish. For comparison, it would be the equivalent of having something the size of a pet rabbit living in your torso. As mentioned above, the only contact the parasite has with the outside world is with a tiny hole through which they breath and release their offspring, and they can reproduce in prodigious numbers - one female isopod was recorded to be carrying 828 larvae. Each catfish was found to (thankfully?) only ever have a single A. trysibia, and it seems that the bigger the host, the bigger the parasite, possibly because a larger host would give the parasite more room to grow.

Artystone trysibia is not alone in its style of parasitism, there are other similar species which are found in both freshwater and marine fish around the world (for example, see here). So the next fish which you come across might not just have a tongue-biter in its mouth, it might also have a ghili in its belly.

Junoy, J. (2016). Parasitism of the isopod Artystone trysibia in the fish Chaetostoma dermorhynchum from the Tena River (Amazonian region, Ecuador). Acta Tropica 153, 36-45.

January 10, 2016

Cardicola orientalis

Tunas are one of the most graceful animals of the sea. These sleek and powerful predators spend their lives in motion, cruising the open seas for prey. But despite being such formidable fast movers, this does not make them immune to parasitic burdens, indeed the parasite we are featuring today are found in the heart these pelagic predators.

Scientists examining Pacific bluefin tuna (Thunnus orientalis) at a tuna ranch at the Wakayama prefecture, Japan came across some unfamiliar-looking flukes living in the heart of the tunas, which they described and named Cardicola orientalis in this study. Aside from feeding on the tuna's blood, the eggs that these parasites produce can become lodged in various tissues, obstructing blood vessels and causing harmful lesions and inflammations. In fact, these wayward eggs are more debilitating to the host than the adult parasite itself.
Photos of Cardicola orientalis from this paper
If you think the fish host have it bad with these parasites, the invertebrate host have it much worse. These parasites have a complex lifecycle that alternates between a vertebrate and an invertebrate host. The adult parasite reproduce sexually in the fish host but the eggs that they produce (if it they don't get lodged in somewherein the host) are released into the environment and hatch into a larva call a miracidium that infects mollusc or polychaete worm (depending on the fluke species). Inside the invertebrate host, the fluke larva transforms into a sausage-shape stage call a sporocyst, which then multiply via cloning inside their body, turning them into a parasite factory (see photo above).

This newly-commissioned biological factory then churn out another larval form of the parasite, called cercariae, which are shed into the surrounding waters where the fish hosts are found. So if you want to stop the tuna from getting infected with blood flukes like C. orientalis, you have to figure out which invertebrate is acting as the parasite factory in the fluke's lifecycle.

Of the 136 known species of fish blood flukes, the full lifecycle is known for a handful of them. Because they are one of the few fluke species that can severely impair or even kill their fish host, fish blood flukes are a major concern to the aquaculture industry. Considering the number of marine invertebrates that can serve as potential host for C. orientalis, it would seem that these scientists had a pretty difficult task at hand. However, based on previously documented lifecycles for tuna blood flukes, they are somewhat different from those other fish blood flukes in that instead of using snail or a bivalve for their clonal stage, they use polychaete worms. Specifically they use a family of worm call the terebellids - also known as spaghetti worms - which live in burrows and crevices.

The research team found many such worms encrusted on the structure of the tuna cage, alongside other invertebrates such as sponge, seashells, and sea squirts. The most abundant species was a marine worm call Nicolea gracilibranchis. They took monthly sample of these worms from the tuna cages from January to May, dissecting 4729 worms in total and finding 349 to be infected with the clonal stage of C. orientalis. Even though the researchers found that most of those worms were living on the floats that surround the tuna cages, it was the worms encrusted on the ropes which held the cage in place that are more likely to be infected with the parasite's clonal stages.

They also noted that infected hosts became more common over course of the sampling period, and while the worms they dissected in January were mostly filled with developing parasite embryos, those sampled after February were ripe with cercariae ready to pop. These pattern seems to indicate that the worms become infected through eggs that were expelled from tunas during winter and the parasite larvae developed over spring.

Since tuna has a reputation for being a fast swimming fish, you'd think their parasites would be equally well-equipped for swimming. But instead, C. orientalis has a tiny stub of a tail which doesn't appear to be good for swimming (or much else for that matter). But somehow, they must be getting to the tuna just fine; either the infected spaghetti worms churn out so much cercariae that at least some manage to encounter their host, or they have some other adaptation that facilitates their rendezvous with a tuna. Or both.

The research team also came across one case of a different tuna blood fluke species - Cardicola fosteri - which has previously been found in Australia and was featured in a post on this blog from 2011. It is worth noting that while in Australia, that parasite infects the worm Longicarpus modestus and the southern bluefin tuna (Thunnus maccoyii), in Japanese water they were infecting a different species of terebellid worm (Amphitrites sp.) and tuna for their lifecycle. So is this ability to switch host common to all fish blood flukes, or is it just this particular group of tuna blood flukes?

This flexibility in host use would be an extremely useful adaptation, especially for a parasite like C. orientalis since its host is an open water animal which is widely distributed across the world's oceans. But this can also be a concern for fish farmers as fish species introduced for aquaculture may exchange parasites with wild fish native to a particular region. As the aquaculture industry incorporate more species to their stock, novel and/or poorly described species will emerge as new problems. The lesson here is that if you are going to farm fish, you better be prepared to come across some flukes.

Shirakashi, S., Tani, K., Ishimaru, K., Shin, S. P., Honryo, T., & Ogawa, K. (2016). Discovery of intermediate hosts for two species of blood flukes Cardicola orientalis and Cardicola forsteri (Trematoda: Aporocotylidae) infecting Pacific bluefin tuna in Japan. Parasitology international 65: 128-136.

December 27, 2015

The Worms of Beasts, Tragic Romances, and Body-Snatchers

It has been yet another year of parasitology, and this year has been my fifth year writing on a regular basis for Parasite of the Day! So what had been on the parasite menu for 2015?  First of all, some of the parasites that made their way onto the blog this year have been various worms that cause misery for everyone's favourite large mammals like dolphins, pandawhales, and baboons. But it is not just large mammals that become unwitting host for parasites, for example, the giant ocean sunfish is also host to a fluke that surrounds itself in a bag of the host's flesh

If that all sounds very snug and cosy, then one might see that some of the posts can be described as love stories, though most of them with a tragic or unsavoury twists. There was a post about treacherous journey undertaken by male pea crabs to answer a booty call, a guest post by Katie O'Dwyer about sexually transmitted infection in ladybirds, and a story of how cicadas' love songs can end in tragedy (chest-burster style).

On the subject of body-snatchers, nature certainly has no shortage of them, and insects are usually the victims - in one case, tapeworms for ants which also seem to affect the behaviour of the host's uninfected nest mates. These body-snatchers also seem to get around as well, one of these well-worn travellers is a species of roundworm that was introduced to New Zealand from Europe via earwigs. That worm is a mermithid nematodes, but its lifecycle is remarkably similar to that of another phylum of worms - the nematomorphs. More commonly known as hairworms, they which share a similar life cycle to the mermithids. This year featured a post about a species which infects and ultimately kills praying mantis, but in male mantis before this parasite takes its life, it take away its junk.

On the subject of that part of the body, there was also a post about frog bladder worms which do not always end up becoming parasitic, and whether they do so depend on its circumstances during the earliest part of its life. But even if some of those worms do no always end up as parasitising frogs, there are other worms that do, for example, the kangaroo leech. It drinks frog blood, hitches ride on crabs, and takes good care of its babies. There were also other blood suckers which were featured on the blog this year, and a rather unlikely one is the vampire snail.

As for guest posts, aside from the one contributed by Katie O'Dwyer, as usual, the students from my parasitology class also wrote stories on parasitoid wasps that force their host to weave a tangled web, tailor-made for their own purpose, but it seems that different wasps also coerce their spider hosts into weaving different webs. There was also a post about a parasite that causes rabbits to end up with a severe case of Shaft Studio head-tilt, a post about how parasites affect Monarch butterfly migration, another about how these butterflies fight back, and finally to top it off, a steaming pile of hyena poop sprinkled with tapeworm eggs.

In addition to writing about new papers about parasites, I also wrote about my experience attending the joint annual meeting for the New Zealand Society of Parasitology (NZSP) and Australian Society for Parasitology (ASP), which was held in Auckland, New Zealand this year. Among other things, in the first report I wrote about the fascinating story of giant squid parasites and its link to sharks, and in the second, I mused about the near-mythical status that Toxoplasma gondii has attained in the public consciousness.

I also wrote a post about parasite in prehistory to accompany my review paper on fossil parasites which has recently been published in the journal Biological Reviews. As a companion pieces, I also wrote an article for The Conversation which focus more specifically on dinosaur parasites (because everyone loves dinosaurs). So that about wraps it up for 2015. See you all in 2016 for another year of posts about more fascinating research into the world of parasites!

P.S. If you can't wait until next year for your parasite fix, as well as writing this blog, I have also been doing a regular radio segment call "Creepy but Curious" where I talk about parasitic and non-parasitic organisms such vegetarian spiderselectric eelshipwormsPompeii wormssirensvampire squid, brood parasitic cuckoo bees and cuckoo birds, carnivorous caterpillarsgreen sea slugs, the macabre bonehouse wasp, and a pair of unlikely parasites in the form of mussels and bitterlings. You can find links to all these and more on this page here.

P.P.S. Some of you might also know that I also do illustrations (and provide cartoons to accompany those Creepy but Curious segments), some of my drawings are about parasites, but I seem to have gone on a somewhat odd direction with those towards the end of the year...