"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 10, 2016

Cuscata campestris

One of the key characteristic of plants is their ability to produce food using sunlight via the process of photosynthesis. But there are many plants that do not photosynthesise - holoparasitic plants obtain all the nutrient they need from their host. One of the most well-known group of holoparasites are the dodders. There plants have no roots, their leaves have been reduced to tiny scales, and are composed entirely of elongated tendril-like vines. Because of their appearance and what they do to the host, the dodder has also acquired many common names including "Angel Hair", "Witch's Hair" and "Devil's Hair". When the dodder latches on to an appropriate host, it extends structure call haustoria which penetrate into the host plant's tissue to suck out its nutrients. This has led to the dodder also being called "vampire plants".
Dodder seedling in the process of coiling around its host (image from Fig. 1. of this paper)
There are 200 known Cuscuta species and of those, a small handful of them (10-15 species) are considered as serious agricultural weeds. These parasitic plants wrap their vines around their hosts and literally suck the life out of them. Infestation of this plant have been known to cause massive losses in alfalfa, tomatoes, carrots, and cranberry crops, and these "vampire plants" are very difficult to get rid off; since dodder vines often completely smother their hosts, it requires a lot of manual and mechanical labour to remove them. Additionally, dodders can also produce large numbers of resilient seeds that can linger in the soil for a long time, waiting for the next year to erupt in another outbreak.

For dodders, as with most other organism, the first moments of its life are the most critical - the newly germinated dodder seedling must secure a grip on a host plant within two to three weeks of germinating, otherwise the seedling would use up its energy reserves and expire. From the moment they germinate, these parasitic plants have various ways of finding their host. Dodder vines are able to "sniff out" host plants through the chemical they give off (essentially plant BO), but they can also use other senses to find their host. A newly germinated dodder seedling can also detect the specific wavelength of light which are reflected off the surface of plants, and use it to reach a host.

The scientists in this study investigated whether exposing dodder seedling to different spectrum of light can disrupt their ability to find their hosts. They conducted their experiments on the seedlings of two dodder species - Cuscuta campestris (which parasitises tomatoes) and Cuscata gronovii (which parasitises jewelweeds), and exposed them to three light source with different spectrums - the spectrum similar to unfiltered sunlight, mostly red light, and mostly far red light. Far red is a wavelength of light which is barely visible to our eyes, but it is the wavelength which is most reflected by the surface of plants. It is also the wavelength which dodders use to home in on their host.

From the experiment, the scientists found that almost all the dodder seedlings that were exposed to unfiltered light and mostly far red light were able to attach to a host after a week or two, in fact those bathed in far red light grew faster than the other groups. However, most of those that were bathed in red light lost their ways. Only 15% of the C. campestris and 27.2% of the C. gronovii seedling that were exposed to high levels of red light had managed to wrap themselves around a host.

So will it possible to control dodder infestation simply by bathing crops in red light? No, not quite since some dodder still managed to wrap themselves around a host plant and the red light treatment is only effective during the earliest stage of the dodder's life. But at least the findings of this experiments have shown that perhaps light manipulation can be combined with other control methods to control dodder infestation.  Additionally, we have gained an insight about how these parasitic plants sense and find their way through the world.

For this particular vampire, its weakness is not against sunlight, but rather, red light.

Reference:
Johnson, B. I., De Moraes, C. M., & Mescher, M. C. (2016). Manipulation of light spectral quality disrupts host location and attachment by parasitic plants in the genus Cuscuta. Journal of Applied Ecology 53: 794-803.

P.S. I recently drew some dodder-inspired art, yes, it is more Parasite Monster Girls - meet Cassandra the Dodder...

June 23, 2016

Halipegus occidualis

The tongue-biter parasite is infamous for living inside the mouths of fish and replacing their tongue. But that parasitic crustacean is not the only parasite with a predilection for that part of host - the tongue is also the preferred site for some species of Halipegus, a genus of digenean trematode fluke that lives in frogs and toads. While Halipegus doesn't replace the tongue the way the tongue-biter does, but they are very particular about where they hangout. Specifically, the adult fluke attaches itself to the lingual vein under the frog's tongue.

Halipegus occidualis in the gray tree frog (left), Southern leopard frog (centre), American bullfrog (right)
Photos from Figure 1 of the paper 
While that may seem oddly specific on Halipegus' part, in fact many parasites are like that - not only are they particular about what host they infect, but they can also be very picky about which part of the host they live in. To a parasite, the host is a collection of very different habitats and only certain parts might meet the parasite's very specific needs. But not all hosts are equal, and a parasite that infect different species of host might also behave differently in each of them.

A pair of researchers from Oklahoma State University conducted a series of experiments to find out more about Halipegus' specific preferences. They collected seven different species of frogs and toads from various locations in Oklahoma, and infected them with a species of fluke call Halipegus occidualis to observed how the parasite behaved and developed in those different hosts. To control for natural variations and to ensure that the parasites they are using is the right species and not some other similar-looking cryptic species (see this for example), they used parasites from a colony of H. occidualis which they have been maintaining in their lab.

They exposed the frogs to larval H. occidualis by feeding them with seed shrimps which they have previously infected with H. occidualis. This procedure through which the frogs are exposed imitates the process of how frogs in the wild become infected with this parasite. After exposure, they inspected the frogs' mouths everyday for the parasite's presence. When H. occidualis is initially swallowed by a frog, it does so as a tiny larva encased in the body of an arthropod. The digestive action of the frog's stomach free the fluke from the arthropod host, and it then migrate to the frog's mouth over the course of a few weeks to develop into a sexually-mature adult. Or at least that's what happens in most frog species.

For six of the frog species in the experiments, H. occidualis showed up as expected under their tongue as mature, egg-laying flukes about 6-8 weeks after they have been fed with infected seed shrimps. The parasite was most successful at establishing in the American toads (93%) and had comparatively lowest success with the southern leopard frogs (67%), but aside from that, there were no major differences among those six species in terms of how H. occidualis performed. But things were a bit different in the American bullfrog. In that host, H. occidualis never show up under the tongue - instead, they simply stayed in the stomach and developed to full maturity there.

It seems that not only is H. occidualis very specific are where it settles, it will also adjust accordingly if the host is different, to the degree that it would do so even if it has already developed into a fully-fledged adult fluke. When the researchers conducted further experiments where they transplanted adult flukes from under the tongue of gray tree frogs to other species of frogs, the flukes were quick to adjust. When the flukes were transplanted from a gray tree frog to yet another gray tree frog or a green frog, the fluke will move to its usual spot under the host's tongue, even though it is now in a new host. But if those flukes were transplanted to an American bullfrog, the flukes would migrate to the bullfrog's stomach. Furthermore, when those parasites were then extracted from the bullfrog and transplanted back to the tree frog, they went back to living underneath the host's tongue.

So what so special about the bullfrog, or specifically its stomach? At this point, it is not entirely clear. Perhaps the bullfrog stomach has some kind of chemical that encourage the fluke to stay instead of migrating to the host's tongue. While a parasite might be very specific about where it exactly it lives in the host, it might not always behave the same way when it finds itself in different host species. For a parasite like H. occidualis, not all frogs are equal.

Reference:
Stigge, H. A., & Bolek, M. G. (2016). Anuran Host Species Influences Site Fidelity of Halipegus occidualis. Journal of Parasitology 102: 47-53

June 10, 2016

Hexametra boddaertii

Nematodes (roundworms) are common parasites which are found in all kinds of animals. The study featured today is a report on a species of nematode reported for the first time in the false coral snake (Oxyrhopus guibei). The false coral snake is a non-venomous snake which mimics the highly venomous coral snakes. The snake in question had been living in captivity for a week at the National Institute of Tropical Medicine (INMeT) in Argentina before it suddenly died. It had appeared healthy until it just keeled over one day. When researchers dissected it, they found that it was full of parasitic roundworms that were identified as belonging to the species Hexametra boddaertii.
(A) The false coral snake a few days after arriving in captivity, (B) Hexametra boddaertii in the snake's body cavity,
(C) Parasitic roundworms in the bowel lumen of the snake, (D) roundworms extracted from the snake's intestine
Photos above from Fig 1 of the paper
The researchers found a total of 120 H. boddaertii in the snake; 68 of which were dwelling in the body cavity while a further 52 were living in the snake's digestive tract. This species of parasite has been recorded in other snakes before, but this is the first time it has been found in the false coral snake, and the first time that it has been reported from Argentina.

Hexametra boddaertii belongs to a group of parasitic roundworm call Ascarididae which also include roundworms that infect various domestic animals and humans. During the snake's stay in captivity, its carers had attempted to deworm it by giving it Fenbendazole - a de-worming drug which is commonly for treating parasitic infections in various domestic animals. They also tried to disinfect the enclosure, but neither seemed to have had any effects on the snake's parasite burden.

When the researchers performed a postmortem examination of the snake, they noticed that the worms found in the snake's body cavity were significantly smaller those found in the gut. On average, the worms dwelling in the body cavity were about 4 cm in length, while those from the gut were about twice as long. It is most likely that those smaller worms were juveniles - one of the key characteristics of ascaridid parasites (including Ascaris lumbricoides which infects humans) are their habit of travelling through the host's body cavity during their juvenile phase (think of it as their coming-of-age, "find yourself" trip) before entering the intestine to settle down and develop into an adult to start reproducing. And those 52 fully-mature worms in the gut had certainly been pretty busy as the snake's faeces were loaded with nematode eggs

But whether they were adults or juveniles, those parasites' presence certainly took their toll on the snake. Parts of the the snake's body cavity showed signs of calcification, its lungs were filled with excess fluid, and its gut lining were inflamed and congested. Given the number of worms the snake had and how well-developed most of them were, the snake most likely had acquired those parasites long before it was brought into captivity.

In addition to providing a new parasite record, this study also revealed a potential risk associated with handling snakes - larvae of other Hexametra nematodes from snake faeces have been reported to successfully infect the crab-eating macaque, so if given the opportunity, there is some potential for H. boddaetii to jump host into primates (including humans).

Sometimes when it comes handling snakes, it is not necessarily just the snake that you have to be careful of...

Peichoto, M. E. et al. (2016). First report of parasitism by Hexametra boddaertii (Nematoda: Ascaridae) in Oxyrhopus guibei (Serpentes: Colubridae). Veterinary Parasitology 224: 60-64.

May 26, 2016

Opechona olssoni

Jellyfish are one of the most common inhabitants of the sea and while they are soft and squishy, their venomous stinging cells (call cnidocytes) can act as a deterrent for many potential predators. But not all predator though - jellies are on the menu for a wide range of different marine animals including sea turtles and various species of fish - this is a fact which has been exploited by some species of parasitic flukes (digenean trematodes) as a mean of completing their life-cycles.
Clockwise from top left: Fluke larvae embedded in sea jelly tissue, close up of fluke larvae, Opechona olssoni from the gut of a butterfish, a juvenile butterfish collected from the Seto Inland Sea. Image modified from Figure 2, 9, 10 of the paper
Flukes have a rather complicated life-cycle which involves multiple different hosts. While the adult stage lives in vertebrate animals, they lay eggs that hatch into larvae which infect a "first intermediate host" (usually some kind of snail) where the parasite undergo asexual proliferation, producing a different type of clonal larval stages that infect a "second intermediate host" which serves as a vehicle to reach the vertebrate host. This is usually accomplished through the vertebrate host eating the second intermediate host. For some species of flukes, such as Opechona olssoni, the second intermediate hosts are sea jellies, and it is not alone - some of its fellow travellers include Leptotrema clavatum and Cephalolepidapedon saba - all flukes from the Lepocreadiidae family which is commonly known to use gelatinous animals as their second host.

In this study, a team of researchers in Japan set out to examine the seasonal pattern in the abundance of parasitic fluke larvae found in some of the jellyfish inhabiting the Seto Inland Sea. Over the course of two and a half years from March 2010 to September 2012, they collected three different species of jellyfish: the moon jelly (Aurelia aurita), the Japanese sea nettle (Chrysaora pacifica). and the ghost jellyfish (Cyanae nozakii).

They noticed that while the fluke larvae were consistent present in the jellyfishes for most the year, their abundance varied over different seasons. Sea jellies were most heavily infected during the middle of the year when it is the warmest. Since the free-swimming clonal larvae of digenean fluke use temperature as their trigger to leave the first intermediate host, when things start heating up, they come streaming out of their host in large numbers.

Out of the three different species jellyfish species they sampled, the ghost jellyfish was the host with the most. While the moon jelly and the sea nettle on average harboured about 28 and 8 fluke cysts respectively, the ghost jellyfish had on average almost 150 fluke larvae, with some individuals carrying over 400 cysts in their tissue. But why are the ghost jellyfish so much more heavily infected than the other jellies? It is only slightly bigger than those other species, so size alone cannot be the answer.

Well, the ghost jellyfish is also known to prey on other gelatinous animals, and by doing so it could have been accumulating an impressive collection of fluke larvae second-hand (so to speak). Due to its jelly-eating ways, the ghost jellyfish earns itself the dubious distinction of serving as a hub for all these flukes to come together as they wait for a final host to come along and gobble them up.

But if the sea jellies are the second intermediate host for O. olssoni and friends, then what are the final hosts where the parasites mature into their adult form? Flukes from the Lepocreadiidae usually use fish as their final host, so the researchers also collected some fish species which are known to either associated with and/or have jellyfish on their menus, since they are the most likely final host for those parasites. They examined their gut content for bits of half-digested jellyfish and the adult form of the jelly-infecting fluke species.

One of the fish which they found to harbour maturing and/or adult flukes is the Japanese butterfish. Juvenile butterfish are known to hangout around the tentacles of jellies, using them for food and shelter, taking a few nibbles here and there whenever they get peckish.  Alongside the butterfish, another species that hosts those flukes is a filefish known as the black scraper. It eats a variety of different marine invertebrates, but is known to partake in some jellies now and then.

For these small fish, hanging out around jellyfishes is a pretty good deal - the curtain of tentacles act as a shelter from their predators, and the shelter itself is edible too. But because the jellyfish's tissue is also laced with fluke larvae, it means that every mouthful of jellies those fish swallow also goes down with a bunch of parasite larvae. As with everything in life, there is always a catch

Reference:
Kondo, Y. et al. (2016). Seasonal changes in infection with trematode species utilizing jellyfish as hosts: evidence of transmission to definitive host fish via medusivory. Parasite, 23: 16

P.S. Some of you might know through my activities on Twitter (@The_Episiarch) that when I'm not writing these blog posts about new scientific papers about various parasites, I also do illustrations. Some of my drawings are about and/or inspired by parasites, however some may find my more recent parasite-inspired pieces to be slightly unusual - I am of course talking about Parasite Monster Girls...

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?

Reference:
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.

Reference:
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.

Reference:
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

Reference:
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.

Reference:
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.

Reference:
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