"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

March 26, 2015

Emblemasoma erro

During summer the air is filled with the rattling ruckus of cicada songs. Male cicadas produce this summer choir using a pair of noise-making organs located in their abdomen, with the aim of getting attention from any prospective mates. But in some cases, they can also end up with some unwanted attention.

Top: Male Tibicen dorsatus cicada
Bottom: Female Embelmasoma erro fly
Photos from Figure 1 & 2 of this paper
The species we are featuring today is an "acoustically hunting" parasitoid fly - it eavesdrops on the male cicada's flirtatious serenading and uses it to home in on its target. This fly is commonly found on the Great Plains of North America and is a scourge to male cicadas, especially male Tibicen dorsatus.

Most of what is known about such acoustically hunting parasitoids are based on flies from the Tachinidae family - one of which targets crickets (I talked about how crickets on Hawaii evolved to become silent due to the presence of one such parasitoid fly here). But this fly belongs to a different family (Sarcophagidae). Only one species of Emblemasoma is well-studied - E. auditrix- and even though Emblemasoma is widely use in the study of insect hearing, not much is known about how they actually live out in the wild. Until now, the only information available on E. erro are based on two scientific papers - one published in 1981 and the other published in 2009. The paper we are featuring today provides some much-needed update on key aspects of this parasitoid's ecology and life history.

This paper reports on a series of field surveys and laboratory experiments that documented the parasitoid's occurrence, abundance, behaviour, and developmental cycle.

The field surveys were conducted at study sites located across Kansas and Colorado. The surveys found that a bit over a quarter of male cicadas were infected with E. erro larvae, and because of how the flies track down their host, almost all the infected cicadas were male - except for one very unlucky female cicada, which most probably got infected because she was responding to the call of a male, ran into a larvae-ladened E. erro that had the same idea, the latter decided that any cicada will do. Talk about a case of fatal attraction!

And it is indeed the sound of the male cicada's serenade that draws in those flies - a loudspeaker playing the recordings of cicada calls is sufficient to attract the attention of E. erro, but a female fly need more than that to commit to dropping off her precious offspring. In outdoor cage experiments where flies and cicadas were housed together and allowed to mingle freely, the researcher observed that even if an E. erro finds herself perched next to a cicada, she will only attack when the host makes any sudden movements. So E. erro uses two separate signals to track down its prey; an acoustic signal at long range in the form of the cicada's call to guide them in, and a visual signal at close range in the form of cicada movement to confirm the host's identity

Emblemasoma erro larva emerging from a cicada
From Figure 6 of this paper
Once she has confirmed her target, the female fly makes an attack run, and very quickly drops off between 1-6 maggots (usually 3) on the base of the cicada's wings. As soon as the maggots land, they immediately start burrowing between the segments and into the cicada's body. The maggots then start devouring its host alive from the inside. Depending on the temperature and clutch size, they take about 88 hours to reach a large enough size to start pupating. At the end of this period, the maggots use teeth-like "oral hooks" to chew their way to freedom, fall onto the soil below to become pupae, and leaving the cicada an empty husk.

So while the aim of the male cicada's singing is to attract the attention of female cicadas, some of them may instead end up getting attention from females of a very different species, and become reluctant incubators for the broods of some keen-eared, cicada-hunting flies.

Reference:
Stucky, B. J. (2015). Infection behavior, life history, and host parasitism rates of Emblemasoma erro (Diptera: Sarcophagidae), an acoustically hunting parasitoid of the cicada Tibicen dorsatus (Hemiptera: Cicadidae). Zoological Studies, 54: 30.

March 11, 2015

Crassicauda magna

During this blog's first year back in 2010, we featured a parasitic nematode (roundworm) that lives in the placenta of sperm whales of all places. Today, we're featuring a study on another nematode which lives in the sperm whale's cousin - the much smaller and more enigmatic pygmy sperm whale Kogia breviceps.
Photo of C. magna in whale tissue from Fig. 1 of the paper

Crassicauda magna is a parasites that really gets under the skin of the pygmy sperm whale. While most worms in the Crassicauda genus live in the urogential and renal system of whales, C. magna just had to be different from the rest of the pack. Instead of living in the whale's plumbing system, it had opt for a life being sandwiched between layers of blubber and muscle, living snugly under the whale's subcutaneous tissue.

While it can be a tight fit in there, C. magna can grow quite large -the largest known fragment is 3.7 m (about 12 feet) long, but due to where they are found in the body and the relatively cryptic nature of its host, no fully intact C. magna has ever been retrieved. The original species description for C. magna was published in 1939, and was based upon fragmentary remains from the front half of the worm, as the rest of the parasite not recovered.

Even though this parasite appears to have a global distribution (like its host), very little is actually known about it. Only a few anatomical details have been recorded, pieced together from worm fragments which had been collected over the years, and until the publication of the present study, there were no genetic data for C. magna. This is not too surprising considering that much of what is known about the pygmy sperm whale itself (let alone C. magna) had about from examining stranded individuals - which is not exactly a routine occurrence.

The C. magna specimens which were the subject of this new study were retrieved from a dead pygmy sperm whale which was beached at Moreton Bay, Queensland. Most importantly, from a taxonomist's perspective, the research team involved was able to retrieve parts of the tail from male worms. The reason why this was kind of a big deal is that one of the key features used to identify different species of nematodes are the needle-like structures on the male genitalia call copulatory spicules. The male worms use these spicules to pry apart the female worm's vulva for sperm transfer, and it just so happened that each species have distinctively shaped spicules, which can be used to tell them apart.

The researchers were able to compare the worms collected for this study with other specimens of Crassicauda stored at the South Australian Museum, the Natural History Museum in London, and the Muséum national d'Histoire naturelle in Paris. They noted that the spicules on C. magna are remarkable similar to those found on another species that was described in 1966 call Crassicauda duguyi - which was also collected from the neck muscle of a pygmy sperm whale (in this case, it was stranded on the west coast of France). Their conclusion was the C. duguyi is most likely just C. magna instead of being a different species, but the taxonomist who described it was not able make the match because the original species description of C. magna did not have information on the male genitalia.

Unlike previous studies, the researchers responsible for the current one also managed to extract some genetic material from the worms they collected. They sequence a section of the worm's ribosomal DNA which was used to reassess the classification of C. magna in relation to other parasitic nematodes. With such a genetic marker at hand, it can be used in the future to find out more about this enigmatic parasite and its equally cryptic host.

Reference:
Jabbar, A., Beveridge, I., & Bryant, M. S. (2015). Morphological and molecular observations on the status of Crassicauda magna, a parasite of the subcutaneous tissues of the pygmy sperm whale, with a re-evaluation of the systematic relationships of the genus Crassicauda. Parasitology Research 114: 835-841

February 24, 2015

Gelis agilis

It's a bug-eat-bug world out there and the same applies to parasitic wasps - even parasites can themselves become parasitised - which is why some parasitoids recruit their dying host as defence. The parasitoids that go after other parasitoids are call "hyperparasitoids" and the species we are featuring today is Gelis agilis, a tiny wingless wasp that lays its eggs in the cocoons of parasitic wasps such as Cotesia glomerata.
Photo of Gelis agilis by Christophe Quintin

This hyperparasitoid wasp has more to contend with than just overcoming their host's reluctant bodyguard. It is after all a small insect which equates to a handy mouthful for many potential predators. The adult G. agilis is a tiny (3-5 mm) and seemingly defenceless - it doesn't even have wings to fly away from any danger. But G. agilis makes up for that with a clever masquerade

If there is a group of tiny insect which are generally regarded as pretty unpalatable, it is ants (except for animals that specialise on eating ants), so many other creatures have evolved to mimic them in one way or the other. When it comes to playing the part of an ant, G. agilis is a method actor - not only does it look and act like an ant, it even smells the part. When agitated, it emits a volatile chemical call sulcatone, which is the same chemical used by ants as alarm pheromone to rally colony members to their defence.

This "full spectrum mimicry" pays off. Spiders that normally pounce straight onto similarly-sized insects such as fruit flies or parasitic wasps like C. glomerata would hesitate or back right off when confronted with Gelis. A species related to G. agilis - G. aerator - looks and acts like an ant but lacks the distinctive "antsy" smell. When G. aerator was put through experimental trials up against hungry wolf spiders, most spiders back off due to its ant-like appearance. But the lack of matching ant BO was enough for a few more daring (or desperate) spiders to get the jump on G. aerator.

By playing the part to its fullest capacity - behaviour, appearance, and scent - G. agilis is better able to evade its predators to survive another day, and go on to make life a living hell for other body-snatchers

Reference:
Malcicka, M., Bezemer, T. M., Visser, B., Bloemberg, M., Snart, C. J., Hardy, I. C., & Harvey, J. A. (2015). Multi-trait mimicry of ants by a parasitoid wasp. Scientific Reports 5: 8043

February 12, 2015

Trichomonas gypaetinii

What does the cause of pigeon canker, today's parasite, and the most common curable sexually transmitted infection in the world have in common? All of them are parasites from the genus Trichomonas. The species that causes pigeon canker is T. gallinae, a protozoan that lives in the upper gastrointestinal tract of pigeons, and it is currently posing a significant threat California's only native pigeon. While T. gallinae does not always cause disease, when the host is stressed, the parasite multiplies, causing lesions to develop in the throat and mouth of their host. The host eventually dies from starvation as the lesions makes it difficult for them to swallow anything. It is possible that a parasite like T. gallinae might have even brought down the occasional Tyrannosaurus rex over 65 million years ago - though the culprit is most likely to have been a different (but similar) species of parasite given how long ago that it all happened.
Photo composed from Fig. 5 & 6 of the paper

With T. rex being one of the most badass dinosaur of all time, it is appropriate that the species of Trichomonas that we are featuring today - T. gypaetinii - is found in some pretty badass living dinosaurs as well. This parasite was first isolated from a bearded vulture (Gypaetus barbatus) - which I am sure most would agree  is a very handsome and intimidating bird. When T. gypaetini was initially isolated, it was not fully described as a species, as there was insufficient material  to do so. However, this study reports on newer samples obtained from a wide epidemiological study of avian trichomonosis in Spain. The research team managed to obtain isolates of T. gypaetini from another two species of vultures - the Egyptian Vulture (Neophron percnopterus) and the Black Vulture (Aegypius monachus) - and now we have a formal description.

So what differentiates T. gypaetinii from canker-causing T. gallinae? There was nothing about their appearance which separates the two species, but when the research team did some genetic analysis on the parasite, they found that all the Trichomonas samples from vultures were perching on their own branch, far away from T. gallinae. When they search for previously published sequences of Trichomonas from vultures, they hit upon the previously undescribed isolate from the bearded vulture mentioned earlier.

So where does T. gypaetinii sit on the Trichomonas family tree? Genetically, T. gypaetinii is actually more similar to T. vaginalis - a sexually transmitted parasite that infects over 160 million people worldwide each year - most of the time without them being aware of it as most cases show no symptoms. Much like those cases of T. vaginalis infection, T. gypaetinii does not appear to cause any problems to their bird host either.

Furthermore, it seems that T. gypaetini is only found in carrion-feeding birds. Other birds of prey can get infected by T. galinae - the canker-causing species - through eating other birds, especially pigeons. But the vultures' comparatively specialised diet and digestion physiology (especially that of the bone-munching bearded vulture) means that  T. gypaetinii is the only Trichomonas that can successfully make vultures their hosts.

Reference:
Martínez-Díaz, R. A., Ponce-Gordo, F., Rodríguez-Arce, I., del Martínez-Herrero, M. C., González, F. G., Molina-López, R. Á., & Gómez-Muñoz, M. T. (2015). Trichomonas gypaetinii n. sp., a new trichomonad from the upper gastrointestinal tract of scavenging birds of prey. Parasitology Research 114: 101-112.

January 26, 2015

Baylisascaris schroederi

Larva of Baylisascaris procyonis, another
parasitic nematode in the same genus as
 Baylisascaris schroederi. Photo from here
To say that Giant Pandas are totally adorable is probably one of the least controversial statements you can make, I mean, just look them here and here. But if you are a fan of giant pandas, then today's parasite is public enemy number one. Baylisascaris schroederi is a species of parasitic roundworm that live as adults in the intestine of giant pandas, and in large numbers, they can form bowel obstruction and other more serious diseases.

The adult B. schroederi produce eggs which leave the host along with panda faeces. When the panda accidentally ingest the eggs, they hatch into larvae in the intestine and proceed to burrow through various bodily tissues, causing inflammation and scarring in the the intestinal wall, liver and lungs.

After a coming-of-age trip through the panda's various organs, the larvae return to the small intestine to grow up into an adult and get on with the business of being a grown-up parasite - laying lots of eggs. The eggs are really hardy and can stay viable in wet soil for many years, waiting for an unlucky panda to swallow them.

Because B. schroederi can stay viable, a panda can get repeated infected by parasite larvae, inflicting internal damage for years. Baylisacaris schroederi is actually one of the leading cause of death in giant pandas, and depending on the region, half or even all of the pandas in a given population might be infected.

To find out more about the ecology of these parasites, a team of scientists from China used a range of mitochondrial DNA markers to work out the population structure of B. schroederi. They collected adult B. schroederi found in giant pandas from ten geographical regions at three different mountain range in southern central China - Qinling, Minshan, and Qionglai.

They found that despite the geographic isolation of those mountain ranges, the gene pool of this parasite is fairly homogenise, indicating that somehow despite their isolation, cross-breeding is occurring between the parasite populations. Perhaps some pandas are visiting neighbouring mountain range and end up picking up worms and dropping off B. schroederi eggs while they were there. Or it might not be the pandas themselves that are moving around - seeing as the eggs can survive for years in wet soil, they may get transported through other means.

While the scientists found that the parasite population in Minshan has the highest level of genetic diversity, as a whole, B. schroederi has relatively low genetic diversity compared with other organism. But that is common feature with other roundworms in its family - the Ascarididae (of which the most well known species is Ascaris suum - the large pig roundworm) - regardless of their population size or geographical distribution.

Currently there are no vaccines available for B. schroederi and the main way through which this parasite is being controlled is with anthelminthic drugs. Despite their relative low genetic diversity, this has not stopped some populations from evolving drug resistance. That is why it is important for us to understand this parasite's population genetics - because if there is cross-breeding occurring, then the gene(s) for drug resistance can spread very quickly across the different population and pose a threat to the endangered giant panda.

Reference:
Xie, Y. et al. (2014). Absence of genetic structure in Baylisascaris schroederi populations, a giant panda parasite, determined by mitochondrial sequencing. Parasites & Vectors 7: 1591.

January 11, 2015

Pennella balaenopterae

Photo of Pennella balaenopterae embedded on
the side of the porpoise's peduncle (from Fig 2 of the paper)
Most people usually think of copepods as tiny crustaceans which live as zooplankton near the, and for most part that is true. But it might be a surprise to some of you that over a third of all known copepods are actually parasitic and they live on/in all kinds of aquatic animals. One particularly successful family of such copepods is the Pennelidae - not that you would necessarily recognise them as crustacean if you are to ever see one. While most species in this family live on fish, the parasite that we are featuring today has evolved to be a bit different. Instead of infecting fish, it has managed to colonised aquatic mammals - specifically cetaceans (whales).

Whales are among the largest known animals to have ever lived, and P. balaenopterae also happens to be the largest known copepod (most free-living copepod are tiny zooplankton measuring a few millimetres in length). As its name indicates, this parasite was initially found on baleen whales, such as fin whales, but it has been reported from different species of toothed whales as well. Despite being known to science since the 19th century, there is very little information about the biology of this peculiar parasite.

The cephalothorax or the "head" of Pennella balaenopterae
which is deeply buried in the host's blubber
The paper we are featuring today reports this parasite infecting harbour porpoises (Phocoena phocoena relicta) in the Aegean Sea. These parasites each measured over 10 centimetres long and most of it is buried deep in the blubber. In this study, Pennella balaenopterae were mostly found on the porpoises' back and abdominal area, probably because those areas are rich in easily accessible blood vessels that the parasite can tap into.

Even though technically it is an ectoparasite (external parasite) as it can be found dangling on the host's external surface, a significant portion of its body is actually deeply buried in the porpoises' tissue (not unlike the shark-infecting barnacle Anelasma squalicola which was featured last year). Hence some parasitologists call them "mesoparasites"; they are not strictly internal parasites (endoparasites) such as many parasitic worms, but they do interact with the host's internal tissues in some major waya.

Species like P. balaenopterae shows that over evolutionary time, some parasites can make rather radical shifts in their preferred host if given the opportunity to do so. Last year I wrote about an elephant blood fluke which has colonised rhinos because both of its mammalian host share the same habitat. Indeed, both whales and fish that are infected other pennelid copepods are both marine animals, so there have been many opportunity for such a host jump to occur.

However, it is one thing to jump from one large, terrestrial mammal into another, it is quite another to branch off and infect an entirely different class of animal which has a very different anatomy and physiology to the ancestral host. More studies will be needed to find out what makes P. balaenopterae different from its related species, as well as when and how it made the leap from living on scale-covered bony fishes, to burying themselves in the tissue of air-breathing blubbery whales.

Reference:
Danyer, E., Tonay, A. M., Aytemiz, I., Dede, A., Yildirim, F., & Gurel, A. (2014) First report of infestation by a parasitic copepod (Pennella balaenopterae) in a harbour porpoise (Phocoena phocoena) from the Aegean Sea: a case report. Veterinarni Medicina, 59: 403-407.

December 30, 2014

Facts Of Life On Planet Parasite

We've come to the end of yet another year and all that it entails in the field of parasitology. As with last year, we have continued to the feature guest posts by student from the University of New England ZOOL329/529 class of 2014, who wrote about fungus that kills water bears, midges that suck blood from mosquitoes, and wasps that zombifies cockroaches and many more. In addition to student guest posts, there were some conference coverage (Part 1, Part 2) mixed in as well.

As for some of the parasites that were featured this year, we looked under the sea - and found that it was filled with shark-suckers, face-huggers, brood-blockers, and egg-mimics. While they sound like the monsters of science fiction horror, but they are non-fiction of the real world, and they are not monsters, but simply living things trying to get on with their life - admittedly in ways that somewhat terrifies us.

This year, we learned about parasite that can take a reproductive toll on their host, such as a lovecraftian parasitic copepods that infect flamboyant sea slugs, a peculiar barnacle which sticks itself in the flesh of a shark and can castrate its host, a tiny crab that brood-blocks its limpet host, and a copepod that masquerading as a lobster egg so they can feast on the brood of its host.

When they're not killing their hosts' broods one way or the other, they outright disintegrate them. We learn about the parasite that kills a species of "killer shrimp" by dissolving them into shrimp paste, but not before causing the crustacean to bring themselves out into the open to the waiting maw of its cannibalistic cousins. Other parasites like myxozoans do not kill their host outright, but when their fish hosts do die, it cause their flesh melt into mush, much to the dismay of fishermen.

But it's not just aquatic critters that are the target of parasites - they rumble in the jungle too, and are found in larger terrestrial animals like rhinos and monkeys, as well as smaller ones like crickets. In the case of the cricket, some parasites actually bring their terrestrial host into the aquatic realm by manipulating the host's behaviour. Other parasites mess with their host's sense of smell. And some parasites don't alter behaviour directly but just gets in the way - the worm that gets in the eyes of prairie chickens (and other birds), and fish are not faring any better, with a parasite that literally get all up in their face.

And there is no escape from parasitism - parasites are found everywhere, even in deep sea hydrothermal vents. And they do more than just gross us out or cause their host to suffer - they can also cause changes in their hosts that sends a ripple effect into the surrounding ecosystem too. Parasites are ubiquitous, diverse, and a major components of this planet's biological diversity. Parasitism is as much a fact of life as feeding, fighting, and f…reproducing - that is unless a parasite gets in the way of your ability to do that last thing…

We will back next year to bring you more posts on parasite research which you might not have read about elsewhere - so here's to another year of more parasitology science! Bring on 2015!

P.S. If you can't wait until next year for your parasite fix, you can check out some of my other parasite-related writing on The Conversation on the important ecosystem roles played by some parasites here and on parasites that blind their hosts here. 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) like hairworms, emerald jewel wasps, killer sponges, vampire snails, colossal squids, second-hand vampires, and melting seastars. You can find links to all these and more on this page here.

December 14, 2014

Gnathia maxillaris

Today's blog post features a study in which an infestation at an aquarium allowed a group of scientists to work out the life cycle of a common parasite. Now, we are not talking about your lounge room fish tank, but the biggest exhibition tank at Aquarium of Barcelona. The exhibition aquarium, call Oceanarium, measures 37000 cubic metres and is home to over 3000 fish of 80 different species. But amidst those 80 different species, they have a parasite which has made its way into the mix.

Adult female with larval brood (left) and newly-hatched zuphea (right)
Photos from Fig. 1 of the paper
The parasite in question - Gnathia maxillaris - belongs to a family of little blood-sucking crustaceans call Gnathiidae (we have previously featured gnathiids on this blog here). You can think of them as being like ticks of the sea - not only are they blood suckers, but they also alternates between a blood-feeding and a free-living stage during their development (like a tick). The parasitic stage of a gnathiid is called a Zuphea - it needs to attach and feed on a host for a while before it drops off to moult into its next stage call a Pranzia. The pranzia is free-living stage, but it doesn't stay that way for long, as the next step of its development is to grow into a slightly larger zuphea which jumps right back onboard a fish for a blood meal. A gnathiid needs to go through this parasitic-then-not-parasitic-then-parasitic-again development cycle three consecutive times (each successive stages are called Z1, P1, Z2, P2, Z3, P3) before it can become an adult (and you thought going through puberty was bad!)

There are over 190 known species of gnathiids from all across the world, but the full life-cycle has only been described for four of those species, and now G. maxillaris join that very short list. Even though G. maxillaris is relatively well-studied and fairly widespread across the Atlantic Ocean as well as the Baltic and Mediterranean seas, the complete life-cycle of G. maxillaris was unknown until now because much of this parasite's development takes place out of sight on the open sea.

But the infestation at Aquarium of Barcelona provided scientists with a great opportunity to study this life-cycle. They harvested G. maxillaris larvae by exploiting their natural attraction to light; at night, they turned on a set of light installed at the bottom of the aquarium, then pump the sea water through a fine-meshed plankton net that have also been placed there to trap the parasite larvae.

Clockwise from upper left:
Adult female, adult male, female carrying eggs
From Fig. 2 of the paper
With the harvested parasites, they exposed them to different species of potential fish hosts to observe their behaviour. They noticed that newly-hatched zuphea (Z1) cannot feed on blood because their mouthpart is so small the fish blood cells cannot fit through them. Instead, they feed on lymph and have to subsequently grow into the larger zuphea stages before they can incorporate blood into their diet.

They also discovered that G. maxillaris has different preference for specific parts of the fish's body, and this has consequences for the parasite's growth. While they can attach pretty much anywhere on the fish's body, they have a taste for the base of the fins, near the gill covers, or around the eyes - basically areas of high blood flow and where it would be harder for the fish to rub them off. They also noticed zuphea that attach themselves to the fish's fin feed for longer and takes more time to develop into a pranzia, most likely because there is less blood flow there than other parts of the body, so the parasite needs to stick around for longer to get a full meal.

In all, G. maxillaris' entire life-cycle takes about three months to complete, but that is if the water temperature is at 17.5 °C; if the surround temperature is 20 °C, then the parasite would take only two months to complete this cycle. At higher temperature, the female parasites also grew larger and produced more eggs. This is particularly pertinent to the current situation because one of the (many) consequences of increasing ocean temperature might mean in the future, the seas will be filled with more gnathiids that grow faster than ever before, which is bad news for fish. Not only are they blood-suckers, like ticks on land, gnathiids can also act as vectors for various other parasites.

While an infestation of tiny "ticks of the sea" might not be the best news for a national aquarium, when life hands you an infestation - you might as well do some science with it!

Reference:
Hispano, C., Bulto, P., & Blanch, A. R. (2014). Life cycle of the fish parasite Gnathia maxillaris (Crustacea: Isopoda: Gnathiidae). Folia Parasitologica 61: 277-284.

November 24, 2014

Oxyspirura petrowi

Photo by USFWS Endangered Species
The Lesser Prairie-Chicken (Tympanuchus pallindicintus) is a very distinctive bird. During breeding season, the males aggregate to put on an elaborate courtship display composed of raised feathers, a series of rapid stomping followed by "booming" and inflating a pair of bright orange air sacs on the side of their necks. But life as a prairie chicken is not so great these days, since the early 1900s, their population and range has shrunken by over 90 percent, mostly due to habitat loss and fragmentation from agriculture and industrial developments.

On top of that, they have to deal with Oxyspirura petrowi - a nematode (roundworm) parasite that lives in their eyes - on the front and/or behind the eyeballs. And these worms aren't small either, they can grow to more than 15 mm long and they feed on blood too, causing severe haemorrhaging and swelling around the eyes. So being infected with O. petrowi can cause a significant impairment to the host. Based on studies on a related species - Oxyspirura mansoni (which infects poultry) - it is most like that the prairie chicken are infected when they eat arthropods which contain the larval stage of the worm and research is still under way to try and figure out which arthropod is the carrier.
Photo of Oxyspirura petrowi from fig 1 of this paper

The lesser prairie-chicken is not the only bird that gets infected by O. petrowi, this worm also infects various game birds like pheasants and quails, as well as some migratory songbirds. If a bird cannot see properly, then it is not going be very good at flying without eventually hitting something. And some prairie-chickens have been reported to fly into vehicles or even the side of barns. Obviously such birds are not going to be very good at evading predators if they cannot even avoid flying into a barn. So is the worm also contributing to the prairie chicken's decline, or something else?

Mercury and lead are both metals that can contaminate the environment as by-products of burning fossil fuel, spent ammunition, and industrial activities. Both have well-documented toxicity effects on animals including neurological damage that results in sensory impairment, convulsions and behavioural disorders. Another common pollutant is organochloride. While organochloride pesticides have been banned or restricted for years, they can linger in the environment for a long time and accumulate up the food chain. In high enough dosage, such pesticides have been known to cause reproductive impairment as well as convulsion and emaciation in birds.

The researchers behind this study analysed the level of these chemical pollutants in the organs of some prairie chickens from Kansas, and while they found traces of all three in the prairie chicken's organs, they were all below the level at which they would being harmful. The level of organochloride was just as they had expected given the birds were from an area that used to be a farmland. As for the two metals, the lead levels lower than toxicity level and the levels of mercury were below detectable limits.

What they did find was a higher prevalence of O. petrowi than they had expected from the region, and some of the birds they examined had up to 16 worms in their eyes. It is worth noting that the birds these researchers sampled were donated by hunters, so it is likely that the eyeworms made them easier targets. So is O. petrowi playing a role in the prairie chicken's decline? It seems unlike given that birds like bobwhites have been documented to be infected with even higher levels of this worm. But its presences is certainly not helping and may interfere with some conservation practices.

For example, one current conservation practice to put up signs and coloured marking tape around fence lines to reduce bird-fence collisions. The idea is that the fences are clearly marked out so the prairie chickens can avoid running into them. But if they are half-blind from having a bunch of worms in their eyes, they might instead end up using those markers as targets and fly headlong into the fence.

When trying to protect any species in a complex environment, it is important to also take their parasites into account, as their presence might confounds your expectations. To save the prairie chickens, you might first have to understand the eyeworm.

Reference:
Dunham, N. R., Peper, S. T., Baxter, C. E., & Kendall, R. J. (2014). The Parasitic Eyeworm Oxyspirura petrowi as a Possible Cause of Decline in the Threatened Lesser Prairie-Chicken (Tympanuchus pallidicinctus). PloS One 9, e108244.

P.S. You can read my article about other blinding parasites in The Conversation here.

November 11, 2014

Leptorhynchoides thecatus

Photo by Scott Bauer
Life is dangerous for a little crustacean like a freshwater amphipod. There are all kinds of things out there that would like to make a meal out of you, so you would sure want to get out of the way at the first sign of any would-be predator. While our sense of smell is relatively poor, other animals live in a far more aromatic and pungent world, filled all kinds of chemical signals. When it comes to chemoreception (what we would consider smell and taste), amphipods can tell the presence of a predator in main two ways, either smell their presence directly through the kairomones (basically BO) they release, or indirectly from the alarm chemicals of dead compatriots (so essentially, the scent of death).

However, this can be big problem for some parasites of these little crustaceans, as they need to be eaten by a predatory animal in order to complete their life cycles. In that case, some of these parasites have ways of making sure that their host never see (or in other ways sense) it coming when a predator comes knocking.

Proboscis of adult L. thecatus
modified from here
Hyalella azteca is a common species of amphipod that is found in many freshwater habitats in North America. It is also host to the larval stage of a thorny-head worm call Leptorhynchoides thecatus. For this parasite to complete its life-cycle the amphipod host needs to be eaten by a fish - such as a green sunfish - something that the amphipod is certainly not okay with. However, regardless of what the amphipod wants, the parasite needs to reach a fish's gut, and it does so by overriding the crustacean's usual response to alarm chemicals in the water. A pair of scientists conducted an experiment to see this in action.

First they made some scent solutions that correspond to the ones that the amphipods would usually respond to in the wild. Alarm chemical from dead or injured H. azteca was relatively straight forward to make as it simply involved mushing up some amphipods in a bit of water to get this "scent of death". But to get some liquid fish BO, they collected water from a tank housing green sunfish which had been circulating for a day without a carbon filter, so the water has been saturated with the "essence of fish" as it were (I'd imagine neither scent would sell all that well if you release it as a line of perfume or cologne).

To see how the amphipods reacted to the scents they've prepared, the scientists placed each H. azteca individually in an observation chamber which has a small shelter at the bottom. After it has settle down, they either drip a bit of that "scent of death", or some of the "essence of fish", or just plain water into the chamber, and watched the amphipod's response.

When uninfected H. azteca catch a whiff of fish BO or the scent of their dead companions, they hid in the shelter and try to keep still (especially at the scent of dead amphipods). But not the amphipods infected with L. thecatus - regardless of what's in the water, they just stayed completely oblivious and carried on with whatever they were doing as usual, as if the scientists had just added plain water to the chamber. If it had been in the wild, those infected amphipods would have been quickly snapped up by a hungry sunfish (and made L. thecatus really happy, if worms are capable of being happy...).

Being visual animals, we humans tend to take more notice when parasites manipulate their hosts in a flashy way that catches our eyes. But there are other ways that parasites can manipulate the sensory world of their hosts in order to complete their life cycle. We have not paid as much attention to those other senses - perhaps it is time that we do so.

Reference:
Stone, C. F., & Moore, J. (2014). Parasite-induced alteration of odour responses in an amphipod–acanthocephalan system. International Journal for Parasitology 44: 969-975.