Philometroides paralichthydis

Philometroides paralichthydis is a filarial nematode that parasitises the southern flounder. The fish becomes infected by eating copepods carrying the larval stage of the parasite and the adult female worm lives embedded in the inclinator muscles at the base of the dorsal and anal fins. There she feeds on blood and produces larvae that are released into the surrounding waters. The physical presence of P. paralichthydis lodged in that part of the host can lead to muscular degeneration, especially when the parasite becomes bloated with baby worms.

photo of Philometroides paralichthydis from here.

The inclinator muscles, where adult P. paralichthydis are found, usually act as pulleys that control the side-to-side motion of the flounder's fins, which in turn affect the fish's swimming performance. So it would make intuitive and common sense that a parasite that damages muscles controlling an animal's movement would cause the host to become physically impaired. For example, Curtuteria australis (a parasite that I have worked with) is a fluke that has larval stages that lodge themselves in the foot muscle of the New Zealand cockle and by doing so they impair the bivalve's burrowing ability.

But, mere common sense is not science and whatever our expectations might be, they must be tested against the real world. To find out just what effects this muscle-dwelling parasite can have, researchers collected a group of flounders from estuaries in South Carolina and put them through a series of physical exercises in a custom-built swim track and an aquarium filled with sand and water. It was a blinded experiment; the researchers did not know the parasite load of the flounders before or during the trials, so they would not be subconsciously biased about the outcome one way or the other. During the trials, they observed and recorded the flounders' ability to perform actions that are key to their survival - swimming, accelerating, and burying themselves in sand - all of which require the use of the dorsal and anal fins.

After those physical trials, they counted the number of worms found in each flounder and in contrary to what "common sense" may lead us to believe, the presence of P. paralichthydis did not seem to make much difference. Regardless of where they are located in the host, they did not compromise the flounder's ability to accelerate, or cover themselves with sand. The parasite was found to have some affect on swimming speed, but only in smaller juvenile fish and not adult fish. On average, juvenile fish with P. paralichthydis swum thirty percent slower than uninfected fish of the same size class.

So why does this parasite only affect juveniles? Perhaps fully-grown fish are better able to compensate for any pathology incurred by the parasite simply by being larger, which provides a buffer against any damage caused by the parasite. It seems that once they are large enough, the flounders are fairly safe from the damaging effects of this muscle parasite. Juvenile fish are generally more vulnerable to the injurious effects of parasites which can contribute significantly to juvenile mortality in fish and affect recruitment by making them more vulnerable to predators or simply through the pathology they can cause (see this post on how juvenile reef fish run the parasite gauntlet before settling down.)

There are two caveats to this study that we need to consider before drawing any final conclusions. It is possible that the researchers had only collected those fish that had managed to survive well enough despite having the parasite, and fish with greater morbidity from P. paralichthydis had naturally died from starvation or predation and were not represented in the sample. Also, none of the flounders in the trial were infected with more than twelve nematodes. Maybe there was simply not enough worms to have a noticeable affects on the flounder's behaviour. Even with Curtuteria australis, there needs to be a critical number of larval flukes in the cockle's foot before its burrowing ability is impaired.

Reference:
Umberger, C. M., de Buron, I., Roumillat, W. A., & McElroy, E. J. (2013). Effects of a muscle‐infecting parasitic nematode on the locomotor performance of their fish host. Journal of Fish Biology 82: 1250–1258

Asobara japonica

Drosophila suzukii is a fruit fly like no other. Native to Asia, it is related to that common lab workhorse(fly) Drosophila melanogaster, but unlike most Drosophila, which lay their eggs on overripe and rotting fruit, D. suzukii has a saw-like ovipositor that allows it to lay its eggs in fruits that are still ripening. Recently D. suzukii has been spreading its wings over the American and European continents, earning the title of being a pest species as it attacks a wide range of soft-skinned fruits including strawberries, cherries, grapes, nectarines, pears, and peaches.

The usual adversary of fruit flies is Leptopilina heterotoma, a parasitoid wasp that can devaste Drosophila maggots. It is such a threat that some maggots resort to imbibing alcohol to stave off this parasitoid. But while L. heterotoma is a menace to most fruit fly maggots, the maggots of D. suzukii is the first Drosophila found to stop that wasp in its tracks. The secret lies in the fly's blood. Insects and other invertebrates have blood cells called hemocytes that patrol their bodies, clotting wounds and entombing foreign invaders in hardened capsules. Leptopilina heterotoma disables those defensive cells by unleashing a virus that destroys them.

Asobara japonica photo from here 

However, this feat of biological warfare doesn't seem to work on D. suzukii. In the study we are featuring today, researchers exposed a group of D. suzukii to some L. heterotoma that were eager to lay their eggs in some suitable victims. But while the wasps readily injected their eggs into D. suzukii as they would with any other fruit flies, most of the eggs ended up being trapped in hemocyte coffins and none of the parasitic larvae ever made it out of a D. suzukii maggot alive. When they looked at the blood of D. suzukii, they found that it has five to ten-fold more hemocytes than D. melanogaster, making it a tough adversary for any would be parasite. Furthermore, not only were the hemocytes of D. suzukii not destroyed by L. heterotoma's "pet" virus, their numbers actually increased in response.

But D. suzukii is by no mean invincible; it has its own parasitoid to watch out for and it is also the species we are featuring today - the parasitic wasp Asobara japonica. This wasp is one nasty customer; it would have to be seeing as it has coevolved with D. suzukii. When the researchers unleashed egg-bearing A. japonica upon both D. suzukii and D. melanogaster, the exposed D. suzukii were able to entomb very few of the A. japonica eggs - a quarter of them at most. In comparison, D. melanogaster did not stand a chance - none them were able to entomb the A. japonica eggs that had been laid inside them.

While both L. heterotoma and A. japonica are both parasitic wasps of fruit flies, they have very different methods for subduing their host's immune system. Whereas Leptopilina heterotoma wages biological warfare on its host, A. japonica is a chemical warfare specialist. It injects at first glance what appears to be a peculiar cocktail into its host; a deadly venom and its antidote. Yet this mixture allows A. japonica to manipulate the host's physiology, but only when both serums are injected in combination. The venom alone will disrupt the host's immune system, and then induce paralysis, which is followed by death. But A. japonica also injects the antidote along with it which mitigates some of the venom's effects - it keeps the host alive, but at the same time allows the immune system to be ravaged. So in effect this wasp brings its host to the edge of death, enough to disable its defences, then cures it - but only so that it can then act as a living incubator for its babies.

Reference:
Poyet, M. et al. (2013). Resistance of Drosophila suzukii to the larval parasitoids Leptopilina heterotoma and Asobara japonica is related to haemocyte load. Physiological Entomology. 38: 45-53.

Duplicibothrium minutum

In June 2010 at Folly Beach, South Carolina, the local community was shocked and dismayed by the sight of millions of dead dwarf surf clams (Mulinia lateralis) that carpeted the beach with rotting shellfish. During the course of an investigation into what might have caused this die-off, researchers discovered some tapeworm larvae in the clams which had not been previously reported from that area.

image of Duplicibothrium minutum from figures in the paper

Because the larval stages of tapeworms have few of the morphological features that usually serve as diagnostic markers to identify different species, the researchers looked to their DNA for clues on their identity. They sequenced a section of the tapeworms' DNA and compared it with known DNA sequences of tapeworms (we have previously featured a study which used the same technique, know as DNA barcoding, to figure out the life cycle of a Great White shark tapeworm.) They were able to determine that the most common species of parasite in those clams was the tapeworm we are featuring today; Duplicibothrium minutum. Out of the 200 clams that the researchers dissected, 150 of them were infected with D. minutum, while four of clams were infected with another species of tapeworm - Rhodobothrium paucitesticulare (three of which were also infected with D. minutum).

photo of Rhodobothrium paucitesticulare 
from figures in the paper

The larvae of these two tapeworms occupied different part of the clam's body - whereas D. minutum were often found in pairs in the bivalve's digestive glands, R. paucitesticulare larvae tucked themselves away at the gap just beneath the clam's fleshy mantle. Both tapeworms are gut parasites of the Cownose ray, (Rhinoptera bonasus) which commonly feed on mollusks and other invertebrates that they suck up from sandflats and crush with their hard dental plates. Rhodobothrium paucitesticulare only infects the dwarf surf clam and one other species of clam (Donax variabilis), while D. minutum has a much wider host range and has also been found in two other species of clams (Donax variabilis and Angulus versicolor) as well as the Florida crown conch (Melongena corona).

Unlike some parasitic flukes that can alter the burrowing behaviour of clams and other bivalves, neither tapeworm caused much noticeable harm to the host clams. The presence of D. minutum caused some minor enlargement at the opening of the digestive glands, but there were no signs of inflammation, and the R. paucitesticulare larvae seem to be completely benign and did not affect the clam's health at all. So while those tapeworms seems to be very common in the clam population, they were not causing nearly enough harm to be considered responsible for the mass die-off.

Reference:
de Buron I, Roth PB, Bergquist DC, Knott DM. (2013) Mulinia lateralis (Mollusca: Bivalvia) die-off in South Carolina: discovery of a vector for two elasmobranch cestode species. Journal of Parasitology 99: 51-55

Ieredactylus rivulus

As you can probably tell from the name, asphalt lakes are not nice places to live. Also known as tar pits, they are natural deposits of bitumen that leak up to the surface, filling the water above with all kinds of nasty substances including volcanic ash, hydrocarbons, sulphur, and metal compounds. There are only five such natural asphalt lake sites in the world, one of which is the well known La Brea tar pits.

The largest asphalt lake in the world is Pitch Lake on the southwest coast of Trinidad and surprisingly, it is actually home to a variety of organisms. Not just bacteria and other hardy microbes, but animals such as aquatic insects, a species of frog (Pseudis reticulata), and some fish have also made it their home sweet home. Despite the inhospitable surroundings, there might be a perk to living in an asphalt lake. Such an harsh environment might also be intolerable for parasites, especially any external parasite which would be exposed to the asphalt-contaminated water.

Ieredactylus rivulus
image from here

In their natural habitat, guppies are commonly plagued by many parasites, especially ectoparasitic flatworms call monogeneans in the genus Gyrodactylus, and in heavily infected populations as many as three-quarters of the fish will be infected. The guppies living Pitch Lake are almost completely free of parasites - except the parasites that we are featuring today - Ieredactylus rivulus. While it is the only parasite to infect Pitch Lake guppies, it is not very abundant and they are found on fewer than five percent of the fish in any given population. Apart from Pitch Lake guppies, this parasite is only found on the giant rivulus Anablepsoides hartii (previously known as Rivulus hartii); another hardy inhabitant of Pitch Lake. Furthermore, the giant rivulus is also known for wandering onto dry land every now and then, so a parasite that lives on the skin of such a fish must be pretty robust.

In the paper we are featuring today, a group of scientists conducted a series of experiments to see how the asphalt lake environment affected the guppy's parasites. In one experiment, they tested whether the Pitch Lake guppies are innately resistant to infections by placing some Pitch Lake guppies in a tank filled with dechlorinated aquarium water. Within a week, seven out of the ten guppies in the aquarium water became infested with various bacterial and fungal infection, whereas all but one of the guppies kept in the original Pitch Lake water were free from infections.

In another experiment, they tested the effect of exposure to Pitch Lake water on monogenean parasites. They collected guppies that are naturally free of monogeneans parasites from a site at the Upper Naranjo, and experimentally infected them with Gyrodactylus by exposing them to parasite-laden guppies from the Lower Aripo, a site with high parasite prevalence. After those guppies had acquired some parasites from their infected cousins, the scientists transferred one group of the newly-infected guppies into a tank filled with water they collected from Pitch Lake that has been diluted to a quarter of its original concentration, and another group into tank of dechlorinated aquarium water. Within 48 hours, the guppies transferred into the diluted Pitch Lake water had lost their newly-acquired parasites, whereas those transferred into the aquarium water were stuck with their new parasites.

Both of those experiments showed that the Pitch Lake water was playing a key role in keeping the Pitch Lake guppies free from (most) infection, and that I. rivulus must have some special adaptations which allows it to survive on fish swimming in a pond filled with bitumen. So if I. rivulus can survive on asphalt lake guppies, what is to stop them from taking on guppies living in less noxious surroundings? Perhaps in the extreme environment of the Pitch Lake, I. rivulus does not face competition from other parasites and can have the host all to itself, whereas in other guppy populations they will be competing with rapidly breeding parasite like Gryodactylus and get shoved aside.

So while asphalt lakes might not be attractive places to live, such extreme environments can provide their inhabitants with a refuge from all but the most hardy parasites.

Reference:
Schelkle B, Mohammed RS, Coogan MP, McMullan M, Gillingham EL, van Oosterhout C, Cable J. (2012) Parasites pitched against nature: Pitch Lake water protects guppies (Poecilia reticulata) from microbial and gyrodactylid infections. Parasitology 139:1772-1779

Bivitellobilharzia loxodontae

Blood flukes from the genus Schistosoma are found in over 77 countries, infecting at least 230 million people, and second only to malaria as the most socioeconomically crippling parasitic disease in the world. But the majority of flukes from the family Schistosomatidae do not infect humans; they parasitise other species of mammals, as well as birds. There are about 100 known species of schistosome flukes around the world. Understandably, those species from the genus Schistosoma are the most extensively studied due to their public health importance. However, there are many other blood flukes for which very little is known on even the most basic aspect of their ecology.

Photo by Thomas Breuer from here

Meet Bivitellobilharzia loxodontae; a schistosome that parasitises African forest elephants (Loxodonta cyclotis). It holds the distinction of probably being the most poorly known of all the schistosomes. The first and only adult specimens of this fluke were retrieved from an elephant that had died in an animal park in Hagenbeck, Germany. To this day, almost everything known about this parasite had come from those samples which were described in 1940. The elephant that was hosting those blood flukes was likely captured from the region now known as the Democratic Republic of Congo.

Because B. loxodontae is an endoparasite (internal parasite) of elephants, adult specimens are hard to come by as they can only be retrieved via "destructive sampling" (dissecting the circulatory system of a dead elephant). And despite extensive sampling in the area where the forest elephant resides, the snail host (where the asexual larval stages of this parasite reside) has not yet been identified. Documenting the life-cycle of these parasites is a labour-intensive and time-consuming task as it requires finding all the different larval stages and demonstrating that all those different stage do indeed belong to the same species by performing experimental infections. Performing experimental infection on an animal like an elephant is out of the question due to its large size and rarity.

Photo of B. loxodontae egg
from the paper

With the advent of molecular techniques, it is now possible to confirm the identity of parasites at different stages of their life cycle without experimental infection (even though experimental infections are still useful for working out other aspects of a parasite's ecology). This can be done by sequencing specific sections of DNA which can serve as markers that identify the species and differentiate it from other parasites which might look similar. In the paper we are featuring today, the researchers extracted DNA from B. loxodontae eggs that were retrieved from samples of elephant dung in order to work out how this blood fluke fits into the schistosome family tree.

Their analyses showed that out of all the schistosome blood flukes, it is most closely related to Bivitellobilharzia nairi -  a species known from the Asian elephant (Elephas maximus). Taxonomically, the genus Bivitellobilharzia sits near the base of a branch within the schistosome family that contains mammal-infecting species (including those species from the Schistosoma genus). The pattern of branches in the schistosome family indicates that at some point in the past, the mammal-infecting group evolved in a divergent direction (in terms of host use) to the rest of the family, which is composed of species that infect birds. This raises intriguing questions about the deep evolutionary history of this group of parasites.

Reference:
Brant SV, Pomajbíková K, Modry D, Petrželková KJ, Todd A, Loker ES. (2013) Molecular phylogenetics of the elephant schistosome Bivitellobilharzia loxodontae (Trematoda: Schistosomatidae) from the Central African Republic. Journal of Helminthology 87: 102-107.

Drepanocephalus spathans

Aquaculture is one of the fastest growing food production industries in the world; it is already responsible for supplying half of fish consumed by the world's population and will soon account for the majority of fish on people's dinner plates. But like other forms of animal production, outbreak of infectious diseases in aquaculture can result in massive die-offs (such as the recent virus outbreak at oyster farms near Sydney, Australia). But even at lower levels of infection prevalence, having infected and sickly animals can result in a loss of production due to reduction in growth and/or the diseased animals simply become unmarketable.

Photo by Ryan Somma

Fish can be infected by all kind of parasites and pathogens ranging from microparasites such as viruses and bacteria, to macroparasites such as flukes, worms, and fish lice (which are actually crustaceans). The parasite we are looking at today - the fluke Drepanocephalus spathans - was found at a fish farm rearing channel catfish. Channel catfish is a very popular angling species in the United States and it has also become a very popular commercial aquaculture species over the last few decades.

Incidentally, catfish farms also make ideal habitats for the rams-horn snail (Planorbella trivolvis) and they are commonly found at fish farms. These snails also happens to be host to a variety of trematode flukes, some of which happen to infect fish as the next host in their life-cycle. In the study we are looking at today, a group of researchers at Mississipi State University examined rams-horn snails from a catfish farm and found the snails there were shedding at least four different species of trematode flukes, with D. spathans being the most abundant species.

Photo of D. spathans from the paper

Drepanocephalus spathans has a typical fluke life-cycle with three hosts - in this case snails, fish, then fish-eating birds. The researchers had known from an earlier study that some of the ram-horn snails at the site did shed larval stages (cercariae) of D. spathans, but the parasite was not previous thought to cause affect the health of the catfish. But when they conducted experimental infections where they placed some catfish in tanks with cercariae-shedding snails, some of the fish died within a week of being exposed to infected snails. When they dissected the surviving catfish (which showed no external signs of disease), they found that the parasites form cysts that congregated mainly around the head of the catfish, particularly at the base of the gills arches. Their presence can possibly interfere with oxygen uptake and had been the cause of death for the fish that had died from exposure to the parasite.

Other animals that frequent catfish farms are fish-eating birds, and ram-horn snails become infected by D. spathans from the parasite's eggs which are shed in the faeces of such birds carrying the adult fluke - in this case the Double-crested cormorant (Phalacrocorax auritus). The parasite undergoes asexual proliferation inside the snail to produce the larval stages that then go on to infect the catfish. So the cormorant is a key source of the parasite; but it is also a protected species under the Migratory Bird Treaty Act, so getting rid of birds "terminally" was not really an option. They also usually feed at night when no one would be around to try and scare them off.

Therefore, the key to breaking the life-cycle of this parasite lies with finding a way of controlling the population of snails at the farm. Flukes with similar life-cycles are common to fish farms and also cause fish diseases in other parts of the world such as Taiwan, Vietnam, and Finland, therefore this is not a problem that is restricted to the fish farms of United States. Understanding the life-cycle of the parasites and how they use each of their hosts is an important step in figuring out how to control disease outbreaks - whether in aquaculture or other contexts where infectious disease is a major problem.

Reference:
Griffin MJ, Khoo LH, Quiniou SM, O'Hear MM, Pote LM, Greenway TE, Wise DJ. (2012) Genetic sequence data identifies the cercaria of Drepanocephalus spathans (Digenea: Echinostomatidae), a parasite of the double-crested cormorant (Phalacrocorax auritus), with notes on its pathology in juvenile channel catfish (Ictalurus punctatus). Journal of Parasitology 98: 967-972.

P.S. Speaking of aquaculture, I have a new paper on the global pattern of disease outbreak in aquaculture in Journal of Applied Ecology. It has been selected as the Editor's Choice; you can read a summary of the findings and a link to a free copy of the paper here. 

Strelkovimermis spiculatus

If you don't like mosquito bites, then you will like today's parasite as it is a mermithid nematode which infects mosquitoes. We have previously featured worms from that family of nematodes - they all infect arthropods as larvae but have adults that live free in the environment. The larval worm develops inside the host's body and once it is fully mature, it exits its host by drilling a hole through the body wall, killing the host in the process (you can see this in action here).

Image taken from a screenshot of this video by Manar Sanad

Not only is Strelkovimermis spiculatus a scourge for mosquito larvae, it is also easily cultured in laboratories, and is not very picky about which species of mosquito it infects. So not surprisingly, S. spiculatus is also currently being considered as a biological control for mosquitoes. But how does S. spiculatus infect the mosquito in the first place? Mosquito larvae can live in just about any waterbodies, ranging from permanent ones such as lakes or dams, to more ephemeral bodies of water such as puddles or rainwater that collect in cavities such as car tires. Because mosquitoes can breed opportunistically in just about any pool of water, parasites that infect their larvae must also be prepared to be equally opportunistic and S. spiculatus has adaptations to do just that.

Firstly, the eggs of S. spiculatus are able to survive being dried out; this allows them to simply wait in empty ponds or puddles for them to fill up and become colonised by mosquito larvae. Secondly, even in more permanent water bodies S. spiculatus eggs can stay dormant in the environment for several months. The reason is that once the parasite larvae hatch, they have a very short window of 24-48 hours to find and infect a host before they die, therefore they have evolved to hatch only if suitable hosts are available. So what is it about the presence of mosquito larvae that trigger these dormant eggs into hatching at just the right moment?

To find out, a group of scientists exposed some S. spiculatus eggs to both chemicals and vibrations that are associated with mosquito larvae. They exposed S. spiculatus eggs to "mosquito-conditioned water" (which is basically water which had mosquito larvae in them for a while) as well as vibrations generated by artificial mosquito larvae that mimic the behaviour of real ones. To make these artificial mosquito larvae, they took tiny strips of iron wire and coated them in hot glue. They then placed those coated bits of wire in the assay container and rested the container on a magnetic stirring plate (common in laboratories) to make their little "artificial mosquito larvae" move.

They found that while the vibrations generated by those fake mosquito larvae did not provoke the eggs into hatching, the scent of mosquito larvae in the water induced about a third of the eggs to hatch. But that was not as good as the presence of actual live mosquito larvae that send the eggs into a hatching frenzy. Furthermore, the eggs of S. spiculatus are also most likely to be set off by the presence of second-instar mosquito larvae and they trigger almost twice as many eggs into hatching than any other mosquito stages. It just so happens that mosquito larvae at this developmental stage are also the most vulnerable to infection by S. spiculatus.

By having hardy eggs that can survive being dried out, remain dormant for extended periods of time, and hatch only when presented with the right signals, S. spiculatus can simply lurk in the environment, ready to launch into action whenever mosquito larvae might appear.

Reference:
Wang Y, Lutfi Z, Dong L, Suman DS, Sanad M, and Gaugler R. (2012) Host cues induce egg hatching and pre-parasitic foraging behaviour in the mosquito parasitic nematode, Strelkovimermis spiculatus. International Journal for Parasitology 42: 881-886

Riouxgolvania kapapkamu

Nematodes are commonly referred to as "roundworms" as that describes their cross-section if you were to cleave one right across its mid-section. Other names for nematodes (particularly parasitic ones) include whipworms and threadworms and this reflects their general shape. Unless you examine their anterior or posterior end really closely, most nematodes are quite... boring looking really... basically, a long thin cylinder that is tapered at both ends. So at least on a superficial level, it's a case of "seen one, seen them all" - but not the nematode we are featuring today.

This species of nematode, which has been newly described from some Japanese bats, is quite literally a round worm - as in it is rather rotund (the picture taken from paper shows an adult worm on the left [fig. 1] and a young adult on the right [fig. 2]). They were found while a group of researchers were studying the ecology of Japanese large-footed bats (Myotis macrodactylus) in Esashi, Hokkaido, Japan and they noticed some peculiar swellings on the head and ears of two male bats they were examining. Interestingly, from the way it was described, the method with which the researchers extracted these globular worms sounded not unlike how one might pop a zit or a pimple...

With their stumpy, globular appearance, they look a bit like another species of nematode we featured back in 2010 - Tetrameres sp. But whereas Tetrameres belongs in their own family (Tetrameridae) and lives in the proventriculus (a part of the bird's stomach just in front of the gizzard) of birds, R. kapapkamu belongs to the family Muspiceidae and lives under the skin of bats

There is very little known about nematodes in the muspiceoid family. Once they become infective, the larvae are assumed to burrow their way out of the body of the female worm and escape to the surface of the host skin. There, they might await contact with another host. However, the larvae of muspiceid nematode have also been found inside blood-sucking ticks and insects, but it is unknown if those arthropods play a role in the parasite's transmission, or if they were simply ingested incidentally while the arthropods were taking a blood meal.

Hasegawa H, Satô M, Maeda K, Murayama Y. (2012) Description of Riouxgolvania kapapkamui sp. n. (Nematoda: Muspiceoidea: Muspiceidae), a peculiar intradermal parasite of bats in Hokkaido, Japan. Journal of Parasitology 98: 995-1000.

Six-legged, fur-covered, sea-faring and conferences - all packed full of parasites!

It looks like we've made it through another year of parasites, filled with posts on new research that was published this year on all manners of parasitic and infectious organisms. Among many other things, this year we covered some parasitological going-ons in the insect world with Zombee parasitoids, a story of parasitoid wasp, aphids and their symbionts, a wasp that can manipulate the colour of berries, and a cricket-infecting horsehair worm which has abandoned sex.

We also wrote about parasites that are infecting our furry friends including reindeer roundworms, a flea of desert rodents, echidna gut parasites, anteater parasites, and a caring, maternal bat tick.

There were a lot of parasite action under the sea too, with jellyfish parasites that provide a floating buffet for some fish, a thorny-head worm which infects krill as a way of getting itself into whales, a leech that lives on shrimps, a prickly worm that lives in the stomach of dolphins, and a story of death, sex and fish guts.

Those are just a few example of post from this year; browse through the archives for a lot more parasitological tales.

Also for the first time on this blog, Susan and I had decided to report from conferences that we had attended on our respective continents! I wrote up a series of blog posts from the Australian Society for Parasitology annual conference (Part 1, Part 2, Part 3, Part 4), and Susan also wrote a few posts reporting from the American Society of Parasitology annual meeting (Part 1, Part 2).

We will be back next year to bring you more posts about the latest development in fields relating to parasitology and just like this time last year, I have already lined up a few which I am going to be writing about... See you all next year!

P.S. If you can't wait until next year for your parasite fix, I was interviewed in a Google Hangout On Air as a part of DeSTEMber - in it I talked about parasites, science, and art. You can watch the interview "Living with Body-Snatchers" here. 

Metarhizium anisopliae

Today, we are featuring the insect-killing fungus Metarhizium anisopliae. I have previously written about a related species that specialises on orthopterans (grasshoppers, locusts) and all species in the Metarhizium genus are dyed-in-the-wool insect killers - some of them are used as biological insecticides. There is even ongoing research looking into ways of loading them with scorpion venom to fight mosquitoes which spread malaria

Metarhizium anisopilae growth from termite cadaver
Image from Fig. 1 of the paper

Metarhizium anisopliae infects a variety of insects and in the study we are featuring today, the host they were presented with were termites. But M. anisopliae is not alone in their taste for these blind social insects. Termites can also fall victim to Aspergillus nomius - a fungus that usually lives as a saprophyte (feeding off dead things), but can sometimes be a parasite when the opportunity arises. Aspergillus nomius can grow very well by feasting on dead termites, but it has one problem; being an opportunistic "sometime" parasite, it is not very good at actually killing termites - in fact it is very bad at it.

When healthy termites are exposed to the spores of A. nomius, they are unaffected. Termites only succumb when exposed to an extremely high dose of spores (five million spores per gram of sand in the enclosure the termites were housed in) and even then, after more than 10 days, only a tenth of the exposed population died. However, when exposed to M. anisopliae at a much lower dose (five hundred thousand spores per gram of sand), the termites died in droves, as expected. When the termite population was exposed to a fifth of the dose of M. anisophliae as had been tested with A. nomius (one million spores per gram of sand), the entire experimental population was wiped out after a week

Aspergillus nomius growth from termite cadaver
Image from Fig. 1 of the paper

In additional experiments where termites were exposed simultaneously to equal doses of spores from both fungi, they died at the same rate as those exposed to the equivalent dose of M. anisopilae sans A. nomius, showing that the M. anisopliae was the true killer and A. nomius did not contribute to bringing down the termites. But despite its role in mixed infection, the dedicated parasite M. anisopliae did not get to reap all the reward for its work in mixed company. Instead, it is out-competed by the opportunistic A. nomius, with termites cadaver killed by mixed infections sprouting more A. nomius.

This study illustrates the context-dependency nature of harm and competition. Ecological competition between parasites often involves trade-offs in a number of traits, and traits that allow a parasite to successfully overcome a host's defences do not necessarily makes it a good competitor when confronted with other parasites. In this particular case, the usually saprophytic A. nomius can't take down a healthy termites on its own, but given the chance through a true killer M. anisopliae, it'll step in and take over completely.

Reference:
Chouvenc, T., Efstathion, C.A., Elliott, M.L., Su, NY. (2012) Resource competition between two fungal parasites in subterranean termites. Naturwissenschaften 99: 949-958