"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
Showing posts with label acanthocephalan. Show all posts
Showing posts with label acanthocephalan. Show all posts

September 16, 2012

Bolbosoma balaenae

Image from Figure 1 of the paper
Today's parasite is an acanthocephalan (also known as a thorny-headed worm) and its name should be a clue to what it infects - baleen whales. And what do most baleen whales eat? Krill - lots and LOTS of it. The authors of the study I am writing about in this post found Bolbosoma balaenae larvae infecting krill that were caught during a plankton trawl off the coast of Ría de Vigo, Spain in the NW Iberian Peninsula.

The krill serve as hosts for larval B. balanae and from there, they proceed to infect the next host of their life-cycle, which as mentioned above, are baleen whales where they develop into adult worms. Acanthocephalans as a whole generally only have two hosts in their life-cycle - a small arthropod intermediate host where the larval worm resides, and the vertebrate definitive host where the adult lives and reproduces. But many of the thorny-headed worms that infect marine mammals add another host into the life-cycle between the crustacean host and the vertebrate host - this extra host is known as a paratenic host. The paratenic host is different from the intermediate host, and here's why.

For parasites with complex, multi-host life-cycles, the intermediate host is an obligate component for successful completion of the cycle. It is where the larval parasites gather resources to undergo development into the next stage, and at the same time, the intermediate host also serves as a mean of transporting the larvae into the definitive host (usually by getting itself eaten by the said host). It is in the definitive host where the parasite reaches sexual maturity. In contrast, a paratenic host serves only as a transport, and while the parasite has to infect an intermediate host to complete its life-cycle, infecting the paratenic host is optional. Seeing how the parasite can technically go through its life without ever hopping inside the paratenic host, why do it at all?

Image from Figure 1 of the paper
In the case of other acanthocephalans that infect marine mammals (such as Corynosoma cetaceum), if they are accidentally ingested by their marine mammal hosts while still inside the tiny crustacean intermediate hosts, they will still reach adulthood. But because the chances of that happening is negligibly slim compared to the likelihood of the crustacean host being eaten by a fish, which itself is then eaten by the said marine mammal, incorporating a paratenic host greatly enhances its chances of completing its life-cycle.

However, all this is unnecessary for B. balaenae, as their next host - fin whales and minke whales - do in fact feed on those tiny crustaceans. The authors of this study found that the infection prevalence of B. balaenae in krill is very low - only one in every thousand krill was infected with B. balaenae. But considering that a fin whale gulps down about 10 kg (22 lb) worth of krill with every mouthful and eats about 1800 kg (4000 lb) of those little crustaceans each day,  they can easily pick a few hundred worms very quickly even though the infection level is relatively low in krill.

Just like another acanthocephalan we have previously featured on this blog, Acanthocephalus dirus, instead of simply shedding eggs that are released into the environment with the host's faeces, the female worm actually leaves the gut once she is filled with fertilised eggs (see this paper). So even though the whale is constantly being infected with new worms with every mouthful, there is also a constant turnover in the population in the form of mature female worms exiting the host.

Gregori, M., Aznar, F.J., Abollo, E., Roura, Á., González, Á.F. and Pascual, S. (2012) Nyctiphanes couchii as intermediate host for the acanthocephalan Bolbosoma balaenae in temperate waters of the NE Atlantic. Diseases of Aquatic Organisms 99: 37-47.

June 19, 2012

Corynosoma cetaceum

image from here
In the last post we met Acanthocephalus rhinensis - an acanthocephalan which lives a pretty normal life (for a thorny-headed worm) - it spends its adult life anchored to the intestinal wall of its eel host, absorbing the nutrient-rich slurry of the intestinal content through its body surface. Today, meet Corynosoma cetaceum - it is yet another acanthocephalan, but that's about where its similarity with A. rhinesis ends. Corynosoma cetaceum lives inside the stomach of dolphins, and it is one prickly customer. As well as having the signature thorny proboscis (see the lower right picture), its entire body is covered with a spiky coat of wickedly-sharp spines (see picture on the upper left showing spines extending well pass the proboscis) which would put a hedgehog to shame.

Whereas in other acanthocephalans the proboscis plays the main attachment role, in C. cetaceum uses its entire body to cling on. The study which forms the basis of today's post looked at differences in the spines of male and female C. cetaceum, and found a high degree of divergence between the sexes. While female worms are smaller, overall they have much longer spines than males. In fact only in females do the spines grow significantly during maturation from larva (known as a cystacanth) to adult. In contrast, the body spines of adult male C. cetaceum remains more or less the same length as they were as cystacanths.

image composed from here and here
This seems odd, because being smaller, the females are actually at less risk of being dislodged (less surface area exposed to the dragging flow of the stomach content) - so why the longer spines? One possibility raised by the researchers is that perhaps the males simply depend upon attachment mechanisms other than body spines - but compared with females, the male worms have smaller proboscis and hooks too. Alternatively (and more likely), perhaps female worms need to stay in the host for longer than the males in order to produce and release eggs. There are indirect data which indicates female C. cetaceum live longer than their male counterpart - this is inferred from what is known for other acanthocephalans, and the sex ratio of C. cetaceum populations found in the stomach of dolphins which is skewed towards having more females.

There are further, as yet unsolved mysteries relating to C. cetaceum. As mentioned at the start of this post, the stomach is a very different habitat to the intestine. The life of parasites living in the intestine is fairly leisurely, being bathed a steady flow of nutrient-rich slush composed of finely-digested food infused with a cocktail of the host's bodily secretions. In stark contrast, the stomach is an extremely harsh environment. It is where early stages of digestion takes place - where chunks of food are mashed up and soaked in harsh digestive juices. The content of the stomach is composed largely of chyme - an acidic mixture of partially digested food and acid which is not all that nutritious for parasites like acanthocephalans which absorb nutrients through their body surface. In addition, carnivorous marine mammals consume huge quantity of food whenever the opportunity arises; this results in unpredictable and heavy flows of food through the stomach which makes for an extremely turbulent environment that can easily dislodge any parasitic worms (see this paper).

Of all the places in the digestive tract that C. cetaceum can occupy, why has this species evolved to live in such an inhospital environment?

Hernández-Orts, J.S., Timi, J.T., Raga, J.A., García-Varela, M., Crespo, E.A. and Aznar, F.J. (2012) Patterns of trunk spine growth in two congeneric species of acanthocephalan: investment in attachment may differ between sexes and species. Parasitology 139:945-955.

P.S. Attention parasite appreciators! Both Susan and I will be attending parasitology conferences happening on our respective continents in July and we will be tweeting about them. So as if this blog isn't already enough, you can your 140 characters or less fix of parasitology goodness on Twitter - you can find me on Twitter @The_Episiarch and Susan @NYCuratrix. I will be tweeting the Australian Society for Parasitology conference 2-5 July, while Susan will be tweeting the American Society of Parasitologists conference 13-16 July. 

June 7, 2012

Acanthocephalus rhinensis

image from figure 1 of the paper
The study which forms the basis of today's post features an acanthocephalan - also known as a thorny-headed worm - which lives in the intestine of European eels in Lake Piediluco in central Italy. Acanthocephalans spend their adult lives like tapeworms, clinging to the wall of their host's intestine, and absorbing nutrients from the pre-digested gut content. But unlike tapeworms, which mostly use suckers and small hooks to cling to the intestinal wall, an acanthocephalan has a formidable bit of armament which puts the tapeworms to shame. As its name indicates, at the front of the acanthocephalan is a hook-laden proboscis (see the picture on the right) to stab into the intestinal wall and firmly anchor themselves in place.

In Lake Piediluco, some eels were found to be infected with up to 350 Acanthocephalus rhinensis, though most eels had fewer than 50 worms. The eels become infected through eating little shrimp-like crustaceans called amphipods. The amphipods live mostly amongst the aquatic vegetation at the edge of the lake, and they are parasitised by the larval stage of A. rhinensis. If you thought the idea of having dozens of prickly-headed worms clinging to your intestinal wall with their nightmarish probosces is bad, A. rhinensis is downright brutal to the amphipod host.

image from figure 3 of the paper
The larval worm (called a cystacanth) occupies a large part of the little crustacean's body (see picture on the left), displacing many of its internal organs. About one in ten amphipods at Lake Piediluco are infected with A. rhinensis, and each amphipod had one or two worms inside them (probably because there wouldn't be much room for more). Acanthocephalus rhinensis imposes a massive burden on the little crustaceans - infected females can only successfully produce half as many eggs as uninfected females.

Armed with that formidable anchor, you would think that A. rhinensis would be able to establish itself in the gut of just about any fish it finds itself in. But it appears to be remarkably faithful to eels, which are the only fish found to have A. rhinensis in their intestines. Perhaps there are other immunological or ecological reasons that prevent this species from successfully infecting other fish.

In addition to establishing the life-cycle of A. rhinesis, another discovery made by the researchers actually served to amend an existing error in the scientific literature. In the original description of A. rhinensis, which was made based on nine specimens, this species is supposed to have a distinctive band of orange-brown (think spray-on tan) pigment just behind their proboscis, a feature that apparently distinguishes it from all the other Acanthocephalus species. However, the researchers who wrote this paper examined a total of over a thousand worms and not a single one had the supposed distinguishing band. But what gave those worms that orange-brown collar? The researchers suggested that this was caused by discolouration from being jammed so deeply into the intestinal wall that the worms inadvertently absorbed pigment from host's intestinal vessel which gave them a distinctive tinge just behind their proboscis.

So in addition to working out the life-cycle of A. rhinensis, this study also served to clarify old mistakes, which will help out any future researchers who work on this species.

Dezfuli, B.S., Lui, A., Squerzanti, S., Lorenzoni, M. and Shinn, A.P. (2012) Confirmation of the hosts involved in the life cycle of an acanthocephalan parasite of Anguilla anguilla (L.) from Lake Piediluco and its effect on the reproductive potential of its amphipod intermediate host. Parasitology Research 11: 2137-2143.

February 16, 2012

Acanthocephalus dirus

The word parasite has a lot of connotations associated with it, and "maternal" is certainly not one of them. To most people, the term "freeloader" comes to mind (hopefully, this blog will show you that parasitism is actually a very challenging way of life). They also have a reputation as being pretty lousy parents. In most textbooks, parasites are usually considered as "r-strategists" - which produce many, many offspring and don't take good care of them (as opposed to a K-strategist which produces fewer offspring, but invest a lot into parental care - like an elephant). But not all parasites are bad parents, and today, I am going to tell you about a study on a maternal parasite which sacrifices everything (literally) for her offspring.

Acanthocephalus dirus has a reproductive strategy that is unusual for its group - the acanthocephalans or the thorny-headed worms (Acantho = "thorns", Cephala = "head"). In fact it is unusual compared to most intestinal parasites. Unlike some tapeworms, which profligately cast off segments (each containing hundreds of eggs) into the wilderness with abandonment, A. dirus has rather different approach. The impetus that spurred on this piece of research were two separate observations: (1) fish that are infected with A. dirus do not have any worm eggs in their feces (unlike most animals infected with intestinal parasites) and (2) perfectly healthy and intact female worms were often expelled from the definitive host. What the researchers found was that instead of simply laying eggs that are expelled from the worm and from the host, a female A. dirus actually retains her eggs until she become completely bloated with them - at which point she exits gracefully from the host fish's digestive tract. Some readers might recall a nematode that has a similar reproductive strategy, and that both lineages have evolved such a reproductive strategy independently. So why has A. dirus evolved such an extreme strategy instead of just laying eggs normally like other thorny-head worms?

One reason could be that A. dirus infects creek chub - which, as its name indicates - lives in flowing creeks. The chub acquire the worm through eating infected isopods in the stream (the picture shows the light-coloured infected isopod on the right, and the darker uninfected individual on the left), which become infected when they ingest worm eggs resting on the creek bed. Acanthocephalan eggs tend to float - so if the eggs are simply expelled into the environment, they would get washed away downstream and deposited where the isopods do not occur. Whereas with A. dirus, the worm's own body can act like a weight belt which would carry the eggs down to the sediment layer, so by the time the worm herself decays, the eggs are already in the sediment where isopods can pick them up.

Furthermore, laboratory tests showed that isopods like to eat egg-filled female worms as much as their usual food - leaf litter - and the worm body itself actually enhances the infection success of the eggs. Researchers found that when exposed to fresh eggs alone, fewer than one in four isopods became infected, whereas when exposed to gravid females, over 80% became infected (natural infection comes somewhere in between those at about 60%). By making the ultimate maternal sacrifice, A. dirus gives her offspring the best possible start in life.

Image from figure in: Seidenberg (1973) Journal of Parasitology 59: 957-962

Kopp, D.A., Elke, D.A., Caddigan, S.C., Raj, A., Rodriguez, L., Young, M.L. and Sparkes, T.C. (2011) Dispersal in the acanthocephalan Acanthocephalus dirus. Journal of Parasitology 97: 101-105

July 28, 2011

Acanthocephalus galaxii

The brown trout (Salmo trutta), a popular angling species, was introduced to the waters of New Zealand in 1867 and has become very well established in the local freshwater system. The trout have made New Zealand their own all-you-can-eat buffet, feeding on many of New Zealand's native freshwater fishes. But other native fauna have also been getting intimate with the trout in a different way. It turns out that during its time in Aotearoa, the brown trout has also picking up a new parasite - Acanthocephalus galaxii, which normally infects a little native fish call the roundhead galaxias (Galaxias anomalus).

Furthermore, the parasitic worm has actually become more abundant in the introduced trout than in the native galaxids - presumably because when compared with the tiny native fish, the much larger trout gobbles up more amphipods (the crustacean which carries the larval stage of A. galaxii). But this isn't necessarily good news for the parasite. Once they get into the trout, because of physiological incompatibility with the introduced host, the parasites are unable to reach maturity. So the trout actually acts as a kind of dead-end sink for the worm, which in turn reduces parasite burden on the native fishes.

So even while the trout might be chomping up native galaxids by the mouthful, they also are inadvertently reducing their parasite burden - though I doubt that would give much comfort to the little galaxids fleeing from a hungry trout!

Paterson, R.A., Townsend, C.R., Poulin, R. and Tompkins, D.M. (2011) Introduced brown trout alter native acanthocephalan infections in native fish. Journal of Animal Ecology 88: 990-998.

November 23, 2010

November 23 - Transvena annulospinosa

Today's parasite is an acanthocephalan (thorny-headed worm) which lives in the Blackback Wrasse (Anampses neoguinaicus), a species of fish found on the Great Barrier Reef of Australia. The picture shows the anterior hook-lined proboscis that the worm uses to anchor itself firmly in the intestinal wall of the host. The photo is actually that of a male worm, and interestingly the males of this species have a pair of paddle-like protrusions at the posterior end of the body. The function of the protrusions are completely unknown. Because it is a purely male characteristic, it is possible that they play a role in sexual competition, though that is purely speculative. However, it has been well established that sexual competition is particularly fierce among the thorny-head worms - male acanthocephalans (including the species in today's post) are armed with a "cement gland" that secretes a substance that they use to block up the female's reproductive tract post-mating. This ensures that she cannot receive future sperm from rival males.

Pichelin, S. and Cribb, T.H. (2001) The status of the Diplosentidae (Acanthocephala: Palaeacanthocephala) and a new family of acanthocephalan from Australian wrasses (Pisces: Labridae). Folia Parasitologica 48: 289-303.

Contributed by Tommy Leung.

October 26, 2010

October 26 - Echinorhynchus salmonis

Echinorhynchus species are acanthocephalan parasites belonging to the family Echinorhynchidae. Like other acanthocephalans we’ve already seen (e.g. Neoechinorhynchus emyditoides; Moniliformis moniliformis; Pseudocorynosoma constrictum, these thorny headed worms parasitize the intestines of fish and amphibians. The species shown here is probably Echinorhnychus salmonis, an acanthocephalan with a Holarctic distribution, occurring in fresh and brackish waters and commonly parasitizing salmoniform and other fishes (intermediate hosts include amphipods such as Monoporeia affinis). Echinorhynchus are often the topic of research projects including effects on host feeding ecology, anti-predator behavior, and host spawning. Here are two links about Echinorhynchus species:
First Site
Second Site

Contributed by Jessica Light.

September 4, 2010

September 4 -Corynosoma enhydri

This photo of today's parasite,Corynosoma enhydri, illustrates the origin of the term “thorny-headed worms” for the Acanthocephala. This species, like Profilicollis altmani, that you met last month, uses sea otters as its definitive host. It is fairly obvious that once this proboscis is embedded in the wall of the small intestine of a sea otter, it could not easily be dislodged. In rare cases, the proboscis can perforate the wall of the intestine, leading to peritonitis. The number of rows of hooks on the proboscis and the number of hooks per row are important characters in identifying species.

Contributed by Mike Kinsella.

August 18, 2010

August 18 - Profilicollis altmani

Parasites that have complex life cycles involving marine creatures really baffle me - the odds of them completing their life cycle just seems so unlikely - and yet they do. Profilicollis altmani is a species of acanthocephalan (thorny-headed worm) that uses mole crabs (Emerita spp.) as its intermediate hosts and then infects shore birds like Herring Gulls as the definitive host. The adult parasite attaches to the intestines of the bird and then will release eggs into its feces where they somehow make their way to new foraging crabs. This parasite is also of recent interest because it appears to have jumped hosts into sea otters, where it can cause fatality. The otters are not normally hosts of these parasites, but perhaps are becoming infected as a result of eating prey that they normally do not.

Photo by Tricia Goulding, Romberg Tiburon Center for Environmental Studies, San Francisco State University.

July 21, 2010

July 21- Pseudocorynosoma constrictum

Pseudocorynosoma constrictum is an acanthocephalan parasite of North American waterfowl. Eggs are released into lakes, where they are ingested by the first host, amphipods. After about a month of development, the infective cystacanth stage is reached and the worm can be transmitted to birds. The cystacanths are bright orange and clearly visible through the cuticle of the intermediate host (see picture). Several acanthocephalan species have orange cystacanths, and there has been much debate about the function of this pigmentation. Hypotheses include increased conspicuousness to final host predators, protection against UV radiation, or that it is just a byproduct of larval physiology.

Contributed by Daniel Benesh.

June 5, 2010

June 5 - Macracanthorhynchus hirudinaceus

Today's parasite has quite a handle of a Latin name - perhaps you'll prefer the common name - "Giant Thorny-Headed Worm of Swine." As that moniker suggests, this is an acanthocephalan, and like its relatives has a life cycle that alternates between an invertebrate and a vertebrate. The eggs of this parasites are eaten by beetles where they will develop into juveniles or cytacanths. These insects then get eaten by pigs (or occasionally dogs or even humans in very rare cases). The adults attach themselves to the small intestinal wall and can get quite large - up to 65 centimeters. The eggs pass out with the pig's feces and interestingly, if a bird accidentally eats them if they are on something that they're gobbling up, they pass right through unharmed to wait for the right (beetle) host.

March 21, 2010

March 21 - Plagiorhynchus cylindraceus

Often times endoparasites will alter the behavior of a host to complete their lifecycles. The acanthocephalan, Plagiorhynchus cylindraceus, is a common parasite of songbirds in North America, typically robins (Turdus migratorius) or Europeans starlings (Sturnus vulgaris). While inside the bird, the worm produces eggs that pass out in the bird’s feces and are consumed by pillbugs (Armadillidium vulgare, shown in photo), the main intermediate host. This worm is capable of activating a suicidal behavior in the pillbugs to propogate its own lifecycle. Once infected with the acanthocephalan, the pillbugs become more active and frequent uncovered, light-colored areas on the forest floor while avoiding hiding underneath objects, such as leaves. By exposing themselves, the pillbugs are more likely to be eaten by a predator, such as robins or starlings. Once consumed by the bird the worm is free to reproduce, thus completing its lifecycle. Pillbugs are not the only animals to become infected with this worm. Some North American shrews (Soricidae) have been found with these worms encapsulated in their intestinal mesenteries, although this becomes a dead-end for the parasite because it cannot be passed on to a songbird from the shrew’s intestines. The proboscis of this parasite has many hooks that it imbeds in the host’s intestinal walls and prevent it from passing through with a host’s meal. Nutrients from such a meal are absorbed through the body surfaces of the parasite; the only way the worm receives nutrition since it lacks a gut tract.

Contributed by Anna Phillips.

March 9, 2010

March 9 - Moniliformis moniliformis

Moniliformis moniliformis is an acanthocephalan, or thorny-headed worm. Like others in this group. M. moniliformis alternates between two hosts. The first is usually an insect such as a cockroach or a beetle, and then the definitive host is often a rodent such as a mouse or rat. Janice Moore and colleagues have used M. moniliformis to conduct a variety of studies on the manipulation of host behavior and have found that some cockroaches that are infected with M. moniliformis move more slowly, though other cockroach species do not have detectable changes in behavior. Humans can become infected if they ingest the intermediate hosts accidentally (but so far there is no evidence that they turn into couch potatoes who like dark rooms.)

January 20, 2010

January 20 - Neoechinorhynchus emyditoides

Neoechinorhynchus emyditoides is a species of acanthocephalan, or thorny-headed worm. These parasites often have very complex life cycles involving multiple trophic levels. The vertebrate host of this species is a turtle and, as the picture shows, a single turtle can have hundreds of worms – in some cases, more than 1000! - filling its intestine. The acanthocephalan eggs are expelled in the turtle’s feces and are eaten by ostracods, tiny crustaceans, where they develop into a stage called an acanthella. When fish eat the ostracods, the acanthella travel to the fish’s liver and await the fish’s ingestion by a turtle. There are 10 species in this genus and they are extremely difficult to tell apart. This photo likely contains a mix of both N. emyditoides and Neoechinorhynchus pseudemydis. The host in this case was a red-eared slider (Trachemys scripta) collected from Reelfoot Lake, Tennessee.

Nomination and photo by Mike Barger.