Stenurus globicephalae

Whales are big animals, even the "smaller" cetaceans such as dolphins and pilot whales are large animals in the range of body sizes across the animal kingdom. The thing about big animals is that they provide a lot of room for parasites. The myriad array of spacious organs found in an animal like a whale provide ample opportunities for parasites to become very specialised not just on a particular species of host, but on a very specific niche within the host's anatomy. Unsurprisingly, there are a wide range of parasites that make their home inside whales. With so many different species inhabiting different parts of the whale anatomy, it is perhaps not surprising that there are worms that specialise in living within the voluminous lungs and sinuses of whales. One of them is a genus named Stenurus.

Numerous Stenurus worms in the pterygoid sinuses of a short-finned pilot whale (Globicephala macrorhynchus).
Photo from Fig. 1 of the paper. 

There are nine known species of Stenurus, all of them live in the respiratory system and sinuses of whales. Each of those worms differ in the particular cetacean species that they infect, as well as the part of the host's respiratory system they inhabit. The study being covered in today's blog post were based on samples collected from fourteen toothed whales that had been stranded along the Galicia coast between 2009 and 2019. There were a mix of six different whale species in total, and considering the population of parasites that each whale could support, it provided researchers with plenty of material to work with. Unlike most other parasitological studies where the dissection takes place in the controlled environment of a laboratory, you can't exactly bring a dead whale back to your lab. So instead, the parasites were collected via on-site necropsies.

The researchers found many different species of lung parasites from the different whales, including three species of Stenurus. But out of them, the most abundant was Stenurus globicephalae, which was found in three host species including Risso's Dolphin, Short-finned pilot whales, and Long-finned pilot whales. Each whale harboured between 18 to over 1700 of those worms, which were mostly found in the pterygoid sinus, located deep within the nasal passage near the back of the whale's throat. Previous studies have also recorded S. globicephalae dwelling in the lungs as well as other canals and cavities in a whale's head including the middle ear cavities and cranial sinuses. These worms seem to like dwelling in soft, moist tubes.

Stenurus globicephalae was found to be particularly prolific in short-finned pilot whales, which indicates that while it is capable of infecting other whale species, the short-finned pilot whale just so happens to be a particularly good "fit" for it. Each Stenurus species differ in their host preferences, and it seems to be related to how they get transmitted to the whale in the first place. Like with many other nematodes that infect whales, they do so by hiding in their host's food.

The researchers noticed a certain pattern of association between different species of Stenurus with the diet of the whale species they infect. For example S. globicephalae was associated with species that mostly ate squid, whereas those that ate fish or have a mixed diet tend to be infected with  S. ovatus and S. minor. So it is a case of "you are infected by what you eat".

Stenurus was not alone in its preference for whale respiratory structures. In some of the whales, the researchers found Stenurus cohabiting those airy passages with other parasites. One of those co-inhabitants was Halocercus - a different genus of whale lungworm which anchor themselves in place by plunging their head firmly into the host tissue. Another co-inhabitant was Nasitrema -  a fluke that lives in the air sinuses of small whales and has a nasty tendency to wander into the host's brain.

While having worms in your sinuses sounds uncomfortable, Stenurus lead a relatively peaceful existence within their wet, cosy homes, and their presence causes surprisingly little to no inflammatory responses. Or at least they do once they settle down as adult worms, because the larval worms can potentially cause focal pneumonia, while the adult worms have a more relaxed relationship with their surroundings. It is as if the whales progressively grow used to their presence, or the worms have grown to tame the fiery response of their host's immune system.

Reference:

Saldaña, A., López, C. M., López, A., Covelo, P., Remesar, S., Martínez-Calabuig, N.,García-Dios, D., Díaz, P., Morrondo, P., Díez-Baños, P., & Panadero, R. (2022). Specificity of Stenurus (Metastrongyloidea: Pseudaliidae) infections in odontocetes stranded along the north-west Spanish coast. International Journal for Parasitology: Parasites and Wildlife 19:148-154.

Anisakis physeteris

Being on top of the food chain sounds like it'd be pretty awesome - all the other animals in the ecosystem are potentially your food and nothing else hunts you. In reality, it also means that there are many parasites out there that see you as prime real estate, a nice place to settle down and start a family. And there's no way for you to avoid them since many of those parasites would be climbing their way up the food chain via the prey animals you have been eating. Nowhere is that more obvious than in the ocean.

The oceans are filled with parasites - not that you'd necessarily know since the vast majority of them are hidden out of plain sight within the body of their hosts. Many of them are parasitic worms that treat the oceanic food web like a transit system, using predator-prey interactions to get from one host to another. This post is about a study on two nematodes that cross path inside some oceanic squids

Left (a, c): Lappetascaris larva (top) embedded in squid mantle muscle, (bottom) viewed under the microscope.
Right (b, d): Anisakis physeteris larva (top) in squid testis, (bottom) viewed under the microscope
Photos from Figure 1 of the paper

A group of researchers from Italy looked at parasitic roundworms that are found in the umbrella squid and the reverse jewel squid. Both of them belong to a group of squid called the "cock-eyed squids", which are commonly found in the mesopelagic zone. The squid that the researchers examined were caught as by-catch from commercial trawling vessels that were operating off the coast of Italy and Naples, and every squid that they looked at were infected with some kind of nematode larvae. 

Most of the nematodes were of a genus called Lappetascaris, along with another species which was identified as Anisakis physeteris. While both of those parasites look superficially similar and sometimes co-infect the same squid, there are some key life history and life cycle differences between them. 

For parasites, a host is not a single homogenous entity, but a collection of different microhabitats, and each parasite species has their own taste when it comes to fine-scale real estate. In this case, the researchers found that A. physeteris mostly settled in the squid's testis whereas Lappetascaris preferred embedding itself in the firm mantle musculature (the part of squid which are sold on the market as "squid tubes").

But these worms don't just differ in the part of the squid they prefer, but also which species of squid they infect. While Lappetascaris was found in both the umbrella squid and the reverse jewel squid, A. physeteris was choosier, and was only found in the umbrella squid. Finally, the two worms complete their life cycles in totally different animals. Lappetascaris reaches maturity in the gut of large teleost fishes such as swordfish and billfish, whereas A. physeteris needs to get into the stomach of a sperm whale - as denoted by its species name (the genus name for sperm whale is Physeter).

This may explain why A. physeteris was only found in the umbrella squid. Compared with the reverse jewel squid, umbrella squid venture into much deeper water which overlaps with the sperm whale's usual hangouts. And this exposes them to infective stages that are being released from sperm whales which have hundreds and thousands of adult Anisakis worms in their gut.

While the popular perception of the sperm whale often depict them as duking it out with the giant squid, the majority of their diet is composed of more modestly sized cephalopods, and the umbrella squid seems to form a major part of their diet. That's not to say umbrella squid is not on the menu of other large oceanic predators like swordfish and billfish too (hence it is also infected by Lappetascaris larvae), but if you are a parasite that is looking for the ideal ride to get you into the belly of a sperm whale, you can't do much better than the umbrella squid. 

What about the Lappetascaris which are sharing that squid with A. physeteris? Well they better hope a swordfish would come along and snatched it up before it ends up in the belly of a marine mammal - an environment that it is ill-equipped to live in.

So while these two worms may sometimes meet in the same squid, they eventually have to go their separate ways - and reaching their respective final hosts would unfortunately spell doom for the other worm in the shared squid. As for the sperm whales, a belly full of yummy squid must inevitably lead to a stomach full of wriggly worms.

Reference:

Palomba, M., Mattiucci, S., Crocetta, F., Osca, D., & Santoro, M. (2021). Insights into the role of deep-sea squids of the genus Histioteuthis (Histioteuthidae) in the life cycle of ascaridoid parasites in the Central Mediterranean Sea waters. Scientific Reports 11: 7135

Neocyamus physeteris

Today we're featuring a guest post by Sean O’Callaghan - a student from 4th year class of the Applied Freshwater and Marine Biology' degree programme at the Galway-Mayo Institute of Technology in Ireland. This class is being taught by lecturer Dr. Katie O’Dwyer, who has previous written guest posts about salp-riding crustaceans and ladybird STI on this blog. This post was written as an assignment on writing a blog post about a parasite, and has been selected to appear as a guest post for this blog. Anyway, I'll let Sean take it from here.

Sperm whales are the largest toothed animal alive and they are capable of diving down to depths of 1200 m to feast on cephalopods (including the planet's largest cephalopods, the colossal and giant squids), but despite their size and abilities, these leviathans can fall victim to a range of cunning ectoparasites, including…Whale Lice!

Line drawing of adult female Neocyamus physeteris from Fig. 2 of this paper, SEM photograph from Fig. 2 of this paper

Three species of whale lice are known to target sperm whales, and from this trio there is a divide of preference between male and female whales. Neocyamus physeteris is one such example - they would rather live on a female whale than a male one. While the exact reasoning behind why there is such a divide in parasite species targeting opposite sexes, the answer may be due to the habits of male whales, which frequent the polar waters more often than the females who seek out the warmer waters around temperate zones.

Whale lice are not really lice in a taxonomic sense. Instead, they are classed as amphipods, crustaceans related to the so-called "lawn shrimps" which are found in some back gardens, but with more specialised features for hanging on to a free-swimming whale. Neocyamus physeteris’ body is flattened like a leaf but largely segmented and have legs tipped with hooked edges that act like crustacean crampons to ensure a consistently ample footing. Otherwise the lice would find itself cast adrift without a home or food supply to die alone in the deep. They also possess sharpened mandibles to munch through the host whales epidermis (top skin layer) while for breathing it has two pairs of gills lining its underside towards the front half of the body. Neocyamus physeteris’ head is quite small in comparison to the rest of its body and is dotted with a pair of tiny eyes along with two antennae. Their white colouration almost gives off a dandruff-like appearance against the whale’s darker complexion (though they would be well camouflaged on Moby Dick if it had existed and was also female!).

They are so intertwined with their host that their life cycle that they lack a free-swimming larval phase or active transmission to other whales, offering limited opportunities to move between hosts (unless during social activities where the whales may rub against one another). So it is fair to say that they live, feed and breed on top of their own biological ark, from the sea's clear surface waters to dark depths of the twilight zone, quite a dependent but extreme lifestyle!

Like most whale lice, little is known about the habits of N. physeteris, but it is so specialised for its life-style that whenever the whale dies, the lice would also kick the can as they require a live host. Hanging onto a host may not seem like an exciting lifestyle, but it is a highly beneficial strategy (for the lice at least). Given its tendency to devour sperm whale skin mainly in areas that are sheltered from water movements like the genital slits, body creases or injured skin, this allows the lice to take advantage of a lifetime supply of renewable food. In other words, the lice won’t starve while on a whale, however there will be an increase demand for firm footholds as the parasite population increases, so the species' overall success is not necessarily always good for the individual louse. The whale probably doesn’t suffer too badly when only a handful of lice are present however a colony must surely be highly irritating to say the least.

The strain imposed on N. physeteris at different depths due to the varying degrees of pressure imposed between the surface and abyss would far exceed our own limits. Undoubtedly there must be a risk posed by potential fishy predators on occasion given the lack of cover afforded by a whale’s skin. However, the benefits appear to outweigh the risks - otherwise they would cease to exist as a species. There is still much to learn about these fascinating parasites but until new means of studying the movements and behaviours of these small, somewhat inconspicuous amphipods on top of a large mobile host like a sperm whale are developed, it could take a while to unravel the intricacies of this skin serrating invertebrate!

References
Hermosilla, C., Silva, L.M.R., Prieto, R., Kleinertz, S., Taubert, A. and Silva, M.A. (2015). Endo- and ectoparasites of large whales (Cetartiodactyla: Balaenopteridae, Physeteridae): Overcoming difficulties in obtaining appropriate samples by non- and minimally-invasive methods. International Journal for Parasitology: Parasites and Wildlife. 4, 414-420.

Leung, Y. (1967) An illustrated key to the species of whale-lice (Amphipoda, Cyamidae), ectoparasites of Cetacea, with a guide to the literature. Crustaceana 12, 279-291.

Oliver, G. and Trilles, J.P. (2000). Crustacés parasites et épizoítes du cachalot, Physeter catodon Linnaeus, 1758 (Cetacea, Odontoceti), dans le golfe du lion (Méditerranánée occidentale). Parasite. 7, 311-321.

This post was written by Sean O’Callaghan

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

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.

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.

Reference:
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.

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?

Reference:
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. 

November 17 - Tetrabothrius sp.

The parasite for today is a tapeworm recovered from the intestine of an Andrews' Beak Whale (Mesoplodon bowdoini). This worm belongs to the genus Tetrabothrius, and while this particular species infects beaked whales (obviously), other species of Tetrabothrius have been found in a range of dissimilar marine hosts ranging from baleen whales to fish-eating birds such as albatross and penguins. Tapeworms like Tetrabothrius often have species-specific morphology of their scolex (which is the organ they use to attach themselves to the intestinal wall), and such features can be used to distinguish different species. Larval stages of tapeworm often lack such distinguishing characteristics, making their identification practically impossible. However, the advent of molecular biology technique has enable scientists to use DNA sequences from the larval stages and match them up to those taken from adult worms, allowing their full life-cycle to be mapped out.

Contributed by Tommy Leung.

May 24 - Anisakis simplex

If you suddenly experience a sharp abdominal pain and have recently eaten sushi, ceviche, or pickled herring, there's a chance you might have just become infected with Anisakis simplex. This species is a nematode that primarily uses marine mammals as its hosts. Eggs are excreted in feces, where they will then infect small crustaceans. There they mature into what are called L3 stage larvae. When the crustacean is eaten by a squid or a small fish, they will migrate to the muscle tissue and wait there for a mammal to eat that fish. If another fish eats it, they just repeat the process, and wait in the muscles of the second fish. When a whale, seal or dolphin eats the fish, the larvae mature into adults, mate, and lay eggs to begin the cycle all over again. Because a human body is - at least to these nematodes - about the same as a seal's, the worms can infect a person who eats an infected fish or squid. The result is pain, nausea, and/or vomiting, but as we humans are dead-end hosts, the treatment is usually just to relieve those symptoms and wait for the adult worms to die and pass. However, if individuals are sensitive to Immunoglobin E, however, ingestion of Anisakis can cause anaphylactic shock. Luckily these worms are fairly rare in the U.S., but cases do occur more frequently in Scandanavia, Japan, and western South America.

The photo is of L3 larvae in a herring.

April 25 - Cryptococcus gattii

The last parasite from "Whale Week" is Cryptococcus gattii, which both infects whales and dolphins, but also has been all over the news lately because it has been spreading across the Pacific Northwest and killing people as well. Thought to be native to the tropical regions of Australia, South America, and Africa the fungus invaded western North America (perhaps via a eucalyptus?) in the mid- to late-1990's and has worked its way as far south as Oregon now. Several whales and porpoises have been found to be infected, though how the fungal spores make their way to those marine creatures remains a mystery. Other animals such as dogs, cats, ferrets, llamas, and alpacas have also tested positive for the fungus. The recent deaths of 5 people in the Pacific Northwest have been creating a bit of a panic, but doctors and news agencies have been warning people to relax and not change their habits. Like other fungal infections, C. gattii cannot be spread from person to person.

A good paper on the recent outbreak in the U.S./Canada can be found here and if you'd rather listen than read, the CDC has a podcast here.

Photo is from this site.

April 24 - Halocercus delphini

Living in the respiratory tract of a whale presents certain challenges that are not faced by those inhabiting the respiratory tract of a terrestrial mammal. Whales and other cetaceans are well-known for their dramatic expiration when they surface to take take a breath, so if you are going to be a parasite that lives in the respiratory tract of a cetacean, you better have a way to hold on tight! Fortunately for the whale lungworms (and unfortunately for the whales), there are some parasites that can do just that. Halocercus delphini is a parasitic nematode which lives in the lung of dolphins. To ensure that it won't be dislodged and expelled when its host takes a breath, the worm plunges its anterior end into the host tissue, forming a capsule which acts an anchor that holds the worm firmly in place. Halocercus delphini is just one species of many from a family of parasitic nematodes (Pseudaliidae) that infect the respiratory, circulatory, and auditory systems of cetaceans.

Contributed by Tommy Leung.

April 22 - Anisakis nascettii

Anisakid nematodes are well-known as gastrointestinal parasites of various marine mammals. They utilise crustaceans like krill and amphipods as intermediate hosts, and when these crustaceans are eaten by fish or squid, they migrate into the muscle tissue where they await ingestion by a marine mammals. Humans can become accidental host of anisakids when they eat raw or undercooked fish or squid. While the worm cannot survive in humans, they can induce a severe allergic reaction.

There are many species of anisakid nematode and recently a new species, Anisakis nascettii, was found in an Andrews' Beak Whale (Mesoplodon bowdoini) stranded off the east coast of South Island, New Zealand. Using morphological and molecular identification, a team of researchers was able to match the worms found in the New Zealand-stranded whale to specimen found in beaked whales on the coast of South Africa and Australia. They also found that those worms actually belong to an undescribed species that has only ever been recorded as larval stages in squid. This goes to shows that while these days, the prospect of discovering a new species of large vertebrates is very unlikely, new species of parasites are being uncovered literally everyday!



Reference:
Mattiucci, S., Paoletti, M., Webb, S.C. (2009). Anisakis nascettii n. sp. (Nematoda: Anisakidae) from beaked whales of the southern hemisphere: morphological description, genetic relationships between congeners and ecological data. Systematic Parasitology, 74:199-217.

Contributed by Tommy Leung.

April 21 - Cyamus ovalis

Some of you might be familiar with the New Zealand movie "Whale Rider" - well, the parasite featured today is a real whale rider.

Cyamus ovalis belongs to a family of crustaceans call Cyamidae that specialize as ectoparasites of cetaceans. Despite being called "lice", whale lice are actually amphipods, and unlike most amphipods (such as sand hoppers or "scuds") which have bodies that are laterally flattened (narrower when viewed from top), the whale lice have dorsally flattened bodies, like a crab, better suited to the life-style of clinging onto the surface of an oceanic behemoth. Because of their lack of free-swimming stages, whale lice can only be transferred from whale to whale on contact. As a result, they have a very close co-evolutionary relationship with their host, and different species of cetaceans have different species of whale lice

Whale lice have been used to track the population genetic structure of their whale hosts. In the case of Cyamus ovalis, their hosts are the right whales Eubalaena spp. Swarms of C. ovalis cover the raised pieces of roughened skin (call callosities) on the head of the whale. Studies into their population genetic structure have revealed that, just like their whale hosts, the Northern and Southern Hemisphere populations of lice have been isolated from each other for several million years.

For more information, on the coevolution of whale lice with their host, see this link . The photograph for Cyamus ovalis was taken from the same webpage as the one above - Photo Credit: Vicky Rowntree, University of Utah.

There is also a trailer on YouTube for an upcoming documentary about researching whale lice. If this doesn't make being a parasitologist look like the most exciting career out there, what will?

Thanks to Tommy Leung for all of this.

April 20 - Nasitrema globicephalae

Helpless whales and dolphins stranded on a beach are always a dismaying sight for any animal lover, and the causes of stranding can often be varied and mysterious, but who would have thought a little worm, hidden from view could be a contributing factor?

Nasitrema globicephalae and other species of that genus are trematodes that inhabit the heads and air sinuses of small cetaceans such as dolphins and pilot whales. It is unclear how dolphins become infected by these endoparasitic flukes, though seeing how it is a trematode, it is quite likely the host become infected through eating prey items which contain the larval stages. Nasitrema is definitely not a very well-behaved parasite because once it is inside the host, it tends to roam around a lot, ending up in all kinds of organs it is not supposed to and causing terrible damage in its path. Sometimes Nasitrema ends up in the brain tissue causing massive necrotic lesion and inflammation that can lead to secondary infections.

It is unknown why Nasitrema would migrate to the brain, and even though the worm can develop to full maturity and even produce eggs within the brain (see picture) this does not benefit the parasite in any way as the eggs have no way of leaving the host through the brain tissue. A study found that high percentage of the cetaceans stranded along the Southern Californian coastline were found to harbour massive infestation of Nasitrema, with mature, gravid (egg-bearing) worms in the brain tissue. Because of the injuries this parasite can cause to its host, it has been suggested as a contributing factor to the stranding of small cetaceans.

It must be noted that Nasitrema is not responsible for all or even most strandings. While they are frequently found associated with stranded dolphins and porpoises, the actual role they play in contributing to that outcome is still uncertain within the context of other factors. And fortunately, at least for dolphins in captivity, it has been found that the same anthelminthic drugs used for treating human lung fluke infection can also be use to treat bottlenose dolphins with Nasitrema infection.

References:

Dailey, M.D. and Walker, W. A. (1978). Parasitism as a factor (?) in single strandings of southern California cetaceans. Journal of Parasitology, 64: 593-596.

O'Shea, T.J., Homer, B.L., Greiner, E.C. and Layton, AW (1991). Nasitrema sp.-associated encephalitis in a striped dolphin (Stenella coeruleoalba) stranded in the Gulf of Mexico. Journal of Wildlife Diseases, 27: 706-709.

Contributed by Tommy Leung.
Photo by William Walker, from "Diseases of Marine Animals" Volume 4 edited by Otto Kinne.

April 19 - Placentanema gigantissma

The parasite featured today is the longest known parasitic nematode, appropriately, its host is also one of the largest known living animal - the sperm whale (Physeter macrocephalus). The name of this parasite is Placentanema gigantissma, and it is indeed a gigantic worm. The female worm can reach up to 8.4 m long, while the smaller male reach "only" 3.75 m in length.

Its scientific name also indicates its peculiar microhabitat - this nematode has only ever been found in the uterus and placenta of female sperm whales. Even though they are relatively common, very little is known about this species. How this parasite transmit from host to host is currently unknown, though it is likely that this is facilitated by the expulsion of the placenta (with the female worm within) at birth, and eggs are released as the female worm decomposes. It has been suggested that larval worms infect the female whale prior to sexual maturity and remain dormant until the whale becomes pregnant.

The photo is of the host, in this case a young sperm whale.
For further details, see p. 839 of Diseases of Marine Animals Vol 4 Part 2 (free to download from here)

Contributed by Tommy Leung.