Sulcascaris sulcata

Shellfish such as oysters, mussels, and whelks are popular fares among seafood lovers, but we are not the only ones with a taste for those molluscs. Despite being heavily-armoured, many of the animals that we consider as "shellfish" are also food for a variety of larger marine animals. But their status as prey to these larger animals also make them attractive intermediate hosts for a wide range of parasites, which use these shellfish as vehicles to reach their final hosts. And sometimes humans end up being the unintended destination.

Anisakidae is a family of nematode worms commonly found in some seafood, and it is responsible for anisakiasis - a type of seafood-borne illness. While their usual hosts are mainly marine mammals, when anisakid nematodes get in humans, they nevertheless try to burrow through the stomach or intestinal wall, causing a great deal of pain. Additionally, their tissue and protein secretions may also cause a severe allergic reaction, including acute onset anaphylaxis.

Most studies on anisakids and anisakiasis focus on the genera Anisakis and Pseudoterranova which are often found in fish. But there are many other lesser-known genera and species in the Anisakidae family. Sulcascaris sulcata is one such species and unlike other anisakid nematodes which use marine mammals or birds as their final hosts, Sulcascaris infects a marine reptile - specifically the loggerhead sea turtle - as its final host.

Left: Photo of a Purple-dye Murex by Holger Krisp, used under the Creative Commons (CC BY 3.0) license
Right: (top) SEM close-up photo of Sulcascaris larva's head, (bottom) a fourth-stage Sulcascaris larva  
(Photo of the nematode from Fig. 2 and Fig 4. of the paper)

Larvae of Sulcascaris have recently been reported from scallops and mussels - which raises some concerns since both are popular shellfish that are often eaten only lightly cooked or not at all. A recently published study adds another shellfish to that list - the purple dye murex, Bolinus brandaris. These large predatory snails are so-called because they used to be harvested to obtain a special type of purple dye. But in addition to their historic use in the textile industry, they are also commonly eaten in many parts of the Mediterranean.

A group of researchers in Italy obtained a haul of purple murex from fishermen on the coast of Baia Domizia, Italy, and brought the snails back to their laboratory to dissect them for parasites. Upon detailed examinations of the snails' organs, they found that 9 out of the 56 snails they obtained were infected with Sulcascaris larvae. However, infection intensity was very low, with most of the infected snails being parasitised by just a single nematode larva. These larval worms measured between one to five centimetres long, and were mostly lodged at the base of the snail's proboscis, with a few others found in the mantle cavity - the fleshy bag in a mollusc's body which houses its gills and other organs. 

Because of where those parasites are located in the snails, they can easily get overlooked during routine sanitary inspections, which only involve examining the outer appearance of the snail. The reason why those worms were mostly situated in those parts of the snail's anatomy might be due to their infection pathway. When the eggs of Sulcascaris are released from the turtle host, they settle onto the seafloor where they hatch into larval stages that lie in wait for an encounter with an unlucky murex. As the predatory sea snail moves across the sea floor, searching for prey with its proboscis, those larvae are sucked in via the inhalant current which transport them right into the snail's proboscis and mantle cavity.

Sea turtles with their strong beak and jaws can crack into these tasty snails which are off-limits to other animals, but it also means they end up with Sulcascaris in their gut. While this and previous studies on Sulcasacris have found that most shellfish carried only one or two individual nematodes, a turtle can eat a lot of shellfish, and over time may end up accumulating dozens or even hundreds of those worms in their stomach. When present in large numbers, these nematodes may cause ulcerous gastritis in sea turtles. But aside from that, not as much is known about this worm compared with its more famous, mammal-dwelling relatives, such as Anisakis.

So what does this mean for people who love eating shellfish? Based on prior experiments, it seems that Sulcasacaris can only infect sea turtles, so it is unlikely to become a zoonotic infection if it ends up being ingested by humans. Also, as mentioned above, when they are present, it's only one or two worms in each shellfish, and since purple murex are usually eaten after being cooked, this would kill the worm in the process. So the health risks presented by Sulcasacaris to any seafood consumers are relatively minimal.

However, like other anisakid worms, their tissue and secreted proteins may still potentially cause allergic reactions in some people, even after cooking. But not much is known about that possibility. The researchers suggested that at the very least, commercial fishermen should avoid harvesting snails from areas with sea turtles, since they are likely to be infected with Sulcascaris. This could be a win-win situation for both turtles and people - the turtles get to keep their feeding grounds to themselves, and seafood lovers can safely enjoy some worm-free sea snails. 

As the consumption of fish and other seafood increases around the world, there is a greater need for more studies on the wide variety of parasites that are found in seafood, along with people who have the skills and expertise to identify them - so we can continue to enjoy seafood without unintentionally barging into the life cycle of a parasite (and suffer its associating consequences).

Reference:

Santoro, M., Palomba, M., & Modica, M. V. (2022). Larvae of Sulcascaris sulcata (Nematoda: Anisakidae), a parasite of sea turtles, infect the edible purple dye murex Bolinus brandaris in the Tyrrhenian Sea. Food Control 132: 108547.

Anoplotaenia dasyuri

Tasmanian Devil is a cute marsupial that packs a mean bite. This charismatic carnivore is found throughout the island of Tasmania and is the largest living carnivorous marsupial. However, it is also currently under threat from the Devil Facial Tumour Disease (DFTD) - which is caused by a peculiar lineage of cancer cells that have evolved to be infectious, able to transmit from host to host, and reproduce itself in each new host along the way. Genetically speaking, this transmissible cancer is essentially a very weird Tassie devil mutant that has evolved to be a single-celled, asexually reproducing, highly virulent pathogen that specifically targets Tassie devils.

But this blog post isn't about the DFTD, instead it is about a unique tapeworm that has been living quietly in the Tassie devil's gut. Unlike the transmissible cancer which is a recently evolved mammalian cell line that is highly lethal and cause grotesquely visible pathology at later stages of infection, this tapeworm has coevolved and cohabited with the Tassie devil for a very long time, and despite its abundance, it is rather innocuous to the host, and is completely hidden from plain sight.

Left: Tassie devil photo by Mathias Appel in Public Domain,
Right: Photos of the Anoplotaenia dasyuri tapeworm provided by and used with permission from Dr Diane Barton

Anoplotaenia dasyuri is one of six species of tapeworms which have been reported from the Tassie devil, two of which are native to Australia, and A. dasyuri is one of them. The other one is Dasyurotaenia robusta - a rare tapeworm which has the distinction of being one of the only parasites listed as a protected species. In contrast to D. robusta, A. dasyuri is a rather common tapeworm, often found in the Tassie devil in huge numbers. In addition to the Tassie devil, the adult stage of this tapeworm is also occasionally found in the spotted quoll, and with the introduction of cats and dogs to  Australia, A. dasyuri has adopted them as hosts as well. However, it seems the Tassie devil is still the tapeworm's preferred host, as they are only ever present in low numbers in those other host species, and A. dasyuri that grew up in dogs were found to be underdeveloped and emaciated. Only in the Tassie devil can these tapeworms thrive and flourish to their full potential.

Like other tapeworms, A. dasyuri needs to infect different host animals to complete its life cycle, and the larval stage are usually found in various macropodid marsupials including wallabies and pademelon, where it resides mostly in the heart muscles. On one occasion, there was a wallaby that was found to have 85 tapeworm larvae in its heart. These animals act as ideal intermediate hosts for the tapeworm's larval stages, as pademleon and other medium-sized macropods are commonly eaten by the Tassie devils. Additionally, old museum specimens indicates that larvae this tapeworm might have even infected the muscles of the extinct thylacine, though it is unclear what role (if any) the thylacine played in the life cycle of this tapeworm. But they were never found to host any adult A. dasyuri worms, indicating the tapeworm treated the thylacine as a stopover on its journey to the Tassie devil.

In this study we're featuring today, researchers from Charles Sturt University examined the innards of Tasmanian Devil carcasses which have been collected over the last ten years and stored in museums. They were all from roadkills which had been donated to museums for scientific studies. From those frozen carcasses, the researchers were able to retrieve jars worth of tapeworms. In total, they were able to pull out 8100 tapeworms from just six infected Tassie devils, which means on average each host was home to about a thousand tapeworms, though the actual numbers in each individual host varied from just two worms to over 4000 worms.

And these researchers had to count and examine each worm individually - that's right, all 8100 of them. They did so in order to check if there were any D. robusta in the mix. Anoplotaenia dasyuri and Dasyurotaenia robusta look very similar to each other, and the key difference between them is the size and shape of the suckers on their respective scolices (attachment organ) - which can only be distinguished under a microscope. So in order to have an accurate count of the tapeworms' numbers and abundance they had to make sure that they were counting the right species.

Perhaps somewhat surprisingly, considering how numerous they can get in the Tassie devils, prior studies reported that these tapeworms cause their hosts very little or no pathologies, even when they occur in massive numbers - which they often do in the thousands. Previous studies have found that even host animals that harboured over fifteen thousand worms seemed remarkably healthy. But then again, unbeknownst to most people, many wild animals are getting through life just fine with an entire colony of parasites inside of them.

Aside from simply recording the number of tapeworms in those Tassie devils, the researchers also used this opportunity to figure out the evolutionary origin of this unique tapeworm. They sequenced sections of the tapeworm's DNA, and compared them with those of other tapeworms in the Cyclophyllidea order. Based on the tapeworm DNA sequences which are available, the closest living relatives of A. dasyuri are tapeworms in the Paruterinidae family, in particular a species from the Cladotaenia genus which was found in a steppe eagle from China.

That doesn't necessarily mean the ancestor of A. dasyuri is from East Asia - very little is known about tapeworms from Australian birds of prey, and there aren't many specimens of tapeworms from Australian raptors available to provide a source of DNA or morphological comparisons. After all, the phylogenetic analysis could only be run against other DNA sequences which are available on Genbank - the global genetic sequence database.

So it is quite likely the actual closest relatives of A. dasyuri are found among Australian raptors. It is worth noting that the diet of a large Australian raptor - the wedge-tail eagle - is rather similar to that of the Tassie devil. So it is possible that at some point in the distant past, the Tasmanian Devil picked up the ancestor of A. dasyuri from sharing a meal with those birds of prey. Those tapeworm larvae might have been waiting to catch a flight in the gut of an eagle, but they ended up finding an equally hospitable home in the gut of the Tassie devil.

Jumping into taxonomically disparate hosts seems to be a common way for parasites like tapeworms to evolve, for example, another tapeworm featured earlier this year on the blog seem to have made a jump from birds to electric fishes, and a few years ago, I wrote a post about a thorny-headed worm which jumped host from sea lions to penguins.

Anoplotaenia dasyuri is not alone in having an interesting evolutionary history - in fact the Tasmanian Devil appears to be home to a peculiar suite of parasites, each as unique as the host itself. Aside from the very abundant A. dasyuri and the very rare D. robusta tapeworms, the Tassie devil is also host to some unusual roundworms, such as a species of pinworm - a family of nematodes which are usually found in herbivorous or omnivore animals with hindgut fermentation, and a species of Baylisascaris - a genus of roundworm which is usually associated with placental carnivore mammals such as bears, raccoons, and mustelids.

So protecting the Tassie devil isn't just about protecting a lone species of marsupial, it is an evolutionary treasure trove that is home to a menagerie of evolutionary unique weirdos and misfits, hailing from a continent known for its unique fauna. Saving the Tassie devil also means saving its posse of worms, each of them representing a disparate legacy of evolution.

Reference:

Barton, D. P., Zhu, X., Lee, V., & Shamsi, S. (2021). The taxonomic position of Anoplotaenia dasyuri (Cestoda) as inferred from molecular sequences. Parasitology 148: 1697-1705. 

Fasciola nyanzae

Lake Kariba is the largest artificial lake in the world - Initially created in the late-1950s and early-1960s, it has since become inhabited by a wide variety of both endemic and introduced species. One of those introduced species is the water hyacinth (Eichhornia crassipes) which have made themselves quite at home in this giant artificial lake. These floating plants can proliferate at an alarming rate, clogging up the shorelines and sucking up vast quantity of water, nutrient and oxygen. They in turn are prime real estate for a variety of aquatic snails, and serves as breeding ground for many native and introduced snail species in the region.

Left: (a) Liver fluke Fasciola nyanzae, and (b and c) stomach flukes from hippos, from Fig. 1 of the paper.
Right: Photo of hippopotamus in Maasai Mara by Markrosenrosen, used under Creative Commons (CC BY-SA 4.0) license

So what does that have to do with parasites? For those who have been reading this blog for a while or know something about fluke biology would know the vital role snails play in the life cycles of digenean flukes - specifically the asexual part of their life cycles. Snails can get commandeered by flukes to act as parasite factories that churn out a stream of free-swimming clonal fluke larvae, and an infected snail can end up with 14-39% of their body mass being converted into parasite tissue. This means any place that is hosting an aquatic snail party would inadvertently become a fluke party too.

This study looked at how Lake Kariba has affected the transmission and infection of flukes in hippopotamus. Like any other large animals, hippos are host to a wide variety of parasites, but because they spend so much of their lives in water, this makes them an especially attractive host to a wide range of different flukes. In this study, researchers collected a variety of different aquatic snails from the northeastern shoreline of Lake Kariba, and examined them for fluke infections. They ended up finding six different species of snails that harboured hippo-infecting flukes.

Additionally, they were able to collect flukes directly from a male hippo which was culled near Kariba Town as part of the local wildlife management program. That hippo turned out to be home to a trove of flukes, ranging from long-bodied liver flukes dwelling in the bile ducts, to several hundred bright red stomach flukes that crowded the hippo's stomach. Using some good old-fashion morphological comparisons combined with DNA sequencing, they were able to identify those flukes and match them with the asexual stages which were found in some of the snails they collected.

The liver flukes, which were particularly large for flukes measuring at about 5 centimetres long, were identified as Fasciola nyanzae, a species known to occur in the region and are commonly recorded from hippos. But what's significant here is that whereas the usual snail host for F. nyanzae is Radix natalensis, a snail species which is native to the region, at Lake Kariba this liver fluke has also recruit two additional introduced snail species to serve as parasite factories to do its asexual reproductive biddings. This includes Radix plicatula from Asia, and Pseudosuccinea columella from North America. In fact, the invasive P. columella snail seems to be an even more receptive host to the hippo liver fluke than the native snails. What's more, F. nyanzae is not the only fluke that has taken a liking to the introduced snails. It seems that those bright red hippo stomach flukes also readily use both native and introduced snails for their asexual reproduction.

In addition, by examining the aquatic snails, the researchers were able to detect other flukes species which they didn't record from that male hippo they dissected, but are likely to be circulating in the local hippopotamus population. One of which was a species of hippo blood flukes (Schistosoma edwardiensis). Unlike the liver fluke or the stomach fluke, this blood fluke has only been found in native snails so far, but two of those snails were from the Bulinus genus, which has never recorded as hosts for S. edwardiensis before. This indicates that the fluke may be more prevalent and flexible about what snails it uses as hosts than previously thought.

While the human-infecting species of blood flukes such as Schistosoma mansoni and Schistosoma haematobium are extensively studied due to their public health significance, very little is known about blood flukes which infect wildlife, and information on species such as the blood flukes of hippos, including their ecology and life cycles, are very limited.

The conditions at Lake Kariba have created a haven for snails, which in turn makes it a hotspot for fluke infections, and the presence of introduced snail species exacerbates that. Because on top of contending with the native snails pumping out fluke larvae as they usually have, the hippos now have to deal with the flukes coming from the introduced snails as well. The introduced snails increase the overall parasite load in the environment - this phenomenon is known in disease ecology as "spillback", where an exotic host organism that was introduced to a new area might have started out largely free of its original parasites, but over time, they pick up some of the local parasite species and turn out to be even better hosts for them than their original hosts, thus amplifying the amount of parasite propagules in the environment.

Data and records on how environmental changes affect the epidemiology of wildlife diseases is severely lacking. And in order to obtain such information, it would require collaborations of researchers from many different fields including parasitology, veterinary science, ecology, and conservation biology. This study is but one small piece in a much larger story of how human activities impact the spread and transmission of infectious diseases on this planet.

Reference:

Schols, R., Carolus, H., Hammoud, C., Muzarabani, K. C., Barson, M., & Huyse, T. (2021). Invasive snails, parasite spillback, and potential parasite spillover drive parasitic diseases of Hippopotamus amphibius in artificial lakes of Zimbabwe. BMC Biology 19: 1-21.

Caledoniella montrouzieri

Mantis shrimps (Stomatopoda) are some of the most formidable crustaceans in the sea; armed with trinocular colour vision, and a pair of powerful raptorial limbs that punch so hard, it generates supercavitating bubbles which collapse with such energy, the vapour within them briefly turns into white-hot plasma. Arguably, one of the most impressive animals in the sea. But to Caledoniella montrouzieri - a mantis shrimp is simply a big juicy host. 

Underside of a parasitised mantis shrimp, showing the male, female, and egg capsules of Caledoniella montrouzieri. 
Photo from Fig. 1 of the paper, taken by Ryutaro Goto.

Most parasitic snails belong to the Eulimidae or Pyramidellidae family, and both of them parasitise slow-moving or sedentary invertebrates such as echinoderms, molluscs, and polychaete worms. But Caledoniella has taken a different, independent route down to parasitism town. It doesn't belong to either of those families, and instead of a slow life feeding on some barely mobile hosts, it lives life in the fast lane, clinging to the belly of a nimble, predatory crustacean.

Such a lifestyle requires some specialised anatomy. In most snails, the foot is a flat muscular organ that is used for crawling over various surfaces. But in Caledoniella, the foot has been transformed into a big suction disc that allows it to cling firmly onto its very agile host. During the course of its evolution, it has also lost one of the key diagnostic characteristics of molluscs - the rasp-like radula in the mouth which snails use to scrape bits of food, be they algae or the flesh of other animals, into their mouth. Instead, it has a mouth that is more suited for suction feeding, and has highly developed salivary glands to facilitate its liquid diet. This snail is a vampire of mantis shrimp, sucking on their host's gill filament for that sweet, sweet hemolymph.

Caledoniella has some noticeable sexual dimorphism with the female snail being much larger than the males. When these snails mature, they pair up as a monogamous couple, living out their lives together on the underside of a mantis shrimp. But this happy couple likes to keep themselves to seperate parts of the mantis shrimp, with the female living near the tail of the shrimp, and the male living near the middle of the abdomen. Sitting between them are all the egg capsules they have been busily making together. This gastropod couple takes their toll on the mantis shrimp, which experience stunted growth, reduced moulting, and infertility.

So how did Caledoniella ended up with its unique way of life? The closest living relatives of Caledoniella are snails that live as roommates with mantis shrimps, hanging on the walls of the crustacean's burrow. While these snails are frequently in the presence of the burrow's main tenant, that's as far as their relationship with the mantis shrimp goes. They are strictly commensals that never lay their foot on the mantis shrimp, and it is likely that was the lifestyle of Caledoniella's ancestors. But at some stage, after living in such close quarters with mantis shrimps for so long, some of those meek wallflower snails just couldn't resist getting more intimate and started taking a bite of its crustacean roomie, thus giving rise to the clingy blood-sucking Caledoniella. 

But when you trace its evolutionary history even further back, it seems those mantis shrimp roommates have themselves evolved from snails that originally lived in the burrows of an entirely different animal - spoon worms! So the ancestors of Caledoniella switched from sharing quarters with spoon worms, to living with mantis shrimp, to living on mantis shrimps

Perhaps somewhat surprisingly, Caledoniella is not the only mollusc that spend their lives clinging to the mantis shrimp - there are also a few species of tiny clams from the Galeommatoidea family called yoyo clams that live attached to the mantis shrimp's belly.  But unlike Caledoniella, they don't go as far as to feed on their host's blood. These clams receive protection from living on the belly of this heavily-armed crustacean, and the host's agile movements provide it with plenty of water flow for all their respiratory and filter-feeding needs. While they aren't blood-suckers, they seem to have followed the same evolutionary pathway as Caledoniella, evolving from ancestors that originally lived as commensals in the burrows of mantis shrimps.

For molluscs, it seems that sharing room with a marine benthic invertebrate is a surefire gateway to becoming a clingy parasite.

Reference:

Goto, R., Takano, T., Eernisse, D. J., Kato, M., & Kano, Y. (2021). Snails riding mantis shrimps: Ectoparasites evolved from ancestors living as commensals on the host’s burrow wall. Molecular Phylogenetics and Evolution, 163:107122.

Unikaryon panopei

Like any living things, parasites can themselves become host to other infectious agents as well, and parasites that specialise in parasitising other parasites are called hyperparasites.  The paper we will be looking at today is about some microsporidian parasites that have evolved to parasitise flukes. Microsporidian are a group of single-celled parasites which are somewhat related to fungi, and they infect a wide range of invertebrate animals - including many parasitic animals.

Left: Fluke metacercaria infected with Unikaryon panopei surrounded with smaller, uninfected flukes
Right: Swollen fluke cell, filled with spores. Photos from Fig. 1. of the paper.

The species featured in this post - Unikaryon panopei - infects flukes which parasitise crabs. More specifically, these flukes were found in black-clawed mud crabs from Tampa Bay, Florida. The researchers who conducted this study collected a relatively small number of crabs - fifteen in total - but that was more than enough to find some which were infected with flukes, because all of them were absolutely loaded, with some crabs harbouring up to 250 fluke larvae. 

The fluke's free-swimming larval stages are able to get through the crab's tough exterior with a microscopic, scalpel-like structure called a stylet, which they use to slice their way through the vulnerable parts of the crab's cuticle, such as the leg joints and gill filaments. Once inside, they crawl to the hepatopancreas (also known as the digestive glands), where they curl up and transform into spherical cysts called metacercariae, and wait for the crab to be eaten by a bird. The flukes essentially use the crab as a temporary stopover and transport to get a ride into shorebirds.

At least that was the plan - until Unikaryon came along to completely ruin their lives, and some unlucky flukes found themselves becoming incubators for microsporidian hyperparasites. Fluke larvae which are heavily infected with Unikaryon swells to twice their usual size, and become filled with spores which are packaged in brown ovoid throughout the fluke's body. While in moderately infected flukes, the spores are mostly concentrated in the intestine and the still developing reproductive organs, in heavily-infected flukes, the hyperparasite replaces all of the fluke's internal tissue and organs, turning it a spore-filled husk.

When the researchers examined the evolutionary lineage of U. panopei in relation to other microsporidian parasites, they found that these hyperparasites might have evolved from microsporidians that originally parasitised crustaceans. For whatever reason, over time, they switched to targeting the parasites of said crustaceans instead. In addition to U. panopei, a handful of other Unikaryon species have also been reported from various species of flukes, and even one species from fish tapeworms. 

In addition to infecting the metacercariae cyst stages as found in this study, Unikaryon has also been found infecting other life stages of flukes, including the asexual stages in snails, and the free-swiming stages which are produced by infected snails. Yet despite being present in those other life stages, Unikaryon has never been found to infect adult flukes.

Given how Unikaryon has been able to insinuate itself into different parts of the fluke life cycle, while remaining strangely absent in the adult stage, this raises the question of how the flukes even get infected with these hyperparasites in the first place. Do they pick it up from the environment? If so, how - given the fluke stages they infect are situated deep in their host's bodies? How do they get released into the surrounding environment, and how are they transmitted to new hosts? Or is the hyperparasite inherited at birth, and just gets passed down each subsequent generation? If so, how could that be possible since it is absent from the adult stage of the fluke's life cycle? 

There are so many questions relating to some of the most basic aspects of this hyperparasite's ecology. Since most groups of parasites are severely under-studied, it is not surprising that we know even less about some parasites' own hyperparasites. These microsporidians are single-celled mysteries, packed in the bodies of animals, which themselves dwell in the armoured bodies of unassuming crustaceans.

Reference:

Sokolova, Y. Y., Overstreet, R. M., Heard, R. W., & Isakova, N. P. (2021). Two new species of Unikaryon (Microsporidia) hyperparasitic in microphallid metacercariae (Digenea) from Florida intertidal crabs. Journal of Invertebrate Pathology, 182, 107582.

Heterobilharzia americana

Heterobilharzia americana is a species of blood fluke which is native to North America. It is mainly a parasite of raccoons, but is also capable of infecting a wide range of other mammals. Its broad taste for different hosts brings them into contact with various domesticated animals, in particular, dogs. Indeed, it is more commonly known as the canine schistosome.

Left: Cercaria of H. americana, Right: Adults (left = male+female pair, right = single female fluke) 
Photos from Fig. 2 and Fig. 4 of the paper

When the aquatic larvae of these flukes come into contact with humans, much like those of bird schistosomes,  they get intercepted and killed by the immune system as they burrow into the skin, and the death throes of these larval parasites manifest themselves as an itchy rash. But in dogs, not only is the parasite able to establish itself and grow to sexual maturity, it also causes far more severe symptoms than merely an itchy rash.

Dogs infected with H. americana exhibit a host of serious pathologies including vomiting, bloody diarrhoea, weight loss, and lethargy. Additionally, the vast number of microscopic eggs produced by the adult flukes are transported via the circulatory system to various internal organs where they can cause inflammation, and form pockets of mineralization. While the parasite can be treated with prazinquantel and fenbendazole, their presence often go under-diagnosed as the disease they manifest are rather nonspecific, and the existence of this parasite is not as commonly known. 

Some of those eggs that are carried in the circulatory system eventually make their way to the outside world via the host's faeces. If an egg reaches a water body such as a pond, it hatches into a ciliated larva which seeks out a suitable snail host. Snails play an important role in the life cycle of digenean flukes like H. americana, for this is where asexual reproduction takes place. Through commandeering much of the snail's internal organs, the parasite raises an entire clone army of free-swimming larval stages called cercariae. A single infected snail can produce and release hundreds of infective cercariae into the surrounding waters on a regular basis.

The usual snail host for H. americana is Galba cubensis, a pond snail mostly found in the warmer parts of the Americas including Mexico, South America, and south-eastern parts of the United States. But a new study indicates that it has recruited a new snail host for the asexual stage of its life cycle, one which would allow it to spread further across North America. 

This study was based on a two-year long investigation into a small outbreak of H. americana in east-central Moab, Utah, where two severely ill dogs were euthanised after exhibiting symptoms associated with canine schistosomiasis. A necropsy revealed many of their internal organs were riddled with inflammation and mineralization caused by the presence of blood fluke eggs. Examination of faecal samples from other dogs in the neighbourhood found that some of them also contained the parasite's eggs.

Given the life cycle of H. americana, the researchers determined that the most likely source of the infection was a nearby irrigation pond which was regularly visited by dogs living in the neighbourhood, including the two deceased dogs. The pond was filled with many different species of aquatic snails, but there was just one species that was shedding schistosome cercariae - an amphibious snail called Galba humilis, which lived along the banks and waterline of the pond.

Galba humilis is widely known to serve as a host for liver flukes (Fasciola hepatica),  but this is the first time it has been recorded to host the canine schistosome as well. While these tiny snails are barely a centimetre in length, each can produce thousands of infective cercariae over its lifetime. The researchers found that on average, an infected snail can release about 800 cercariae during each shedding period. Furthermore, to increase their chances of encountering a host, they mostly come out of the snail at night between 6:00 pm and 7:30 pm, which overlaps with the active period of their main mammalian hosts - raccoons. 

Based on the faecal samples the researchers found in the area, the animals which introduced H. americana to the snails in that pond in Moab were most likely to be the aforementioned raccoons. And they were able to confirm this via experimental infection of snails from a captive-raised colony.

The most worrying implications from this study is that by acquiring a snail like G. humilis as a host, H. americana would be able to spread to more temperate regions. Galba humilis is widely found across the United States and is a common snail in human-built habitats like irrigation dams and ponds. These habitats also attract various animals like raccoons and other mammals which are viable hosts for this parasite. Thus these water bodies can bring together everything H. americana needs to complete its life cycle and reproduce.

With landscape changes due to agriculture, urbanisation and climate change, raccoons have become more abundant and are expanding their range across North America. In addition to raccoons, dogs are also common in urban areas and can serve as key reservoirs and means of dispersal for the parasite. Whenever they visit a pond with snails, infected dogs can introduce and establish a new H. americana hotspot for the local mammal population.

All these factors contribute to further the spread of this parasite across North America, and possibly elsewhere too. Raccoons have become a prolific invasive species in many parts of the world, and since H. americana has already switched its snail host once, it may do it again to whatever amphibious or aquatic lymnaeid snail it encounters. For the canine schistosome and other parasites, what played out at that pond in Moab is a sign of things to come in our changing world.

Reference:

Loker, E. S., et al. (2021). An outbreak of canine schistosomiasis in Utah: Acquisition of a new snail host (Galba humilis) by Heterobilharzia americana, a pathogenic parasite on the move. One Health, 100280.

Echinophthirius horridus

Lice are common parasites of mammals. Humans alone are host to three different species of lice, and it's not just humans or land mammals that can get infected with lice. Pinnipeds - seals and sea lions - also have to contend with being covered in those ectoparasites. Unlike many other ectoparasites in the sea which have been bestowed with the name of "lice" such as salmon lice, tongue-biter lice, or whale lice (all of which are crustaceans), seal lice are true lice, in that they are parasitic insects belonging to the order called Phthiraptera.

Left: An adult seal louse, Right: two opened seal lice eggs (nits) glued to a strand of seal fur hair
From Fig. 1 of the paper

When the ancestors of modern pinnipeds took to the sea some time in the Oligocene about 30 million years ago, the lice followed them into the water, and in the process, they have to deal with all the challenges associated with living on an aquatic host. Seal lice belong to a family of lice called the Echinophthiridae and they have some specialised adaptations for living on hosts that spend most of their time immersed in sea water. This include elongated spiracles (the opening insects use to breathe) with mechanism for closing, a dense covering of spines and scales, and stout clamp-like claws that allow them to grip tightly onto their hosts' fur.

Blood-sucking arthropods such as ticks, fleas, and lice are often responsible for transmitting a wide variety of parasites and pathogens. And it seems that seal lice can also play a similar role in the sea. While performing routine diagnostics on 54 harbour seals and a very heavily-infected grey seal pup that were hospitalised at the Sealcentre Piteterburn (a seal rehabilitation and research centre in Netherlands), a group of scientists were able to use that opportunity to collect a massive number of seal lice from those marine mammals. They ended up collecting 200 lice from the harbour seals, and another 1000 from that one very heavily infested seal pup.

Those researchers divided the lice into batches of 1-20 lice, based on the individual host that they came from (the lice from the heavily infected seal pups were divided into multiple batches of 15 lice), then grind them up, and examined the lice slurry by subjecting it to polymerase chain reactions that amplified the DNA of known seal parasites and pathogens.

The DNA analyses showed that the seal heartworm (Acanthocheilonema spirocauda) was the most commonly found parasite, with it being detected in about one-third of the lice samples. While most people would associate "heartworm" with the dog heartworm (Dirofilaria immitis), that species is just one out of many different filarial roundworms that live in the heart of mammals. The findings of this study corroborates with previously published research which have found heartworm larvae dwelling in the gut of seal lice, demonstrating that these ectoparasitic insects play a key role in the transmission and life cycle of these nematodes.

Alongside the heartworm, there were also some bacterial pathogens lurking in those lice. Some of the lice from the grey seal pup were also carrying Anaplasma phagocytophilum, the bacteria which causes tick-borne fever and as their name indicates, are usually carried by ticks. Additionally, a few of the lice from that seal pup and some of the harbour seals were also carrying a species of Mycoplasma bacteria. This microbe is commonly found in seals and other marine mammals, but when it gets transmitted to humans, it is also associated with a disease known as "seal fingers". However, unlike the heartworm, it is unclear if the lice actually play a role in the transmission of these bacterial pathogens, or if they were incidental infections that simply came with living on a seal host.

It is worth noting that while pinnipeds had retained an heirloom of their terrestrial ancestry in the form of lice, another group of marine mammals - the whales - have acquired their own unique suite of ectoparasites which are unlike that of any other mammals. They have "whale lice" which are actually crustaceans in the same group as sandhoppers, along with pennellid copepods - a family of parasitic copepods that usually infect fish, with the exception of one species which has evolved to parasitise whales.

So why are there no "true lice" on whales? Well, for all their adeptness at clinging to their host, lice ultimately depend on the presence of hair or similar structures to hang onto their host. When a seal dives underwater, the layer of fur forms a covering that the lice can shelter underneath. But no such shelter exists on the smooth, hair-free surface of a whale. As a result, while whales have escaped the lice (and have picked up other parasites in the process), pinnipeds have kept their fur, and along with it, their lice and the worms that they carry.

Reference:

Hirzmann, J., Ebmer, D., Sánchez-Contreras, G. J., Rubio-García, A., Magdowski, G., Gärtner, U., Taubert, A & Hermosilla, C. (2021). The seal louse (Echinophthirius horridus) in the Dutch Wadden Sea: investigation of vector-borne pathogens. Parasites & Vectors 14: 96.

Allokepon hendersoni

Crabs have some pretty scary parasites infecting them. They range from worms that use them as vehicles to complete their complex life cycles, to parasitic dinoflagellates that turn their muscles into bitter slurry, and on top of those, there are also other crustaceans that can take over their body, and in some cases, castrate them in the process. These body-snatching crustaceans come in two main types - bopyrids and rhizocephalans. 

Top: Bopyrid isopod and an infected crab, Bottom: Rhizocephalan barnacle with infected crab.
Photos modified from the graphical abstract of the paper

Bopyrids are parasitic isopods in the same suborder as the infamous tongue biter parasite, but instead of going into a fish's mouth, they go inside the body of crabs and make themselves at home, often causing a characteristic bulge on the infected crab's carapace. And then, you have the rhizocephalans, which are freaky barnacles that have a body composed of a network of roots which wrap themselves around the crab's internal organs.

Each of them inflict their own respective flavour of pain on their crab hosts.

This study looked at the effects that these parasitic crustaceans have on the two-spotted swimming crab (Charybdis bimaculata), which is host to both parasitic isopods and barnacles. Here representing the bopyrid isopods, we have a species named Allokepon hendersoni. And fronting for the barnacles, is an as yet undescribed species of rhizocephalan. As hinted at earlier, these two body-snatcher parasites seem to have different effects on the crabs - but what exactly are they?

When scientists compared infected crabs with uninfected crabs, they found the effects to be most pronounced in male crabs, with both species of parasites causing a reduction in weight and claw size of their hosts. This is most likely due to the energetic drain associated with hosting these crustaceans, since they can grow to alarmingly large sizes when compared with their hosts. 

While reducing the claw size may leave the host crab less able to compete with uninfected males, on another hand (or claw as the case may be), it would not be in the interest of the parasite for its host to be getting into too many fights and risk injuries anyway, so it can be a beneficial side-effect from the parasite's perspective

But there were some changes which were more specific to particular parasite species. Male crabs infected with Allokepon had a narrower abdominal flap (the triangular flap on the "belly" of a crab). In male crabs, this flap would usually serve to protect the gonopods - which are specialised appendages that arthropods use in reproduction - but given the crab is already hosting such a demanding resident in its body, it wouldn't be getting up to any of that any time soon.

In contrast, the rhizocephalan barnacle had the opposite effect on the crab and widened that flap - this is part of a whole suite of changes that these parasites induce in male hosts. Male crabs that are infected by rhizocephalans develop characteristics which are associated with female crabs in both appearance and behaviour. In female crabs, the wider abdominal flap serves to cradle and brood the eggs before they hatch. So the barnacle essentially "feminise" the male crabs so that they can become better babysitters for the barnacle's offspring.

Fortunately for this crab population, infection rate was very low. Of the 2601 crabs the scientists examined, only 14 were infected with the isopod, and 21 infected with the rhizocephalan barnacle, though the isopod seems to have a preference for infecting male crabs, whereas the barnacle was less discriminate.

But if you are the unfortunate crab that gets infected, you are in for a bad time either way.

Reference:

Corral, J. M., Henmi, Y., & Itani, G. (2021). Differences in the parasitic effects of a bopyrid isopod and rhizocephalan barnacle on the portunid crab, Charybdis bimaculata. Parasitology International, 81: 102283.

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

Pterobdellina vernadskyi

If you've ever been out hiking in the wilderness, you would know that there is no shortage of tiny animals out there that love nothing more than to feast on your blood. They range from mosquitoes to midges, from fleas to ticks, and of course, let's not forget about leeches - a group of animals so synonymous with blood-sucking that its name is also used as a term for exploiting the life blood of others.

But leeches aren't just found out in the bushes, there are hundreds of species of blood-sucking leeches that are actually aquatic, feeding mostly on amphibians and fishes. In fact, spare a thought for the fishes, which have a whole family of jawless leeches called Piscicolidae that are after their blood.

Left (top) Antarctic toothfish with P. vernadskyi leeches on its body surface, and (bottom) in its mouth. Photos by Gennadiy Shandikov from Fig. 1 of the paper.
Right: Two live specimens of P. vernadskyi (the left leech has a spermatophore in its clitellum) from Fig. 2 of the paper
 

For fish, there is no escape from these leeches as they are found in a wide range of aquatic habitats, ranging from freshwater to the marine environment. They target a wide range of hosts, from trout and carp, to rays and sharks. Even in the inky depths of the deep sea, there are hungry leeches waiting for a tasty fish to swim by, and it is one of these deep sea leeches that is featured in today's post.

This post is about a newly described species of fish leech - Pterobdellina vernadskyi - which has been found parasitising the Antarctic toothfish (Dissostichus mawsoni) in the cold dark waters of Antarctica. The researchers who described this species collected them from fish that were caught by longline commercial fishing vessels - Antarctic toothfish are highly valued on the commercial market, where they are often sold alongside the Patagonian toothfish as "Chilean Sea Bass".

While most of the fish that the researchers encountered only had one or two leeches, some were afflicted with ten or more, and one unlucky fish was covered in twenty eight leeches all over its body. They tend to favour attaching to either the dorsal surface of the fish, or inside the mouth, where they are more sheltered. And P. vernadskyi can grow rather large compared to other fish leeches, reaching about 8 cm in length - so roughly the size of your finger.

Aside from its sheer size, another thing that differentiates it from other leeches are series of distinct, zig-zag ridges along its back and fin-like projections along its sides. It is not entirely clear what purpose those structures serve for the leech, though there are other deep sea ectoparasites which also have some unusual external structures. The researchers suggested that perhaps they serve some kind of sensory function that allows to leech to find their host, or they might be adaptations to the low oxygen levels of its environment, increasing the leech's surface area so it can absorb more oxygen from the surrounding waters.

In additional to those external features, it is worth mentioning that this leech's host, the Antarctic toothfish, is notable for producing antifreeze glycoproteins in its blood, which allows it to dwell in such frigid waters. But this additive would surely have some implications for the digestive system and physiology of P. vernadskyi compared with other fish leech that feed on hosts with more conventional blood.

Since the Antarctic toothfish can be found dwelling as deep as 2600 m below sea level, this would make P. vernadskyi the deepest Antarctican leech that has ever been recorded. However, it is NOT the deepest depths that a leech has ever ventured. That title goes to Johanssonia extrema which has been found in the hadal zone over 8700 m below sea level in the Kuril–Kamchatka Trench, where the waters are still and the pressures are crushingly immense.

Pterobdellina vernadskyi is just one out of two dozen different species of fish leeches that have been recorded from Antarctica, and there are a number of other leeches which have been reported from deep sea habitats. It would be safe to say that P. venadskyi, and other marine leeches that have been described in the scientific literature, represents only the tip of the iceberg. Where there are fish, there are leeches.

Reference:

Utevsky, А., Solod, R., & Utevsky, S. (2021). A new deep-sea fish leech of the bipolar genus Pterobdellina stat. rev.(Hirudinea: Piscicolidae) parasitic on the Antarctic toothfish Dissostichus mawsoni (Perciformes: Nototheniidae). Marine Biodiversity 51: 15.