Myxobolus medusae

Myxozoans are a group of single-celled parasites which had evolved from jellyfish-like ancestors, thus making them a type of single-celled animal. There are about 2400 known species and they mostly infect fish, with a handful of them infecting other kinds of vertebrate animals including amphibians, turtles, ducks, and even shrews. The species being featured in this post, Myxobolus medusae, infects fish like most other myxozoans, but not just any fish, it's one with a notorious, but overblown, reputation - the red piranha (Pygocentrus nattereri). Despite its fearsome reputation, the red piranha are commonly caught and regarded as a regular food fish, so researchers in this study were able to obtain the piranha from local fishermen around Lake Sacaizal, and describe a previously undocumented species of myxozoan.

Left: Myxobolus medusae cyst (indicated by arrowhead) in the eye of a piranha, Right: Illustration of M. medusa spore.
From Fig. 1 and 2 of the paper.

While myxozoan infections are often visible as white cysts in the host's tissue, the spores themselves are actually microscopic and come in various different shapes. Some myxozoans produce spores that have a pair of long wispy tails, but the spores of M. medusae are far more unique and extravagant, with multiple branching tendrils, like the medusa of Greek mythology, in unicellular form (hence its species name). But why have such an elaborate structure in the first place? The researchers suggested those appendages might help the spores disperse in water where they act like a web that catches the current and carry the spores far and wide.

But this parasite also has another connection to its medusa namesake, namely where M. medusae lives in its host. The medusa in Greek mythology can turn someone into stone with a stare from her eyes - and that's where M. medusae lives in the piranha. Myxozoans can occur in various different parts of the host's body, and the genus Myxobolus is an exemplar of that. With almost a thousand known species, they inhabit just about every part of a fish's anatomy including the gills, kidneys, liver, ovaries, muscles, and even the cartilages of the skull and spine, where they constricts and compress the fish's spinal cord and brainstem, resulting in symptoms called "whirling disease". In the case of M. medusae, they appear as a white cyst lodged in the eye's interior.

Myxobolus medusae is not the only parasite to inhabit fish eyes, they are also the favoured infection site for other species of Myxobolus, and a number of trematode flukes. But why the eye though? For the aforementioned fluke, hanging out in the eye would hinder a fish's ability to see, which makes it more vulnerable to birds - the next host in the flukes' life cycle. But it wouldn't do any good for M. medusae if its host gets eaten by a predator, because its spores need to make their way to worms, not the belly of a hungry bird.  However, the eyes are still considered prime real estate for any would-be parasites because along with the rest of the central nervous system, the eyes are "immune privilege sites" which are mostly off-limits to the immune system, thus they can act as potential parasite shelters.

This also applies to those eye flukes too, scientists have found that flukes which infect the fish's eyes are able to infect wider range of fish species than those infecting other parts of the host's body.

Since each species of fish have a slightly different immune system, for the body-dwelling flukes, they are more limited in their host choice because their tricks for overcoming one fish species' immune system might not work for another. But since eye flukes don't have to deal with the immune system, they are free to infect a wider range of fish. So M. medusae might also be hiding in the eye for the same reason.

So while beauty might be in the eye of the beholder, in this case, a medusa is found in the eyes of a piranha.

Reference:

de Sena, N. M., Eduard, J., Pereira, C. M. B., Neto, J. L. S., & Velasco, M. (2025). Myxobolus medusae n. sp., a new species of Myxozoa with dendritic appendages. Parasitology International 109:103106.

Acanthamoeba spp.

Today we're featuring a guest post by Sally O'Meara - 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 and this post was written as an assignment about writing a blog post about a parasite, and has been selected to appear as a guest post for the blog. Some of you might remember Dr. O'Dwyer from previous guest post on ladybird STI and salp-riding crustaceans. I'll let Sally take it from here.

This blog post today is dedicated to all you visually impaired contact lens wearing folk out there! Before I begin, I just want to say that I truly hope all of you adhere to the instructions your optometrist gives you with regards to using contact lenses (washing hands before and after handing them, taking them out while showering/bathing). If not, I’m afraid you are running the risk of meeting my new acquaintance; Acanthamoeba spp., also known as the cornea guzzling free-living protozoa from hell!

Acanthamoeba in its two forms: (A) trophozoite, (B) impenetrable cyst
Image by Jacob Lorenzo-Morales, Naveed A. Khan, and Julia Walochnik, used under CC BY 2.0

Acanthamoeba spp. are microscopic organisms that can be found just about anywhere, from soil to water, to the air we breathe. They are the direct culprits of Acanthamoeba keratitis (AK) a relatively rare but sight-threatening disease which is actually caused by at least eight species of Acanthamoeba: A. castellanii, A. culbertsoni, A. polyphaga, A. hatchetti, A. rhysodes, A. lugdunesis, A. quina, and A. griffin. Ocular trauma and contaminated water are also associated with AK infections but it has been found that contact lens wearing accounts for > 80% of the cases. If found early the infection can be cured, but this gets progressively more difficult the longer it remains untreated. The difficulty lies with the life cycle of the Acanthamoeba species which consists of two stages: the trophozoite and the cyst.

The trophozoite is the vegetative form which feeds on organic matter and ranges in size from 10 to 25µm. When the going gets tough, the tough get going... tough being the trophozoite. When conditions become unfavourable, like under extreme heat or lack of nutrients, the trophozoite transforms itself into a double walled cyst which is almost invincible. The cyst remains unscathed by repeated cycles of freeze-thawing, and incredibly high doses of UV and even GAMMA RADIATION. Cue the Terminator and his infamous catchphrase…. “I’ll be back”.

Characteristics of AK include eye pain, redness, itchiness, and a general feeling of something being stuck in your eye. Sounds like most eye infections, right? One extra feature is the presence of a stromal ring-like infiltrate in the eye. Basically, an ulcer forms on the cornea of the infected eye as a result of the hungry Acanthamoeba. It has been discussed that contact lenses serve as vectors for transmitting Acanthamoeba trophozoites, and to make matters worse studies have shown that wearing lenses results in mild corneal trauma which alters the surface of your eye making it even more susceptible to infection!

Healthy human eye (left) vs infected eye with Acanthamoeba keratitis (right). Arrow indicating stromal ring-like infiltrate.
From Figure 1 of the paper

Scientists have tried to create vaccines to prevent AK by terminating the Acanthamoeba trophozoite or the cyst, but these have proved unsuccessful. However, it was discovered that using a vaccine composed of dead trophozoites stimulates the production of antibodies in the tears, and these block adhesion of the trophozoites to the ocular surface which in turn prevents the development of AK.

Now, before you all go destroying your contact lenses in a panic-stricken state let me inform you that over 30 million Americans wear contact lenses, yet remarkably the incidence of AK in contact lens wearers is less than 33 cases per million. Acanthamoeba species are found in virtually every environmental niche on our planet ranging from thermal springs to solid ice, yet why are AK cases so far and few between? Scientists believe the host’s immune system plays an important role in successful AK infections.

Serological analysis of IgG and tear IgA (both of which are antibodies found in blood) revealed that 50-100% of healthy individuals with no history of AK possessed antibodies against Acanthamoeba antigens. What’s more, the serum IgG and tear IgA levels were significantly lower in patients with AK compared to the cohort of normal individuals with no history of AK, suggesting a prominent role of the mucosal immune system in preventing AK.

In 1939, Winston Churchill referred to Russia as “… a riddle, wrapped in a mystery, inside an enigma” … one might classify Acanthamoeba and the infections it produces in the same way! Although scientists have a clearer understanding of Acanthamoeba keratitis and the parasite which causes it, there is still much to be learned about its cunning and conniving ways.

References:
Neelam S. and Niederkorn J.Y. (2017) Pathobiology and Immunobiology of Acanthamoeba Keratitis: Insights from Animal Models
. The Yale Journal of Biology and Medicine. 90:261-268.

This post was written by Sally O'Meara

Tylodelphys sp.

There are many examples in nature where parasites are able to alter their host's behaviour in some way. More recently, some scientists have been investigating just how the parasite are altering or controlling host behaviour. Most of them had looked at the chemicals secreted by the parasites to lull the host into compliance, but the study we're featuring today looked at something different - how the behaviour of the parasite itself can affect the behaviour of the host.

Left: Histology section of an infected bully's eye from Fig. 1. of the paper (r = retina, l = lens, m = metacercariae [flukes])
Right: Tylodelphys in the eye of a common bully from this video

The star of today's post is Tylodelphys - a parasitic fluke which infects a small freshwater fish in New Zealand call the common bully. In order for this parasite to complete its life cycle, Tylodelphys must enter the gut of a fish-eating bird, which would naturally involve the unfortunate fish being eaten by the said bird. While it is in the common bully, Tylodelphys dwells in its host's eyes in the vitreous liquid between the lens and the retina (see video here).

Unlike other species of flukes which turn into dormant cysts at a similar stage of development, Tylodelphys stays active and free to roam around inside the fish's eye - which provides it with plenty of opportunity to get up to all kinds of parasitic hi-jinx. When Tylodelphys larvae are crawling around inside a fish's eye and happen to get in between the retina and the lens, this can partially blind the fish and prevent it from being able to notice incoming predators such as birds.

To examine what Tylodelphys gets up to during the day, researchers at the Otago Parasitology Lab collected some common bullies and gave them eye examinations using an opthalmoscope (yes, like the one used for your eye exam). Using the opthalmoscope, they captured a series of short videos of the infected fish's eyes at different time of day, and watched what eye flukes got up to. They also performed histology to examine if the flukes are damaging the fish's eyes.

The bullies they examined varied in how heavily infested their eyes were - this range from just having a single fluke in the eye, or it can be up to seventeen flukes, with the average being about seven. Living in a crowded eye is not good for the parasites either, as the researchers found that flukes from heavily infected fish are comparatively smaller. But despite being found in a vital and sensitive part of the host body, Tylodelphys was otherwise a relatively benign tenant - they didn't mess up any eye tissue.

Compare this with other species of eye flukes which can cause cataracts in the eye of their fish host, Tylodelphys seems rather well behaved. However, that does not mean Tylodelphys isn't bad news for the bully - just that its modus operandi is more subtle. Instead of impairing the fish's sight by damaging the eye, as mentioned the above, when the flukes position themselves in front of the retina, they act like internal blinkers. Surprisingly, fish that are more heavily infected didn't have their retina more covered up by the flukes than less heavily infected fish, which means it's not simply the sheer number of flukes that blinds the fish - it's something else the flukes are doing.

Tylodelphys  has a daily routine and shifts its position in the eye throughout the day. During day time, the flukes sit between the lens and the retina, blocking the host's line of sight. But at night, they settle down to the bottom of the eye, allowing the fish to see properly again. But if Tylodelphys is trying to get its host eaten by a predator, why doesn't it just stay in front of the retina all the time? That is probably because not all predators are the same for Tylodelphys.

During the day, the main predators of bullies are fish-eating birds (which are Tylodelphys' final host), whereas at night, the main predators are longfin eels (which are not suitable as host), so it'll be good for to the fluke if their host fish can still see and avoid the incoming predators at night. So the flukes keep this fish's eyes covered during the day, but move aside to not get in the way at night, and this seems to follow a circadian rhythm.

While this helps the fluke reach its final bird host, the reason why this behaviour manifests probably has nothing to do with trying to change the host's behaviour. As mentioned above, unlike other flukes that become a dormant cyst at this stage of development, Tylodelphys keeps growing so it needs to feed - and the only thing around to eat in the eye of a fish is the fluid in the eye's vitreous body. The partial blinding of the fish host during daytime might simply be a side-effect of the parasite's feeding routine.

So while the fluke moves around in the fish's eye to get its daily dose of eye jelly, this also produce a useful side-effect by making the host more vulnerable to fish-eating birds. Such "useful side-effects" could be how many parasite host manipulation tactics have evolved. Indeed, that is often how evolution often work; co-opting preexisting features and behaviours into new roles. To understand how a parasite affect the behaviour of its host, sometimes perhaps it is best to start with studying the behaviour of the parasite itself.

Reference:
Stumbo, A.D., and R. Poulin. 2016. Possible mechanism of host manipulation resulting from a diel behaviour pattern of eye-dwelling parasites? Parasitology 143: 1261-1267

Diplostomum pseudospathaceum

Most animals are infected by multiple species of parasites or multiple strains of the same parasite species that are not close kin. Rarely does an individual parasite (and its close kin) gets to monopolise the resources of an entire host. So do parasites like to share? The answer to that depends on the host (and parasite) in question. Because different species often exploit a single host in different ways, a parasite can share a host with many other species without ever coming into conflict with them. But, when a parasite finds itself sharing a host with members of its own species, competition is more likely to occur as they are all going to be after the same (limited) resource (whatever that may be) from the host.

For some parasites, competition arising from coinfection can favour the most virulent strains, leading to greater overall harm to the host. But more recent studies have shown that the most competitive strains are not always the most virulent. In some cases, competition between co-occurring parasites can actually be beneficial for the host as the parasites end up mutually suppressing each other. How this plays out depends on how the parasite uses its host, and in parasites that have complex life-cycles, this can change from host to host.

The parasite we are looking at today is Diplostomum pseudospathaceum - more commonly known as the eyefluke. As with most parasitic flukes, D. pseudospathaceum uses a snail for the clonal stage of its life-cycle to make thousands of larval stages (called cercariae), which are then released into the water to infect any nearby fish. In the fish, the parasite migrates to its eye and uses it as a temporary vehicle to reach its next host - a fish-eating bird. Diplostomum pseudospathaceum essentially turns its snail host into a parasite factory, using the snail's bodily reserves as raw material to produce its army of clones. There is only so much to go around inside a snail, and when a particular D. pseudospathaceum strain has to share a snail with other strains, it ends up producing many fewer cercariae than if it had the whole snail to itself. But while it is detrimental for D. pseudospathaceum to have close company in the snail, it is a different story in the fish.

While D. pseudospathaceum undergoes a resource-hungry spree of rampant asexual multiplication inside the snail, it is relatively dormant inside its fish host. All it needs from the fish is a space to tuck into within its eye - and there is plenty of room in the fish's eye for these flukes. In fact the more the merrier given that at high numbers the eyeflukes can induce the formation of cataracts - and a half-blind fish is more likely to be eaten by a bird. But there is also another reason for D. pseudospathaceum to be more welcoming to strangers in the fish host.

The immune system of the fish is remarkably adept at responding to intruding parasites, and the immune system can be quickly "primed" towards recognising and destroying particular parasite strains. To reach the fish's eye, D. pseudospathaceum has to complete a treacherous journey through the fish's body, all while under close scrutiny and assault by the fish's immune system. In fish that have previously been exposed (and thus "primed") to D. pseudospathaceum cercariae, the chances of the parasite successfully reaching its destination are greatly diminished. But while a fish might be "primed" towards cercariae of a particular strain, a double- or even triple-prong attack is likely to overwhelm its finely-tuned targeting system. When exposed to simultaneous attacks from multiple, genetically-diverse strains of D. pseudospathaceum cercariae, the fish's immune system becomes overwhelmed, which in turn allows more eyeflukes to slip by.

In the life-cycle of D. pseudospathaceum, there is a time and place for everything - while there is a time to be on the look out for number one, there is also a time to love thy neighbour.

image by Tina Loy, modified from here

Karvonen A, Rellstab C, Louhi KR, Jokela J. (2012) Synchronous attack is advantageous: mixed genotype infections lead to higher infection success in trematode parasites. Proceeding of the Royal Society B 279: 171–176

Philophthalmus gralli - update on the "Parasite of 2010"

Last year the most “yuck” votes were cast for a photo of Philophthalmus gralli in the eyes of a rhea at the Phoenix Zoo. Melanie Church, the vet who treated the rheas, gave me a back-door tour of the zoo in January. The three rheas are doing fine. She removed most of the flukes manually, treated the eyes with an anthelmintic ointment, and the birds are now virtually parasite-free. The rheas have been moved to a pen where the snail intermediate hosts are not present to prevent re-infection.

Contributed by Mike Kinsella.

October 30 - Philophthalmus gralli

Continuing our theme of nasty places to have worms, how about under the eyelid?! Flukes of the genus Philophthalmus are found in the conjunctival sac of the eyes of many species of birds and rarely in humans. In this case, a rhea (Rhea americana) in a zoo had an unusually heavy infection of several hundred flukes in each eye. The intermediate hosts are aquatic snails. When the cercariae escape from the snail, they encyst on vegetation or occasionally at the surface of the water and the cysts are ingested by the final host, where they hatch and migrate to the eye.

Contributed by Mike Kinsella, photo by Melanie Church.

August 30 - Oculotrema hippopotami

The hippopotamus is notorious for being one of (if not the most) dangerous large animals of Africa, as they are extremely aggressive and unpredictable, and are responsible for killing more people than some of the iconic predators of Africa such as lions and crocodile. However, even this fierce giant is ailed by a tiny irritant - Oculotrema hippopotami - a species from a group of ectoparasitic flatworms known as the monogeneans.

Most monogeneans live on the skin or gills of fish, however there is an unusual family of monogeneans called the polystomatids that live mostly in the bladder of frogs and turtles. O. hippopotami can be considered even more of an oddball out of a family of oddballs. Not only has it colonised a mammal, it also lives in a peculiar part of its host. As its name implies, it lives in proximity of the hippo's eye, more specifically, under the eyelids. So for all its brazen brawn and strength, the mighty hippo is not immune from being parasitised!

Photo is from this site.

Contributed by Tommy Leung.

August 7 - Caligus oculicola

The parasite Ommatokoita elongata probably made a few of you squeamish. If so, then you probably want to just close this page, too. Caligus oculicola is a recently described species of copepod that lives on the surface of the eye of tiger sharks (Galeocerdo cuvier). Although most species of Caligus infect teleost (bony) fish, not sharks, this copepod seems to have found an ideal habitat of the eyes. They have specialized structures that allow them to adhere, suction-cup style, to the eye and then they proceed to feed on the host's tissue through a scraping and swallowing kind of fashion.

Drawing of a male Caligus oculicola, modified from the original paper.

Thanks to Laurence Frabotta and Colleen Ingram for nominating this parasite.

July 22 - Ommatokoita elongata

If you find the idea of having something lodged in your eye distressing (ok let's face it, who doesn't?), then today's parasite is probably your worst nightmare. Fortunately for you, it is not a human parasite. The hosts for today's parasite are Greenland sharks (Somniosus microcephalus) and Pacific sleeper sharks (Somniosus pacificus) - both large deep water sharks. Ommatokoita elongata is a parasitic copepod, approximately 5 cm in length (almost 2 inches) with a very specific and truly cringe-worthy preference about where it attaches on to the host.The adult female copepod attaches herself to the shark's eye with an anchoring structure call the bulba, and grazes on the surface of the cornea (see photo, black arrow indicates attachment point), hanging off the eyes of the shark like a grotesque tassle

There are two possible reasons for the copepod's attachment site. Shark skin is covered in microscopic, teeth-like structures call denticles which can make it difficult for parasites to attach themselves to skin (though some species of parasitic copepods
manage). Secondly the eye is considered to be a "immunologically benign environment" for parasites, thus such an attachment is less likely to illicit an immune response.

While the parasite can cause significant damage to the cornea and result in blindness for the host, most sharks seem unaffected by the presence of the parasite and many sharks have the copepod in both eyes, strangely enough. This goes to show when considering the virulence (harmfulness of a parasite to its host) of a parasite, it is worth taking into account the perspective of the host involved - what may seem debilitating to us may not necessarily be the case for the actual organism in question.


Photo source: Borucinska, J.D., Benz, G.W. and Whiteley, H.E. (1998) Ocular lesions associated with attachment of the parasitic copepod Ommatokoita elongata (Grant) to corneas of Greenland sharks, Somniosus microcephalus (Bloch & Schneider) Journal of Fish Diseases, 21:415-422

Also some good photos of live Greenland sharks with the parasite can be seen in Caloyianis, N. "Greenland Sharks." National Geographic 194, no. 3 (1998): 60–71.

Contributed by Tommy Leung.

May 9 - Thelazia callipaeda

Oriental eyeworm, Thelazia callipaeda, is kind of a gruesome parasite. These nematodes are transmitted by flies - but not biting flies - just those that come to lap up tears from the eyes of vertebrate animals. The larvae are swallowed by these flies where they will go through a couple of developmental stages and then move to the mouthparts of the fly, where they can be released the next time the insect feeds from the tears or secretions. This species has an almost world-wide distribution and can infect many canids, cats and rabbits - and, in some cases, humans. On top of the gross factor (sorry about the photo), they can cause health issues such as conjunctivitis, visual impairment and corneal scarring.

April 18 - Demodex folliculorum

One of the things that grosses non-parasitologists out (and probably many parasitologists!) is the fact that little tiny mites live on our eyelashes. These are Demodex folliculorum, and they can actually inhabit many different follicles on humans' faces. In fact, they're a bit social - as many as 10 of them can co-exist in a single follicle (party on the forehead - spread the word!) There are no known pathogens that they spread, but they can cause people to lose hair and they can make pores larger. They don't bite people and instead they mostly just munch up the secretions from our sebaceous glands. One of the coolest things is that they seem to be so incredibly efficient at digestion that they don't produce waste - so much so that they don't even have an excretory pore for defecation. So, if you have to deal with the fact that right now there are potentially a bunch of tiny spider-like things living on your face, at least you can take a little comfort from the knowledge that they are clean house guests.

March 30 - Loa loa

Loa loa, also known as the “eye worm,” gets its name from an affinity for subcutaneous tissue like the tissue found in human eyes. This nematode parasite finds its way into humans through the deer fly, in which the Loa loa worm larvae develop. Once in its third stage, a larva can enter a human when a deer fly bites. When larvae mature within subcutaneous tissue, they produce microfilariae – an uninfective stage of the Loa loa, which are then picked up by other deer flies. These parasites can be removed surgically, but drugs are required to kill microfilaria within the bloodstream. Adults can survive in subcutaneous tissue for up to 17 years, and take one year to fully mature. The first recorded case of Loa loa dates back to the year 1770 when a surgeon failed to remove a worm in a woman’s eye in the Caribbean. Loa loa was again found in slave ships coming to America. Loa loa has not been diagnosed in the United States in almost a century, but the parasite is still endemic to western and central Africa. While not fatal, Loa loa can be a complication for patients with other diseases. These microscopic worms are hard to see, unless they’re in your eye.


References:
1. Loa loa: A cutaneous filarial parasite of humans. Filarial Biology.
2. Muller, R. Worms and Human Disease. New York: CABI, 2002.

Contributed by Prath Devre, Bucknell University.