Parasite Lost

Ernst Haeckel's illustration of parasitic worms from the 1904 edition of Kunstformen der Natur. A Taenia solium rectangular proglottid and round scolex can be seen in the middle right of this image.

Ernst Haeckel’s illustration of parasitic worms from the 1904 edition of Kunstformen der Natur. A Taenia solium rectangular proglottid and round scolex can be seen in the middle right of this image.

While parasites are fantastically well-adapted to life in their specific hosts, that usually means they are somewhat lacking in the “living literally anywhere else” department. Thus, when natural selection has nixed the ability to read a map and a parasite finds itself in unfamiliar territory, it is rarely very good at improvising.

Years of co-evolution with their hosts have helped many parasites find a happy medium between harming its host and causing just enough damage to happily feed, reproduce, and spread to a new host. After all, it’s ill-advised to destroy your meal ticket. However, if a parasite finds itself in a host that is not a usual part of its lifecycle – an incidental infection – the usual rules no longer apply.

CREEPING ERUPTION

As a parasite’s attempts to MacGyver its way out of an incidental host to a more familiar one often aren’t pretty, incidental infections can be more severe, or simply more bizarre, than the normal course of infection.

Take, for example, the case of cutaneous larval migrans. Cutaneous larval migrans, also known by the appetizing names “creeping eruption” and “sandworm,” occurs when a human becomes infected with canine hookworm larvae. These parasites have evolved to infect the intestines of dogs and other canids, and so become very confused when they find themselves in a two-legged, hairless ape instead of a loveably dopey quadruped.

A leg with the red, itchy trails characteristic of cutaneous larval migrans. Image: Grook Da Oger

A leg with the red, itchy trails characteristic of cutaneous larval migrans.
Image: Grook Da Oger

Rather than stopping to ask for directions, the now very lost larvae short-circuit and meander in the upper layers of the skin, trying to find their way to the gut. This aimless rambling gives the disease its name – though it uses more SAT words than necessary to say so, “cutaneous larval migrans” just means “worm larvae that wander in the skin.”

The result of the larvae’s migration through the skin is an angry, red, very itchy rash. The little worms actually leave behind tiny, visible trails on the skin in their search for a familiar environment (someone call Kevin Bacon.)

On the bright side, since the parasites are not well-adapted to life in a human host, they cannot complete their life cycle and survive in people. As a result, the larvae die within a few weeks and the rash clears up on its own, though treatment with standard anthelmintic drugs can speed up the process.

An 1831 illustration of Taenia solium by Johann Gottfried Bremser

An 1831 illustration of Taenia solium by Johann Gottfried Bremser

MUTUALLY-ASSURED DESTRUCTION

Sometimes meeting a familiar host at an unfamiliar time can be equally as disastrous for the parasite and the host.

The pork tapeworm, Taenia solium, is a very common human intestinal worm. If a person eats undercooked pork that is infected with hard, spherical cysticerci – encysted T. solium larvae – the digestive proteins in the intestines propel the worm into the next stage of its lifecycle.

During this stage, the worm latches onto the intestinal lining, feeds itself on the nutrients in its surroundings, and sheds eggs in the feces of their human host. Pigs, who are not particularly known for their discerning palates, become infected upon eating these eggs, completing the worm’s lifecycle.

While not pretty, a normal Taenia infection is not severe; if any symptoms show up at all, the worst of them include indigestion and mild anemia. However, when it comes to parasites, timing is everything. Even a relatively harmless parasite can be thrown for a loop if a host enters the mix at the wrong stage of its lifecycle.

If a human ingests T. solium eggs instead of its larvae, that person effectively takes the place of the pig in the lifecycle. This means that cysticerci form in the tissues of the human instead of the pig.

This MRI image shows the brain of a patient suffering from Neurocysticercosis. Each dark round spot is an encysted Taenia solium larva.

This MRI image shows the brain of a patient suffering from Neurocysticercosis. Each dark, round spot is an encysted Taenia solium larva.

This isn’t ideal for the parasite; as cannibalism is typically frowned upon in polite society, a human host at this stage of the lifecycle is a dead-end for the worm. As a result, pork tapeworms haven’t had a chance to evolve to become as good at forming cysticerci in humans as they are in pigs.

These misplaced, inexperienced tapeworms have a penchant for forming cysts in brain tissue in people, as opposed to muscle tissue in pigs. The resulting disease – called neurocysticercosis – causes severe neurological problems.

Though neurocysticercosis is preventable through proper sanitation and hygiene practices, the lack of basic sanitation infrastructure and sufficient medical care has allowed this disease to persist as one of the leading causes of epileptic seizures in the developing world.

STRANGER IN A STRANGE LAND

West Nile Virus is an arbovirus of simple tastes – give it a warm environment populated by birds and mosquitoes, and it will be happy to cycle back and forth between those two hosts. It doesn’t feel the need to make life difficult with an overly complex lifecycle like some pathogens (…we’re looking at you, Dicrocoelium dendriticum.)

Sadly, you know what they say about the best-laid plans of mice and Flaviviridae. If a West Nile Virus-infected mosquito takes a blood meal from a human instead of a bird, the virus enters this strange host and finds itself trapped.

Though it passes into this new host easily enough, it cannot reproduce efficiently in it, and so only achieves low levels of virus in the bloodstream. This means that even if another mosquito feeds on one of these incidental hosts, it cannot pick up enough of the virus to become infected.

A transmission electron micrograph of West Nile Virus. This image has been colored - the original would have been in black and white. Image: Cynthia Goldsmith (CDC)

A transmission electron micrograph of West Nile Virus. This image has been colored – the original would have been in black and white.
Image: Cynthia Goldsmith (CDC)

Despite the fact that West Nile Virus does not replicate well in people, the virus still causes disease in about 20% of infected human hosts. Most people who develop West Nile fever experience mild symptoms that soon resolve themselves, such as headache and fever.

However, in some rare cases the virus causes encephalitis – inflammation of the brain – or meningitis – inflammation of the brain, spinal cord, and surrounding tissues. Encephalitis and meningitis cause serious neurological problems, including paralysis in the limbs or, worse, the muscles needed to breathe.

A PARASITE WITHIN THEE, HAPPIER FAR.

In the end, things rarely go well for a parasite that has been cast out of its intended host and into unfamiliar territory. Whether they end up in the right place at the wrong time, or simply go where they never should have gone in the first place, parasites that have lost their way are far from paradise.

Should Auld Acquaintance Be Forgot

Ms. Pelletier's American Literature and Knife-Wielding 101. Image: AMC

Ms. Peletier’s American Literature and Knife-Wielding 101.
Image: AMC

I think we can all agree that there are many drawbacks to life in the post-apocalyptic world of the Walking Dead. The lack of resources, collapse of basic infrastructure, limited medical care, and the ever-present, ravenous undead make even the simplest tasks a struggle. Let’s not even get started on traveling up Atlanta’s Downtown Connector – between the buildup of abandoned cars and flamed-out tanks, traveling through Atlanta could only be harder if there was an inch of snow on the ground.

While adjusting to the new world can be tough for many survivors, there are some surprising benefits for the children of the apocalypse: an education that includes both Tom Sawyer and knife-wielding skills, the opportunity to develop a pretty forgiving palate, and, surprisingly, an immune system that’s trained better than a Navy SEAL.

What gives little Judith Grimes a biological edge over even Michonne and her katana? Simply put, she will have grown up caked in dirt and, well…grime.

violet

Farmer Rick’s pig, Violet, taking immune health very seriously.
Image: AMC

NEXT TO GODLINESS

In the modern world, if something is dirty, you clean it. Whether your home, your body, your car, or your pet, clean is the norm and dirty should be an exception that is remedied as soon as possible. Dirt is the enemy, and cleanliness is healthy and good.

In the last quarter century, however, doctors have begun to realize that this idea is not necessarily true. Though the incidence of childhood infections has decreased since the advent of modern medicine and sanitation, more and more people are developing health problems like allergies, asthma, and autoimmune diseases.

JUDGE, JURY, EXECUTIONER

We so often talk about threats our immune systems need to recognize and fight that we tend to forget an equally important part of the immune system’s job is to not recognize other things that are harmless. The question is, how do we teach our immune system to tell the difference between the good guys and the bad guys?

Enter what scientists call “immune tolerance.” Immune tolerance is the process by which the immune system keeps the immune cells that respond to actual threats and get rid of those that respond to things that should be ignored, like the body’s own cells.

By developing proper immune tolerance, we avoid both autoimmune diseases and chronic inflammatory diseases caused by a poorly trained, trigger-happy immune system. If the immune system constantly activates in response to things that don’t actually need to be killed, the collateral damage to the body’s own tissue is very high.

Nasty but nice - even microbes that can cause mild disease, like tapeworms, are considered "old friends" microbes. Image: Hubert Ludwig

Nasty but nice: even microbes that can cause mild disease, like this tapeworm, are considered “old friends” microbes.
Image: Hubert Ludwig

OLD FRIENDS MAKE THE BEST FRIENDS

When the human immune system was first evolving back in our hunter-gatherer days, we were far from clean. People would have had regular contact with their microbial neighbors that live in the soil, water, and everywhere else, for that matter – so-called “old friends” microorganisms.

By reducing our regular contact with these “old friends,” the modern urban lifestyle that predominates in most of the developed world is associated with the uptick in both chronic inflammatory diseases, like asthma and hay fever, and autoimmune diseases, such as multiple sclerosis and Crohn’s disease.

In fact, treatment with antibiotics within the first six months of life, effectively killing off an infant’s microbes, is associated with an increased risk of such diseases. Even being born by Cesarean section increases the chances a child will develop these diseases by interfering with the colonization of the infant by its mother’s microbes during natural birth (so maybe Judith is at a slight disadvantage there…)

It appears that these interactions between humans and microbes in the dirt were so constant that as our immune system developed, it evolved a special role for these “old friends.”

Are they dirty because they're badasses, or are they badasses because they're dirty? Yes. Image: AMC

Are they dirty because they’re badasses, or are they badasses because they’re dirty? Yes.
Image: AMC

THE HYGIENE HYPOTHESIS

The biological reasoning behind this idea, better known as the Hygiene Hypothesis, is fairly simple.

Much like Rick’s mental state, the immune system exists in a delicate balance. One arm serves to call immune cells to action, while the other arm activates the production and release of antibodies. If one arm or the other gets too active, either autoimmune diseases or allergies can develop.

The improbable rise of autoimmune and inflammatory diseases in the 20th century puzzled scientists for some time, as they indicated problems on both arms of the immune system. How could one problem affect each arm differently from person to person?

It turns out the problem wasn’t with these different arms themselves, but with the immune cells in charge of keeping them in line. Regulatory T cells are the police force of the immune system, playing an important role in immune tolerance. When these regulatory cells find an immune cell that responds to harmless things, it hog-ties and gets rid of it before it can do more damage.

That constant contact with our “old friends” microbes gives the regulatory cells the guidebook they need to tell the Governor from good-guy Aaron. Without being able to sample these microbes and learn from them, regulatory cells don’t develop properly and can’t recognize immune cells that need to be stopped.

“LI’L ASS KICKER”

While proper hygiene and sanitation are very important at blocking infection with enemy microbes, they are also keeping out friendly ones that we need. In order to get around this Catch-22, many scientists are considering purposefully introducing our old friends to polite society via a different route, such as in our food or as a medication (think probiotics!)

In the meantime, our old friends will ensure little Judith and other kids rolling around in the post-apocalyptic dirt are well equipped to handle whatever the Walkers, Kirkman, or Nicotero throw at them.

 

Jurassic Pests

Drs. Sattler and Grant treat the sick triceratops in the film adaptation of Jurassic Park (1993).   Image: imdb.com

Drs. Sattler and Grant treat the sick triceratops in the film adaptation of Jurassic Park (1993).
Image: imdb.com

When considering the practical issues surrounding opening Jurassic Park, several obvious areas of concern immediately come to mind: finding an isolated chain of tropical islands, building immense electric fences, hunting versus feeding regimens, and kitchen-oriented velociraptor escape plans. Equally as important as containment of the island’s inhabitants, however, is the prevention of dino diseases that could quickly put the park out of business.

Fortunately for Jurassic Park’s veterinarians, we already have a pretty good sense of some of the major diseases that could afflict the park’s main attractions. Through careful analysis of the fossilized clues dinosaurs left behind the last time they roamed the earth, paleobiologists have discovered that the Land Before Time was crawling with the microbial ancestors of many bugs that plague tropical regions today.

A sauropod coprolite, with external surface above and cut and polished surface below.  Source: Graham Young, The Manitoba Museum

The prettiest poo you’ll ever see: a sauropod coprolite, with external surface above and cut and polished surface below. 
Image: Graham Young, The Manitoba Museum

DIAMONDS IN THE ROUGH

Dinosaurs were kind enough to leave ample clues as to what plagued them in the form of coprolites – Latin for “dung stones” and English for fossilized dinosaur poo. Left behind in the coprolites are indicators that ancient forms of the very same worms and protozoa that infect humans and other modern vertebrates were also a problem for dinosaurs.

Though remains of adult parasitic worms did not survive the intervening years, fossilized eggs from trematodes, commonly known as “flatworms” or “flukes,” and three types of nematodes, or roundworms, were found in coprolites. Preserved cysts of the protozoan Entamoeba antiquus, a cousin of the modern-day gastrointestinal parasite Entamoeba histolytica, have also been seen entrapped in coprolites.

Though their eggs and cysts were shed in the dinosaur’s feces, the mature forms of all four parasites would have resided in the dinosaurs’ intestines, just like their modern-day descendants. The forms of the parasites found fossilized in the dinosaur dung are the toughest stages of these parasites’ life cycles, and help them endure the harsh environment outside their cozy host long enough to infect another individual.

CLEAR TO THE BONE

Evidence of other prehistoric parasites has been found coprolites’ more popular fossil cousins, dinosaur skeletons.

For years, paleobiologists have hypothesized that lesions seen on the jawbones of Tyrannosaurus rex and its cousins were bite wounds due to fighting. However, recent investigations have shown that the lesions were actually caused by an ancestor of the protozoan Trichomonas gallinae, which is best known for causing similar disease in the beaks of modern birds.

A Tyrannosaurus rex mandible with multiple trichomonosis-type lesions (indicated by white arrows).  Image: Wolff et al., PLoS One September 2009

A Tyrannosaurus rex mandible with multiple trichomonosis-type lesions (indicated by white arrows).
Image: Wolff et al., PLoS One September 2009

In case you needed another reason to play nice with your neighbors, it turns out that the paleobiologists’ first guess actually wasn’t too far off. Though these particular bone lesions are due to disease rather than bite wounds, scientists now hypothesize that fighting and even cannibalism within tyrannosaurs were instrumental in spreading the disease.

VEXING VECTORS

Especially considering the park’s tropical location, of particular concern to Jurassic Park’s vets are vector-borne diseases, which are transmitted from host to host by another living organism. During the Cretaceous period (around 120 million years ago), many insects that would be familiar to us today made an appearance, bringing with them diseases that evolved to be carried by these new species.

The most prevalent vector-borne diseases are spread by blood-feeding arthropods like mosquitoes and ticks. Dinosaurs had very tough, thick hides composed of tuberculate scales, which sit next to each other but don’t overlap. Like biting insects feed off of large reptiles today, paleobiologists believed their ancestors likely fed from dinosaurs by biting the bits of skin exposed between scales.

This mosquito trapped in amber still contains the blood from its last meal in its stomach.  Source: Didier Desouens

This mosquito trapped in amber still contains the blood from its last meal in its stomach.
Image: Didier Desouens

If any bugs playing taxi to a pathogen found themselves stuck in tree sap, the fossilized sap – called amber – would freeze the bug and the contents of its gut, providing modern-day scientists with a snapshot of what that bug ate. Looking at amber-imprisoned mosquitos and sand flies under a microscope has revealed that (fortunately for Jurassic Park’s geneticists), not only did these insects feed on dinosaurs, they carried with them several familiar diseases.

Leishmania and malaria are two vector-borne protozoan parasites found in amber-preserved sand flies and mosquitoes, respectively. Today, there are a whopping 198 million cases of malaria worldwide every year, most occurring in sub-Saharan Africa; leishmania comes in behind it with 1.3 million cases annually.

Though it’s not completely clear how the disease progressed in dinosaurs, in humans, leishmania takes several different forms, from a painful, disfiguring skin disease to an often-fatal enlargement of the spleen and the liver. Malaria infects and destroys red blood cells, causing severe anemia, and, in severe cases, neurological problems and pregnancy loss. It’s likely these diseases manifested in similar ways in ancient reptiles.

“I’LL BE DAMNED…”

With many of the diseases that burdened dinosaurs in their heyday still around today, Jurassic Park’s chief veterinarian has a lot to look out for. Fortunately for him, his charges gave him plenty of advance notice of what to expect – about 200 million years’ worth.

Afflictions of Classical Fiction

A cough reveals a splash of red on the soft whiteness of the handkerchief. The orchestra swells as the heroine turns her waif-like form to hide her dismay: she has consumption, or in other words – she’s a goner. Though the bloody handkerchief has been used to foreshadow a character’s imminent demise for centuries, consumption has fallen onto a list of illnesses commonly seen in the literary world, but which seem to no longer exist in the real one.

From Mimi’s consumption to Magwitch’s ague, the diseases that afflict our favorite characters are largely neither fictional nor extinct, but are simply known by different names in modern society. Whether thanks to an evolution in language or in our understanding of the disease, these names for common ailments may have fallen out of current fashion, but they are still alive and well in classic films and literature.

CONSUMPTION

The death of Mimi, from King Vidor's "La Bohème" (1926)

The death of Mimi, from King Vidor’s La Bohème (1926)

Rodolfo:
Una terribil tosse               A terrible cough shakes
l’esil petto le scuote           her chest and weak frame,
e già le smunte gote          and yet her pinched
di sangue ha rosse…         cheeks are flushed…

      – Giacomo Puccini, La Bohème, Act III

Consumption, a particular favorite of bohemians and prima donnas, is better known today as tuberculosis, a disease caused by rod-shaped bacteria of the genus Mycobacterium. Though tuberculosis infections can develop in other parts of the body, including the nervous system, bones, and joints, “consumption” refers to pulmonary tuberculosis, which occurs in the lungs and appears in 90% of active tuberculosis cases. In addition to the weight loss and fatigue that gave it its name, consumption also causes chest pain and a severe, mucous cough that will sometimes bring up blood.

Tuberculosis has been infecting humans for thousands of years; scientists have found evidence of tuberculosis in the remains of Egyptian mummies dating back as far as 2,000 BCE, and in the mid-400’s BCE, Hippocrates called it the most widespread and deadly disease of his time. In Europe, the incidence of consumption began to rise in the 1600’s, and by the 1800’s it accounted for 1 in every 4 deaths, particularly among the working poor.

Though campaigns existed to improve living and work conditions and reduce the incidence of tuberculosis, the first antibiotic capable of effectively fighting it, streptomycin, wasn’t developed until 1946. It’s no wonder so many characters from this time period worried about consumption – it was felling their real-life counterparts left and right.

 

Death of the Duke of Orleans of apoplexy, December 1723

Death of the Duke of Orleans of apoplexy, December 1723

APOPLEXY

“The effect of the present revelation was stunning; he trembled and was on the verge of apoplexy.”

– Alexandre Dumas, The Count of Monte Cristo

Until the late 19th century, apoplexy was used to describe what today we call a stroke. A stroke is the sudden loss of brain function when a hemorrhage or clotted blood vessel interrupts blood flow to the brain. The resulting neurological damage can be permanent or even fatal. In fact, stroke was the world’s second leading cause of death in 2012, accounting for 6.7 million deaths.

Though apoplexy is often used to literally indicate a cause of death, “apoplectic” is also a common way to describe someone who is red-faced, spitting, and speechless with rage. It may not be a pretty image, but it must be a good one, as this sense of the word is still in use today.

A late 19th century advertisement for "Ague Conqueror," a purported antimalarial drug.

A late 19th century advertisement for “Ague Conqueror,” a purported antimalarial drug.


AGUE

He shivered all the while so violently, that it was quite as much as he could do to keep the neck of the bottle between his teeth, without biting it off.
“I think you have got the ague,” said I.

Charles Dickens, Great Expectations

Although the word “ague” (pronounced “ay-gyoo”) can be used to mean any fit of fever, chills, and pains, it specifically referred to malaria well into the 1800’s. The name “malaria” derives from the Medieval Italian “mal aria” – bad air – due to the once commonly held belief that the disease was caused by toxic fumes from marshes and swamps. This belief survived until the late 1800’s, when it was shown that the disease is caused not by swamp gas, but by mosquito-borne single-celled parasites.

When a malaria-infected mosquito draws blood, the parasites enter the blood stream and invade red blood cells. Once there, they grow and divide until the cell bursts and the parasites are released to infect new cells. This cycle of bursting and re-invasion causes regularly timed waves of fevers and bone-rattling chills.

Malaria is currently primarily a tropical disease, but back as recently as the early 20th century, malaria ranged as far as England and the southeastern United States. It was common enough in Elizabethan England that Shakespeare mentioned the ague in nine different plays. Bonus: “Ague cake” referred to a hard, enlarged spleen resulting from a prolonged malaria infection. Yum.

 

Gerrit Dou, 1663 - "The Dropsical Woman" The physician in this painting (the man on the right) is examining the patient's urine in a specifically-designed flask called a "matula." Examining urine was one of the only diagnostic tools available to doctors in this era.

Gerrit Dou, 1663 – The Dropsical Woman

DROPSY

Beside him, a man with the dropsy was getting rid of his swelling, and making four or five female thieves, who were disputing at the same table, over a child who had been stolen that evening, hold their noses.”

– Victor Hugo, Notre-Dame de Paris

Dropsy is the buildup of fluid in the space just beneath the skin and between tissues, currently known as edema. Body fluid outside of cells is normally stored either in blood vessels or in the spaces between the cells that make up tissues, called the interstitial space. Edema occurs when excessive amounts of fluid leak from the blood vessels and build up in the interstitial spaces, causing swelling.

Edema isn’t necessarily a bad thing; fluid leaving the blood vessels for the interstitial space contains immune cells that can help fight infections. However, it usually is a sign or symptom of other problems in the body, including allergic reactions, obstruction of blood flow, and kidney, liver, or heart disease.

 

Jane Bennet rides through the rain to Netherfield in Niroot Puttapipat’s illustration of Pride and Prejudice. This rainy ride landed Jane in bed with a sore throat, fever, and head ache.

CATCH YOUR DEATH OF COLD

“Two delightful twilight walks on the third and fourth evenings of her being there…had–assisted by the still greater imprudence of sitting in her wet shoes and stockings–given Marianne a cold so violent as…would force itself by increasing ailments on the concern of every body, and the notice of herself.”

– Jane Austen, Sense and Sensibility

It’s unclear where the “Sense” comes into Jane Austen’s tendency to afflict her heroines with “putrid” fevers as a result of walks in the rain or dewy grass. It’s not actually possible to catch an illness just by getting wet. In fact, current scientific literature is having trouble finding evidence even for the idea that exposure to the cold can make it significantly harder for your immune system to fight an infection – even if you are so imprudent as to sit in your wet shoes and stockings. Sorry, Miss Dashwood.

The Marching Dead

A pleasant stroll through the countryside from Romero's "Night of the Living Dead" (1968)

The dead take a pleasant stroll through the countryside in Romero’s Night of the Living Dead (1968)

Ever since Romero’s 1968 classic Night of the Living Dead revolutionized the zombie film, popular fiction has been flooded with stories of a world brought to its knees by the undead. Whether at the hands of the slow re-animated corpses of The Walking Dead or the lightning-fast, virus-infected cannibals of 28 Days Later, the way in which a zombie apocalypse could start holds a strange fascination for fans worldwide. The existence of zombies is so improbable that most believe they can’t exist outside the worlds of science fiction and fantasy. Fortunately, the real world is a very weird place.

The reality is much more complex than the mindless, shuffling hordes would have you believe. Here and now – outside the world of science fiction – zombie ants actually exist, driven not by unconscious cravings, but by parasites. Through millennia of adaptation and evolution, these parasites have honed their survival skills into an art form, developing intricate life cycles at the ants’ expense. The most amazing part? No single parasite is responsible for this zombie ant phenomenon; many diverse species, including fungi, flies, and worms, have independently evolved the ability to control ant behavior in order to survive.

One notable zombifying parasite is Dicrocoelium dendriticum, more commonly known as the “lancet river fluke.” Flukes are a group of parasitic flatworms (although not all are of the mind-control variety) whose life cycles include a vertebrate definitive host, such as fish or mammals, and a snail. Lancet river flukes were first identified in 1819, but their life cycle is so complex that it wasn’t fully unraveled until the early 1950’s.

The lancet river fluke is thought to be endemic in countries on every continent, living in grazing animals and causing such a mild infection that it usually goes unnoticed. That seems harmless enough, but here’s where things get interesting: once the grazer excretes the fluke’s eggs, land snails eat them and get infected.

"BRAAAIIINS!" Image: Shutterstock

“BRAAAIIINS!”
Image: Shutterstock

As a self-defense mechanism, the snail goes into overdrive on slime production in order to wall off the flukes, which have since grown into larvae called “miracidia,” from the rest of the snail’s digestive tract. The snail drops the miracidia-filled slime balls on the grass, and continues on its way. Fortunately for the fluke, a much sought-after ant delicacy is – you guessed it – snail slime balls. The ant follows the slime trail snails leave behind, munching on slime balls along the way.

Most flukes that have made their way into the ant stay in the ant’s haemocoel, where blood bathes its organs, and wait until they can find their way into the next host. However, one adventurous fluke works its way up to the ant’s brain. From there, the fluke goes into mind-control mode, waiting for evening.

When the sun begins to set and the air cools off, the fluke manipulates the ant’s nerves, compelling it to move away from its colony and up to the top of a blade of grass, where it firmly clamps down its mandibles. The ant stays there, paralyzed, until dawn. When the sun begins to rise again, the fluke allows the ant to return to the colony and go about its daily formic business; none in the colony suspects there is anything wrong with their zombified neighbor.

While this series of events seems weirdly specific, the lancet river fluke isn’t simply making a power play. By forcing the ant to move to the top of the blade of grass, it makes it much more vulnerable to consumption by a cow or sheep, which would complete the fluke’s life cycle. Prolonged direct sunlight would kill both the ant and parasite, however, so the fluke ensures that they are only exposed during the coolest parts of the day.

Zombie ants are found all over the world, but human infections with the lancet river fluke are exceedingly rare and only cause mild, non-neurological symptoms, if any at all. In other words, you won’t be seeing human zombies climb trees to watch the sunrise any time soon.

For those who get excited at the possibility of a zombie apocalypse – don’t be too disappointed. Modern science may not be able to support mindless reanimated corpses, but nature has provided us with the next best thing: a horde of zombie animals.