Parasitic life cycles involve one organism, a parasite, which exploits and benefits from another organism, the host. Generally, the host does not gain from this relationship and often suffers as a result. In many cases, the parasite changes the host physically or behaviorally; in extreme examples known as parasitoids the organism consumes, sterilizes or kills their host. Many parasites depend on human hosts to facilitate their life cycles – there are a huge number of organisms that exploit our bodies, some benign, some rather more unpleasant and harmful.
Parasites exhibit a remarkable degree of specialization with life cycles that can involve multiple stages with a main ‘definitive’ host, and may also involve one or more secondary ‘intermediate’ hosts. Parasites have developed their lifestyles and evolved in tandem with their host species, resulting in life cycles that can be incredibly complex, fascinating, disturbing and often extremely bizarre.
1. Guinea Worm
Dracunculus medinensis, better known as the guinea worm, is a roundworm parasite that has been preying on humans throughout our history – Egyptian medical papyrus dating from 1550 BC mention the infection and specimens of the worm have been found within the bodies of calcified Egyptian mummies. Dracunculiasis, the Latin term for guinea worm disease, derives from the phrase ‘afflicted with little dragons’, describing the terrible burning sensation as the worm wriggles under a victim’s skin.
Guinea worm is found in several countries in Africa and Asia and particularly afflicts those with no access to clean water – the parasite’s life cycle relies on a mammalian host drinking infected water.
Guinea worm larvae live inside microscopic fleas in water; a human (or other mammal) drinks the unfiltered water and ingests the flea along with the parasites inside its body. Stomach acid dissolves the flea leaving the larvae free to borrow through the stomach lining into the host’s body cavities. The worms grow and reproduce, after which the male is absorbed by the body while the fertilized female continues to grow and moves into connective tissues in the host’s limbs. After approximately one year – by which time the female may have grown to two or three feet in length and as thick as a spaghetti noodle – a painful blister appears on the host’s skin, usually on a leg or foot, the blister bursts and the end of the worm appears.
The victim feels a searing, burning pain which can only be relieved by dipping the blistered area into water – as soon as the worm’s exposed end touches water thousands of larvae are released. The larvae are eaten by microscopic fleas and the cycle starts once more.
Exposed guinea worms can be removed by gently coiling around a stick, and the best way to prevent infection in the first place is to only drink clean, filtered water. Prevention through behaviour change has been a great success in recent years, and the Carter Centre has predicted that guinea worm disease will be the ‘first parasitic disease to be eradicated and the first disease to be eradicated without the use of vaccines or medical treatment.’
2. Filarial Worm
Filariasis is another parasitic disease caused by roundworms, in this case thread-like filarial worms, a family of parasites that is divided into three groups depending on which part of the human body they occupy. Lymphatic filariasis is caused by worms living in the host’s lymphatic system, and can lead to dramatic cases of elephantiasis, as we see in the above photograph. Subcutaneous filariasis is caused by worms living under the skin and similar tissues, including the worm that causes river blindness, one of the world’s main causes of infectious blindness in humans. Serous cavity filariasis is caused by worms that live in serous cavities in the abdomen.
The lifecycle of filarial worms in humans is a complex process, but can roughly be divided into five stages. Adult worms living in the host’s body tissues mate, the female then releases thousands of live microfilariae, which make their way into the host’s lymph and blood systems. A black fly or mosquito takes a blood meal from the human host, ingests the microfilariae and becomes the intermediate host. The microfilariae move to the mosquito’s muscles, molt and become infective larvae. The infective larvae move to the mosquito’s head and proboscis and are injected into another human whilst the mosquito is taking another blood meal. The larvae penetrate the human host’s body tissues, mature into adult worms, reproduce and the life cycle continues.
3. Emerald Cockroach Wasp
The beautiful metallic blue-green bodied emerald cockroach wasp is an example of entomophagous parasites, which are insects that are parasitic on other insects. The wasp, which is found in the tropics of Africa, South Asia and Oceania, is known for it’s strange parasitic reproductive process involving a cockroach host.
When a pair of wasps have mated, the female finds a specific type of cockroach that she stings twice (the female possesses a stinging organ; the male does not). The first sting accurately injects venom in a specific site in the thorax, temporarily paralysing the cockroach’s front legs. The second sting precisely targets and disables a site in the cockroach’s brain which is responsible for the escape reflex.
Lacking the instinct to escape, the cockroach is now at the mercy of the wasp, who trims the roach’s antennae and then leads it to the wasp’s burrow as if on a leash. Once there the wasp lays an egg on the cockroach’s abdomen and then buries the insect.
Underground, a wasp larva hatches from the egg, eats its way into the cockroach’s body and starts to slowly devour the vital organs from the inside – the pacified cockroach hopefully stays alive long enough for the larva to grow and cocoon within the roach’s body. Finally, the larva completes its metamorphosis into an adult wasp, emerges from the roach’s remains and goes on to live, reproduce and repeat the process.
Sacculina is another example of a bizarre parasitic life cycle from the animal kingdom, a type of parasitic barnacle that depends on crabs for its growth and reproductive processes. A female Sacculina larva finds a crab, sheds its hard outer layer and injects her soft inner body through a joint in the crab’s shell, entering the crustacean’s body and getting to work at her parasitic behaviour.
The Sacculina larva grows inside the crab and develops a sac which hangs outside the crab’s body where the crab’s eggs would normally incubate. Sacculina renders the crab infertile – the parasite wants the energy that the crab would invest in reproduction to be conserved for its own growth and life cycle. Infected crabs are also prevented from moulting their shells and re-growing lost limbs, further conserving energy for the Sacculina.
In male crabs the parasite makes some remarkable changes to the crab’s body and behaviour. Sacculina releases hormones that chemically castrate the male crab, change the crab’s body to resemble a female of the species and even make the crab execute female mating dances.
Male Sacculina find an infected crab and fertilize the eggs in the female’s sac dangling from the crab’s thorax. During reproduction in healthy crabs, the female finds a high rock and releases fertilized eggs from its brooding sac. Parasitized crabs perform the same behaviour, but inadvertently release a cloud of Sacculina eggs. The crab nurtures the Sacculina eggs as if they were its own offspring, and once the larvae hatch from the eggs and are released into the sea the process begins once again.
5. Green-Banded Broodsac
Leucochloridium paradoxum – the green-banded broodsac – is a type of parasitic flatworm whose life cycle involves birds as definitive hosts and snails as intermediate hosts. The adult parasites live in the digestive systems of birds, reproduce and release eggs that the bird leaves in its droppings.
Snails eat the bird droppings and ingest the parasites, which form a sporocyst, or ‘broodsac’, which contains hundreds of larvae. The broodsac grows in one or both of the snail’s eye tentacles, usually preferring the left tentacle, and eventually causes the eye tentacle’s appearance to dramatically transform into that of a large, brightly-coloured, pulsating caterpillar.
A bird spots this deliciously juicy ‘caterpillar’ meal, bites off the snail’s eye tentacle along with the broodsac and ingests its larval contents. The larvae develop into adults within the bird’s digestive system and the green-banded broodsac’s life cycle continues once more.
6. Cymothoa exigua
Cymothoa exigua is a type of parasitic tongue-eating louse that lives in the gills and mouths of fish. Currently, biologists know little of the louse’s life cycle, although it is thought that the young of the species enter a fish’s head through its gills. Once inside they reproduce; following this the males remain living in the gills while the female makes her way to the fish’s mouth. If there are two males and no females, one of the male lice transforms into a female and moves to the mouth.
Inside the mouth, the louse attaches herself to the fish’s tongue by its front claws and begins drinking blood, causing the tongue to shrivel away. The louse then latches herself to the tongue stub and sits in its place, replacing the organ. The louse lives by parasitically feeding on the fish’s blood or its mouth mucus, while the host fish appears to live a normal life and can use the louse as it would its own tongue.
7. Horsehair Worm
Spinochordodes tellinii is a type of parasitic horsehair worm that uses grasshoppers and crickets in its life cycle and is able to alter its host’s behaviour. The adult worms live and reproduce in water, generating microscopic larvae. Crickets and grasshoppers ingest the larvae when drinking, the larvae then develop into worms within the insects’ bodies. The worms can grow to be as much as four times longer than their host’s body.
Once the parasitic worm has reached maturity it is able to influence the host’s behaviour to complete its life cycle. The exact mechanism that the worm uses to influence the host is currently unknown, but the implications can be catastrophic for the cricket or grasshopper. The insect is compelled to find a body of water and throw itself in; once submerged the parasite exits its host’s body, while the host usually drowns. The worm goes on to live its adult life in the water, reproduce, release larvae and continue its parasitic life cycle.
8. Ophiocordyceps unilateralis
Ophiocordyceps unilateralis is a parasitoidal fungus found in the tropical forests of Africa, Brazil and Thailand. The fungus uses ants in its life cycle, specifically the Camponotus leonardi ant, although it has been known to parasitize similar ant species.
The fungus releases spores that enter the ant’s body through the cuticle and once inside begin eating away at non-vital tissue. This means that the ant stays alive while the parasite receives nourishment to grow. The fungus spreads through the ant’s body and, in its yeast stage, uses an unknown mechanism to change the insect’s brain function and behaviour, taking over its body and creating a so-called ‘zombie’ ant.
The zombie ant climbs up a plant and uses it mandibles to secure itself to a leaf vein, usually about 25 cm off the ground, north-facing and in a temperature range of 20 to 30 degrees centigrade. In these ideal conditions the fungus continues to grow and kills the ant, eventually producing a mushroom that erupts from the back of the ant’s head, releasing spores sometime between four to 10 days after appearing. Some of the spores settle on the bodies of other ants in the forest, pass through the cuticle and continue the parasitic process.
Interestingly, parasitized zombie ants leave a characteristic bite mark in leaves when infected by the fungus – scientists have studied the fossil record and found evidence of these bite marks on leaf specimens that are 48 million years old.
9. Lancet Liver Fluke
Dicrocoelium dendriticum, commonly called the Lancet liver fluke, is a parasitic fluke that displays one of the most complex parasitic life cycles currently known. The Lancet liver fluke lives mainly in cows and sheep, but is also found in other herbivorous mammals and, in rare cases, has been known to infect humans.
Adult Lancet liver flukes spend their adult lives in the livers of ruminants such as cows and sheep. When Lancet liver fluke reproduce their eggs are released in the host’s faeces. The first intermediate host is a land snail, which eats the animal faeces containing the parasite’s eggs. It is only when a specific type of snail ingests the eggs that they hatch larvae. The larvae then make their way into the snail’s body tissues. The larval parasite irritates the host snail, which walls the growing parasites off in a mucus cyst, eventually coughing up a slime ball containing the larvae.
Next, an ant, the second intermediate host, eats the slime ball and ingests the hundreds of larval flukes contained in the cyst. The larvae distribute through the ant’s body – one larva attaches itself to a bunch of nerves and begins influencing the ant’s behaviour in a remarkable fashion.
In the evening, as the temperature drops and the rest of the ant colony retreats underground, an individual zombie ant infected by the larvae walks off on its own, climbs a blade of grass and firmly attaches itself to the grass tip with its mandibles. It remains there until the sun rises at dawn, when the zombie ant returns to the colony and resumes its normal daytime activities with the other ants. The following evening the infected ant again climbs and attaches itself to a grass tip, and continues to do so night after night until it is eaten by a grass-eating animal such as a cow or sheep.
The ant and the many larval flukes it contains are ingested by the mammal, the larvae burrow into the animals stomach wall, make their way to the animal’s liver, develop into adult Lancet liver flukes, reproduce and continue this astonishing parasitic life cycle.
10. Toxoplasma gondii
Finally, Toxoplasma gondii is a type of parasitic protozoa that infects cats, which are its definitive hosts, but is also carried by other warm-blooded animals, mainly rodents and ruminants, but is also notably present in humans – it is thought that between one-third and half of all people alive today carry the parasite.
T. gondii lives in cats, sexually reproducing in the host animal’s tissues, but can also reproduce asexually in intermediate hosts in the second phase of its lifecycle. Host animals can be infected with T. gondii by either coming into contact with soil containing cat faeces, such as eating unwashed vegetables, or by eating uncooked meat containing the parasite. T. gondii enters the host’s cells and form cysts in body tissues, which transmit the parasite if the host animal is eaten. Eating animals such as sheep, cows and pigs is thought to be a source of infection for humans, as is close contact with cat excrement.
A fascinating aspect of the T. gondii lifecycle is its ability to influence the behaviour of host animals. Healthy, uninfected rodents are naturally frightened by the scent of cat urine, meaning they avoid places inhabited by cats and reduce the risk of being eaten. Rodents infected with the T. gondii parasite, however, lose this fear of cat urine, and may even be attracted to the scent, resulting in much greater chances of being eaten by a cat. This behavioural change makes it much easier for cats to catch infected rodents, and benefits T. gondii by making it more likely that the parasite is transmitted to its definitive host, the cat.
With so many humans infected with T. gondii, there could be important implications for our species. Most people infected with the parasite may experience a brief period of mild flu-like symptoms followed by no apparent further ill-effects. In certain cases however, such as during pregnancy or in people with poor immune systems, the symptoms can be much more serious, or even fatal.
The parasite’s ability to alter behaviour may have profound effects on human hosts, influences scientists are only beginning to understand. T. gondii has been linked to schizophrenia, suicidal behaviour and more subtle personality changes. The parasite also reduces reaction time, and has been implicated in road accidents – Czech professor of biology Jaroslav Flegr has suggested that globally the infection could be the cause of at least one million traffic accidents per year.
Parasites such as T. gondii demonstrate the complexity, importance for humans and the often downright weirdness of parasitic life cycles. With so many people infected with the parasite, and the potential for it to alter our behaviour, it could be speculated that parasites could actually influence human culture, along with our evolution.
Parasites are fascinating organisms that seem to exist in every place where life thrives and have developed ways of interacting and exploiting other species that are complex, sometimes unpleasant, and often simply bewildering when you consider how these relationships may have evolved over time.