Did you know that around a million people each year die from parasitic infections, let alone all the other unfortunate animals that fall prey to parasitism?
These organisms may be very small, but they can be very deadly. But what exactly are parasites?
What are parasites?
Put simply, parasites are any organism (living thing) that lives in or on another organism - known as the host. They rely on hosts for nutrition and survival, feeding on the living body of the host, either by directly eating their tissues or absorbing nutrients from their body.
Eminent evolutionary biologist E.O. Wilson described parasites as "predators that eat prey in units of less than one". As well as feeding on them, many parasites use their hosts as breeding grounds. They may reproduce sexually or asexually inside the body, and lay eggs that hatch inside, creating the next generation of parasites within…
Unlike other, mutually beneficial, symbiotic relationships, such as the relationship between a plant and a pollinator, parasitism is one-sided and harmful to the host. Parasitism is by definition an exploitative relationship, with the parasite taking resources that the host does not want to give up. The harm caused may be slight, or it may be extreme, resulting in death.
Given parasites depend on their hosts for nourishment, however, killing the host is generally not a successful strategy for a parasite, and most find a way to coexist - taking the resources they need but not at the expense of their host's life. The ones on this list do often kill however, whether by accident or design.
Deadliest parasites in the world
Malaria Parasite (Plasmodium)
Not only a killer parasite, but one of the world’s biggest killers, the malaria parasite is responsible for around 600,000 deaths a year. Their hosts and carriers, female mosquitos of the anopheles genus, are consequently considered to be one of the deadliest animals to humans and the world's deadliest insect.
The disease-causing parasite itself is a single-celled microorganism called Plasmodium. Plasmodium are obligate parasites, meaning that they cannot complete their life cycles without finding and exploiting a suitable host- namely mosquitos and humans. Malaria does not spread from person to person by itself, instead, it is spread through infected blood that is carried from one mosquito bite to the next.
Could climate change bring malaria back to the UK?
Once a plasmodium parasite has entered a person’s bloodstream it travels to their liver, where it then matures and reproduces. The resulting next generation then invade the person’s red blood cells, and in doing so leave a trail of destruction in their wake, rupturing liver cells and disrupting the blood cells' ability to transport oxygen around the body. If left untreated the damage caused by the parasites can be fatal.
One of the first treatments found to be effective against malaria was quinine, a compound extracted from the bark of the South American cinchona tree, brought to the attention of disease-stricken European colonialists in the 17th Century. British officers in India used it in tonic form, but mixed it with gin and citrus to disguise it’s bitter taste - and hence the famous G&T was born!
Schistosoma (Blood Flukes)
Like malaria, schistosomiasis is a parasitic disease that particularly effects those in the developing world, particularly in the tropics. The disease agent in this case is something called a blood fluke - a genus of flatworm.
These tiny worms (just a little longer than a grain of rice) are spread through contact with contaminated freshwater - such as through swimming or fishing. In addition to humans (and other mammals), these parasites infect and develop within freshwater snails, and are released as free-swimming larvae by these snails into water sources.
These larvae then burrow into the skin of unfortunate humans and enter their bloodstream. Once they are inside they make their way to the bladder or intestines where they will mature into adult flatworms. Infection by Schistosoma can cause many issues, from kidney failure to internal bleeding. Although it does not typically cause death directly, in severe cases the damage it causes, and the resulting knock-on effects, can be fatal.
Trypanosoma brucei (Sleeping Sickness Parasite)
Unlike other single-celled parasites, this deadly freeloader does not infect the blood and tissue cells but exclusively inhabits the blood plasma and other bodily fluids outside of cells.
The disease that Trypanosoma brucei causes is known as Sleeping Sickness or African trypanosomiasis. This nasty tropical illness first causes fevers and headaches, then at a later stage confusion, comas, and extreme daytime sleepiness (hence the name).
The parasite lives within tsetse flies and is transferred from person to person through the fly’s saliva when it bites. Without treatment, this disease is generally fatal, and unlike bacteria and viruses, parasitic diseases cannot be prevented by vaccines. The only way to prevent it is to avoid getting bitten by tsetse flies. Thankfully, however, if you are unlucky enough to receive an infectious bite, the parasites can be treated and eliminated with medicines.
Leishmania (Leishmaniasis Parasite)
Another parasite spread by a flying insect is Leishmania, a single-celled organism that is responsible for the nasty tropical and subtropical disease leishmaniasis. The parasite is transmitted and spread between its victims by the bite of infected female sandflies of the phlebotomine genus.
There are a few different forms of the disease, with differing degrees of severity. The deadliest form is Visceral leishmaniasis. If left untreated it is fatal in more than 95% of cases. Fortunately, leishmaniasis is both treatable and curable, however it does require a well-functioning immune system. Leishmaniasis is at its most deadly in poorer countries, where people don’t have access to good treatment and malnutrition causes weakened immune systems.
Guinea Worm (Dracunculus medinensis)
This next one is liable to make you feel very squeamish. Guinea worms are the largest tissue parasites that affect humans (as opposed to intestinal parasites such as tapeworms). When adults they can measure 600 to 800 mm in length and 2 mm in diameter, and their lifecycle is like something from a horror film.
A person becomes infected when they drink water contaminated by guinea worm larvae, residing in the body of small Cyclops crustaceans (the intermediate host). The cyclops is then dissolved in the stomach, releasing the worm larvae which then migrate through the intestinal wall and escape into the body. About a year later, after mating with the host, the now fully grown female will travel down the body, through muscles, to find an exit site. The worm will emerge, usually from the feet, erupting from the skin in an intensely painful blister. Horrific!
Toxoplasma gondii (Toxoplasmosis Parasite)
As well as making you squeamish, this one may make you think differently about your feline friends. Toxoplasmosis is spread to humans from infected cats, specifically via their poo.
Most of the time the parasite does not cause any symptoms, and people don’t even know they have it, but it can rarely cause serious problems, particularly in people who are pregnant. Fascinatingly, infection by Toxoplasma gondii is known to change the behaviour of rats and mice to make them more likely to be eaten by cats! Rodents who are infected show a lessened aversion to cat urine and more exploratory behaviours. In this way, the parasite ensures it is passed on to the host in which it breeds.
Fasciola hepatica
So far we have mainly discussed parasites that are deadly to humans, but what about other animals? Also known as the common liver fluke, Fasciola hepatica is a species of flatworm that infects the livers of various mammals, but primarily sheep and cattle, which can then pass them on to humans.
It causes the, thankfully rare, disease Fascioliasis. Because the parasite lives and develops within the liver and bile ducts of its hosts, the disease can cause damage to these areas and result in symptoms like inflammation, abdominal pain, and jaundice.
In itself, fascioliasis is rarely fatal in humans, though severe untreated cases can lead to life-threatening complications. In cattle, sheep, and goats however the death rate is high, and global losses in animal productivity due to fasciolosis constitute an expensive and livelihood-threatening problem.
Leucochloridium paradoxum
Another parasite that is deadly to a non-human animal is Leucochloridium paradoxum, also known as green-banded broodsac, and the creator of zombie snails!
This parasitic flatworm infects the eyestalks of land snails, reproducing and creating large broodsacs that pulsate in a way that imitates the movement of a caterpillar – the prey of their primary host, birds. Even more impressively, the parasite manipulates the behaviour of its host, making the snail stay out in the open and on higher vegetation so that they are more conspicuous.
This ensures that the snails (and the parasites within) are eaten by birds and the parasites can move on to the next stage in their lifecycle.
Ampulex compressa
From zombie snails to zombie cockroaches! This next on our list is the small but deadly solitary tropical wasp, Ampulex compressa, also known as the jewel wasp.
These mind-bending parasites are of little consequence to humans, but instead exert their gruesome powers on unfortunate cockroaches. In a sequence of behaviours that was the inspiration for the classic chest-bursting scene in Alien, the female jewel wasp first jabs the victim with her stinger delivering a powerful neurotoxin.
The toxin is not injected at random, but very deliberately into the ganglia - the cockroach’s equivalent of a brain. This causes a rapid change in behaviour of the cockroach. First, it will meticulously groom itself, then it will become lethargic and sluggish, all normal escape responses put on pause.
Whilst it is in this state the wasp takes the opportunity to chew off half of each of the roach's antennae, drag it to a pre-prepared dark burrow by the antennae, and then lay her eggs on its body. She then buries it alive. When the eggs hatch the wasp larvae start feeding on the roach from the outside and as they grow will chew their way into its abdomen and live inside. Eventually, a fully formed wasps will burst out and look for new hosts to exploit…
Cordyceps fungi
Finally, we move on to perhaps one of the most talked about parasites in popular culture. Unlike in a recent popular TV show Cordyceps does not infect humans (and lead to zombie apocalypses) but mainly exploits insects. In contrast to the rest of the parasites on this list - primarily single-celled organisms and various worms - Cordyceps is a fungus.
They also do not need to enter their host’s body through ingestion as many parasites do but can directly breach the exoskeleton of their insect victim. Spores of the fungus attach themselves to the outer body and bore through the skeleton as they germinate. Once within, the mycelium (fungal threads) invades and eventually replaces the body tissues.
In a dramatic and gory finale, the fungus’ fruiting bodies will erupt from the unfortunate (and now dead) insect, using their emptied corpse as a launchpad from which to disperse more spores. As with Ampulex compressa and Leucochloridium paradoxum, there is a mind-control aspect to their parasitism. Infection causes the hosts to seek higher ground, benefitting spore dispersal.
We named Cordyceps one of the world's weirdest fungi
Read about more fascinating parasites
- Ugh there's a tongue-eating parasite? Afraid so, and it grows as big as a cockroach. Meet Cymothoa exigua
- Wriggling 8cm-long roundworm larvae found in Australian woman’s brain
- It’s a slimy, prehistoric parasite with the mouth of an alien. And two fearless photographers are obsessed with finding it
- Meet the fairy wasp, a particularly deadly parasitic wasp
