These organisms may be very small, but they can be very deadly. But what exactly are parasites?<\/p>
What are parasites?<\/h2>
Put simply, parasites<\/a> 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. <\/p> 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\u2026<\/p> 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. <\/p> 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. <\/p> Not only a killer parasite, but one of the world\u2019s biggest killers, the malaria parasite is responsible for around 600,000 deaths a year. Their hosts and carriers, female mosquitos of the anopheles<\/em> genus, are consequently considered to be one of the deadliest animals to humans<\/a> and the world’s deadliest insect.<\/a> <\/p> The disease-causing parasite itself is a single-celled microorganism called Plasmodium. Plasmodium <\/em>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<\/a> that is carried from one mosquito<\/a> bite to the next. <\/p> Could climate change bring malaria back to the UK?<\/a><\/p> Once a plasmodium parasite has entered a person\u2019s bloodstream it travels to their liver, where it then matures and reproduces. The resulting next generation then invade the person\u2019s red blood cells, and in doing so leave a trail of destruction in their wake, rupturing liver cells and disrupting the blood cells<\/a>‘ ability to transport oxygen around the body. If left untreated the damage caused by the parasites can be fatal. <\/p> 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\u2019s bitter taste – and hence the famous G&T was born!<\/p> 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. <\/p> 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. <\/p> 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.<\/p> 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<\/em> of cells. <\/p> The disease that Trypanosoma brucei <\/em>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). <\/p> The parasite lives within tsetse flies and is transferred from person to person through the fly\u2019s 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.<\/p> 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. <\/p> 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\u2019t have access to good treatment and malnutrition causes weakened immune systems.<\/p> 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. <\/p> A person becomes infected when they drink water contaminated by guinea worm larvae, residing in the body of small Cyclops <\/em>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!<\/p> 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. <\/p> Most of the time the parasite does not cause any symptoms, and people don\u2019t even know they have it, but it can rarely cause serious problems, particularly in people who are pregnant. Fascinatingly, infection by Toxoplasma gondii <\/em>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.<\/p> 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<\/em> is a species of flatworm that infects the livers of various mammals, but primarily sheep<\/a> and cattle<\/a>, which can then pass them on to humans. <\/p> 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.<\/p> 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.<\/p> Another parasite that is deadly to a non-human animal is Leucochloridium paradoxum, <\/em>also known as green-banded broodsac<\/a>, and the creator of zombie snails! <\/p> 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 \u2013 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. <\/p> 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.<\/p>Deadliest parasites in the world<\/h2>
Malaria Parasite (Plasmodium)<\/h3>
Schistosoma (Blood Flukes)<\/h3>
![\"schistosoma\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/GettyImages-1124683548.jpg\")
Trypanosoma brucei <\/em>(Sleeping Sickness Parasite)<\/h3>
Leishmania (Leishmaniasis Parasite)<\/h3>
![\"\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/Leishmaniasis-Parasite.jpg\")
Guinea Worm (Dracunculus medinensis<\/em>)<\/h3>
![\"\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/Guinea-worm-emerging-from-infected-foot.jpg\")
Toxoplasma gondii<\/em> (Toxoplasmosis Parasite)<\/h3>
![\"\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/Toxoplasma-gondii.jpg\")
Fasciola hepatica<\/em><\/h3>
![\"\"](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/Fasciola-hepatica.jpg\")
Leucochloridium paradoxum<\/em><\/h3>
![\"green](\"https:\/\/c02.purpledshub.com\/uploads\/sites\/62\/2024\/06\/Screenshot-2024-06-10-at-12.31.38.png\")
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