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Hidden parasites of the human brain

TW: disturbing imagery, psychiatric disorders, suicide 

This article is not suitable for hypochondriacs 

Beyond the realm of science fiction, there are several creatures out there that can actually change the behaviour of the humans and animals they infect. 

I will present two parasites in particular: Toxoplasma gondii, a protozoan parasite that causes toxoplasmosis, and the prion that causes Kuru disease. 


Toxoplasmosis, the endemic plague

T. gondii is a protozoan, a single-celled eukaryote, whose spores are excreted in cat faeces. Once excreted, the spores can be picked up by other mammals, and even birds, where they develop in the host’s digestive tract. T. gondii can also be picked up by eating undercooked meat and seafood. From there the protozoa migrate to the eyes, muscles and brain of the host, where they can lay dormant for decades. In most humans, infection doesn’t cause any noticeable symptoms, although those with weakened immune systems may experience flu-like symptoms and damage to brain and eyes.

It is the most prevalent human infection in the world, with an estimated 30-50% of all humans carrying T. gondii in their brains, usually asymptomatically. Until the protozoa wake up. 

You may be surprised to hear the highest global prevalence of T. gondii infection is actually in Europe and South America, with an average incidence rate of 10-60% and up to 90% in some areas of Europe. Asymptomatic infection is linked to a range of common conditions such as epilepsy, OCD, autism, Down’s Syndrome, psychotic disorders, as well as an increased risk of attempting suicide.

A sharp increase in human schizophrenia prevalence coincides with the rise in domesticated cats kept as pets in the early 1900s. Although toxoplasmosis does not directly cause schizophrenia, there is strong evidence that it could be a trigger for those that are genetically predisposed to it.

Pregnant women are advised not to clean up cat litter due to the risk of birth defects toxoplasmosis can cause in a developing foetus that has an undeveloped immune system. 

Vaccine development for both cats and humans is ongoing.



Kuru, the Zombie disease

Kuru disease is another disease that infects the human brain, and is a type of spongiform encephalopathy, a group of diseases that includes Mad Cow’s Disease.

Kuru disease is caused by misfolded proteins called prions that, although not technically alive, act like viruses in that they trigger normal proteins to misfold when they come into contact. The prions “eventually misfold enough proteins to kill pockets of nerve cells in the brain, leaving the cerebellum riddled with holes, like a sponge.” This is how it gets the name spongiform encephalitis, from sponge-shaped “enkephalos” meaning brain in Greek.

In contrast to toxoplasmosis, kuru is extremely rare and thought to be isolated to the Fore people in Papua New Guinea. However, it’s fascinating in that it is probably the origin of our modern day zombie tropes. Kuru is spread by eating brain matter, a funeral rite formerly practiced by the Fore tribe in the belief that it would free their dead ancestors’ spirits. 

Symptoms of kuru disease will be familiar to zombie fans: uncoordinated movement and unsteady walk, difficulty speaking known as dysarthria, tremors, uncontrollable laughter, ulcers and skin necrosis (i.e. rotting skin), dysphagia which causes drooling, vomiting, and difficulty controlling the mouth, and becoming unresponsive to their environments. 

The clinical stage, the time between which the symptoms begin and when they end, is 12 months, meaning infected victims would have been walking around their communities for quite some time. As kuru is transmitted by cannibalism of relatives, it tended to run in families and villages, especially in women and children as they were the ones that would traditionally eat the brains. 

Since the ban on cannibalism by Australian colonizers in the 1950s, kuru prevalence decreased dramatically. However, with an incubation period of up to 50 years or more, the last reported case died as recently as 2005.

Although these two diseases can lay dormant for several decades completely unnoticed, they can have dramatic and lethal effects on human physical and cognitive health. In particular, it will be interesting to see how future research will shed more light on the effects of toxoplasmosis on human behaviour. With up to half of the Earth’s eight billion potentially infected, the consequences are far from trivial. Could the crazy cat lady trope hold some truth?

What do you think? Is the human race run by parasites?

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