Slaves to parasites
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Invisible to the naked eye, deceitful micro-organisms deploy subtle strategies to proliferate by hijacking your behaviours or those of the animals you live with. You may have already come across one of them during your lifetime, without even knowing. Their purpose is to survive to the detriment of their host, not for lethal purposes but in order to proliferate and continue to infect. Welcome to the world of parasites.
My pact with felines
My name is Toxoplasma gondii, and I cause toxoplasmosis. This is my story1. As a clandestine passenger, I have developed numerous strategies to infect you. Have you ever heard of the dystopian tale of The Last of Us? In this TV series adapted from a video game [7], fungi transform their victims into zombies. In nature, these fungi are restricted to insects, because your body temperature of 37°C renders access to your body impossible. However, where they fail, I win. Let me reveal the secrets of a silent invasion.
First of all – no offence meant – you should know that even if I do infect you, my hosts of choice are actually cats. Indeed, it is only in their intestines that I am able to achieve my sexual reproduction and share my genes with those of my own species. This genetic mixing is vital – it makes me more adaptable and resistant before my offspring can be disseminated into nature as oocysts, through cat faeces. This type of egg is extremely resistant to environmental pressures and will wait patiently for someone to inadvertently ingest it so that we can colonise you.
The art of infiltration
Once ingested, I turn into a lively and proliferative organism – a tachyzoite. Without your permission, I infiltrate your cells by creating my own portal of entry. I hack into your membrane proteins and then wrap myself in them to form, at the heart of the cell, a new structure called a parasitophorous vacuole. This vacuole offers me a cloak of invisibility that prevents your lysosomes (the “stomachs” of your cells) from digesting me.
From this safe haven, I can deploy my most sophisticated arsenal to re-programme the control centre of your cells. My rhoptries (tools for molecular hijacking) inject proteins that inhibit the warning signals of inflammation. My dense granules then take over. They strengthen my vacuolar wall and pierce it with holes to suck in your nutrients. Your cells thus become a passive larder where I can safely proliferate2.
From cell to secret conveyor
My development takes a decisive step when I infect your dendritic cells – the sentinels of the immune system [11]. Although they should sound alarm bells in your lymph nodes, I am able to hijack their capacity for migration. I thus turn them into clandestine transporters that enable me to travel via your blood throughout your body. However, the latter is not a passive fortress. So despite all my efforts, the immune system ultimately manages to detect the activity of my living forms – the tachyzoites – and to halt the acute infection. That is why I need to establish myself in the only place where your immune defences can no longer reach me: your brain.
Without arousing the least suspicion, I will therefore breach the blood-brain barrier [12], this reputedly insurmountable wall that separates the blood from the nervous system. By forcing through this barrier, I can reach your brain and will be able to implement my plan for cerebral manipulation3.
The sanctuary of the brain
Once in this sheltered environment, I change strategy and form. From a tachyzoite I become a bradyzoite, a latent and patient form. Comfortably in place, I cloister myself in cysts where I remain metabolically inactive. Scientists suspect that my presence in this form could trigger epileptic seizures, psychiatric illnesses such as schizophrenia or neurodegenerative disorders like Alzheimer’s disease [14]. And indeed, by living in your neurons, I am able to modify the production of dopamine, a key molecule in motivation…
But the fact remains that settling in your body is a biological deadlock for me. Remember my true objective: the only place where I can achieve my sexual reproduction is the intestine of cats. And because you will never be on the menu of your pet, I and my line are condemned to die with you.
In fact, it is in nature that my talents as a puppet master are fully expressed, to the detriment of smaller animals. To reach cats, I therefore hijack the brains of mice. By secreting a molecule that mimics dopamine, I am able to modify their behaviour and suppress their instinctive fear of predators. In practice, instead of fleeing the smell of cat [15] urine, mice are now attracted by it and deliberately follow the path towards a deadly end.
From puppet master to slave
However, I have to admit – albeit reluctantly – that humans have discovered a means of exploiting my skills as a manipulator of the brain. Scientists have focused on my ability to cross the blood-brain barrier and settle in your neurons. So they have in turn manoeuvred me, changing me into a biological messenger and forcing me to transport therapeutic proteins to treat Rett’s syndrome4 directly in the brain5.
You could say that’s giving me a taste of my own medicine, and it may be true. This fragile boundary between technological feat and formidable weapon has fascinated one of the most famous thriller authors. In Franck Thilliez’s novel À retardement (“Delayed action”), I am used and hijacked in order to become the trigger for true violence, turning ordinary individuals into compulsive killers. Although science shows us that I am not this fictional weapon, these stories highlight how much my links with your brain continue to captivate human imagination.
I am one of the parasites that most enthralls the world of research, although not the only one. My phylum, the Apicomplexa, encompasses some 6000 species capable of infecting warm-blooded animals – not only birds but mammals, which of course include human beings.
While you read these lines, perhaps one of my fellow parasites is quietly sleeping in the folds of your brain. It will not turn you into a zombie, because the evolution of such hijacking may take millions of years. But a question remains: in this arms race6, who really is in the lead?
- 1. This work of popularisation is based on the research of, and interviews with, two CNRS specialists whose studies have thrown light on the mechanisms of survival, transformation and movement of micro-organisms: Frédéric Bringaud, CNRS research professor in the MFP laboratory (CNRS / Université de Bordeaux), and Mathieu Gissot, CNRS research professor at the Centre for Infection and Immunity of Lille (CIIL – CNRS / CHU Lille / INSERM / Institut Pasteur de Lille / Université de Lille).
- 2. Bringaud F., et al. “Energy metabolism of trypanosomatids”, Molecular and Biochemical Parasitology, 2006: https://doi.org/10.1016/j.molbiopara.2006.03.017 [17]
- 3. T. Mouveaux, et al. “Primary brain cell infection by Toxoplasma gondii reveals the extent and dynamics of parasite differentiation and its impact on neuron biology”, Open Biology, 2021: https://doi.org/10.1098/rsob.210053 [18]
- 4. A rare disease that alters the development of the central nervous system.
- 5. S. Bracha, et al., “Engineering Toxoplasma gondii secretion systems for intracellular delivery of multiple large therapeutic proteins to neurons”, Nature Microbiology, 2024: https://doi.org/10.1038/s41564-024-01750-6 [19]
- 6. Permanent evolution of a species to maintain its abilities when faced with evolutions in the species with which it is co-evolving.
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