Dear friends, today we will enter an invisible universe, a microcosm populated by creatures as small as they are extraordinary: parasites. Don’t be fooled by the name, which conjures up images of repulsive and dangerous beings. Parasites are much more than simple enemies: they are key players in an eternal comedy of nature, a dance of survival and adaptation that has been unfolding for millions of years and which continues to surprise us with discoveries.
Imagine an immense stage where every living being plays its part. In this theatre, parasites are the protagonists of a silent drama, a story of complex and fascinating interactions with their hosts, the organisms that host them. And just like in a good drama, there is no shortage of twists and new revelations, as the latest news from the world of science shows.
But what exactly are parasites?
They are somewhat invasive tenants who settle inside or on the surface of another organism, drawing nourishment and shelter from it. Think of a tiny worm that nestles in the intestine or a protozoan that multiplies in the blood. Their presence can go unnoticed or manifest itself with more or less severe symptoms, depending on the type of parasite and the response of the host organism.
An endless battle: mutual adaptation
The relationship between parasite and host is a continuous challenge, an evolutionary tug-of-war in which both contenders deploy sophisticated strategies. The host develops an arsenal of biological weapons to defend itself: the immune system, with its white blood cells and antibodies, is at the forefront of the fight against invaders. But parasites do not stand idly by: over millennia, they have evolved incredible mechanisms to evade host defences, camouflaging, hiding, or even manipulating the immune system to their advantage.
It is a never-ending arms race, where each adaptation on one side pushes the other to evolve new countermeasures. An example? The genes of the major histocompatibility complex (MHC), fundamental for recognizing pathogens by the immune system, show extraordinary variability in populations living with high parasite pressure. It is as if the host, faced with the threat, diversified its genetic locks to make it more difficult for parasites to enter.
Invisible messengers: extracellular vesicles
But communication between parasite and host is not limited to a simple clash. Silent messengers also come into play: extracellular vesicles (EVs). Think of them as tiny spaceships released from the parasite’s cells, carrying information into the host organism. This information can influence the immune system, attenuating the defensive response or even promoting the parasite’s survival.
When the parasite takes control: behavioural manipulation
Sometimes, the parasite’s strategy is even more subtle. Some parasites can manipulate the host’s behaviour, transforming it into a sort of puppet. Please think of the case of Toxoplasma gondii, a protozoan that infects mice, making them attracted to the smell of cat urine, its definitive host. In this way, the parasite increases its chances of completing its life cycle. A recent study published in “Nature” has shown that Toxoplasma gondii can also alter human behaviour, increasing impulsivity and propensity for risk. [Link to the article in Nature]
A crowded apartment building: coinfections
In reality, a host organism can be colonized by multiple parasites simultaneously. It’s like a crowded condominium, where different species coexist, sometimes in harmony, sometimes in competition. Coinfections can have unpredictable consequences: sometimes, one parasite can facilitate the infection of another, and other times, it can hinder it.
Take, for example, malaria, helminths, and intestinal worms. Malaria stimulates a Th1 immune response, while helminths induce a Th2 response. The presence of a coinfection can alter this balance, influencing the course of both diseases. In some cases, helminths can even protect against severe malaria, while in others, they can worsen the situation. A recent study conducted in Africa has shown that coinfection with helminths can reduce the effectiveness of antimalarial drugs. [Link to article on study in Africa]
An immune system under attack
Parasites, as we have seen, are masters in manipulation. Not only can they evade the host’s defences, but they can also influence the host’s immune system in profound ways. Some parasites, for example, induce a Th2-type immune response, which favours their survival but can make the host more vulnerable to other infections. Other parasites alter the function of macrophages, immune cells essential for the body’s defence. An article published in “Science” recently highlighted how some intestinal parasites can “reprogram” the immune system, inducing tolerance towards allergens. [Link to Science article]
From co-evolution to medicine: new research perspectives
The study of parasites and their interaction with hosts is not just a matter of scientific curiosity. Understanding co-evolution mechanisms, immune manipulation, and coinfection can open new avenues for fighting infectious diseases. Knowledge gained in this field could lead to developing new vaccines, drugs and pest control strategies. For example, researchers at the University of Oxford study EVs released by malaria parasites to create a new vaccine against this disease. [Link to news about Oxford researchers]
A world to protect
Parasites, as unwelcome as they may seem, are integral to our planet’s biodiversity. Their presence, although sometimes problematic, contributes to the balance of ecosystems. Therefore, It is important not to demonize them but to study and understand them and learn to live with them sustainably.
And now, dear friends, I leave you with a question: what will be the next chapter in this fascinating story of co-evolution between parasites and hosts? Only scientific research will be able to reveal the secrets of this hidden world, offering us new solutions to protect our health and that of the planet.
References
- ** Mardahl, M., Borup, A., & Nejsum, P.** (2019). A new level of complexity in parasite-host interaction: The role of extracellular vesicles. Advances in Parasitology, 104, 39-112.
- ** Woolhouse, M. E. J., Webster, J. P., Domingo, E., Charlesworth, B., & Levin, B. R.** (2002). Biological and biomedical implications of the co-evolution of pathogens and their hosts. Nature Genetics, 32(4), 569-577.
- ** Ebert, D., & Fields, P. D.** (2020). Host-parasite co-evolution and its genomic signature. Nature Reviews Genetics, 21(12), 754-768.
- ** Cattadori, I. M., Boag, B., & Hudson, P. J.** (2008). Parasite coinfection and interaction as drivers of host heterogeneity. International Journal for Parasitology, 38(3-4), 371-380.
- ** Hughes, D. P., & Libersat, F.** (2019). Parasite manipulation of host behaviour. Current Biology, 29(2), R45-R47.
- ** Klein, S. L.** (2005). Parasite manipulation of host behaviour: mechanisms, ecology, and future directions. Behavioural Processes, 68(3), 219-221.
- ** Viney, M. E., & Graham, A. L.** (2013). Patterns and processes in parasite coinfection. In Advances in Parasitology (Vol. 82, pp. 321-369). Elsevier.
- ** Mabbott, N. A.** (2018). The Influence of Parasite Infections on Host Immunity to Coinfection With Other Pathogens. Frontiers in Immunology, 9, 2579.
- **Gómez-Arreaza, A., Haenni, A.-L., Dunia, I., & Avilán, L.** (2017). Viruses of parasites as actors in the parasite-host relationship: A “ménage à trois”. Acta Tropica, 166, 126-132.
