Last updated March 15, 2018 at 10:47 am
The colonisation of a host by a pathogen – whether that be a virus, a bacterium or a large, complex parasite – involves a gruesome trade-off.
If colonisation of a host by a pathogen is fast, furious and uncontrolled – as in the case of Ebola, for instance – the host dies in a very short period of time. This, in turn, results in the death of the pathogen and severely diminished opportunities to find a new host while the old one is still capable of walking around.
So several studies into pathogen virulence have suggested that, over time, bugs should become less lethal, because it is in their evolutionary interests to extend the lifespan of a host for as long as possible.
The contention is broadly supported by evidence in many cases, although there are some confounding factors. Rapid expansion within a host, for instance, is adaptively useful, because it allows a pathogen to out-compete other nasties vying for the same territories, even though this strategy can have lethal consequences.
Now, researchers from the University of Manchester in the UK have identified an unusual survival mechanism employed by a multicellular parasite known as a whipworm.
The worms (Trichuris muris) are one of the most common human parasites in the world, infecting perhaps as many as one in four people. They cause an infection known as trichuriasis, and have pretty much nailed the virulence-survival trade-off.
Whipworms face a numbers game
Despite infecting an estimated 795 million people, trichuriasis is rarely fatal. The real number of infections is probably very much higher, but people harbouring only a small colony of the worms are unlikely to show any symptoms. At higher infestations, drawbacks primarily involve diarrhoea – which can cause severe problems for children, especially those without adequate nutrition or hydration.
Whipworms spread through eggs in the faeces of their host, which are then consumed either directly (transferred by poor handwashing practice, for instance) or on the surface of unwashed vegetables (in places where human dung is used as fertiliser).
Once hatched inside the human gut, the whipworm potentially faces a numbers game. A small population means a long and happy life for both parasite and host, but if the population grows, the viability of the host is adversely affected.
In a paper published in the journal Science Advances, a team led by Richard Grencis from Manchester University’s School of Biological Sciences reveals how whipworms prevent overcrowding by sabotaging the eggs of their own species.
Grencis and colleagues discovered that once hatched the worms co-opt human gut microbes to form their own digestive flora. This was quite surprising.
Shining a light on parasites
What was even more unusual, however, was how the whipworms then established a feedback loop between its own, now substantially modified, microbiome and that of its host. The result was that the host’s gut became antagonistic towards newly arrived eggs, preventing them from hatching and thus keeping the parasite population stable.
“We already know that our intestinal microflora underpins all aspects of our health,” says co-author Ian Roberts. “But we were amazed to find that whipworms have their own distinct microflora – and that it does the same for them, something we think is unique for these types of parasites.
“This shines a light on the fascinating relationships between the parasites, the host and their intestinal dwelling bacteria.
“But is also shows how the parasite can exploit this relationship to its own ends promoting its own existence.”
Whipworms are increasingly resistant to existing treatments. The scientists hope that their research will provide new targets for drug development.