As a concert organizer, it's a difficult crowd.
More than a billion people harbor parasitic worms that reside in their intestines. For most of them, the stay is short, the immune system expelling worms in a few days or weeks and leaving no trace of the presence of parasites.
However, in a small percentage of people and other animals, worms have a permanent mouthing and can remain in the intestines for years. This persistence often leads to malnutrition, which tightens the vise of infection and triggers a spiral difficult to escape.
The question of why this small percentage struggles to eliminate parasites has blocked researchers for a while. But a new mathematical model developed at the University of Nebraska-Lincoln and at the Royal Netherlands Institute for Sea Research offers an answer while reinforcing a saying: security in numbers.
Clay Cressler, Anieke van Leeuwen, and their colleagues concluded that when a large number of worms invade the intestinal tract in a host, they are usually able to attach to it in the long term. In contrast, the model suggests that isolated worms, or small groups of them, are submerged by the immune system and rapidly removed from the gastrointestinal tract.
"Some hosts get sick for a long time – some (parasites) very quickly," said Cressler, assistant professor of biological sciences in Nebraska. "This is not something that could be explained with the theory we had before."
The team model can simulate host-parasite interactions by taking into account several variables: number of parasites, availability of food, body mass of the host, which part of this mass is it accessible to parasites compared to the immune system. Crucially, this also takes into account that parasitic worms can manipulate the host's ability to digest food and trigger an immune response.
In a recent study, the team simulated whipworm infections in 1000 mice while varying the number of worms infecting their colon. The researchers deduced from these results that a large group of whipworms can trigger a cycle that is perpetuated and that almost guarantees the survival of parasites.
Although the details are not entirely clear, previous research has led the team to believe that a critical mass of worms could trigger a barrage of molecules that would scramble or suppress the immune system and allow parasites to start extracting nutrients. of a host.
"All that a parasite does to increase its own access to resources indirectly deprives the host," Cressler said. "The more parasites there are, the better they manipulate the resources, the better they handle, the better the parasites that develop, and the better they handle, so you get that positive feedback loop that allows them to do that. establish a chronic infection.
When a moderate number of worms infects a host, the length of their stay becomes less certain and seems to depend more on other factors: What is the magnitude of the host's fat reserves? Has his immune system already been prepared to react? How much does it eat?
On this last question, the team found evidence that more food increases the chances of staying in the long run.
"Historically, the prospect is that food really only counts for the host," Cressler said. "Only recently, in the last decade or so, has there been more focus on the effects of diet on the parasite. Is there a chance that what you do is just feeding the parasite more? In this case, that's what happens.
"If (biological) systems are frequently found in this place where you can go in one direction or the other, then small changes in other things to which we are not (historically) attentive may be those that determine whether you are going one way or the other. "
However, when all other factors are equal, the model indicates that the amount of parasitic worms alone can determine whether they persist in a host. This is a potentially important advance, Cressler said, since previous approaches had mainly treated the duration of the parasite as a fixed and independent variable rather than dependent on other factors.
And when the team compiled the fate of each mouse in its simulation, the resulting distribution resembled the pattern commonly observed in nature: most hosts are encumbered only by a few parasites, some are loaded with many .
"Most of the theory we use to understand immune-parasite interactions is insufficient to address the issue of the origin of this type of variation," Cressler said. "If you have the capacity to (allow) long or short, you will find that many hosts continue to contract short-term infections and have a very small workload, and some hosts will continue to contract these infections. long-term infections and end up with a very heavy burden.
"So, this hypothesis could potentially help explain two major trends in parasitology: this widespread variation in the duration of infections and this widespread variation in the health status of each host."
Although the team model should first of all withstand the scrutiny of experimentation and further research, it could possibly help to inform the treatment of parasitic or parasitic infections. to calibrate the forecasts of chronic cases. This before potentially life-threatening infections lead to malnutrition – and reduce the usefulness of drugs that eliminate pests – could prove particularly useful in developing countries.
"If we could (eventually) say," That's what you need to measure to see if a person is going to be chronically infected, "it may tell you something about how you should treat or who you must treat, "Cressler said." Where is the high-risk area, where is the environment such that people will be sensitized to this chronic infection rather than to acute? "
"This variation in duration is a real trend and it has real consequences for people – anything we can do to help us understand how these infections work could potentially have therapeutic value later on." We do not know what it's now, but the goal. "
The researchers reported their findings in the journal Acts of the Royal Society B.
Parasitic worms also need their intestinal microflora
Anieke van Leeuwen et al. The manipulation of parasite resources results in a bimodal variation in the duration of infection, Acts of the Royal Society B: Biological Sciences (2019). DOI: 10.1098 / rspb.2019.0456
Discovery could explain mystery of long-term parasites (May 22, 2019)
recovered on May 23, 2019
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