In countries like the USA and the UK, inflammatory diseases – like allergies, type 1 diabetes, inflammatory bowel disease, autoimmune disorders, and even some kinds of atherosclerosis, depression and cancer – are all on the rise. Despite ever-increasing access to healthcare and medical technology, the rate of these diseases, which are caused by our bodies waging war with ourselves, is still increasing. Meanwhile, these same developed nations have almost completely eradicated parasitic infections like they’re the plague. We’re grossed out by tapeworms and botflies, and pity the suffering countries that aren’t able to provide the sterile, parasite-free lifestyle that we desperately try to maintain.
Three papers published in the journal Immunology might have an unexpected answer that brings together these seemingly disconnected trends: it might be a lack of parasites that is aiding the increase of inflammatory diseases in our hygiene-obsessed, over-developed world.
What are Helminths?
To understand how parasitic infections might benefit our immune systems, you have to understand how they work. Parasitic worms from the order Helminthes are worms that cause long-standing internal infections that our bodies are generally powerless or refuse to fight off. They’re broken down into three groups:
- the cestodes (tapeworms)
- nematodes (roundworms)
- trematodes (flukes)
They feed off of our bodies, either by stealing the nutrients we ingest or actually eating our tissues and cells. Many grow and live inside us without our knowledge. For example, it’s estimated that 40 million Americans are infected with pinworms, a kind of roundworm, whose only symptom is a little itching.
Keep Friends Close and Your Enemies Still Closer
To stay alive inside another body requires a very specific battery of weapons to fight off the human immune system – and this is where the Helminths may have cracked the code of coexisting within humans. By presenting and releasing a barrage of different compounds to trick and calm our immune responses to their presence, the Helminths are allowed to stay. This is true of most parasites we tend not to notice – after all, we don’t notice them because our immune systems are not causing the symptoms they normally do with other infections, like fevers and congestion.
The idea that the immune system intentionally avoids a strong response to Helminth infections is sometimes called the “Old Friends Hypothesis“. Since worm infections were so common in our past and often weren’t that devastating, trying to rid ourselves of the parasites caused more damage than putting up with them.
Put it this way: if our systems try to attack and kill every foreign thing that enters our bodies, we run the risk of making mistakes – attacking our own cells instead of offending intruders. This is especially true of parasitic infections where there is an evolving organism involved, constantly changing what it looks like, and mimicking our own cells if it can.
Can something that steals our food and eats our flesh be good for us?
Whether its because of our own immune systems ignoring parasitic infections or immune-suppressing compounds released by parasites to stay alive, the most common response in humans to Helminth infection is a release of anti-inflammatory compounds which down-regulate immune responses – quite a good thing if your immune system is over-responding, as is the case with Type 1 Diabetes.
This kind of diabetes is caused by immune cells killing off the pancreatic B cells that produce insulin. We’re not entirely sure why some people’s immune systems plan this seek-and-destroy against their own pancreas, but it’s a terrible disease. Logically, shutting down the over-zealous immune system helps prevent and even reverse Type 1 Diabetes.
Anne Cooke, from the department of Pathology at the University of Cambridge, reviews the mounting evidence that early infections, particularly with helminths, can help prevent or delay the onset of diabetes1. Studies have shown that mice that are genetically engineered to get this type of diabetes don’t get it if they’re injected with the adults, eggs, or compounds produced by the flatworm Schistosoma mansoni at an early age. Infection increases the immune regulatory circuits, delaying or completely preventing the diabetic onset.
Parasitic infections have also been shown to slow or stop the development of Crohn’s disease, inflammatory bowel disease, Grave’s disease, and autoimmune diseases. Below is a neat video explaining some aspects of Helminthic Therapy – used for autoimmune diseases:
The idea put forth in these three articles from Immunology is that by essentially eradicating many of the parasitic worms from developed nations, we’ve made our immune systems more likely to over respond and malfunction. In the past, infection with these parasites was so common that we co-evolved to live with them, and by removing them, we’ve taken away one of the regulatory mechanisms that our bodies once couldn’t avoid.
As Joseph Jackson et al. explains:
“there would have been selection for immunoregulatory systems that work well in the context of worm infection” because infection “is, and (probably) has been so ubiquitous.”2
By eradicating our pests chemically and by excessive hygiene, we’ve thrown our immune systems out of whack. As Graham Rook writes in his review, we have:
“increasing failure in the rich, developed nations of immunoregulatory mechanisms that should terminate inappropriate inflammatory responses.”3
He goes on to explain how these mechanisms are kick-started by parasites, and how a lack of them can lead to all kinds of pathology, from allergies to cancer.
But it’s not just self-inflicted diseases that helminths help prevent. Other infectious agents, like aggressive forms of malaria and the flu, are worsened when our immune systems go into overdrive trying to kill them off. So having worms might alleviate the symptoms of other infections, too.
The downside, of course, is that a depressed immune response, while great for preventing and treating over active immune diseases, also means we have less response when we’re supposed to or need to. Indeed, parasitic infections have also shown to make other pathogens, particularly bacteria, much worse.
What do we do with this knowledge?
What’s important to take away from these kinds of interactions is that by removing certain stimulus from our immune systems, we’ve knocked them off balance. Future research might be able to find compounds or vaccines that get the balance back in check without having to infect people with parasites. We might be able to mimic the positive effects of parasitism without exposing ourselves to the negative ones. In the meantime, such research stands to show that we need a healthy understanding and appreciation for the delicate balance between organisms and the different roles each living creature plays, especially the ever-loathsome and undervalued parasites.
1. Cooke, A. (2009). Review series on helminths, immune modulation and the hygiene hypothesis: How might infection modulate the onset of type 1 diabetes? Immunology, 126 (1), 12-17 DOI: 10.1111/j.1365-2567.2008.03009.x
2. Jackson, J., Friberg, I., Little, S., & Bradley, J. (2009). Review series on helminths, immune modulation and the hygiene hypothesis: Immunity against helminths and immunological phenomena in modern human populations: coevolutionary legacies? Immunology, 126 (1), 18-27 DOI: 10.1111/j.1365-2567.2008.03010.x
3. Rook, G. (2009). Review series on helminths, immune modulation and the hygiene hypothesis: The broader implications of the hygiene hypothesis Immunology, 126 (1), 3-11 DOI: 10.1111/j.1365-2567.2008.03007.x