(Ladies and Gentlemen, today marks the first guest post on Parasite Ecology!! We’re going to go a bit more medical this week, with this freelance article from Gemma Corden.)
Parasites are incredibly ubiquitous but to us humans they’re still mysterious creatures in many ways. And on the whole, autoimmune diseases and their specific triggers and causes are as yet not entirely understood, so it’s somehow fitting that a very specific kind of parasitic infection can be an effective therapy for certain types of autoimmune disease. Specifically, helminthic treatment for autoimmune disease has been under investigation for more than a decade by several research groups, with some promising results that are interesting for more than just their therapeutic implications.
Helminthic Therapy Under Investigation
Just a couple of decades ago, helminthic therapy as a field of investigation was limited to purely observational studies, and relatively speaking, it’s been a rapid advancement from those beginnings to the clinical trials that are now underway and already showing promising results. In part this is due to the fact that the sum of immunological knowledge has vastly increased in the same amount of time.
Helminthic therapy is being tested in patients with a range of inflammatory and immunological disorders, including multiple sclerosis, Crohn’s disease, Celiac disease, inflammatory bowel disease, ulcerative colitis, and allergic rhinitis. The results have been mixed; in Crohn’s disease, for example, helminth therapy has been highly successful for many people, effectively treating many of the symptoms, and even reducing the need for and reliance on pain medications. In one study, patients with Crohn’s disease were treated with a single dose of Trichuris suis; twelve weeks later, 75% were in remission. However, with a 66% relapse rate for this single-dose therapy, there is still more information to be uncovered. Another study recorded a 100% remission and 33% relapse rate for patients with ulcerative colitis following a single-dose therapy. Results have been largely negative in the case of allergic rhinitis, with most studies recording no significant improvements in symptoms such as airway responsiveness, asthma control, and allergen tests. And while positive results are scarce for Celiac disease, one study on people with MS showed that T. suis therapy was able to temporarily reduce CNS and neurological lesions.
Despite the fact that helminthic therapy is still experimental, unregulated, and unapproved for use in any country, organisms are already being sold for therapeutic use; the relative ease with which this mode of treatment can be manufactured outside of a laboratory setting unfortunately means it’s ripe for exploitation.
Why does it Work? Hygiene Theory and Immunome Selection
In the last four or five decades, most industrialized countries have seen huge increases in the development of allergies, inflammatory disorders of idiopathic origin, and autoimmune diseases. Similar trends develop in modernized regions of countries that are less industrialized. Previously it was thought that this trend might be accounted for in large part as the result of lifestyle and environmental changes—in particular the reduced exposure to infectious agents that is experienced by people in industrialized countries. This is the “hygiene theory,” which posits that lack of exposure to pathogens, particularly in childhood, prevents the immune system from developing normally, and increases the likelihood that a given individual might develop an allergy or autoimmune disorder. Essentially, the immune system needs this early pathogen exposure to become primed to react normally to infectious agents and to differentiate correctly between pathogen and self.
Recent research indicates that there may be more to the story than this, however, as it seems likely that underlying the hygiene theory, there are highly influential genetic factors at play—specifically, that as humans and their pathogens have evolved alongside one another, human pathogens have exerted evolutionary pressure on the human genome. In other words, the immunome—the genes that code for immune system components—that humans have today is the result of coexistence with pathogenic agents, and as such it can be inferred that pathogenic exposure is necessary for normal immunome expression, and normal immune system function.
Interestingly, it seems that helminths in particular may be “master selectors of the immunome,” that have exerted more evolutionary pressure on the human immunome than any other type of pathogen, in part due to the incredibly ubiquity of these organisms. Another contributing factor is that helminths tend to infect humans prior to puberty and most often cause chronic infections, meaning that people who survive to adulthood and parenthood with such infections are more likely to have immunomes with advantageous mutations in terms of resistance to helminth infection.
Jabr, Ferris. “For the Good of the Gut: Can Parasitic Worms Treat Autoimmune Diseases?” Scientific American.Accessed 28 November, 2014.
Lisenborg, Jori. “Helminthic Therapy.” Accessed 28 November, 2014.
Matisz, Chelsea E., McDougall, Jason J., Sharkey, Keith A., and McKay, Derek M. “Helminth Parasites and the Modulation of Joint Inflammation.” Journal of Parasitology Research. Volume 2011 (2011). doi: 10.1155/2011/942616 Accessed 28 November, 2014.
Wamms, Linda J., Mpairwe, Harriet, Elliot, Allison M., and Yazdanbaksh, Maria. “Helminth therapy or elimination: epidemiological, immunological, and clinical considerations.” The Lancet Infectious Diseases, Volume 14, Issue 11, Pages 1150 – 1162, November 2014. doi: 10.1016/S1473-3099(14)70771-6. Accessed 28 November, 2014.