Can A Parasite Cause Autoimmune Disease?

First of all, it is important to define what is an autoimmune disease. Actually, in this type of diseases, the body produces antibodies against its own antigens or proteins, meaning that it mistakes its own proteins as an intruder and treats them like one. These antibodies are then responsible to tag the antigens or proteins for further destruction by the cells from the immune system. As these self-proteins are normally needed for different body functions or organs, their destruction by the immune system caused a disease. Furthermore, it is noteworthy that the immune system can also recognise its own proteins as an intruder if an external agents, such as chemicals or microorganisms, had altered them. In plain words, the body’s immune system attacks itself and progressively destroys its own tissues and/or organs. Well-known autoimmune diseases on the rise in developed countries include type 1 diabetes, multiple sclerosis, rheumatoid arthritis, and Crohn’s disease. However, the triggers of many of these diseases are still unknown or not well understood.

Even though the relationship between protozoan parasites and autoimmune diseases has been less extensively studied, there is actually no strong scientific evidence implicating parasites as a leading cause of autoimmune diseases. However, it is not excluded that further studies shed a new light on these complex relationships, especially in the case of the less studied protozoan parasites. In the meantime, there are scientific evidences linking some helminth infections with a decrease of autoimmune diseases. In fact, a correlation has been established between the dramatic decreasing rates of helminthic infections in developed countries since the past thirty years or so with the increasing rate of autoimmune diseases. Furthermore, it has also been observed that the occurrence of autoimmune diseases is very low in developing countries where helminthic infections are still widespread.

The main scientific theory explaining this fact is derived from the «Hygiene Hypothesis». This hypothesis originally stipulates that the lack of exposure to infectious agents, especially in the early childhood, can predispose to allergies later in life. It has then been extended to cover autoimmune diseases in general. Recently, this hypothesis has been refined by many research involving helminthic infections as a protection against autoimmune diseases.

Some studies suggest that the softening of the immune response induced by the worms explains the effects of helminths on autoimmune diseases incidence. In fact, helminths are able to deactivate some immune system cells in order to promote their own survival. In turn, this phenomenon softens the immune system and inhibits the response to harmless antigens produced by the body itself. This positive effect has been suggested to exist as a result of the coevolution for millions of years of primates, including humans, and parasitic worms. However, the weakening of the immune system caused by worms seems to have some downsides. For example, helminthic infections decrease the efficiency of vaccination against other diseases, as the vaccination process must induce a strong immune response in order to work. Furthermore, there are evidences that infection with some worms could worsen or predispose to other diseases, such as viral diseases like HIV and hepatitis, and bacterial diseases like tuberculosis.

Consequently, many studies in the past few years have focused on helminths as a potential therapeutic agent for autoimmune diseases. This new type of experimental therapy is called helminthic therapy. It involves a deliberate infection with specific worms or their eggs in order to treat autoimmune diseases. It is important to note that worms are able to persist and establish infection in the human body and important side effects caused by the parasitic infection itself are susceptible to occur. Intestinal nematodes are the most often used type of worms in helminthic therapy. For example, therapy with eggs from the porcine whipworm Trichuris suis has been approved as an investigational medicinal product for humans by the American Food and Drug Administration, meaning that it can now be used in human as part of clinical studies and research programs only. As Trichuris suis is not able to colonize human and only stay transiently in the intestine for about 2 weeks, it is considered as an excellent candidate for helminthic therapy. It has been shown so far to be effective to treat Crohn’s disease and suspected to also work as a therapy for autism. However, more studies will be required before health regulation authorities officially approve this treatment.

Finally, it is interesting to mention that some scientific studies identified last year very promising specific peptides released by parasitic worms and thought to be responsible for the modulation of the immune system. This discovery paved the way for the development of a new drug able to treat autoimmune diseases. This drug would be indeed a better way to treat autoimmune diseases than to deliberately infect people with helminths as it would eliminate all the potential side effects of a parasitic infection. However, many more years of research will be required before the arrival of this promising therapeutic agent on the market.

Source: Merck Manual; ZACCONE, P., FEHERVARI, Z., PHILLIPS, J. M., DUNNE, D. W., & COOKE, A. (2006). Parasitic worms and inflammatory diseases. Parasite Immunology, 28(10), 515–523.

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