In medicine, microbes are notorious for causing disease. In neurology particularly, infection is the direct cause of serious diseases such as meningitis and encephalitis. Infections may also act as catalysts for neurological disorders, for example when they trigger Guillain Barre syndrome (GBS). Infection is therefore a villain, a scoundrel to be apprehended and disarmed whenever it rears its head. But this picture may not hold true when it comes to multiple sclerosis (MS) where a contrary story is emerging, a narrative which holds infection as the hero, the daredevil that will save the day. The premise is simple: MS has very little presence in the regions of the world where infections reign supreme. Just look at any world prevalence map of MS to be convinced.
How strong is this inverse relationship between infection and MS? It all boils down to the so-called hygeine hypothesis of autoimmune diseases. This suggests that the human immune system becomes dysregulated when it is not primed by infections, and this dysregulation results in autoimmune disorders. This point was strongly argued by Aakanksha Dixit and colleagues in their paper published in the International Journal of Molecular Science, titled Novel Therapeutics for Multiple Sclerosis Designed by Parasitic Worms. They contend that the relationship between parasitic infections and autoimmune diseases is “most compelling“, going on to assert that helminthic infections “may be the protective environmental factor against the development of MS”.
To support the hygiene theory of MS, that helminthic infections play a role in banishing MS, three levels of evidence are offered.
- The prevalence of MS steadily increases when the frequency of infections in a community is reduces.
- People with MS who also have helminthic infections have fewer relapses and slower disease progression.
- MS patients who are treated for their helminthic infections develop more relapses and have a more active disease course.
Toxoplasma gondii, the cause of toxoplasmosis, is perhaps the major parasite investigated in relation to MS. Asli Koskderelioglu and colleagues, for example, reported that exposure to T.gondii is less frequent in people with MS than in healthy control subjects. In their 2017 paper titled Is Toxoplasma gondii infection protective against multiple sclerosis risk?, published in Multiple Sclerosis and Related Disorders, they found that MS subjects who have higher toxoplasma antibody levels experience fewer relapses and less severe disease courses. This finding is corroborated by a 2015 paper in the Journal of Neuroimmunology titled Toxoplasma gondii seropositivity is negatively associated with multiple sclerosis.
Neurologists are however very cynical people, and they never believe what single trials tell them. After all, many microbes, such as Ebstein Barr virus (EBV), are touted as MS risk factors. For the sceptical neurologist, only systematic reviews and meta-analyses will do; these are the stuff of our dreams, the essence of our daily existence. So it is with a huge cheer that neurologists welcomed a 2018 systematic review and meta-analysis published in the Journal of Neuroimmunology and titled Is toxoplasma gondii playing a positive role in multiple sclerosis risk? The paper poured very cold water on the beautiful hygiene hypothesis. Whilst the authors, Reza Saberi and colleagues, confirmed that MS subjects had a lower risk of exposure to T. gondii, they found no relationship between this parasite and the development of MS. Wither a theory when it hits the reality of cold statistical analysis.
Notwithstanding the systematic review, the helminth hypothesis marches on. It has even reached the stage of therapeutic trials where, as distasteful as it sounds, subjects ingest parasites by mouth! And the fancied parasite is not Toxoplasma gondii but Trichuris suis ova (TSO). It all began with a small observational trial in 10 people which proved that TSO is safe and well-tolerated (phew), but it had no value whatsoever in treating MS. Not discouraged, the hypothesis entered the slightly larger HINT 2 trial; this again confirmed good tolerability in 16 subjects, but any benefit in reversing MS was questionable. Undeterred, the hypothesis has gone for a bigger study in the form of the TRIOMS trial. This is a randomized, placebo-controlled study of 50 people with MS or clinically isolated syndrome (CIS) in which subjects will be ingesting 2,500 Trichuris suis eggs every two weeks. We wait with bated breaths for the results.
Before leaving this subject, we must know that helminths are not the only game in town; they have strong competition from their cousins, bacteria. And the standout character in this arena is Helicobacter pylori. We learnt this from a study published in 2015 in the Journal of Neurology Neurosurgery and Psychiatry. Titled Helicobacter pylori infection as a protective factor against multiple sclerosis risk in females, the paper reported that people with MS were less likely than controls to have been exposed to H. pylori. Two meta-analyses have also reviewed the relationship of H. pylori and MS, arguing strongly that, in Western countries, there is an inverse relationship between H pylori and MS. And they assert that H. pylori may be protective against MS. If it feels like déjà vu, it is.
We have surely not heard the last of bugs and MS. However, for now, the foundations of the hygiene theory are a bit shaky, and the future rather hazy.
