The study, “Toxoplasma gondii and multiple sclerosis: a population‐based case-control study,” was published in the journal Scientific Reports.
Exposure to certain infectious agents early in life may serve as a protective factor for autoimmune diseases such as MS, according to a theory called hygiene hypothesis. This hypothesis theorizes that living in an environment with a high incidence of infectious diseases builds a resistance of sorts, protecting inhabitants in that area against allergic and autoimmune diseases.
According to scientists, this theory could explain why western countries, which have higher sanitation levels and thus less such exposure, also have a higher incidence of autoimmune diseases and allergies.
T.gondii, as Toxoplasma gondii is known, is a parasite that is able to infect humans. Domestic cats and their relatives are the only known hosts for the parasite, which can infect most species of warm-blooded animals, including humans, according to the Centers for Disease Control and Prevention.
Estimates suggest that at least one-third of the world’s population is infected by T.gondii, yet this infection is generally asymptomatic in people with a healthy immune system. However, for people with compromised immunity, severe symptoms — like encephalitis, or inflammation of the brain — may occur.
To learn more about that association, researchers at the University of Catania, in Italy, collected blood samples from 129 MS patients (mean age 44.7 years) and 287 healthy individuals (mean age 48.1 years), who served as controls. About two-thirds of the individuals in both groups were women.
Among the MS patients, disease onset occurred around the age of 32.7, on average, and the mean disease duration was 12 years. The most common type of MS was the relapsing-remitting (RRMS) form of the disease, present in 103 patients (79.8%).
To determine whether someone had been infected with T. gondii, the researchers examined blood samples from each participant for the presence of antibodies against the parasite. These antibodies were detected in 38 MS patients (29.5%), and in 130 of the healthy controls (45.4%).
The fact that more healthy individuals had been infected suggested that this parasite worked to protect against MS development. A statistical analysis demonstrated that people who had been infected were about 50% less likely to develop the autoimmune condition than those with no history of infection.
Notably, a double infection by T. gondii and Toxocara canis, a roundworm parasite found in dogs, did not significantly affect the likelihood of MS.
After adjusting for confounding factors like age and sex, T. gondii infection remained significantly associated with a reduced likelihood of MS. Meanwhile, more years of schooling and a history of mononucleosis — an infection characterized by fever, sore throat, and fatigue, caused by the Epstein-Barr virus, which is a known risk factor for MS — were positively associated with the condition.
Additional analyses also demonstrated that T. gondii infection was associated with older age and being female, but was less likely to occur in people with more years of education.
Finally, consistent with prior associations between T. gondii infection and autoimmune conditions, the team found that infection by this parasite in the healthy controls reduced those individuals’ odds of having other autoimmune conditions, particularly after considering age and sex in the analysis.
Overall, “we found a negative association between T. gondii and MS, suggesting a possible protective role of this parasite and supporting the hygiene hypothesis,” the researchers wrote.
This negative association, they suggest, is likely due to a switch in immune responses in the presence of the parasite. Specifically, the immune signaling used to fight this invader is less likely to cause autoimmune conditions than other kinds of immune responses, they said.
However, “further clinical and experimental studies are needed to confirm our results and to understand the underlying pathophysiological [disease] mechanisms,” the investigators concluded.
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