Sometimes the best way to fight one disease is with another. This week, scientists found evidence that the parasite that causes toxoplasmosis—a neurological disorder that, among other things, makes rats lose their fear of cats, but is usually harmless in adult humans—may help slow or even reverse the progress of Alzheimer’s disease.
Last year, Turkish researchers found that patients with Alzheimer’s were nearly twice as likely to have also shown an immune response to the toxoplasmosis parasite. The more recent study, however, showed that when mice with Alzheimer’s were injected with the parasite, the characteristic plaques of Alzheimer’s were actually reduced.
These plaques are the result of a chemical called beta amyloid, which is part of the immune response to toxoplasma gondii. It may be that the initial toxoplasmosis infestation builds up the plaques, but if the infestation—which can suppress the immune system—comes later, it reduces them; another possibility is that the beta amyloid activity that causes Alzheimer’s created false positives in the Turkish study, appearing to be an immune response to toxoplasmosis when in fact they were not.
Toxoplasmosis affects about one in five Americans, most of whom show no symptoms. The parasite is often acquired by handling cat feces or eating contaminated meat. It is fairly harmless in most adults, but it can be quite serious in people with weakened immune system and in small children and infants, especially infants who contract it prenatally.