But is there any evidence that infection with Toxoplasma gondii is actually harmful except to restricted groups (in whom, indeed, it can be very serious indeed)? The Lancet editorial refers to two pieces of research, one from Denmark and the other from the United States, that suggest that infection might do neuropsychiatric harm.
Danish researchers found that women who had antibodies against the parasite were more likely than those who did not to make suicide attempts, particularly by violent methods. (Although Denmark is a small country with a small population, it has a remarkable record of medical research on large numbers of people.) Altogether 45,788 women had their blood checked for antibodies against Toxoplasma while they were pregnant, and it was found that those who tested positive were 1.53 times as likely to attempt suicide as those who did not; moreover, the risk increased with higher levels of antibody.
In the United States, researchers found that people who suffered from bipolar affective disorder (both mania and depression) had an increased prevalence of seropositivity for Toxoplasma.
As the editorial points out, correlation is not causation; it could be, for example, that people with certain behavioral propensities are more likely to become infected with Toxoplasma. But it also points out that the parasite, which enters the human brain, has genes that can modify the production of the neurotransmitters that according to modern neuro-scientific doctrine play a determining role in our mood and conduct.
Dogs can be infected by Toxoplasma as well as humans, and the infection sometimes causes them serious symptoms. They are infected by cat feces, and indeed without cats there would be no toxoplasmosis. No wonder that dogs act on a variant of Dick’s famous advice in Henry VI Part 2: The first thing we do, let’s kill all the cats.
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