The single-celled parasite toxoplasma gondii is known to structurally change the brains of infected mice to cause them to lose their fear of cats. This transformation aids the fitness of the pathogen essential for the pathogen to complete its life cycle, because it can reproduce sexually only in cat guts. The fungus Ophiocordyceps unilateralis infects carpenter ants, and then
it grows through the insect’s body, draining it of nutrients and hijacking its mind. Over the course of a week, it compels the ant to leave the safety of its nest and ascend a nearby plant stem. It stops the ant at a height of 25 centimeters—a zone with precisely the right temperature and humidity for the fungus to grow. It forces the ant to permanently lock its mandibles around a leaf. Eventually, it sends a long stalk through the ant’s head, growing into a bulbous capsule full of spores. And because the ant typically climbs a leaf that overhangs its colony’s foraging trails, the fungal spores rain down onto its sisters below, zombifying them in turn.
The rabies virus is well known to induce aggression in its hosts, leading them to bite others and so transmit the virus in its saliva.
Is any of this relevant to humans? Toxoplasma infection is found in around 30% of UK residents — acquired from contact with pet cats — and there is evidence that it may contribute to schizophrenia. There is strong evidence that prenatal maternal infection raises the risk of the child going on to develop schizophrenia. But this is presumably just a byproduct of the essential neuropathogenicity that promoted the pathogen’s fitness in mice.
I was thinking of this, though, when I saw this new study:
People who had previously suffered a Covid infection “reported a significantly higher number of symptoms of executive dysfunction than their non-infected counterparts”. Executive dysfunction, according to Wikipedia, is “a disruption to the efficacy of the executive functions, which is a group of cognitive processes that regulate, control, and manage other cognitive processes… Executive processes are integral to higher brain function, particularly in the areas of goal formation, planning, goal-directed action, self-monitoring, attention, response inhibition, and coordination of complex cognition.”
Perhaps coincidentally, we have seen, since the start of the pandemic, an upsurge of seemingly inexplicable emotionally overwrought rejection of measures that might prevent the individual from spreading the virus, or from catching it again oneself, especially masking and vaccination. Could it be that this is itself a neurological sequela of a Covid infection, that manipulates the sufferer’s brain, like the carpenter ant’s, to maximise the spread to conspecifics? Or that, like a hacker “backdooring” a compromised system, the virus has evolved to make its host pliable to future infection, once the immune response has waned?
I’m just asking questions.