A parasite that needs cats appears to make rats lose their fear of them
Toxoplasma gondii can live in almost any warm-blooded animal, but it can only reproduce sexually inside a cat. Infected rats become less repelled by cat urine and infected mice grow bolder, changes that tend to deliver the parasite straight back to a feline gut. Up to half of humanity may carry it.
Cats hold a unique position in this parasite's life. Their intestines lack an enzyme called delta-6-desaturase, so linoleic acid builds up in their bodies, and that surplus turns out to be essential for the parasite's sexual stage. Inside a cat's gut lining it produces millions of tough oocysts that leave in faeces and can stay infective for months in soil or water, especially in cold, dry conditions. Every other host, from birds to pigs to people, supports only asexual multiplication. Cattle and horses appear to resist significant infection.
In an intermediate host the parasite first multiplies rapidly as tachyzoites, spreading through the bloodstream to every organ, including the brain. Immune pressure then pushes it into a slow, semi-dormant form sealed in tissue cysts, mostly in brain, eye and muscle, which form around 7 to 10 days after infection. Cysts measure roughly 5 to 50 micrometres across and can persist for life, apparently by periodically bursting and re-forming. People are infected mainly by swallowing oocysts, for instance on unwashed vegetables or from litter, or by eating cysts in undercooked pork or lamb.
The behavioural tricks in rodents are thought to be adaptations favouring the parasite, and they work through epigenetic changes in the neurons controlling those behaviours. Whether anything similar happens in people is far less settled. Most infected adults have no symptoms beyond a brief flu-like spell, though infants and people with weakened immunity can become seriously ill. Latent infection has been linked to a reduced aversion to cat urine and higher risks of schizophrenia, but many of these associations are disputed, and a 2016 study found them weak.
Serological surveys suggest exposure rates differ sharply by country, with Ethiopia recording 64.2 percent in 2018. The parasite reaches the sea too: an exhibit in San Diego notes that runoff carrying cat faeces can kill sea otters. Despite having a sexual phase, North American strains are dominated by a few clonal types, with 85 percent belonging to three genotypes.
Source: Toxoplasma gondii
