Parasite hijacks cells, dulls fear

Cats, such as Poppy here in Oakland, are the definitive hosts of Toxoplasma gondii / Kamala Kelkar

By John Upton

Toxoplasma gondii is a wily parasite that’s beautifully adapted to the urban environment. The protozoans pass between rats, house cats and humans with aplomb.

Once inside a rat, the single-celled stalker diminishes its host’s fear of cats, which helps it spread from the hunted to the hunter. From the cat, the parasite passes into kitty litter, where it can infect new rats and enter humans through litter-tainted food or licked fingers.

Perhaps one quarter of the world’s human population is infected. Consequences are seemingly slight: Temporary flu symptoms and a lifetime of benign infection. But inside the central nervous system the disease could trigger depression and schizophrenia and reduce reaction times. Infected humans are more likely to crash their vehicles than those who are uninfected.

Researchers have honed in on a deft trick used by the parasite to spread through the body and commandeer parts of its hosts’ brains. By affecting brain function, the protozoans could help cats catch infected rats.

T. gondii under a microscope / Flickr: OmarCerna

The protozoans bust into dendritic cells, which play major roles in the immune system. Research published last week in PLOS Pathogens reveals that the protozoans trick the infected cells into producing a compound dubbed GABA.

Production of GABA excites the infected cell, encouraging it to move around the body and aiding in the parasite’s migration into the brain.

GABA is also a neurotransmitter; it stifles sensations such as fear and anxiety.

“For toxoplasma to make cells in the immune defence secrete GABA was as surprising as it was unexpected,” said Antonio Barragan, researcher at the Center for Infectious Medicine at Karolinska Institute and the Swedish Institute for Communicable Disease Control, “and is very clever of the parasite.”