The cat-loving parasite with a creepy reputation for hijacking its host's behavior is using the infected body's own immune system for transportation, but also to juice it with a neurotransmitter that can suppress fear. There's a growing body of research about the extent to which Toxoplasma gondii can affect the behavior of its host, and a recent study shows the single-celled organism has a sinister ability to hijack the very immune system cells meant to defend against it. It also causes the cells to secrete a neurotransmitter that makes the cells travel faster, but is also associated with reduced fear and anxiety.
Toxoplasma is known for reproducing in cat digestive systems, spreading to the bodies of living rats through cat feces, then steering the rats toward other predators to complete the life cycle. When the protozoa travels from cat poop to rodents, it can enter the brains of its hosts and affect behavior. Primarily, it makes the rat fearless or attracted to the smell of cat urine. Since the organism can only reproduce in another cat, this is Toxoplasma's clever way of getting back to its breeding ground.
The pathogen also exists in an estimated 30%-50% of humans, although for the vast majority infected, the parasite doesn't pose a threat because the immune system keeps it from becoming active. It can be a threat to people with weakened immune systems, and can to fetuses if it infects pregnant women.
While it's long been considered otherwise harmless, there are increasingly credible theories that even in its dormant stage within humans, it could have a similar behavior-warping effect, causing a high affinity for cats, reckless or suicidal behavior, and even contribute to onset of schizophrenia. Keep in mind, this is a common infection that has been co-existing with humans for centuries, so this is not some kind of new zombie epidemic. But the abilities of Toxoplasma and its sneaky ways continue to unfold.
Now scientists in Sweden have discovered it has a similar talent for taking control of certain cells in the immune system. Dendritic cells are the messengers of the immune system, carrying instructions from the outer realms of the body to other immune system cells to inform the body's response to invaders. In the case of Toxoplasma, the messengers act as shuttles. "Dendritic cells are considered the gatekeepers of the immune system but can, paradoxically, also mediate dissemination of the parasite."
Even more sinister, however, the pathogen causes the cells to secrete GABA, a neurotransmitter normally found in the brain. This heightened GABA in the system causes the immune cells to become hyperactive and spread the pathogen faster. But GABA is also associated with the fear response, and related disorders involving anxiety, schizophrenia and bipolar disorder. The study's results (tested in mice) suggest that this production of GABA may be causing the host rats to become fearless of cats. It may also explain how the parasite can control the brain if it can create a similar response once invading upstairs.
Photo: My cat, whom I continue to love unconditionally.