Toxoplasmosis is caused by the Toxoplasma gondii parasite, which experts estimate infects 30 to 50 percent of the global population.
The infection is also found in animals, especially domestic cats and people contract the parasite mostly by eating the poorly cooked flesh of infected animals or through contact with cat feces. The infection causes mild flu-like symptoms in adults and otherwise healthy people before entering a chronic and dormant phase, which has previously been regarded as symptom-free. It is, however, known that toxoplasmosis in the brain can be fatal in people with depleted immune defence and in fetuses, which can be infected through the mother. Because of this risk, pregnant women are recommended to avoid contact with cat litter trays.
A number of studies have been presented in recent years showing that the toxoplasmosis parasite affects its host even during the dormant phase. It has, for example, already been observed that rats become unafraid of cats and even attracted by their scent, which makes them easy prey. This has been interpreted as the parasite assuring its survival and propagation, since the consumed rat then infects the cat, which through its faces can infect the food that other rats might then proceed to eat.
A number of studies also confirm that mental diseases like schizophrenia, depression and anxiety syndrome are more common in people with toxoplasmosis, while others suggest that toxoplasmosis can influence how extroverted, aggressive or risk-inclined an individual's behavior is.
So it isn't that crazy old cat ladies acquire a lot of cats, it's that a lot of cats turn normal ladies into crazy ones.
"We've not looked at behavioral changes in people infected with toxoplasma, as that's been dealt with by previous studies," says Antonio Barragan, researcher at the Center for Infectious Medicine at Karolinska Institutet and the Swedish Institute for Communicable Disease Control. "Instead, we've shown for the first time how the parasite behaves in the body of its host, by which I mean how it enters the brain and manipulates the host by taking over one of the brain's neurotransmitters."
In one laboratory experiment, human dendritic cells were infected with toxoplasma. After infection, the cells, which are a key component of the immune defense, started secreting the signal substance GABA. In another experiment on live mice, the team was able to trace the movement of infected dendritic cells in the body after introducing the parasite into the brain, from where it spread and continued to affect the GABA system.
GABA is a signal substance that, amongst other effects, inhibits the sensation of fear and anxiety. Disturbances of the GABA system are seen in people with depression, schizophrenia, bipolar diseases, anxiety syndrome and other mental diseases.
"For toxoplasma to make cells in the immune defence secrete GABA was as surprising as it was unexpected, and is very clever of the parasite," says Barragan. "It would now be worth studying the links that exist between toxoplasmosis, the GABA systems and major public health threats.
"At the same time, it's important to emphasize that humans have lived with this parasite for many millennia, so today's carriers of Toxoplasma need not be particularly worried."
Citation: Fuks JM, Arrighi RBG, Weidner JM, Kumar Mendu S, Jin Z, et al. (2012) GABAergic Signaling Is Linked to a Hypermigratory Phenotype in Dendritic Cells Infected by Toxoplasma gondii. PLoS Pathog 8(12): e1003051. doi:10.1371/journal.ppat.1003051