Earlier this year researchers writing in the Archives of General Psychiatry suggested a parasite associated with cats might raise the suicide risk of women who live with the animals as pets.
The parasite, the researchers found, was spread through contact with cat feces as well as eating undercooked meat or unwashed vegetable. Now, in a follow-up, Swedish researchers believe they know how the parasite enters the brain and influences the behavior of its victim.
"We believe that this knowledge may be important for the further understanding of complex interactions in some major public health issues, that modern science still hasn't been able to explain fully", said Antonio Barragan, researcher at the Center for Infectious Medicine at Karolinska Institutet and the Swedish Institute for Communicable Disease Control. "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."
In other words, the parasite is widespread and has been for a long time. Yes, cleaning a cat's litter box may bring you in contact with it but that doesn't necessarily mean you're going to harm yourself.
The current study, which is published in the scientific journal PLoS Pathogens, suggests between 30 and 50 per cent of the global population is infected. The infection is also found in animals -- especially domestic cats.
People contract the parasite mostly by eating the poorly cooked meat from infected animals or through close-up contact with cats. 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 defense and in fetuses, which can be infected through the mother. Because of this risk, pregnant women are recommended to avoid contact with cat litter boxes.
Even dangerous while dormant
Other studies have shown that the toxoplasmosis parasite can affect the victim 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. And here it gets a little creepy -- almost like a science fiction movie.
Some scientists believe this is the parasite's way of assuring its survival and propagation, since the consumed rat then infects the cat, which through its feces 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.
"We've not looked at behavioral changes in people infected with toxoplasma, as that's been dealt with by previous studies," Barragan said. "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 cells infected with toxoplasma secreted what's known as a "signal substance." Among its effects is reducing the sensation of fear and anxiety. It's similar to the effects seen in people with depression, schizophrenia, bipolar diseases, anxiety syndrome and other mental diseases.
Battagan says the findings convince him that scientists should closely study the link the toxoplasmosis parasite and major public health threats.