...Their findings suggest that this microbe is particularly insidious-the changes it makes may be permanent.

To replicate, Toxoplasma must invade a cell. Stanford parasitologist John C. Boothroyd has dubbed this process "kiss and spit." The parasite first attaches to the host cell (the kiss) and then releases an arsenal of foreign proteins into that cell (the spit). Toxoplasma then enters the host cell, and the injected proteins help it redecorate its new home. The parasite's first act is establishing a protective bubble in which it can divide in peace without attacks from host cell proteins. (Later, during the infection's dormant stage, these bubbles thicken to become tissue cysts.) The parasite then moves the mitochondria, which serve as the cell's powerhouses, to be adjacent to the protective bubble. It also acts on the cell's DNA, inhibiting the expression of some host genes while activating others. Finally, Toxoplasma modifies host proteins to alter their function and inhibit the immune response.

Altogether, these modifications ensure that the host cell will live a long time and supply energy to the parasite, without alerting immune cells that a parasite has moved in. Although these findings have principally been made with rodents, work with human cell cultures suggests that the same changes probably take place in the human body. In our labs, we are studying how Toxoplasma replicates and interacts with its host in an effort to develop new drugs to treat this infection.

Remarkably, a study that Boothroyd's group published in 2012 showed that Toxoplasma not only spits into the cells it invades but also spits into cells that it does not infect. This behavior-spitting proteins in passing without lingering in the cells-is a recent discovery in the microbial world. Consequently, cells that are not harboring Toxoplasma contain parasite proteins that can co-opt and reprogram that cell. In the brains of infected mice, cells that have been spat into but not invaded are even more common than ones containing parasites. This widespread scattering of proteins means Toxoplasma can affect its host at a global level, making it easier to imagine how the parasite might manipulate the activity of an entire animal. In 2013 biologist Michael Eisen of the University of California, Berkeley, and his colleagues found that a rodent's strange attraction to cat odors may be permanent, even if there are no longer signs of infection. In one study, Eisen exposed mice to a mutant strain of the parasite that does not appear to form brain cysts. Four months later the infected mice had no detectable parasites in the brain, yet they were still attracted to cat odors instead of repelled.

--"Common Parasite Could Manipulate Our Behavior: Toxoplasma gondii, a parasite inhabiting the brains of an estimated three billion people, could tweak its host's behavior", SciAm