Toxoplasma is a genus of parasitic protozoa whose definitive host is cats but which can be carried by the vast majority of warm-blooded animals, including humans.
Toxoplasmosis, the disease it causes, is usually minor and self-limiting but can have serious or even fatal effects on a fetus whose mother first contracts the disease during pregnancy or on an immuno-compromised human or cat. It can also cause severe birth defects.
Many other humans are also thought to carry the parasite, without any ill effects but Stanford University scientists have now written that the behavior of a single protein determines what happens.
The study published in Nature suggest that Toxoplasma is specially dangerous for people with weakened immune systems, such as those with HIV and can also lead to brain damage or even death.
According to BBC reports, the scientists say that the parasite's normal lifecycle starts in cats. It is then passed, usually via the cat's feces, to rats and then back to cats when they catch and eat the infected rats.
It can spread to humans when they come into contact with cat feces, or by eating undercooked mutton, as sheep can also become carriers.
According to the scientists at Stanford, the parasite also has the ability of Toxoplasma to adapt genetically to a wide variety of hosts.
The latest study reveals how the parasite injects a single protein into a cell it wants to invade, and how this protein makes its way to the nucleus of the cell and interferes with the ability of the cell to trigger an immune defense.
According to Susan Coller, one of the researchers leading the project, "The nucleus is the heart of the cell, the ultimate prize. If you want to affect the cell in a dramatic way, go straight there."
Furthermore, the scientists also found that there were subtle differences in this key protein between different types of Toxoplasma. It is believed that each different strain could infect different types of host cell with the minimum damage.
The severest form of toxoplasmosis occurs when the "wrong" strain, one not suited to infecting humans, tries to invade human cells with the protein either too powerful.
The result is that it destroys them, or if ineffective starts a massive immune system response.
According to Professor John Barrett, a senior lecturer in parasitology at the University of Aberystwyth, "The effect on the fetus can be very serious and there is a reliable test for Toxoplasma, so in these higher risk groups, it may be worthwhile."
