Cat Parasite Hijacks The Identity Of Immune Cells To Spread Inside The Human Body, Study Finds

The cat parasite Toxoplasma infects immune cells and hijacks their identity to spread inside the host.

Did you know that cats and other felines are the only hosts in which the parasite Toxoplasma gondii can undergo sexual reproduction? A large portion of the human population carries the cat parasite Toxoplasma. It is capable of infecting all warm-blooded animals, and causes a disease called toxoplasmosis. Now, researchers at Stockholm University have shown how Toxoplasma successfully spreads in the body of the host. The cat parasite infects immune cells and hijacks their identity to spread inside the host. The study describing the findings was recently published in the journal Cell Host & Microbe. 

The different roles of immune cells in the body are strictly regulated to fight infections. A mystery which has always intrigued scientists is how Toxoplasma manages to infect so many people and animal species and spread so efficiently. 

In a statement released by Stockholm University, Arne ten Hoeve, one of the authors on the paper, said the researchers have discovered a protein that Toxoplasma uses to reprogram the immune system of the host.

How Toxoplasma changes the identity of immune cells

The parasite injects the protein into the nucleus of the immune cell, and thus, changes the cell's identity, the study says. Toxoplasma tricks the immune cell into thinking the parasite is another type of cell. This causes the gene expression and behaviour of the immune cell to change. 

Toxoplasma causes infected cells which normally do not travel in the body to move very quickly. As a result, the parasite quickly spreads to different organs. 

This phenomenon is described as Toxoplasma turning immune cells into Trojan horses or wandering 'zombies' that spread the parasite, the statement says. The new study provides a molecular explanation for the phenomenon, and also shows that Toxoplasma is much more targeted in its spread than previously thought. 

Professor Antonio Barragan, the lead author on the paper, said it is astonishing that the parasite succeeds in hijacking the identity of the immune cells in such a clever way. He added that the team believes the findings can explain why Toxoplasma spreads so efficiently in the body when it infects humans and animals. 

More about Toxoplasma and the disease toxoplasmosis

Toxoplasma causes toxoplasmosis, which is the most common parasitic infection in humans globally. The parasite also infects many animal species, including pets. At least 30 per cent of the world's human population is a carrier of Toxoplasma, the World Health Organization (WHO) estimates. About 15 to 20 per cent of the Swedish population carry the parasite, and a vast majority of them carry it without being aware, studies have found. In several other European countries, the incidence of Toxoplasma is even higher. 

Felines, not just domestic cats, have a special place in the life of Toxoplasma because it is only in the cat's intestine that sexual reproduction takes place. In other hosts, including humans, reproduction of Toxoplasma takes place asexually, by division of the parasite. 

Toxoplasma is spread through food and contact with cats, and in nature, it spreads preferentially from rodents to cats to rodents and so forth. 

Toxoplasma 'sleeps' in the rodent's brain and when the cat eats the mouse, the parasite multiplies in the cat's intestine and comes out through faeces. Toxoplasma ends up in the vegetation, and when the rodent eats the vegetation, it becomes infected. Consumption of infected meat and contact with cats can cause humans to become infected with Toxoplasma. 

When a human is infected with Toxoplasma for the first time they experience mild flu-like symptoms that resemble a cold or a flu. The parasite transitions to a 'sleeping' stage in the brain after the first infection phase. Then, it begins a chronic silent infection that can last for decades or for life. While chronic infection usually causes no infection in healthy individuals, Toxoplasma can cause a life-threatening brain infection called encephalitis in people with a weakened immune system, and can be dangerous to the foetus during pregnancy.

However, Toxoplasma can cause eye infections even in healthy individuals.