Brain parasite spread from cats rarely gets humans sick – here's why

The parasite, Toxoplasma gondii, is found worldwide, and is particularly associated with cats. In humans, infection by the parasite can lead to developing a potentially fatal illness, toxoplasmosis.

By AARON REICH
Toxoplasma gondii tissue is seen in a cyst in a mouse brain. (photo credit: Wikimedia Commons)
Toxoplasma gondii tissue is seen in a cyst in a mouse brain.
(photo credit: Wikimedia Commons)
A brain parasite that infects millions of people worldwide and infects meat will almost never show symptoms, researchers from the University of Virginia (UVA) School of Medicine have found.
The parasite, Toxoplasma gondii, is found worldwide, and is particularly associated with cats. This is because the parasitic life form is only known to be able to reproduce in cats. It is often found in other animals too – most notably rats, which the parasite has been shown to alter the behavior, essentially shutting down fear responses in order to ensure the rats are eaten by the cat.
However, the parasite is one of the most common among humans in developing nations. Many countries have different rates of infection, with the US having around 30 million. In 2000, studies singled out France for the highest number of infected citizens, which constituted 84% of the population.
In humans, infection by the parasite can lead to developing a potentially fatal illness, toxoplasmosis.
However, the disease is very rare, and often only shows up in people who are immunocompromised, such as HIV/AIDS patients. In fact, most infected humans will never be affected by the disease, and the new study from UVA has found out why.
According to the research, which was published in the academic journal Nature Communications, the parasites are kept in check by a type of immune cell in the brain called the microglia. What these cells do is release a unique molecule, called IL-1a, to allow immune cells in the blood to essentially control the parasites. This process is very effective, and explains not only why so few humans contract toxoplasmosis, but also why immunocompromised patients can – as their weakened immune system renders this ineffective.
The implications behind this finding are groundbreaking, as knowing how the brain reacts to one infection like this can help find ways to motivate microglia to react to other conditions. This can range from strokes, multiple sclerosis, Alzheimer's disease and even brain injuries.
This is merely the latest in a series of groundbreaking discoveries about the brain's relationship with the immune system made by UVA researchers in recent years, which have essentially upended widespread scientific consensus. It was previously widely held that the brain is completely disconnected from the immune system, but the UVA studies on the role played by the microglia have debunked this misconception.
A major part of the confusion is that microglia are nearly indistinguishable from other immune cells, but UVA researchers discovered a means to tell them apart, as they react to infections differently than other immune cells.
“Microglia must die to save the brain from this infection,” Tajie Harris, PhD, of UVA’s Department of Neuroscience and the interim director of the Center for Brain Immunology and Glia (BIG), said in a statement. “Otherwise the IL-1a remains stuck inside the microglia and wouldn’t alert the immune system that something is wrong.”

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The researchers now seek to understand how the microglia detects the parasites, and hope that this could allow them to understand reactions to different diseases.
“The immune system must enter the brain to fight dangerous infections,” Harris explained. “We now understand how microglia sound the alarm to protect the brain. We suspect that similar signals are missed or misinterpreted in Alzheimer’s disease, opening up an exciting new research avenue in the lab.”


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