Infection with a mysterious parasite carried by cats seems to help the immune system target tumor cells.

A parasite that lives in the intestines of house cats and causes infected rodents to be less wary of their feline predators, called Toxoplasma gondii, has even been blamed for strange behavior in humans. But it turns out that this single-celled parasite may offer powerful benefits, too.

Although T. gondii cannot reproduce without a feline host, the parasite can live in other mammals. About a third of all people worldwide may be infected. Most people have no symptoms, but some experience flu-like symptoms. (Some very tentative research also links T. gondii infection with unusual and potentially even suicidal behavior in humans.)

However, the reaction the human body produces in response to the parasite is much like the immune response needed to destroy cancer cells. That means that T. gondii could be a building block for a new cancer vaccine.

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“We know biologically this parasite has figured out how to stimulate the exact immune responses you want to fight cancer,” David J. Bzik, an immunology professor at Dartmouth medical school, said in a news release.

Cancer suppresses some natural immune responses, particularly the work of dendritic cells and macrophages. T. gondii gets inside these cells and somehow reactivates their fight against cancer. They destroy cancer cells and teach the body’s T-cells to recognize and target them.

Bzik genetically altered the parasite to make it safer and and stripped it of its ability to reproduce, even in people whose immune systems are compromised.

“Once it’s inside the cell, it cannot replicate, not even once. It modifies the cell but doesn’t kill it,” he said.

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In government-funded research published last year, Bzik tested his version of the parasite, dubbed cps, in mice with very aggressive melanomas and ovarian cancers.

Miraculously, 90 percent of the melanoma-infected mice survived. The mice infected with ovarian cancer survived much longer with the treatment than they otherwise would have.

“It’s the first stand-alone immunotherapy that’s been successful — and trust me, there are a million things that have been tried,” Bzik said.

The researchers say they still need to understand the biochemical mechanism that cps uses, once inside immune cells, to rekindle the body’s attack on cancer.

While the sanitized version of the parasite shouldn’t cause infection, it will still have to be tested for safety before the researchers can turn it loose on human cancers.

In the meantime, don’t rush to adopt a cat. A cat’s litter box rarely leads to a T. gondii infection in its owner. Eating infected meat is the most common source of infection. 

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