Watch a brain-hijacking parasite sneak its way in

You may have read last week that the single-celled parasite Toxoplasma gondii can hijack a chimpanzee’s brain, luring them to the urine of their only natural predator, the leopard.

Today an American study, published in the journal Nature Microbiology, claims to have found the pesky parasite’s path from blood to brain, and even made a microscopy video showing an infection taking hold.

T. gondii, or Toxo for short, can infect loads of different warm-blooded animals – including humans – but needs a cat to complete its life cycle.

It’s well documented that infected mice lose all fear of felines – even running straight up to their mortal enemies, almost begging to be eaten. And Toxo is common in humans, with around 30-50% of people harbouring the parasite.

When healthy adults and children ingest Toxo, whether it is in undercooked meat, through cleaning the litter box or playing in a sandpit, they usually fend off the initial infection with only a few fluey symptoms.

Those with a weaker immune system, though, may need antibiotics to stave off the disease. And if a woman becomes infected for the first time while pregnant, it can cross the placenta and harm the foetus.

Usually, though, once the initial symptoms disappear the microbe nestles in brain cells, lying dormant and dampened by the immune system.

But how the microbes manage to bust their way through the body’s blood-brain barrier, which is usually quite good at keeping microbes out, has been a mystery. Do they squeeze their own way through, or hop inside a human “Trojan horse” cell, such as an immune cell, to ferry them across?

Turns out it’s neither.

Using mice, Christoph Konradt from the University of Pennsylvania and colleagues showed the parasite penetrates the very cells that create the blood-brain barrier – endothelial cells.

Once inside, they replicate like mad. In just two weeks, the host cell bursts, releasing the newly “born” parasites straight into the brain.

The researchers also found that replication seemed to be key to the parasite’s success. When they infected mice with mutant Toxo, which couldn’t replicate, they didn’t see any of it make it through the blood-brain barrier.

“That could mean a drug that blocks replication could be effective at preventing dissemination,” Konradt said.

The mice were specially bred to express a green fluorescent molecule on their endothelial cells. You can watch the parasite’s path in the video below or read further on the subject with our feature Microbes with Mind Control.

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