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You can keep building your STEM skills at Brilliant.org/SciShow with 20% off an annual premium subscription! [♪ INTRO] Doctors have a huge collection of tools, tests and procedures to diagnose many known conditions. But when something /unknown/ comes up, it’s a race against time to figure out what’s going on.

A new condition may affect many people, or just a few. Or in the rarest cases, only one. Yet it’s still important to get them care, even in the most extraordinary circumstances.

Both to help them, and anyone else who develops the condition, unlikely as it may be. Which is why doctors and pathologists, the scientists who study disease, had to crack the mystery of the patient with the rarest cancer ever seen; a cancer that as far as we know, has only ever happened /once/. The patient was a 41-year-old man in Colombia.

And the cancer technically wasn’t even human. The malignant masses of cells in his lymph nodes originally came from a tapeworm. But they didn’t look like tapeworms, or act like tapeworms.

So it took about five months for the patient’s doctors in Colombia and researchers from the U. S. Centers for Disease Control and Prevention to puzzle out what was going on.

Let’s look at the case from the beginning. In January 2013, the patient presented with a fever, cough, fatigue, and weight loss. He had been diagnosed with HIV in 2006.

There are therapies available to manage HIV infections, but the patient was not taking any medication when he presented. By 2013, the HIV infection had taken a toll on his immune system. This can be measured in two ways: the amount of virus or viral load in the blood, and the number of immune cells called CD4 cells also present in the blood.

HIV destroys CD4 cells, so as viral load goes up, CD4 counts go down. The patient’s viral load was about 70,000 per milliliter, where 100,000 is considered severe. His CD4 count was 28 per cubic millimeter, where below 200 indicates severe infection..

The doctors checked for other infections and found evidence of parasites including Hymenolepis nana, better known as dwarf tapeworm. Dwarf tapeworm is the most common tapeworm infection. It affects about 75 million people around the world, and it has one trick that makes it particularly nasty.

Unlike most parasites, which need to live in a couple of different organisms throughout their life cycles, dwarf tapeworms can spend their whole lives, and reproduce, in the human small intestine. But there was one other thing that caught the doctors’ attention. The man had clumps of cells up to four-and-a-half centimeters across growing in his lungs, his liver, near his kidneys, and in his lymph nodes.

In many ways, the clumps seemed a lot like cancer. Under a microscope, the cells in the clump all looked like copies of each other, a sure sign of uncontrolled cell growth. And they were growing in places where they obviously didn’t fit in, much like malignant cancer cells that can spread through the body and take root in new places.

Each clump of cells was densely packed and unorganized, like the cells were growing and dividing super fast without any particular function. The cells also had large nuclei, which is common in stem cells, cells that, somewhat like cancer cells, are able to keep growing and dividing. That also pointed toward something malignant.

But the clumps of cells had other features that stopped the team from calling it cancer outright. First, the cells were smaller than most human cancer cells. And when the researchers specifically looked for human cytokeratin and vimentin, proteins that usually show up in human cancers, they didn’t find any.

The researchers started brainstorming: Maybe it’s a slime mold, or amoebas, or something infectious. But they ruled those ideas out one by one. While they ran a DNA test for slime molds and fungi, they received an unexpected result: Dwarf tapeworm DNA.

With a 99% match. The team double- and triple-checked their results because these cell clumps looked nothing like tapeworms, and they were far outside the tapeworms’ usual territory in the intestinal tract. But remember, the patient did have a tapeworm infection.

So the question became: How did it turn to cancer? A close look at the whole DNA sequence of that tapeworm infection revealed significant mutations in at least five proteins. Three of those proteins had known equivalents in mammals, and those had previously been tied to cancer.

So the tapeworm had developed mutations likely related to cancer. And the masses of cells looked like cancer in every sense, except for the part where they were tapeworm cells. Leading the doctors to conclude that the clumps were malignant, cancer-like masses that originated in tapeworms.

It took the research team five months to make the diagnosis. But by that time, the patient’s condition had deteriorated. He gave permission for scientists to study his condition and publish the results.

Three days after DNA analysis confirmed the diagnosis, he died of kidney failure. It took a few more years for the team to finish analyzing the case and publish their results. The question remained: How do tapeworm cells spread around a person’s body?

Scientists think that because HIV had depleted the man’s immune system, the tapeworm population in his gut mutated so much that it could move beyond the small intestine. One of the researchers speculated that the masses started out as regular tapeworm larvae. But being in the lungs or lymph nodes and not the intestine that they’re adapted to, they matured out of control into these cancer-like clumps instead of adult tapeworms.

The good news is that the researchers don’t expect this to be a common result of a tapeworm infection. There are effective treatment options available for tapeworm infections, and there are antiretroviral therapies that can lower an HIV infection to an undetectable level, for those with access to that care. And while the researchers don’t necessarily expect this to happen again, they do suggest that if it does, it could go mis-indentified.

But now that there is a well-documented case, doctors caring for patients in similar circumstances will know to look extra closely at cancer-like lumps. If tapeworms are to blame, then the doctors can start working on the next question: what kind of treatment will help these patients? It’s a curious case.

So thanks for fueling your curiosity and watching this SciShow video. There’s a lot of stuff we’ve learned in school but forgotten until something jogs that memory and reignites our curiosity. And calculus falls into that category for a lot of people because it can seem abstract the first time you learn it.

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Even if it’s been a while since your first calculus lesson, these courses are designed for people of all levels. Click the link in the description down below or visit Brilliant.org/SciShow and you’ll get 20% off the annual Premium subscription. Thanks to Brilliant for supporting this video! [♪ OUTRO]