In a scientific discovery at Dartmouth recently hailed as "highly shareable" by the internet, cat poop is being mentioned in connection with a newly discovered potential cancer treatment.
Toxoplasma gondii, a single-celled parasite found in the guts of cats, has been used in a lab to treat cancer. It might, after enough testing, turn out to be a viable cancer therapy. However, toxoplasma is a strange, shape-shifting organism, and the kind cats poop out won't shrink your tumors one bit. Still, Dartmouth recently publicized the very promising discovery: A modified version of toxoplasma, when injected into mice with certain kinds of cancer, switched on an immune response that the cancer had deactivated, which then allowed the body to fight the disease itself.
David J. Bzik, PhD, of Dartmouth's Geisel Medical School, has been experimenting with toxoplasma for at least a decade. He says the discovery that an altered form of the parasite might cure cancer is a big deal, but that toxoplasma is weird and wonderful microbe that still has surprises in store for humanity, none of which involved ingesting cat poop by any stretch of the imagination.
He also schooled me on some interesting trivia I thought I knew about toxoplasma. What follows is an edited version of my conversation with him.
Via Flickr user Yale Rosen
I'm reading a lot of headlines about cat poop curing cancer.
Oh, of course. They're sensationalist.
What should they be reporting?
We developed this strain of toxoplasma that doesn't replicate.
Could you remind us what toxoplasma is?
It's a protozoan. Its closest relative is malaria; it's in the same phylum.
And what happens when it can't reproduce?
It doesn't cause disease in mice. It's a great vaccine for toxoplasmosis, [which], in AIDS patients, is a really big disease. Also in cancer patients—when their immune systems are suppressed, they're vulnerable to natural infections by toxoplasma. So having a vaccine is a good idea. This has not been tested as a vaccine yet in humans or cats, and we also haven't tested the anti-cancer effects in humans either. This has all been mouse work.
If you've seen Trainspotting, you might remeber what happens when someone with AIDS gets toxoplasmosis (spoiler warning).
But in doing that work, we've realized that there were a lot of immune signatures that were anti-cancer in nature. So we did some trial studies in cancer models in mice. We've now looked at three cancer models, and we find very potent anti-cancer responses. In one, our ovarian cancer model, we showed that we observed 100 percent survival.
You injected the parasite into the tumors, and it treated cancer?
In melanoma cells we injected the parasite right into the tumor in the skin, and in the ovarian we injected it into the peritoneum—into the gut—where the ovarian cancer is growing. If we can understand what the fundamental mechanisms are, that will create new strategies for cancer therapeutics that attack the cancer biology. So you can imagine pharmarmacologically, you may not even need this parasite down the road.
Could these cell responses mean it could do things other than treat cancer?
As a scientist, I'm much more excited about the biology, because what this parasite is telling these innate immune cells is a remarkable piece of biology. So by understanding the network, the signaling pathway and the communication, that the parasite is doing inside of the cell, we'll actually get at more fundamental mechanisms that we don't understand about how to manipulate these cell types to make them do the types of immune responses that we need. Not just for vaccines, but even potentially for other types of diseases that we don't have appropriate treatments for yet.
But as for it being a cancer treatment, should we really be getting that excited?
I'm really excited about it from a practical perspective. And this is based in part on the history of vaccine development. The FDA just approved the first cancer vaccine, just two years ago. That treatment's exact strategy provides one potential mechanism for delivering the cancer vaccine to patients. So we do see that with appropriate safety testing, and phase I, II, III clinical trials showing efficacy in some form of human cancer, that there is some promise for that strategy.
But to be clear, the form that cats poop out, that won't treat your cancer, right?
That's correct. That will give you toxoplasmosis, unless you have it already. What we're using in the laboratory is called the asexual form.
Via Flickr user Goodiesfirst
What does the form that's in cat poop do, if not cure cancer?
When cats are on farms, it gets into the farm mud, into the field, into the rainwater runoff, and it's a very stable form. If it's in a field and it rains, it will float, it'll stick to a blade of grass, and then a grazing animal, like a sheep, will eat it, and then be infected, and if that sheep is infected, then it can cause abortion in sheep. That's an agricultural cost, and that's why we need a vaccine to protect sheep, so we don't lose baby sheep. Humans though, are a dead-end host, because we're not eaten by cats.
No. Not often.
So when that cat poops in a field, and a rodent eats something that's coated in cat poop, it gets reinfected. And then it just goes around and around.
Right. There is that factoid that gets passed around online that says that cats actually benefit from having in their gut.
I know. There are a lot of evolutionary theories out there, and that is indeed one of them. You know, there are other theories also, asking the question as to whether this latent infection with toxoplasma affects the behavior of the infected animal, including humans. I mean there's almost a cult of scientists who are looking into some of these issues.
Does that hold water, in your opinion?
I think some of the science is solid. But if you look at it from a practical point of view, the penetrance, meaning the fraction of that disease—like say for example schizophrenia. Some of the behavioral phenotypes are commonly caused by other things, so teasing out when toxoplasma is causing it, and how much, and why, is going to be challenging.
Thanks, Dr. Bzik!
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