Tomorrow evening zombies take center stage at the California Academy of Sciences' NightLife event. Among other (really fun looking) activities, Patrick House, a PhD candidate in neuroscience in the School of Medicine, will give a lecture on the neuroscience of Toxoplasma gondii, a parasite that, according to the Centers for Disease Control and Prevention, has infected more than 60 million people in the United States.
Curious to know more about House's Zombie NightLife talk, I posed a few questions to him about parasitic mind control and the latest scientific research on Toxoplasma gondii:
How does this parasite manipulate its host to enter a state of zombism and sacrifice itself for the reproductive nature of the organism?
Toxoplasma, for some unknown reason, requires the cat to sexually reproduce - so it spends its time needing to get from one cat to the next. Without legs of its own, it depends on intermediate hosts like rats to ferry it to the next cat. The problem being, of course, that rats are normally and appropriately quite afraid of cats. But, and this is where the zombie lore comes in, if rats have Toxoplasma in their brains they are no longer as afraid of cat odors and, in some cases, they are actually attracted to the odors. This makes the rats more likely to get eaten and thus pass on the parasite.
If you believe in rat free will, and you believe that a hijacking of that free will qualifies as 'zombism', then you don't need George Romero to make this stuff up for you . There are scurrying rat zombies in just about every city block.
In a recent study, you and colleagues tested the brain activity of 36 infected and uninfected rats exposed to either the smell of a cat or that of an estrous female rat to better understand how the parasite may change hosts' behavior. What were the study results indicate about Toxoplasma gondii sabotages the rodents' neuronal pathways?
We found something utterly bizarre. When a normal male rat is exposed to either a cat or a female rat we can look in the brain and see a big difference between the brain regions that respond to emotional stimuli. These are evolutionary, emotionally and behaviorally very different stimuli - one the rat immediately runs from and the other it mates with. We see this reflected in neural activity in the brain. But in the male rats with Toxoplasma, when we expose them to cat odor, we get something that looks like a little bit like a rat that has just smelled a cat and a little bit like a rat that has just smelled a female rat. Potentially this is a neural mechanism for the 'attraction' behavior we see in infected rats.
How do people become infected with Toxoplasma gondii and how prevalent is it in the United States?
Toxoplasma is shed from the cat in feces, which often ends up in one of two places: fertilizer for livestock or in people's homes and litter boxes if they have a pet cat. People mainly get Toxoplasma from either eating raw meat, ingesting little bits of litter box contaminated with cat feces or incidentally the cat feces itself. Often doctors will tell pregnant women to stay away from litter boxes for this very reason, as acquiring Toxoplasma during pregnancy can be quite harmful to the developing fetus.
But cooking meat fully and washing your hands after changing the litter box will prevent Toxoplasma spread. So there are ways to avoid it. Not everyone is so good at avoiding it though. Estimates of Toxoplasma infection in the U.S. suggest somewhere between 12 percent and 20 percent of the entire population has Toxoplasma sitting in their brains.
What does the scientific evidence indicate about the how the parasite affects human brain function and behavior?
Some estimates predict up to 33 percent of humans worldwide have Toxoplasma in their brains. This is mostly not a medically relevant problem, though - Toxoplasma is clinically asymptomatic if you have a sound and healthy immune system. For various reasons, though, over 50 studies now have looked at a relationship between Toxoplasma and schizophrenia and found positive associations in almost all of them - the finding being 'If you have schizophrenia, you are more likely to have Toxoplasma.' We don't know why, though - whether Toxoplasma is somehow accelerating or causing schizophrenia, or whether schizophrenics are more likely through their habits to pick up the parasite.
What is the focus of your current research efforts on the parasite?
Currently, colleagues and I working in the lab of Robert Sapolsky, PhD, in close collaboration with members of the Boothroyd Lab at Stanford, are trying to engineer better tools and ways of tracking the parasite in the brains of infected rodents. We don't have a very good map of where the parasite goes when it gets in the brain, nor do we know what it does when it gets there. But we know it infects neurons and makes cysts in the brain. So we can look for those cysts, look for the infected neurons and ask questions such as: What is the impact of Toxoplasma on a single brain region or a single neuron? How does this relate back to the innate rat fear response to cat odors? And, how is Toxoplasma inspiring it's manipulation?