"Long-Term Effects of Toxoplasmosis Brain Infection"
Of all foodborne diseases, a brain parasite called toxoplasma
is ranked as the fourth leading cause of hospitalization
and the second leading cause of death in the United States.
Nearly a quarter of us have already been infected,
one in three of us by the time we hit our fifties.
Although we may then be stuck with this thing in our brain for life,
our immune system is so good at keeping it at bay
for most healthy people it’s never able to rear
its ugly head—or at least overtly.
“Studies have now revealed associations between [toxoplasma infection]
and the presence of various psychiatric disorders in humans”—
schizophrenia, bipolar, suicide, self-harm,
and memory impairment when we get older.
How can a tiny parasite alter our very behavior?
I talked about how the rabies virus hangs out in the saliva
while specifically targeting the emotions center of the brain
to drive animals into a fury, so they’ll effectively do
the virus’s bidding to bite others to transmit the virus;
or the famous zombie-ant brain fungus that takes over the animal completely.
These are examples of so-called parasitic manipulation,
where the parasite manipulates the host to “[enhance] its own transmission
by altering host behavior.”
And toxoplasma is perhaps one of the most convincing examples
of a manipulative parasite of higher animals, like us.
Since the parasite thrives in cats, chronically infected rodents
no longer respond to cat odor with fear and indeed,
the physical response is reversed to attraction.
Mice become attracted to the smell of cats,
serving the parasite up on a silver platter.
The parasite manipulates the rodent’s brain to turn their innate
aversion to cats into a ‘suicidal’ ‘fatal feline attraction.’
Mice become attracted to cat pee, and such fatal feline attraction
appears specific towards cats.
They don't become attracted to pee in general.
They remain indifferent to rabbit pee and
continue to be turned off by other predator pee.
So, on one hand, the parasitic manipulation appears incredibly specific,
but the parasite doesn’t just want the mouse to seek out the cat,
but get eaten as well.
And so, there are these general effects too:
impaired motor function, slower response times, memory, and co-ordination.
And so, when the cat pounces, the parasite tries to make sure
the mouse doesn’t get away.
It’s like when California sea otters get toxoplasma
they’re more likely to get eaten by a shark.
It’s not that the parasite wants to get into the shark—
it may just be a by-product of the kind of general cognitive deficits
that is so helpful for the parasite in other contexts.
It’s like when humans get toxoplasma,
we start liking the smell of cat pee more too.
Isn’t that wacky?
The parasite knows just what strings to pull.
But it’s the more general effects we’re concerned about.
We don’t need to worry our newfound appreciation
for saber-toothed tiger urine is going to get us eaten,
but mucking with our reaction time – that could be a problem.
That could be why multiple studies have shown more traffic
and worksite accidents among those who are chronically infected.
But it may not just be our slowed reaction time.
The parasite appears to also affect subtle behavioral alterations,
like personality alterations that make us more likely to take risks.
Great for the parasite in the cat and mouse game,
but not so much if we’re driving a car or wondering whether or not
to take that next drink.
Maybe one reason people with this brain parasite
get into so many car accidents is that it may make people engage
in riskier behaviors like excessive alcohol consumption.
We typically think of malaria as being humanity’s greatest killer parasite.
However, when we take into account the hundreds of thousands of deaths
that occur due to the increased probability of traffic accidents,
working accidents, suicides, and possibly other side effects
of the infection, maybe this supposed
“asymptomatic” latent toxoplasma infection
that has infested one in four Americans could easily take malaria down a notch.
Before I get into how to prevent and treat the darn thing,
what might these other side effects be?
How exactly does toxoplasma manipulate behavior?
Well, one clue we got decades ago is the rise in dopamine levels in the brain.
You can show it right in a petri dish of infected brain tissue.
Turns out that these parasites actually have an enzyme
to make dopamine from scratch, which they then release
into the surrounding brain tissue. Why do we care?
Because elevated dopamine is a characteristic of schizophrenia.
That’s how nearly all modern antipsychotic drugs work,
by trying to bring dopamine levels back down—
either inhibiting dopamine receptors or decreasing
the level of dopamine in the brain.
So, is it possible that the increased dopamine accumulation and release
observed during toxoplasma infection might increase
the risk of schizophrenia?
Well that should be easy to figure out.
I mean, do schizophrenics have an increased prevalence of infection?
The increased prevalence of toxoplasmosis in schizophrenics
has been demonstrated by at least 50 studies.