The Wonderful World of Parasites

Dov Michaeli

Posted 1/24/12 on The Doctor Weighs In

Today we are going to go on a circuitous and fascinating journey that will start with, brace yourself…cockroaches, but will end up in the brain of…maybe your own.

I recently came across a mind-blowing article in the Scientist online edition on how parasites manipulate the brain, and behavior, of their victims.

Describing the rich tapestry of host-parasite interactions is close to impossible, so I’d like to highlight only two of them.

The tale of the wasp and the zombie cockroach

The tiny wasp called the jewel wasp (sometimes also called emerald wasp)  is using the cockroach as an incubator for its brood. How she does it will blow not only your mind, but the cockroach’s as well. She stings the cockroach twice in rapid succession: first in the thorax, and then in the brain. The first sting stops the poor cockroach in its tracks, temporarily paralyzing its front legs only. The second goes right into an area in the brain, the subesophageal ganglion, an area that controls the ability to initiate movement, which by the way, is one of the manifestations of Parkinson’s disease.

Now that the poor cockroach cannot move on his own volition, the wasp grasps him by one of his antennae and leads him like a dog on a leash to her nest, and lays her eggs on his abdomen. A couple of days later the larvae hatch, barrow their way into the zombie cockroach, and feed on his innards. Pretty icky, but think how incredibly sophisticated is the brain-control the wasp was exerting. One could understand how natural selection would favor the wasp that could aim its sting at the approximate area of the subesophageal ganglion, however minuscule it is. But how does the wasp know that it is in precisely the right place? It turns out that the wasp has mechanoreceptors on her sting that can sense the consistency of the brain tissue, and only the correct consistency will trigger the injection of the venom that paralyzes the ganglion. In human brain surgery electrodes are inserted stereotactically, using a map of the brain. But the precision of brain surgery is orders of magnitude lower than what evolution endowed the little wasp with.

Interesting, but what does it have to do with us? Read on.

Cat and mouse games

Rodents, such as rats and mice, harbor a protoza called Toxoplasma gondii. In order to for it to reproduce sexually it needs to colonize a cat’s intestine. There they form the oocytes (eggs) that shed in the cat’s feces. From there they find their way to the next rodent. Only one hitch: rodents have an innate fear of the smell of cat urine, which allows them to avoid their mortal enemy. How does this fear work? Like any other fear in mammals: through the brain area that controls basic emotions like fear, rage, and sexual attraction- the amygdala. So what is a Toxoplasma organism to do to enhance its chances of getting to the cat so it can reproduce? It has to make the cat overcome its fear of cat urine. And this is accomplished by the protozoan crossing the bood-brain barrier, and changing the amygdala’s program of fear of the cat’s urine. Turns out that Toxoplasma makes an enzyme (tyrosine hydroxylase) that is the rate-limiting step in the synthesis of dopamine, which is a neurotransmitter that is used extensively in the amygdala. Thus, the parasite enzyme allows extra dopamine synthesis, which switches the rodent’s amygdala from a fear reaction to an attraction to the cat urine as if he was encountering a female in heat (estrus). Speaking of fatal attractions, this one turns the affected rodent into a meal ticket.

Once it ends up in the cat’s bowel, the parasite finds its to way to the cat’s human owner. And as we said, it can cross the blood-brain barrier of mammals, including ours. About 20-40% of the world population is infected with Toxoplasma.

In case you freak out reading it -don’t! As long as you have a functioning immune response it causes no problem. But severe immunodefficiency caused by HIV or chemotherapy may unleash the parasite and wreak havoc on the brain tissue, a condition called Toxoplasmosis.

But there is still a lingering question: although immunity inhibits reproduction of the parasites, the synthesis of extra dopamine  by Toxoplasma is not restrained by the immune response. So how does it affect our amydala? The short answer is that we don’t know, yet. But here is an intriguing factoid: more than three dozen studies have found a positive link between neurological disorders such as schizophrenia and Toxoplasma infection. And schizophrenics are much more likely to harbor Toxoplasma than non-schizophrenics. Not very compelling, because it is merely correlational, but intriguing and definitely deserving further study.

So there we are. As promised, we went through a fascinating journey of mind-control by parasites, starting with the lowly cockroach and ending with our own brain. Who said biology isn’t endlessly amazing?