15. The Stealth Attack: Part 3 – No, Humans Are Not Immune.

In my previous blog, I introduced the notion that humans, like insects and mollusks, may also be the subjects of involuntary behavioral control resulting from parasitic infection. May I now introduce Toxoplasma, a leading candidate for such a human behavior manipulator?

                Toxoplasma is a protozoan, one of the vast Kingdom of single-celled creatures that swarm in the earth’s waters, soils, and yes – within other organisms. Many of you, although you may not know its name, have heeded warnings about Toxoplasma exposure. This is the parasite often found in cat feces and thus the admonitions to be careful when cleaning your cats’ litter box. This is particularly true for pregnant women as the parasite can be transmitted to the fetus with potentially dangerous results. Infection with this protozoan is known as toxoplasmosis and it is common in a variety of mammals. These include cats, rodents, pigs, and humans. By some estimates, nearly thirty percent of the world’s human population is infected with Toxoplasma.  Normally the protozoan, after causing initial flu-like symptoms, resides in the human body without further affect – or so it was thought.

I had been aware for some time of the hijacking of the behavioral instincts of rats by Toxoplasma.  Since rats are a potential prey of cats, it makes sense that they would serve as a link in the completion of the parasites’ life cycle. It seems a cycle such as: protozoan reproduces in cat – protozoan in cat feces – feces in soil – accidental ingestion of protozoan by rat – cat eats rat – would be straightforward enough. But, here again, we are dealing with the strange world of behavioral manipulation by a parasite. Studies have shown that the rat is programmed by Toxoplasma to engage in behavior which makes the rodent much more likely to be eaten by a cat. In other words, the parasite guides rodent behaviors which will increase the likelihood of its return to the gut of a cat host. Here it can reproduce and continue the completion of its life cycle.

For obvious reasons, rats are normally extremely averse to having a meeting with a cat. When these rodents are infected with Toxoplasma however, their behavior changes dramatically. They are more prone to actively expose themselves and their reaction to danger is slowed. Normally, a rat smelling cat urine responds by freezing, analyzing its surroundings, and then scurrying for cover. Rats harboring Toxoplasma do the exact opposite. They seem to actually be attracted to cat urine and are quite content to ignore their instinct to avoid a meeting with their feline nemesis. Research suggests that, upon exposure to cat urine, Toxoplasma is actually biochemically activating a part of the rat brain associated with sexual attraction. This powerful urge then overrides the rat’s inclination to flee from signs of the presence of a predator. In other words, Toxoplasma is altering the behavior of the rat in order to make it more likely that a cat will ingest the parasite and thus will the protozoan be transmitted to other feline hosts.

Could Toxoplasma be causing behavioral changes in humans sheltering the parasite? An article by Kathleen McAuliffe which appeared in The Atlantic in 2012* certainly seems to suggest as much. Recall that most people infected with this parasite experience it in its so-called latent form. After the initial flu-like reaction, it was thought to lie quietly in the neurons of our brain without harm. Some scientists now suspect that Toxoplasma is not such a benign resident.  This was the gist of McAuliffe’s article which profiled a Czech biologist, Jaroslav Flegr, who was himself infected with Toxoplasma. For many years Flegr was puzzled by some aspects of his behavior. He reported in the interview with McAuliffe that he thought nothing of walking into a busy, traffic-filled street. Honks of irritation from oncoming motorists he met with total indifference. Furthermore, he openly criticized the ruling Communist Party, a most dangerous endeavor at that time. Doing research in a war torn area of Turkey, he was surprised that his reaction to nearby gunfire was complete lack of distress and absence of any instinct to take cover. All of these behaviors bore an uncanny resemblance to the daredevil antics of a rat with toxoplasmosis.

After reading articles regarding parasitic mind control among invertebrates, Flegr began to suspect that he might be similarly affected. He had himself tested and discovered that he did indeed have toxoplasmosis. We should think of humans as a dead end in this parasite’s life cycle (cats usually don’t eat humans). However, we are similar enough in genetic and physiological makeup to other mammals that it seems reasonable to assume that Toxoplasma might not “know” the difference. Pursuing this line of reasoning, Flegr’s investigations revealed that humans infected with Toxoplasma showed inattentiveness and delayed reaction times as do infected rodents. His research showed that certain drivers, as a result, were nearly twice as likely to be involved in vehicular accidents if they carried the latent Toxoplasma. Perhaps even more disturbing, science is suggesting that Toxoplasma may be involved in triggering schizophrenia in susceptible humans. Furthermore, a 2012 Scientific American article reports that researchers now also suspect a link between toxoplasmosis and an increased risk of suicide.

I find the thought that parasitic infection could cause such serious psychological manifestations among us humans quite alarming. One cannot help reexamining the extensive list of endoparasites which make humans their home – roundworms, tapeworms, flukes, protozoans. In the process, we are left to wonder just how many other neuroses or psychotic manifestations of Homo sapiens may someday be traced to our eerily creepy stealth attackers.


* https://www.theatlantic.com/magazine/archive/2012/03/how-your-cat-is-making-you-crazy/308873/


I recently became aware of another possible actor in the drama of parasitic mind control. I was reading Oliver Sacks’ fascinating book, The Man Who Mistook His Wife for a Hat and Other Clinical Tales. In his book Sacks, who was a neurologist for almost five decades, recounts case histories of patients with peculiar, often bizarre neurological disorders.

One of his patients was a 90 year old woman named Natasha. She came to Dr. Sacks after noticing a change in her behavior which had begun when she was 88. The doctor, of course, inquired as to what sort of change she had noticed. “Delightful!” she exclaimed. “I thoroughly enjoyed it. I felt more energetic, more alive – I felt young once again. I took an interest in the young men. I started to feel, you might say, frisky – yes, frisky.”

In the course of receiving Natasha’s medical history Dr. Sacks found that she had worked in a brothel over 60 years ago. She revealed that, like most of her coworkers, she had acquired syphilis during this time. As you know, this is a highly contagious, sexually transmitted disease caused by a bacterium named Treponema pallidum. This bacterium is a type of spirochete, so-called because of its helically spiraled shape.

The disease syphilis typically occurs in stages. The so-called tertiary stage may occur years after the initial infection. Although not infective at this time, the victim may experience serious symptoms such as loss of muscle control, depression, mania, and dementia. Death may ensue.

Natasha described her illness as, “something in my body, my, brain that was making me high.” To Dr. Sacks’ surprise, she then self-diagnosed herself as having “Cupid’s Disease”, a term with which he was unfamiliar. “All the girls called it that,” she said. The doctor proceeded to test her spinal fluid. “She was right; the spinal fluid was positive, she did have neurosyphilis, it was indeed the spirochetes stimulating her ancient cerebral cortex.”

I have as yet found no clinical studies affirming that Treponema increases promiscuity or the libido of its host (as in Natasha’s case). But in light of the impact of Toxoplasma on human behavior (as described above), it seems a possibility. After all, what better way could the syphilis bacterium ensure continued species survival than by making its human host feel – shall we say frisky?

Photo Credits:

Cat and pregnant women courtesy of findatopdoc.com

People ignoring danger courtesy of upsplash.com

The Scream by Edvard Munch via Wikimedia Commons [Public domain,

Toxoplamosis gondii courtesy of The Center for Disease Control

Toxoplasmosis life cycle courtesy of researchgate.net

syphilis bacterium by Dr. David Cox @ en.wikipedia.org


14. The Stealth Attack: Part 2 – This Being the Story of How a Snail Became a Zombie

That long-ago encounter with the Cordyceps fungus (described in Part 1 of this series) spurred my interest in determining whether other, similar mind-controlling parasites exist. Not surprisingly, they do. I find such symbiotic interactions between parasite and host to be both fascinating in their intricacy and disturbing in their manifestations.

For example, there is Leucochloridium. This parasite is a fluke, a type of flatworm. Although many flatworms are free-living, the group also contains a plethora of species which have evolutionarily opted for the parasitic way of life. All of the vertebrates, including mammals and birds, are subject to parasitism by some kind of fluke. In the case at hand, it is the alteration of the host’s behavior that makes Leucochloridium yet another fascinating example of the bizarre and creepy world of the endoparasites.

Leucochloridium parasitizes birds. The big problem faced by Leucochloridium, and parasites in general, is how to get from one host to another. Endoparasites are highly and specifically adapted for surviving in the warm, dark, nutrient rich innards of their host. They are not built to function in what for them is the hostile outer world we humans inhabit. Here the atmosphere is highly oxygenated, there is intense sunlight, and ambient temperatures are highly variable. Yet, to continue their species, they must find a way to insinuate their adult progeny into the internal organs of another host.

We must now ask, how does a flatworm (Leucochloridium) which lives in the rectum of a bird ensure that its descendants find a similar warm, fecal-laden home in a different bird? As with many endoparasites, the answer lies in the use of an intermediate host animal within which the larval stages go through their transitional, developmental steps. Flukes commonly use snails as intermediate hosts and Leucochloridium is no exception.

The life cycle begins as the adult flatworm voids its eggs into the surrounding rectum of its bird host. The intestinal wastes of the bird are thus loaded with the fluke’s eggs. A foraging snail, the intermediate host of the flatworm, happens upon the bird feces and begins to feed. The eggs are coincidentally ingested with the fecal material. Subsequently they hatch and go through a series of developmental stages. Having reached a phase at which they are ready to infect another bird, the fluke larvae perform a most interesting migration. Called cercariae at this stage, the larvae migrate through the snail’s body and into the tentacles on the head. You might be familiar with these as the long stalks at the end of which are located the snail’s eyes.  As a result of infiltration by the larvae, the tentacles (or often one tentacle) become engorged and distended. Even more remarkably, they appear to pulsate like a neon sign in response to exposure to light (https://www.youtube.com/watch?v=v0Ytm2U4Ch0). For birds, this banded, animated appearance bears a remarkable resemblance to a caterpillar. Of course caterpillars are a favorite food of many bird species. And remember, these puffed-up tentacles are packed with fluke larvae very much needing to get into a bird.

And now the curious issue of parasitic mind control again enters the equation. Snails normally prefer dark habitats. Here the air is more humid which helps to prevent dehydration of their moist molluscan body. The dark niches of their world also offer protection from predators seeking a nice meal of escargot. But snails infected by Leucochloridium shun the dark, hidden places. They boldly venture out into the daylight. Goaded by biochemical signals from the parasite, they become even more incautious and climb up onto grasses and trees thus making themselves even more conspicuous. Perched here in the broad daylight, with their pulsating, caterpillar-like tentacles the snails are attractive targets for birds. Perceiving what it thinks is a caterpillar, a bird will swoop down and attack the snail’s infected tentacle. In the process, it comes away not with a young insect but a nice package of fluke larvae. These move on into the bird’s digestive tract and, voila, the life cycle of the fluke is completed. Again, the thing that strikes me as so uncanny is the manner in which the parasite changes the behavior of its host. The hapless snail which, under normal conditions, eschews the light and the dry air suddenly behaves in exactly the opposite manner. And, as a result, the parasite’s life cycle needs are brought to a perfect ending. How weird! This would be a campfire tale guaranteed to keep little snails up all night.

How about one more example of the strange world of parasitic mind control? Although it uses venom as a rather more straightforward method for controlling its host, the life cycle of the little emerald cockroach wasp is still a marvel of evolution. As the name suggests, this is a small (a bit less than an inch) wasp with a beautiful metallic greenish-bluish body color. And, it does indeed parasitize cockroaches. As you might guess, it is the manner in which it does so that is dumbfounding.

Upon locating a cockroach the female wasp stings it. Surprisingly (and precisely) she does so twice. The first sting is administered into the abdomen at the exact point necessary to paralyze the cockroach’s two front legs. Now the wasp is able to work around the roach’s head without any counterattack from these appendages. Again bringing her sting to bear, the wasp injects her venom into the portion of the cockroach’s nervous system called the subesophageal ganglion, a sort of insect sub-brain. The result of this sting is that the roach loses its will to escape.  Although capable of movement, the hapless victim behaves like some arthropod robot and passively waits for the wasp to continue its ghastly work. Now, taking hold of one of the cockroach’s antennae with its jaws, the wasp leads its host to a previously prepared burrow in the ground. Like a well-trained dog on a lead, the cockroach obediently allows itself to be ushered, by the wasp, down into its own tomb.

Having arrived in the burrow, the wasp proceeds to lay a single egg on the abdomen of the amazingly cooperative cockroach. Still the roach shows no inclination to flee. The wasp now clambers out of the burrow and backfills it with soil or small pebbles. In a few days, the wasp’s larva emerges from the egg. As with other hymenopterans, the larva resembles a fly maggot. This larva now begins to feed on the ill-fated cockroach and, in so doing, gradually moves into the cockroach’s abdominal cavity. Incidentally, recent research has shown that the larvae of this wasp secrete antibiotic laden saliva which kills bacteria harbored within the cockroach’s body. These are bacteria which might be harmful to the grub. Could this be yet another biological source of antibiotic for human use? Time will tell.

After several days of feasting, the wasp larva forms a pupa inside the cockroach’s body. From here, it soon emerges as an adult wasp. Sounds like something out of a science fiction movie. Anyone else thinking of Ridley Scott’s Alien trilogy? It seems that, in this case, the wasp is using its toxin to block certain insect neurological pathways. Although perhaps less biochemically complicated than the mind altering mechanisms of Cordyceps or Leucochloridium, we still must marvel at the complexity and efficiency of this diminutive wasp’s behavioral repertoire.

One might presume to meditate upon the fact that it is a really good thing that humans don’t fall prey to such insidious parasitic mind control. Of course, in making such a presumption, we would likely be kidding ourselves. We must recognize that humans are biological organisms too – undeniably solid citizens of good-standing within the Kingdom Animalia. As a result, we should be considerably more surprised if we were not considered fair game by such parasites. In fact, research suggests very strongly that humans are far from immune to the mind-bending antics of parasites.

For a glimpse into the disconcertingly disturbing world of parasitic mind control in humans, be sure to keep an eye out for The Stealth Attack – Part 3.

Photo Credits:
Leucochloridium graphic - Intro.to Parasitology by Chandler and Read
Leucochloridium infected snail - Thomas Hahmann at Wikimedia Commons
Emerald Cockroach Wasp - courtesy of bioweb.uwlax.edu
Wasp with Cockroach - courtesy of creativecommons.com