Parasites: definition, examples, consequences for humans and animals

Parasites: definition, examples, consequences for humans and animals

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Book review: “The Psycho-Trojans. How Parasites Control Us ”by Monika Niehaus and Andrea Pfuhl
The title evokes a horror film, a conspiracy myth or a metaphor for people who psychologically exploit others. The biologists Monika Niehaus and Andrea Pfuhl provide an insight into real life forms that control the behavior and psyche of their hosts and trigger not only illnesses but also psychological changes in humans - parasites.

The strategies that syphilis, rabies, or toxoplasmosis have developed in evolution to nest in their hosts are more fantastic than a Hollywood director could think of. "The arms race between parasites and their hosts started before the evolution of the mitochrondria and will only end when the evolution itself ends." James Moore

Better than a thriller

Nature eclipses the plots of the best thriller authors. Unfortunately, only a few biologists are able to tell the abilities of living things to survive and reproduce as excitingly as they are. This does not apply to Niehaus and Pfuhl: from "embarrassing relatives" (lice) to "Little Red Riding Hood and the fear of the bad wolf" (rabies), they take the reader on a journey from the steppes of Central Asia (plague) to the Greek Roman antiquity (elephantiasis) leads to the Renaissance (syphilis), and from werewolves (rabies) to rats that are sexually interested in cats (toxoplasmosis).

Parasites are everywhere

"Parasites are omnipresent, practically no living thing is safe from their unwanted affection," is how the authors begin their cultural history of the pests. The most sophisticated parasites do not force their victim to defend themselves, but cannot be recognized by the victim because they manipulate their host's psyche.

According to the authors, this applies not only to the "usual suspects" who plague people, dogs or cats. Various parasites have been controlling insects, spiders and crabs for at least 500 million years.

Brainwashed to suicide

Gurus who brainwash their believers to suicide have biological role models: A roundworm in the rainforest of Latin America reprograms ants so that birds pick them. To do this, he blushes the abdomen of the infected ant, because the birds love red fruit.

Then he makes the ant climb a tree on which red berries grow. A bird eats the "antberry". If it excretes its faeces, this in turn serves as food for ants, which they feed on their larvae. The next colony of ants is infected and the game starts again.

In the mouth of the enemy

Scratch worms change the serotonin level of amphipods. This distorts the signal transmission from the eye to the brain. The light of the sun appears gloomy, the crab floats out of the mud to the surface of the water, where birds eat it. The bird droppings ensure that the children of the scratch worm again infect crayfish.

Another scratch worm causes its host to swim directly into the open mouth of a predatory fish.

Destruction of body chemistry

Parasites can change complex behavior patterns by directly affecting their victims 'hormones, for example, a virus that infects the caterpillars' caterpillars. It destroys the hormone, which suggests satiety to the caterpillars, whereupon they pupate. Without this signal, the caterpillars continue to eat until they reach the treetops, where they die.

The viruses in the remains of the caterpillar corpses now sail through the air and thus come to other places of the sponge spinner.

Complexity is not a protection

Crayfish and sponge spider caterpillars are simply structured creatures that are easy to manipulate. But the much more complex brains of mammals are not immune to such changes in the psyche.

More than a dozen infectious agents are thought to trigger psychiatric illnesses. This includes rabies and syphilis viruses, including Borrelia, chlamydia, herpes viruses, streptococci and Toxoplasma gondii.

War in the body

The human body is not defenseless against these hostile invaders. The biologists write: “The first barrier is the immune system. It releases so-called cytokines, which allow it to fight the intruder. Then the parasite moves again, influencing the attack in its favor. "

From defense to manipulation

The authors quote psychoneuroimmunologist Shelley Adamo: “There may be only a small evolutionary step between manipulating the immune system (with which the parasite tries to prevent its destruction) and manipulating it to force its host to produce substances that act on the nervous system and influence its behavior. "

According to Niehaus and Pfuhl, evolution promoted the parasites that manipulate the immune system on the one hand, and those that affect the nervous system on the other. If parasites have reached the brain, they are protected because they are less exposed to attacks by the immune system.

Spirit and body

There is no separation between psyche and body. According to the authors, the psyche is not metaphysical, insects behave crazy when they are infested with parasites - so do people.


In the first chapters, the authors focus on the "classic" troublemakers of humans. This includes the felt louse, vulgarly known as the “sack rat”, which affects the beard, eyelash, eyebrows and pubic hair, “which makes it the most embarrassing representative of the triumvirate,” said the authors. The other two in the group are the subspecies head louse and the clothing louse.

They actively climb from host to host and hang around wherever people are, in the bus and train, clothing and bed linen.

According to Niehaus and Pfuhl, the clothing louse is very young in the history of evolution. It only developed when human ancestors lost their fur and wore clothes. She accompanied the ancestors of the Native Americans from North Asia via the Bering Strait.

The lice are easy to control. They rely on human body temperature, so washing clothes contaminated with lice at 60 degrees Celsius is enough.

Nesting place human

Lice need to take blood every few days, otherwise they will dry out. Itchy wheals then form in the clothing louse. After eating, the animals mate, two hours later the females lay their eggs. Head lice in nits lay up to 300 eggs on the host's hair shafts, lice on the seams of the clothes. The “glue” cannot be washed out with water alone. The larvae hatch after 1-2 weeks and are sexually mature after the third moult, i.e. in about four weeks.

Dreadlocks and Vistula Braids

Today around 6% of children between 6 and 12 years are affected by head lice, the unwanted guests prefer long hair. A clear sign is the "Weichselzopf", in which hair shafts matted into a kind of dreadlocks.

This is why dreadlock carriers are often considered unsanitary in many older people, in whose childhood lice were ubiquitous.

The medusa hair

Such “louse curls” were found in a mummy from the 7th millennium BCE as well as in mummies from ancient Egypt. The authors call the chapter "medusa hair" for a reason. Medusa, a mythological figure from ancient Greece, wore snakes on its head instead of hair by a curse. One thesis is that this myth originated from the hair matted by lice. According to the authors, shaving the head and body hair of Egyptians, Greeks and Romans probably has the reason to keep the lice away.

What they don't mention are skinheads. These originally displayed their origin from the English working class: DocMartens were the shoes that their fathers wore at work in the factory; the hair of the working children was generally shaved to prevent plague of lice.

Superstition: a gift for lice

There was superstition around the matted hair, which went hand in hand with folk medicine. The "Weichselzopf" should draw diseases from the body and offer a home to evil spirits that would otherwise have nested in the body.

The notion that harmful forces could be drawn from the inside of the body to the surface of the body in order to render them harmless persisted in academic medicine until the 19th century. The cause of this, which the authors do not mention, could again be parasites: tapeworms or roundworms cause harm to the body, if they leave the body through a worming treatment, they are harmless.

With this superstition, our ancestors ensured that the lice spread all over the place. It was a misfortune to cut off the lousy braids; the evil spirit then became homeless and took revenge on the former braid bearer.

A lousy society

According to the authors, lice were omnipresent until the 19th century, and not only among the poor. The rich dressed in furs, and these offered paradise to the parasites. The bloodsuckers were even seen as a sign of abundant manpower. In the tradition of Galen's humoral teaching, contemporaries believed that lice draw harmful juices from the body.


It was only in the 20th century that medicine recognized that head and clothing lice could transmit dangerous diseases. So countless people infected by the parasites became infected with typhus, which leads to hallucinations and in the past very often to death.

Aztecs and Napoleon

It is believed that two million inhabitants of the Aztec empire died when the Spaniards introduced the pathogens with their lice, and, according to the authors, "responsible for the high losses on Napoleon's Russian campaign was less the military genius of the opposing armed forces than the devastating influence of" General Winter "And" General Laus ".

The transmitters are rickettsia, in turn parasitic bacteria that live in human clothing and hair parasites and use them as vehicles. Borrelia still use lice today and get on human skin when we crush lice.

Ship and dungeon fever

With the lice, the typhus spread everywhere and spread mostly where many people were in a confined space: in prisons, under sailors and in the poor areas.

The ship's doctor James Lind (1716-1794) was the first to recognize that the "ship fever" had to do with the sailors' clothing because it spread when the sailors went ashore and that it was the same disease as the " Dungeon Fever ”acted. Later terms such as "industrial fever" and "Irish fever" indicate where the Rickettsia struck later - in the slums of industrial workers and among the impoverished Irish.


In the further course of the first part, Niehaus and Pfuhl devote themselves to fleas and worms, the plague and elephantiasis, in the second part to address the pathogens that directly change brain functions - syphilis and the Bornavirus, rabies and toxoplasma.

According to the authors, no pathogen shaped the sexual morality of Europe more than a spiral tiny, Treponema pallidum, the causative agent of syphilis. They continue to write: "And Treponema also shaped psychiatry, because for a long time neurosyphilis sufferers still made up the majority of the inmates of" mental asylums "."

Depression viruses

Viruses that trigger mental disorders that medicine knows as bipolar disorders and depression are largely unknown. Bornaviruses settle in the cell nucleus and control the limbic system.

The authors ask whether the diagnosis of depression is not a general store where various symptoms with different causes are dealt with.

The Bornavirus nests in infected people in the limbic system and can break out if they are exposed to emotional stress that weakens the immune system. The longer this stress lasts, the more likely there is an excess of Bornavirus proteins in the cell nucleus that disrupt the brain's neuromitter.

In carriers of the Bornavirus, a weakening of the immune system through chemotherapy, stress or AIDS can lead to recurrent depression.

In this case, the depression is not due to trauma, it has no psychological cause, but a parasite is the culprit. Conversely, trauma in an infected person can cause the immune system to fail, thus activating the parasite.

According to the authors, markers for the virus occur significantly more frequently in people with mental illnesses such as schizophrenia or depression than in people without these illnesses, and this applies to Germany as well as to the Czech Republic, Australia and Iran.

The veterinarian Bernd Iben states: "(The Borna virus) corresponds to the melancholic subtype of major depression."

The Veitstanz

Streptococci are bacteria. A few of their strains can trigger a rheumatic fever. In rare cases, there is an attack on the brain, which leads to uncontrolled twitching of the hands and facial muscles. Based on the mass hysteria of the early modern period, this phenomenon is referred to as “Veitstanz”.

The disorders of this “Veitstanz” also include panic attacks such as compulsive acts and symptoms that are considered classic psychological complaints: separation anxiety, increased irritability and thoughts of suicide.

For example, obsessive-compulsive disorders were shown by Shakespeare with Lady Macbeth, who repeatedly torture nonsensical thoughts and constantly repeat actions by metaphorically washing off the blood that sticks to her hands.

Pediatrician Susan Swedo suspected the same mechanism behind the different complications of streptococcal infection. Accordingly, the bacteria deceive the immune system, which in addition to the bacteria also attacks the body's own proteins. The proteins on the surface of the heart muscle and the heart valves, the inner skin of the joint and the nerve fibers are very similar to the streptococcal proteins.

The tics and obsessive-compulsive disorders that cause streptococci are called Obsessive Compulsive Disorder. A previous infection with the bacteria is related to a sudden change in nature.

Streptococcus pyogenes thus "triggers neurological symptoms via willing helpers by using the immune cells of its victims," ​​according to the authors.

Straight into the brain

Pathogens also penetrate directly into the brain. Niehaus and Pfuhl write: "The poisons of the cholera bacteria and the gas fire exciter work as highly chemical lockpicks by opening the tightly closing doors of the blood-brain barrier (...) to the brain tissue."

Neurologists and psychiatrists

In the United States, a conflict of powers between neurologists and psychiatrists is raging over the consequences of streptococci. By definition, the neurologists take care of the nerves, i.e. the body, while the psychiatrists deal with the psyche.

This dividing line only exists as a construction. Viruses, bacteria and fungi "can find their way into the brain and make it go crazy," said the authors.

Niehaus and Pfuhl note: "The age-old concept that" madmen "are possessed by evil spirits seems more modern than the diagnostic and statistical guidelines for mental disorders with regard to the microorganisms invisible to the naked eye."

However, medicine today expressly recognizes that mental illnesses can be triggered by microorganisms. Prof. Karl Bechter from the University of Ulm says: "Today there is clear evidence that serious mental illnesses can be related to infections or the immune processes they cause."


"It's simple, tiny, and incredibly powerful," says virologist Nathan Wolfe of the rabies virus, because, the authors add, "it kills practically every host it infects."

The rage of rabies sufferers has been known since ancient times and gave the disease its name. Great in the meaning of insane and angry as an expression of uncontrolled frenzy characterizes the psychological changes of those affected by the rabies virus.

According to Niehaus and Pfuhl, the fear of the wolf was due to our ancestors' experience with the disease, also known as "dog frenzy" - a thesis that the author of this review also advocated in his master's thesis.

However, the “great rage” remained unknown for a long time as an infectious disease. According to the authors, this is due to their long incubation period. Weeks, months and even years can pass between animal bite and clear symptoms. In the first century CE Greek doctors then realized that “water aversion in humans” and “dog rage” were the same disease.

Since no one knew the cause, the treatments were wrong and cruel to the animals. From ancient times until the 19th century, "worm cutters" cut a connective tissue strand out of the dog's tongue because they believed that this "rabies" triggered the disease. The virus organizes its reproduction very well. It hijacks the brain and thus influences its host's immune response.

It is established in the limbic system as well as in the thalamus, brain stem and basal ganglia and “provides a fascinating clinicopathological link with alertness, (…) abnormal sexual behavior and aggressiveness. (…) No other virus is so diabolically well adapted that it can infuriate the host furiously and thereby ensure its transmission to another host. ”

Unchecked aggression

The virus changes the level of cytokines, at the same time lowers the serotonin level and triggers unchecked aggression. The pathogen also paralyzes the posterior cranial nerves, so that the victim's throat is paralyzed. It can no longer swallow saliva, but the highly infectious saliva "foams at the mouth". The uninhibited aggression causes the victim to bite wildly - the swallowing paralysis guarantees that the bites transmit the pathogen.

Wolves that travel up to 70 kilometers a day (the authors write 60) are ideal spreaders of the virus. Terrible stories of wolves that bite humans and dogs and then leave “madmen” behind indicate that wolves infected with rabies were a great danger.

The authors conclude: "Our deep-rooted fear of wolf probably goes back to the (historically rather rare) killing of people by wolves, but to the cruel, almost always fatal disease that spread and claimed countless victims."

Schizophrenic cats?

According to the authors, streptococci can be responsible for obsessive-compulsive disorder, the Borna virus for depression, and rabies leads to delusional aggression and hypersexuality. The authors next discuss toxoplasmosis, which cats transmit to humans, a mutable infection caused by a single cell.

Humans are a false host who is infected, for example, by cat droppings, because the parasite can only reproduce sexually in cats. It increases the dopamine level in the brain's pleasure center in rodents and synthesizes dopamine. The smell of a cat now sexually attracts a rat goat. At the same time, the parasite stimulates testosterone production and thus ensures that it spreads further through infected ejaculate.

Chimpanzees infected with Toxoplasma are magically attracted to their predator leopard, which supports the assumption that the pathogen originally specialized in big cats and used primates (like us) as a means of transport.

Niehaus and Pfuhl cite the thesis that even mental illnesses such as schizophrenia can be traced back to toxoplasmosis. This is mainly due to the increased dopamine level caused by the pathogen, which coincides with symptoms of schizophrenia: delusions, paranoia, megalomania, hallucinations. Toxo infections are also exceptionally common in people who are diagnosed with schizophrenia.


Niehaus and Pfuhl do not “only” write about parasitic life forms as biologists. At the same time, they create an unusual view of the cultural history of man, in which microorganisms decided wars (typhus), changed values ​​and norms (syphilis) and anchored deeply in the unconscious (rabies).

At the same time, they critically criticize the web of a separation of psyche and mind on the one hand and the body on the other hand in humans. This construction shapes the human image of Europe - from antiquity to the present day.

Biological parasites trigger psychological disorders and changed hormones lead to changes in personality. The book is a warning sign to look closely to see whether a parasite has taken root in one or the other childhood trauma of people with psychiatric symptoms. That would require a completely different treatment.

Niehaus and Pfuhl have presented a brilliant work with a storytelling art that is unusual in the natural sciences and provide clarification in the best sense. Anyone who is literally interested in naturopathy and does not confuse nature with esoteric gossip is a must for the "psycho-Trojans". (Dr. Utz Anhalt)

Author and source information

This text corresponds to the requirements of the medical literature, medical guidelines and current studies and has been checked by medical doctors.

Dr. phil. Utz Anhalt, Barbara Schindewolf-Lensch


  • Niehaus, Monika / Pfuhl, Andrea: The Psycho-Trojans - How Parasites Control Us, S. Hirzel Verlag, 2016, lgl.bayern.de

ICD codes for this disease: B89ICD codes are internationally valid encodings for medical diagnoses. You can find yourself e.g. in doctor's letters or on disability certificates.

Video: David Roos U Penn Part 1: Biology of Apicomplexan Parasites (November 2022).