Toxoplasmosis – the infection caused by parasites from raw meat, sausage and cat faeces – is particularly risky for pregnant women. Studies show that every second person carries the pathogen. It can lie dormant in the brain for life, promote brain diseases and manipulate our behavior.

This parasite appears to be particularly resilient and prone to spread: the protozoan Toxoplasma gondii has infected around a third of the world’s population, making it perhaps the most successful parasite of all. It is the causative agent of toxoplasmosis, which is a zoonosis, i.e. an infectious disease that is transmitted from animals to humans.

In Germany, around every second person is infected with it. “However, that doesn’t mean that every one of them falls ill with toxoplasmosis,” Uwe Groß clarifies. He is director of the Institute for Medical Microbiology at the University Medical Center Göttingen. Here, the epidemiology of toxoplasmosis is investigated, a working group is concerned with the persistence of the parasite, with how it changes its host, and the scientist and his colleagues have also researched the development cycle of Toxoplasma gondii and diagnosis.

However, the 1:2 infection rate does not remain, it increases in parallel with age: more than 70 percent of those over 70 are already infected. Because the parasite is mainly found in a food that most of us eat frequently: meat.

“Numerous studies show that it is the main source of infection, namely insufficiently heated meat,” explains the expert. These are, for example, ground pork, raw sausages and bloody steak.

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However, beef is only very rarely contaminated, while poultry is more. However, the so-called cysts, the permanent forms of the parasite, were most frequently detected in pork, the meat most commonly consumed in Germany. Cat faeces can also be a source of infection.

Animals for slaughter and cat excrement as a source of infection – at first glance, these hardly fit together. The developmental cycle of Toxoplasma gondii shows the background. The parasite goes through different stages of development:

1. Cats usually become infected with the parasite via prey. “Any feline animal can absorb the parasite and only in her, in her intestines, does sexual reproduction take place,” explains Uwe Groß. From tigers to house cats, cats are the only main host in which the parasite can multiply into extremely contagious oocysts. That’s why cat feces can contain millions of them. The oocysts can survive outdoors for months with sufficient humidity, and can thus enter the water cycle.

2. Rodents, birds, mammals and thus animals for slaughter such as pigs and poultry can become infected through direct contact with contaminated soil, via the water and vegetables, grain and fruit contaminated with it. They are the intermediate hosts of Toxoplasma gondii – as are humans, who ingest the oocysts orally.

3. In humans and other intermediate hosts, the oocysts overcome gastric acid and multiply into tachyzoites in the intestinal wall. The barrier function of the intestinal wall also fails against the rapidly multiplying parasites. They use guard cells of the immune system as a means of transport without being recognized and in this way flood the organism. As the third barrier in the body, they also overcome the blood-brain barrier. This stage of multiplication and spread marks the acute phase of toxoplasmosis.

4. Only now does the immune system begin to attack the tachyzoites – they replicate more slowly and transform into inactive bradyzoites. Bradyzoites congregate in thousands and are protected by a cyst wall, held in check by the immune system. The tissue cysts are deposited in the brain, but also in the retina and muscles. In the case of congenital immunodeficiency, immunosuppression after transplantation, cancer or HIV infection, there is a risk of acute toxoplasmosis, which can ultimately affect all organs.

The decisive factor is whether an otherwise healthy person is infected with the tissue cysts or an immunosuppressed person. “Immunocompetent people react with a slight swelling of the lymph nodes in the neck area,” reports the expert. The other symptoms are similar to those of a flu-like infection and are not very pronounced. Overall, symptoms only occur in less than five percent of those infected, and the infection usually goes unnoticed.

Only if the symptoms are more stubborn will those affected go to the doctor, who will probably first suspect a flu-like infection. However, if the lymph node swelling persists for weeks, it would make sense to also think about toxoplasmosis.

The infection can be detected with a blood test. If there are no complications or if you are not pregnant, the mild swelling of the lymph nodes usually does not require any treatment.

However, dormant tissue cysts after a previous infection, or an initial infection, can lead to a

The inflammation leaves scars on the retina, which can severely limit vision and threaten blindness.

However, immunosuppressed people show a completely different set of symptoms. “In the case of an initial infection, this can lead to pneumonia, possibly hepatitis,” says the professor, describing the difference.

Antibiotics such as spiramycin, pyrimethamine, sulfadiazine or clindamycin are used against toxoplasmosis. “The drugs suppress the inflammation, but they cannot kill the parasite completely,” Uwe Groß qualifies. The pathogens in the form of tissue cysts lie dormant in the body for life and can ultimately break out and lead to an infection whenever the immune system is weak lead, “similar to herpes viruses,” explains the scientist.

The flare-up of infection can be stopped, but the herpes simplex virus does not go away from the body. It is similar with ocular toxoplasmosis. It can keep recurring because the cysts also persist in the retina.

A special case also means pregnancy and toxoplasmosis. If the woman had contact with the pathogen long before the start of the pregnancy, “we assume that the woman is protected from a second infection and thus also the child,” explains Uwe Groß.

However, in the case of an initial infection during pregnancy, the notifiable congenital or prenatal toxoplasmosis, there is a risk that the parasite could be passed on to the child. It is true that the placenta is particularly insurmountable for the pathogen in the first few weeks of pregnancy, but if it still manages to do so, it has dramatic consequences for the child, which can even lead to an abortion.

It is not known how many pregnancies end far too early because of this, and the aborted children are not examined. “We only know something about live births,” says the microbiologist. And there would be a big gap between the reported cases (less than 25 per year) and the calculations of the Robert Koch Institute.

According to this, around 1,300 children a year should be born with toxoplasmosis. “345 of them should have symptoms,” reports Uwe Groß. These can be as noticeable as hydrocephalus, i.e. hydrocephalus, but also only a slight fever. However, eye toxoplasmosis is also common, but this is not recorded in newborns – the child cannot express itself and say that I have vision problems.

The number of unreported cases is therefore enormous and if you included not only all children who are conspicuous at birth, but also those who develop toxoplasmosis through the infection before birth until they grow up, you would easily get to this number. “Most of those affected are only noticed many years after birth and then mostly through ocular toxoplasmosis,” summarizes the researcher.

Will the antibiotics still help? “It definitely makes sense,” states Uwe Groß. Research in countries such as certain regions of Brazil, where severe courses of toxoplasmosis are common, showed that regular treatment significantly reduced the severity and incidence of ocular toxoplasmosis, for example.

That means: low-dose, long-term antibiotics. As a result, there is no second or even third flare-up.

How can you prevent a child from becoming infected with toxoplasmosis before birth? There is no general screening of pregnant women for toxoplasmosis in Germany, as is done in Austria and France. However, pregnant women who suspect that they have been infected should be tested by a doctor. “If an initial infection is discovered in the pregnant woman, targeted toxoplasmosis therapy can significantly reduce the risk of transmission to the child and also reduce the child’s symptoms,” reports the scientist, who conducted a study on this topic.

However, there are indications that toxoplasmosis could have other far-reaching consequences. A study showed that in mice infected with toxoplasmosis, the natural and vital fear of cats as predators decreased significantly. Other studies transfer this possibility of behavioral change to humans and see a connection between toxoplasmosis and aggressive behavior. The parasite should therefore be able to manipulate our behavior.

Other studies link the infection to mental and neurological diseases such as Parkinson’s, Alzheimer’s and schizophrenia.

Uwe Groß counters this and argues that if 51 percent of the population in Germany is latently infected with the parasite, i.e. every second person has the parasite in their brain, it would be responsible for some psychiatric neurodegenerative disease in every second person. Half of the German population should therefore have corresponding problems. However, this is not the case.

The studies presented are primarily association studies. This means, for example, that blood tests for toxoplasmosis have been carried out in Alzheimer’s patients and in healthy people. Toxoplasmosis was found more often among the sick. However, it is incorrect to draw the conclusion that the parasite promotes Alzheimer’s. There are studies that even show the opposite – according to which the parasite would protect against brain degeneration. It would be scientifically correct to divide healthy people into two groups and infect one of them with Toxoplasma gondii and observe whether the brain and/or behavior change over the years – which, however, would not be ethically justifiable.

“On the other hand, the parasite sits in the brain for life, so it could be that there is an interaction,” the expert points out. However, this has not yet been proven by any reliable study. It is quite possible that the parasite influences one or the other neurodegenerative abnormalities. To what extent that is, however, no one can say for sure at this point in time.

However, a certain degree of uncertainty remains – chimpanzees, i.e. large mammals that are closely related to us, lost their fear of their natural predator leopard after being infected with toxoplasmosis, as a study shows. So it could well be that the parasite influences our behavior and certain diseases. However, conclusive evidence for humans is still lacking, the scientist concludes.

What is certain, however, is that it is sensible to avoid becoming infected with the parasite. This is especially true for pregnant women: