Parasites that infect up to 50% of people can decapitate human sperm

Male fertility rates have fallen sharply over the past half century. The 1992 analysis showed a steady decrease in the quantity and quality of sperm since the 1940s. A more recent study showed that male infertility rates increased by almost 80% from 1990 to 2019.

The reasons for this trend remain a mystery, but the frequently mentioned perpetrators include obesity, poor nutrition and environmental toxins.

Infectious diseases such as gonorrhea or chlamydia often overlook the factors that affect fertility in men. The accumulation of evidence suggests that a common unicellular parasite called Toxoplasma gondii can also contribute: a study conducted in April 2025 showed for the first time that "human sperm loses its head in direct contact" with the parasite.

I am a microbiologist, and my laboratory studies toxoplasma. This new study supports new results that emphasize the importance of preventing this parasitic infection.

Many ways to get toxoplasmosis

Infected cats defecate Toxoplasma eggs in the toilet, garden or other places in the environment where they can be picked up by people or other animals. Water, mollusks and unwashed fruits and vegetables can also contain infectious parasite eggs.

In addition to eggs, tissue cysts present in the meat of warm-blooded animals can also spread toxoplasmosis if they are not destroyed when cooked at the right temperature.

While most parasite hosts can control the initial infection with few symptoms, if any, toxoplasma remains in the body for life in the form of sleeping cysts in the brain, heart and muscle tissue. These cysts can reactivate and cause additional episodes of severe diseases that damage critical organ systems.

From 30% to 50% of the world's population is constantly infected with toxoplasma due to many ways of spreading the parasite.

Toxoplasma can be targeted at male reproductive organs

After infection, toxoplasma spreads to almost every organ and skeletal muscles. Evidence that toxoplasma can also target male human reproductive organs first appeared in the midst of the AIDS pandemic in the 1980s, when some patients had a parasitic infection in the testicles.

Although immunocompromised patients are at the highest risk of testicular toxoplasmosis, it can also occur in healthy people. Visual studies of infected mice confirm that Toxoplasma parasites are rapidly transmitted to the testicles in addition to the brain and eyes within a few days after infection.

In 2017, my colleagues and I discovered that toxoplasma can also form cysts in the prostate gland of the mouse. Researchers have also observed these parasites in the ejacula of many animals, including human sperm, which increases the possibility of sexual transmission.

The knowledge that toxoplasma can live in the male reproductive organs prompted the analysis of fertility in infected men. A small 2021 study in Prague with 163 men infected with toxoplasma showed that more than 86% had sperm abnormalities.

A 2002 study in China showed that infertile pairs are more likely to have toxoplasma infection than fertile pairs, 34.83% against 12.11%. A 2005 study in China also showed that sterile men are more likely to have a positive toxoplasma test than fertile men.

However, not all studies provide a link between toxoplasmosis and sperm quality.

Toxoplasma can directly damage human sperm

Toxoplasmosis in animals reflects infection in humans, which allows researchers to solve issues that are not easy to study in humans.

The function of the testicles and sperm production are sharply reduced in toxoplasma-infected mice, rats and rams. Infected mice have significantly less sperm and a higher proportion of abnormal sperm.

In that study in April 2025, researchers from Germany, Uruguay and Chile noticed that toxoplasma can reach the testicles and epididymis, a tube where sperm matures and is stored, two days after infection in mice. This conclusion prompted the team to check what happens when the parasite comes into direct contact with the human sperm in the test tube.

Just five minutes after contact with the parasite, 22.4% of sperm were beheaded. The number of decapitated sperm increased the longer they interacted with parasites. Sperm that supported their head were often twisted and desfalted. Some sperm had holes in the head, which suggests that the parasites tried to invade them, as well as any other type of cells in the organs they penetrate.

In addition to direct contact, toxoplasma can also damage sperm, because the infection contributes to chronic inflammation. Inflammatory conditions in the male reproductive tract are harmful to sperm production and function.

Researchers suggest that the harmful effects of toxoplasma on sperm may contribute to a significant global decline in male fertility in recent decades.

Prevention of toxoplasmosis

The evidence that Toxoplasma can penetrate the male reproductive organs in animals is convincing, but whether this causes health problems in humans remains unclear. Toxoplasmosis of the testicles shows that parasites can invade human testicles, but the symptomatic disease is very rare. Studies to date that show defects in the sperm of infected men are too small to draw firm conclusions at present.

In addition, some reports suggest that toxoplasmosis rates in high-income countries have not increased over the past few decades, while male infertility has increased, so this will probably be just one piece of the puzzle.

Regardless of the potential impact of this parasite on fertility, it is reasonable to avoid toxoplasma. The infection can cause miscarriage or birth defects if someone first acquires it during pregnancy, and it can be dangerous for people with weakened immunity. Toxoplasma is also the leading cause of death from foodborne illnesses in the United States.

Proper care for your cat, surgical cleaning of the toilet and thorough hand washing after that can help reduce the impact of toxoplasma. You can also protect yourself from this parasite by washing out fruits and vegetables, cooking meat to the right temperature before eating and avoiding raw shellfish, raw water and raw milk.