The male fertility rate has declined sharply over the past half-century. An analysis from 1992 recorded a steady decrease in both sperm count and quality since the 1940s.

A more recent study shows that male infertility rates increased nearly 80% from 1990 to 2019. The reasons driving this trend remain a mystery, but common culprits often mentioned include obesity, poor diets, and environmental toxins.

Infectious diseases like gonorrhea or chlamydia are often overlooked factors that affect male fertility. Accumulating evidence suggests that a common single-celled parasite called Toxoplasma gondii could also be a contributing factor: A study in April 2025 first showed that "human sperm loses its head when directly exposed" to the parasite. This new research reinforces emerging findings emphasizing the importance of preventing infections caused by this parasite.

How You Can Get Toxoplasmosis

Infected cats shed Toxoplasma eggs in their litter boxes, gardens, or other areas in the environment, where they can be picked up by humans or other animals. Water, shellfish, and unwashed fruits and vegetables can also contain parasitic eggs.

Besides the eggs, cysts in the meat of warm-blooded animals can also spread toxoplasmosis if not killed by cooking at the right temperature. While most hosts of the parasite can control the initial infection with little or no symptoms, Toxoplasma remains in the body for life as dormant cysts in the brain, heart, and muscle tissue. These cysts can reactivate and cause severe illness, damaging key organ systems.

From 30% to 50% of the global population is permanently infected with Toxoplasma because the parasite can spread in many ways.

Toxoplasma May Target Male Reproductive Organs

When infected, Toxoplasma spreads to almost every organ and muscle. Evidence suggests that Toxoplasma may also target human male reproductive organs, first appearing during the peak of the AIDS pandemic in the 1980s, when some patients were found to have the parasite in their testes.

While immunocompromised patients have the highest risk of Toxoplasma infection in the testes, the disease can also occur in healthy individuals. Imaging studies on infected mice confirmed that Toxoplasma quickly moves to the testes, outside the brain and eyes, within a few days of infection.

In 2017, researchers discovered that Toxoplasma can also form cysts in the prostate glands of mice. They also observed these parasites in the semen of several animals, including human semen, increasing the potential for sexual transmission.

Knowing that Toxoplasma can reside in male reproductive organs has prompted analyses of fertility in infected men. A small study in Prague in 2021 involving 163 men infected with Toxoplasma showed that over 86% had sperm abnormalities.

Toxoplasma Can Directly Damage Sperm

Toxoplasmosis in animals is similar to human infections, allowing researchers to address issues that are difficult to study in humans. Testicular function and sperm production significantly declined in mice, rats, and male sheep infected with Toxoplasma. Infected mice had significantly lower sperm counts and a higher rate of sperm abnormalities.

In the April 2025 study, researchers from Germany, Uruguay, and Chile observed that Toxoplasma could reach the testes and epididymis—the tube where sperm matures and is stored—just two days after infection in mice. This discovery led the research team to experiment with what happens when the parasite directly contacts human sperm in vitro.

After just five minutes of exposure to the parasite, 22.4% of sperm cells had their heads cut off. The number of sperm heads cut off increased with the duration of interaction with the parasite. The sperm cells with intact heads were often twisted and deformed. Some sperm cells had holes in their heads, suggesting that the parasite was attempting to invade them like any other cell it infects.

In addition to direct contact, Toxoplasma can also damage sperm and promote chronic inflammation. Inflammation in the male reproductive tract is harmful to sperm production and function.

Researchers speculate that the damage caused by Toxoplasma to sperm may significantly contribute to the global decline in male fertility over the past decades.

Preventing Toxoplasmosis

Evidence showing that Toxoplasma can infect male reproductive organs in animals is convincing, but whether this causes health problems in humans remains unclear. Toxoplasma testicular disease shows that the parasite can invade human testes, but symptomatic cases are rare. Research so far indicates that the defects in sperm from infected men are too small to draw definitive conclusions at this time.

Additionally, some reports suggest that the rate of Toxoplasma infection in high-income countries has not increased over the past few decades, while male infertility has risen.

Regardless of the potential impact of this parasite on fertility, you should avoid Toxoplasma. The infection can cause miscarriage or birth defects if someone contracts it for the first time during pregnancy and can be life-threatening for those with weakened immune systems.

Proper care of cats, cleaning litter boxes immediately, and washing hands thoroughly afterward can help reduce the risk of exposure to Toxoplasma. You can also protect yourself from this parasite by washing fruits and vegetables, cooking meat at the appropriate temperature before eating, and avoiding raw shellfish and unpasteurized milk.