Male fertility rates have been dropping sharply over the last 50 years. A 1992 analysis showed a steady decline in sperm count and quality since the 1940s. More recently, male infertility rates increased by nearly 80% from 1990 to 2019. Experts are still unsure why, but blame often falls on obesity, poor diet, and environmental toxins.
Infectious diseases such as gonorrhea and chlamydia are less often discussed but also affect male fertility. Now, scientists have identified another potential factor: a single-celled parasite called Toxoplasma gondii. A groundbreaking study published in April 2025 found that this parasite can cause human sperm to lose their heads on direct contact.
As a microbiologist studying Toxoplasma, I see this new research as an important warning. It highlights the need to prevent this widespread infection.
How People Contract Toxoplasmosis
Toxoplasma eggs enter the environment through cat feces, contaminating litter boxes, gardens, and soil. Humans and animals can accidentally ingest these eggs. They also contaminate water, shellfish, and unwashed fruits and vegetables.
The parasite can also spread through tissue cysts found in undercooked meat from warm-blooded animals. Once inside the body, most people show little or no symptoms, but the parasite remains for life, hiding in the brain, heart, and muscles as dormant cysts.
These cysts can reactivate later and cause severe illness affecting vital organs. Due to the many ways it spreads, between 30% and 50% of the global population carries Toxoplasma.
Targeting Male Reproductive Organs
After infection, Toxoplasma can reach nearly every organ, including muscles. Evidence that it invades male reproductive organs first appeared during the 1980s AIDS crisis, when some patients showed parasitic infection in their testes.
Although people with weakened immune systems are at highest risk for testicular toxoplasmosis, healthy individuals can be infected too. Animal studies reveal that the parasite quickly travels to testes, brain, and eyes within days of infection.
In 2017, research showed Toxoplasma cysts can form in mouse prostates. The parasite has also been detected in the semen of various animals, including humans, suggesting the possibility of sexual transmission.
Studies linking Toxoplasma infection to fertility issues have emerged. For example, a 2021 Prague study of 163 infected men found that over 86% had abnormal semen. Earlier research in China reported that infertile couples were more likely to carry the parasite than fertile couples.
However, not all studies agree on a direct link between toxoplasmosis and sperm quality.
Direct Damage to Human Sperm Confirmed
Animal infections often mirror human disease, helping scientists explore hard-to-study effects. Research shows that Toxoplasma reduces sperm production and damages sperm shape in mice, rats, and rams.
In the 2025 study involving teams from Germany, Uruguay, and Chile, researchers found that Toxoplasma reaches the testes and epididymis in mice within two days of infection. They then exposed human sperm to the parasite in lab tests.
After just five minutes, 22.4% of sperm lost their heads. Longer exposure increased this number. Even sperm heads that remained were often twisted or had holes, indicating the parasite was attacking them directly.
Besides direct damage, Toxoplasma may harm sperm indirectly by causing chronic inflammation in male reproductive organs. Such inflammation disrupts sperm production and function.
Researchers suggest that Toxoplasma’s damaging effects on sperm might be a hidden factor behind the global decline in male fertility over recent decades.
Preventing Toxoplasmosis
While animal studies convincingly show Toxoplasma invades male reproductive organs, its impact on human fertility remains unclear. Testicular infections in humans are rare and symptoms often mild or absent.
The human studies showing sperm abnormalities in infected men have been too small to provide conclusive evidence. Furthermore, some data suggest toxoplasmosis rates have not increased in high-income countries, even as male infertility rises. This implies the parasite is likely one of many factors influencing fertility declines.
Still, the evidence highlights the importance of raising awareness about toxoplasmosis and taking precautions to avoid infection.
Related topics:
