The male birth rate is experiencing a significant decline, as highlighted in a study over the last fifty years. Research conducted in 1992 noted a consistent drop in both sperm count and quality, starting from the 1940s. A recent survey established a male infertility rate that rose nearly 80% from 1990 to 2019. Although the reasons for this trend remain obscure, several commonly cited factors include obesity, poor nutrition, and environmental toxins.
Infectious diseases, including gonorrhea and chlamydia, are often neglected factors that diminish male fertility. Accumulated research suggests that a prevalent single-celled parasite known as Toxoplasma gondii could contribute to this issue. A 2025 survey elaborated that “human sperm” shows signs of decay upon direct contact, indicating its parasitic nature.
As a microbiologist, my lab research on Toxoplasma emphasizes the critical need to prevent this parasitic infection.
Various Routes to Contract Toxoplasmosis
Infected cats excrete Toxoplasma eggs, which may be found in litter boxes, gardens, or other locations, potentially ingested by humans or other animals. Additionally, water, shellfish, and unwashed fruits and vegetables also serve as carriers for the infectious parasite eggs.
Besides the eggs, tissue cysts in the meat of warm-blooded animals can also transmit Toxoplasmosis, especially if they are not adequately cooked to the proper temperature.
Despite most hosts being able to manage early infections when symptoms are minimal, Toxoplasma can remain in the body for life as dormant cysts in the brain, heart, and muscle tissues. These cysts can reactivate, leading to severe illness and potentially damaging crucial organ systems.
Related: 32 Alarming Parasitic Diseases
Approximately 30% to 50% of the global population is permanently infected with Toxoplasma owing to numerous transmission pathways.
Toxoplasma’s Impact on Male Reproductive Organs
Patients with compromised immune systems are particularly vulnerable to testicular toxoplasmosis, but it has been known to occur in healthy individuals as well. Imaging studies on infected mice prove that Toxoplasma parasites can infiltrate the testes quickly, reaching the brain and eyes within just days of initial infection.
(Image credit: DPDX Image Library/CDC)
In 2017, my colleague and I discovered that Toxoplasma could induce the formation of cysts in mouse prostates. These parasites were also identified in ejaculation in various animals, including human semen, raising concerns about sexual transmission.
Given that Toxoplasma can persist within male reproductive organs, an analysis of infected men revealed alarming results. A small 2021 study of 163 men in Prague showed that Toxoplasma was present in over 86% with abnormal semen.
A 2002 study in China highlighted that couples facing infertility were 34.83% more likely to have Toxoplasma infections compared to fertile couples, at rates of 34.83% vs. 12.11%. Moreover, a 2005 study indicated that sterile men were more likely to test positive for Toxoplasma than fertile men.
Not every study supports a connection between toxoplasmosis and sperm quality though.
Direct Damage to Human Sperm by Toxoplasma
Animal models of toxoplasmosis provide insights into human infections, enabling researchers to explore issues difficult to study in humans.
Marked declines in testicular functionality and sperm production have been observed in mice infected with Toxoplasma, rats, and rams. Infected mice often present significantly reduced sperm counts and an increased proportion of abnormally shaped sperm.
In a September 2025 study, researchers from Germany, Uruguay, and Chile discovered that Toxoplasma could infect the testis and epididymis—tubes that serve as storage for matured sperm—just two days after infection in mice. This finding prompted the team to investigate the effects of the parasites when they come into contact with human sperm in vitro.
After only five minutes of exposure to the parasites, 22.4% of sperm cells were decapitated, with decapitation rates rising the longer the exposure lasted. The sperm cells that retained their heads frequently exhibited abnormal shapes, such as twisted forms. Some sperm cells showed holes, indicating imminent invasion by parasites, similar to other cells infiltrated by the organism.
Beyond direct contact, Toxoplasma may also compromise sperm quality by inducing chronic inflammation, a state detrimental to the sperm’s production and functionality.
Researchers theorize that the detrimental effects of Toxoplasma on sperm are likely contributing to the global decrease in male fertility observed in recent decades.
(Image credit: Rojas-Barónetal/The Febs Journal, CC by-sa)
Strategies for Preventing Toxoplasmosis
While it is confirmed that Toxoplasma has an affinity for male reproductive organs, it remains unclear whether this leads to significant health issues in humans. Testicular toxoplasmosis signifies the capability of the parasite to infect human testes, although symptomatic cases are exceedingly rare. Previous studies concerning sperm abnormalities in infected males have been too limited in scope to yield definitive conclusions.
Several reports also indicate that the incidence of toxoplasmosis in high-income nations is not on the rise, even as rates of male infertility increase, making it a complex puzzle over recent years.
Nonetheless, safeguarding against this parasite is advisable, irrespective of its potential influence on birth rates. Infection with Toxoplasma can lead to miscarriages or congenital disabilities if contracted during pregnancy and can be life-threatening in immunocompromised individuals.
Properly caring for your cat can diminish exposure by promptly cleaning the litter box and adequately washing your hands. Additionally, washing fruits and vegetables will protect against these parasites. It is also crucial to cook meat to the appropriate temperature before consumption, while avoiding raw shellfish, raw water, and raw milk.
This article has been revised to rectify inaccuracies regarding the frequency of fatalities associated with foodborne toxoplasmosis.
This modified piece has been republished from The Conversation under the Creative Commons license. For more details, please read the original article.
Source: www.livescience.com