New threat to plummeting male fertility rates identified as common but ‘silent’ parasite that ‘DECAPITATES sperm’

FERTILITY rates appear to have been declining in both men and women globally – and a common parasite may be a potential cause.

Male fertility rates in particular have been plummeting over the past half-century, with an analysis from 1992 showing a steady decrease in sperm counts and quality since the 1940s.

And a more recent study found male infertility rates increased nearly 80 per cent from 1990 to 2019.

Declining fertility has been attributed to a combination of factors, including rising costs associated with raising children.

But now accumulating evidence suggests parasitic infections could also be a threat.

A study published in April this year showed for the first time “human sperm lose their heads upon direct contact” with a parasite called Toxoplasma gondii.

Toxoplasma gondii causes toxoplasmosis – an often harmless infection but can cause serious problems in individuals with weakened immune systems, pregnant women, and newborns. 

The parasite is found in the faeces of infected cats and in undercooked meat. 

Writing for The Conversation, Bill Sullivan, Professor of Microbiology and Immunology, Indiana University, said how the “new study bolsters emerging findings that underscore the importance of preventing this parasitic infection”.

He says: “Between 30 per cent and 50 per cent of the world’s population is permanently infected with toxoplasma due to the many ways the parasite can spread.“

Foodbourne transmission and animal to human transmission are the top ways people can get toxoplasmosis.

Eating raw or undercooked meat, particularly lamb, pork, and venison, is a primary source of infection. 

And eating unwashed fruits and vegetables that have been contaminated with cat faeces or soil can also lead to infection. 

Ingesting the parasite through contact with cat faeces (poop) or soil contaminated with cat faeces is a common way to get toxoplasmosis. 

Cleaning cat litter boxes without proper hygiene can also expose people to the parasite. 

While toxoplasmosis is generally harmless for most people with healthy immune systems, often causing no symptoms or mild, flu-like symptoms that resolve on their own, Prof Sullivan said toxoplasma remains in the body for life as dormant cysts in brain, heart and muscle tissue.

And these cysts can re-activate and cause additional episodes of severe illness that damage critical organs.

Prof Sullivan said: “Upon infection, toxoplasma spreads to virtually every organ and skeletal muscle. Evidence that toxoplasma can also target human male reproductive organs first surfaced during the height of the AIDS pandemic in the 1980s, when some patients presented with the parasitic infection in their testes.

“While immunocompromised patients are most at risk for testicular toxoplasmosis, it can also occur in otherwise healthy individuals.

“Imaging studies of infected mice confirm that toxoplasma parasites quickly travel to the testes in addition to the brain and eyes within days of infection.”

Evidence in past studies suggesting toxoplasma can reside in male reproductive organs has prompted analysis of fertility in infected men.

In the April 2025 study, researchyers from Germany, Uraguay and Chile observed that toxoplasma can reach the testes and epididymis – the tube where sperm mature and are stored – two days after infection in mice.

Looking at what happens when the parasite comes into direct contact with human sperm in a test tube, they observed that after only five minutes, 22.4 per cent of sperm cells were beheaded.

The number of decapitated sperm also increased the longer they interacted with the parasites.

Sperm cells that maintained their head were often twisted and misshapen, and sperm cells had hole sin their head, suggesting the parasites were trying to invade them as it would any other type of cell.

Prof Sullivan added: “The evidence that toxoplasma can infiltrate male reproductive organs in animals is compelling, but whether this produces health issues in people remains unclear.

“Testicular toxoplasmosis shows that parasites can invade human testes, but 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 this time.

“Additionally, some reports suggest that rates of toxoplasmosis in high-income countries have not been increasing over the past few decades while male infertility was rising, so it’s likely to only be one part of the puzzle.”