The worry isn’t just that global birth rates are plummeting, it’s that fertility rates are plummeting along with them. Especially male fertility rates. Sperm counts and quality have been on a steady decline since the 1940s. The decline went into overdrive between 1990 and 2019, when male infertility shot up by nearly 80 percent.
The proposed reasons vary widely, from men living more sedentary lifestyles to junk food to the wide variety of chemicals we’re exposed to nowadays. Now, thanks to Indiana University’s Professor of Microbiology and Immunology, Bill Sullivan, we have a new possible explanation to add to the list: a parasite commonly carried by cats, among other things.
As Sullivan explained in The Conversation, Toxoplasma gondii is a parasite that lives in your cat’s poop. The parasite infects a staggering 30 to 50 percent of the global population and spreads through undercooked meat, dirty produce, contaminated water, and those adorable litter box landmines.
Once it’s in you, it doesn’t leave. It just hangs around, all the while forming permanent cysts in your brain, heart, muscles, and even in your testicles and prostate.
Animal studies have been dropping red flags for years. Mice, rats, and rams infected with Toxoplasma show fewer sperm, weirder-shaped sperm, and lower testicular function.
One April 2025 study upped the stakes by exposing human sperm to the parasite in a lab. Within five minutes, over 22 percent of sperm cells were literally beheaded. Others developed twisted tails or Swiss cheese-style holes in their heads, like the parasite was trying to burrow into them. Our sperm cells are being decapitated by cat poop parasites.
We’re still in the early stages of connecting these dots in humans. Several small studies in China and Europe have found that men with toxoplasmosis tend to have lower-quality sperm than those without it. But the sample sizes are tiny, and not every study agrees. Plus, toxoplasmosis rates haven’t been skyrocketing in recent decades, unlike infertility.
Still, the parasite’s M.O. isn’t helping anyone’s reproductive odds. It could be fueling chronic inflammation in the testes, degrading sperm quality over time.
The science is far from definitive, but there is a mounting message that is becoming clearer by the day: if you want to protect your sperm count and quality, wash your produce, cook your meat enough to kill off parasites, maybe don’t drink raw milk like you’re living in an 1800s prairie town, and wear a mask while you scoop your cat’s litter box and definitely wash your hands afterward.
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