Is A Cat Poop Parasite Decapitating Human Sperm Contributing To Rising Infertility?

Is a common parasite contributing to the global fertility decline? This worrying prospect has been raised by researchers who believes the parasite Toxoplasma gondii, which can be picked up from undercooked meat or cat feces, may decapitate human sperm when it infects male testes.

Toxoplasma gondii is an incredibly pervasive parasite that can infect pretty much any warm-blooded animal, taking up residence in various parts of their bodies. It is currently estimated that between 30 and 50 percent of the world’s population is infected with the little blighters, many of whom don’t even realize it.

The routes for infection are numerous. Cats can leave parasite eggs in their litter boxes when they defecate, which can be transmitted to humans when they are cleaned or if they come into contact with them in the environment. The eggs can also be present in water sources, on unwashed fruit and vegetables, or on shellfish. Meat can also harbor the cysts created by the parasite, which can survive if the meat hasn’t been sufficiently cooked.

Once in the body, the parasite can spread to pretty much anywhere, infecting the host’s organs (they love the brain) and muscles and, on rarer occasions, even their sexual organs. Typically speaking, these latter infections tend to occur more in immunocompromised patients, but can occur in other people too.

According to Professor Bill Sullivan, a microbiologist and immunologist at Indiana University who recently produced a Conversation article on this subject, imaging studies of infected mice have shown the parasites quickly travel to the testes as well as the brain and eyes within a few days of infection. Here the parasites form cysts in the mouse prostate and are also present in the ejaculate of many animals they infect. This, as Sullivan notes, raised the possibility that the parasites can be sexually transmitted too.

But what are these little invaders doing when they infect someone’s testes? Soon after they were confirmed in this part of the body, researchers started to investigate their relationship to male fertility. A few studies have found some worrying results. One conducted in 2021 showed that 86 percent of a sample of 163 infected men had semen anomalies. In 2002, a study in China found that couples with infertility were 34.83 percent more likely to be infected with the parasites compared to only 12.11 percent fertile couples.

Although it is important to note that this link between Toxoplasma infection and fertility has not been demonstrated in all studies, a recent study has now shown that the parasites do impact human sperm – specifically, they appear to decapitate them. 

In this study, the researchers examined exactly what happens when sperm comes into contact with the parasites in a test tube. According to their results, within five minutes of contact, the parasites managed to decapitate 22.4 percent of the sperm. This percentage continued to increase the longer the exposure lasted.

“Herein we show, for the first time, that the direct contact between mature human spermatozoa and T. gondii [...] triggers ultrastructural changes in the former, leading to an increase in the percentage of headless sperm cells,” the researchers explain in their paper.

Even those that were not beheaded by the parasites experienced morphological issues that left them misshapen and twisted. Some had little holes in their heads that suggest the parasites were trying to infect them.

The researchers conclude that this destructive interaction between parasite and sperm may be contributing to the fertility decline we have seen in the last few decades. However, this is still far from certain. The research into this troubling topic is relatively new and the existing studies have been too small for us to make any serious generalizations. Also, as Sullivan points out, the rates of infection do not necessarily match the rates of the infertility increase we have seen.

“[S]ome 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.”

More research is obviously needed at this stage, but one thing is clear: we need to avoid the parasite more than ever, so clean your cat’s litter box properly, wash your food, and cook your meat to the proper temperatures. Otherwise, you never know if these opportunistic critters will behead your swimmers. 

The study is published in The FEBS Journal.