Could the answer to the male fertility crisis be lurking in your cat's litter tray?

Telegraph

18-06-2025

It may come as a surprise but there's about a one-in-three chance that a parasite for which there is no cure has invaded your body and is lying dormant inside you.
Your chances of having picked it up are higher if you have a cat, as the parasite only reproduces inside the feline intestinal tract and its eggs end up in the litter tray, or wherever else your pet does its business. From there, it spreads through the environment to other animals, including ones we eat, so those partial to a steak tartare ought to be careful too.
Yet there is no reason to panic. The vast majority of people infected with Toxoplasma gondii, the single-celled organism that causes toxoplasmosis, will never know they've had it. It can be dangerous to pregnant women and the people with compromised immune systems. But for the rest of us, any symptoms that do emerge tend to be mild and flu-like – unless you're a sperm.
A growing body of evidence suggests toxoplasma gondii may be contributing to a dramatic global decline in male fertility counts.
The discovery came by accident.
Dr Zahady Velásquez, a researcher from the Institute for Parasitology at the University of Giessen in western Germany, was investigating how toxoplasma takes control of cells when she peered into her microscope and saw what looked like the aftermath of an execution. Dozens of sperm lay dismembered in the petri dish. They had clearly been decapitated after coming into contact with toxoplasma.
'The first thing that I saw was a lot of heads and tails,' she said of the scene.
Dr Velásquez's initial reaction was that something had gone awry with the experiment. 'I thought: 'Oh my god, I did something wrong!',' she told The Telegraph.
When it happened again, she decided to consult with a colleague who persuaded her that something interesting was going on. Then the work began.
Sperm samples were exposed to the single-celled parasite for differing lengths of time, and the damage done was measured. After just five minutes in a test tube with the parasite, almost a quarter (22.4 per cent) of the sperm had been decapitated, and the number of beheadings rose the longer the sperm were exposed.
Some of the sperm that survived were left misshapen and scarred by the encounter, their tails curled up or bent at right angles. Others bore characteristic puncture wounds to their heads suggesting the toxoplasma had tried to invade them in the same way they do other cells.
The researchers also carried out experiments on mice and found that the toxoplasma were able to reach the testes and epididymis – the coiled tube at the back of the testicle where the sperm are stored – within two days of infection.
It is the Toxoplasma gondii's mostly benign relationship with its hosts and ability to infect almost every warm-blooded animal on Earth that explains why toxoplasma has been so successful.
However, the new research suggests that the parasite may just be acting slowly.
The findings, coupled with the high prevalence of the parasite in the global population, suggest toxoplasma could be having 'an underappreciated impact on male fertility,' the researchers conclude in their report, which was published in the Federation of European Biochemical Societies journal in April.
For years scientists have been grappling with an apparent crisis in male fertility, particularly after a 2017 meta-analysis (a statistical technique for combining the results of multiple studies) estimated that global sperm counts had fallen by 59 per cent since 1973.
To be clear, this is a separate issue from declining birth rates. While the belief that civilisation faces oblivion because people are no longer interested in having children is currently in vogue, in fact, a range of factors including the high cost of parenthood and housing, as well as fears for the future, are to blame for declining birth rates, a recent major UN study found.
The issue of sperm counts is similarly controversial. The reason behind the decline has so far eluded scientists. Numerous culprits have been put forward, from pollution, microplastics, food additives, obesity and even radiation emitted by mobile phones and microwaves.
The discovery by Dr Velásquez and her colleagues has helped to open up a new line of enquiry, building on a handful of previous studies that suggested a correlation between toxoplasma infection and male infertility.
A 2002 study in China, for instance, found that infertile couples were almost three times more likely to have a toxoplasma infection than fertile couples, while a 2005 study, also in China, found that sterile men were more likely to be infected.
'There's enough evidence out there from different independent labs across the world to pique my interest,' said Dr Bill Sullivan, a professor of pharmacology and microbiology at the Indiana University School of Medicine.
'I think we need to be vigilant and look at every possibility – because it's probably not just one thing,' he told The Telegraph. 'I think it's worthy of further investigation.'
Dr Velásquez agrees: 'I really love our results, but I believe that [toxoplasma] is not the only issue that is inducing infertility in men.
'You need to consider that the diet that we have now is deficient in a lot of different minerals and vitamins, also the stress that we live in every day [...] we are not sleeping enough. It's a really big package and I believe that Toxo is just one part of it.'
The discovery by Dr Velásquez and her team is the latest in a series of surprising findings about Toxoplasma. Studies have shown that infected mice lose their fear of cats, making them more likely to become prey, as the parasite causes inflammation in parts of the brain that process anxiety.
Dr Velásquez is currently applying for grants for further research into the effects toxoplasma have on sperm.
Of particular interest is the extent to which the parasite can interfere with sperm production while it is in its dormant stage. After spreading around the body, the organism forms cysts that can survive for long periods within the tissue of the host, including in the testes but also muscle tissue, the heart and the brain.
It's these cysts that are excreted by infected cats, and that then find their way into the bodies of other mammals, including grazing cattle, sheep and domestic cat owners.
The 'spy in your body'
In another sign of how widespread the parasite is, a recent study from the UK suggested that 80 per cent of all sheep in the country carried the infection.
While most people can easily overcome the initial infection and force the parasite into dormancy, removing these cysts entirely and ridding the body of the infection has so far proved impossible, largely because the parasite is so similar to the cells it infects, said Dr Velásquez.
'The Toxo, in this case, is like a spy in your body. It's exactly like your cells – they have almost exactly the same proteins inside, they do division as our cells do. So if I try to cut the life of the parasite, I'm probably going to affect your body a lot, and I could kill you.'
Blood tests can identify whether a person has a latent toxoplasma infection which, as it can be passed from mother to child, can pose a serious threat to pregnant women. The parasite, in its active phase, can cause miscarriages and birth defects.
People with compromised immune systems are also at risk. When the immune system goes down, the dormant parasite can reawaken and attack the body, leading to serious complications – a particularly sinister feature of the parasite that came to the fore during the height of the Aids epidemic.
'There's no cure to get rid of the tissue cysts that could be in your brain and your heart, and those are like time bombs ready to go off if your immune system should falter,' said Dr Sullivan.
For the time being our best course of action to mitigate against the spread of the infection is by fostering greater awareness.
'Educational measures have gone a long way to diminish seroprevalence in more developed countries that are aware of the problem,' said Dr Sullivan.
'In France, the seroprevalence in the past was upwards of 80 to 90 per cent – an outrageous number [which] was believed to be through the consumption of a lot of rare meat, steak tartare, for example,' he said.
But thanks to educational campaigns and improvements to food hygiene, rates of infection have come down to about 50 per cent. 'Still a huge number, but almost cut in half.'
And what about pet cats? Should we be more careful about sharing our homes with these carriers of the parasite?
'There's certainly an element of risk that people need to be aware of in taking care of cat waste properly. But you certainly don't need to throw the baby out with the bathwater, so to speak. You don't need to get rid of your cat.'
In its advice for avoiding toxoplasmosis, the NHS recommends wearing gloves when emptying your cat's litter tray and washing your hands afterwards.
You should also wear gloves while gardening, thoroughly cook meat 'especially lamb, pork and venison' and wash fruit and vegetables thoroughly.
Dr Velásquez would like to see more attention given to the potential effects of toxoplasma infection on male fertility.
We know that it can be dangerous for pregnant women, but no one is paying attention to how it could affect men, she said.
In some countries, including Austria, France and Uruguay, pregnant women undergo mandatory screening to detect Toxoplasma.
But most countries do not test for it in women and none test for it in men, meaning families around the world are having children without ever knowing whether they carry the parasite, potentially putting themselves and their offspring at risk.
'When I was pregnant in Chile, we didn't have any test for us to know – but probably I was infected,' she said. 'I'm a little bit scared now, because what happens if it's in my brain?'