Many people infected with toxoplasmosis will have no symptoms, but others face serious consequences.
Many of us will be infected with toxoplasmosis at some point in our lives without having any idea of it happening. Perhaps we’ll notice flu-like symptoms such as muscle aches and swollen glands but for those blessed with a healthy immune system, there may be no obvious signs.
Toxoplasmosis is caused by a single-cell parasite that is found all over the world and can infect most warm-blooded animals, although it reproduces only in cats. More than a quarter of the world’s human population is believed to have been infected. Drinking contaminated water or eating undercooked meat can be a source, as is exposure to cat faeces perhaps via a litter tray or cat-poo-laced soil in a garden. After the initial infection, the parasite goes into a cyst phase and remains in the body for life as neither our immune system nor antibiotics can eliminate it entirely.
“For most people there are no long-term problems,” says Rachael Niederer, an ophthalmologist at Auckland Eye.
As a retina specialist, she has an interest in the condition because a small percentage of people who get toxoplasmosis will have eye lesions associated with it. “They may notice that they have inflammation in their eyes. They might get floaters, blurred vision or be sensitive to bright light,” says Niederer.
Sometimes this results in retinal tearing, and problems with the eyes may recur years after the original infection if there are flare-ups.
The toxoplasma gondii parasite can also cause low-level inflammation in the brain and there are some conditions that seem to occur at a higher rate.
“Obsessive compulsive disorder occurs at about double the rate,” explains Niederer. “And it is also associated with bipolar disorder and schizophrenia.”
Conversely, some studies have found a protective effect against multiple sclerosis, with a 2021 review from the European Journal of Neurology finding those who had been infected with the parasite were 32% less likely to develop multiple sclerosis than those who had never been infected.
Toxoplasmosis has also been linked to behavioural changes in both animals and human hosts.
“If a mouse is infected with toxoplasmosis, it will actually run towards the smell of cat urine rather than away from it, which would be normal, and spend more time in the open where there is a greater risk of being caught and eaten by a cat. So the toxoplasmosis can then complete its life cycle,” explains Niederer.
“That holds true even for primates. A chimpanzee that has been infected with toxoplasmosis will go towards the smell of leopard urine, rather than away from it which is the natural thing to do.”
In humans, the infection is associated with increased risk-taking and even a higher likelihood of entrepreneurial behaviour. Research led by the University of Colorado found that university students who tested positive were more likely to major in business and that professionals who had been infected were more likely to start their own business at some point. The theory is that this is due to a reduced rational fear of failure.
For those with compromised immune systems, a toxoplasma infection can cause severe disease. HIV-positive individuals, transplant recipients and those undergoing cancer treatment are particularly at risk.
If a woman becomes infected with the toxoplasma gondii parasite while pregnant, she might pass the infection on to her unborn baby which could affect growth and brain development, cause eye problems, and also carries a risk of miscarriage or stillbirth.
“We know that if babies are positive when they’re born then their risk of having problems in the future is high,” says Niederer.
While both mother and child can be treated with antibiotics, that won’t undo any damage that has already occurred, and so taking some basic precautions to avoid the parasite is advisable.
“It’s definitely worthwhile for pregnant women to take care,” says Niederer. “They want to wash vegetables well, wear gloves while gardening so that they don’t get contaminated soil under their fingernails, and heat their meat – make sure it’s over 70°C. If you’re a cat owner, then wear gloves when you’re changing the litter, or better still make it your partner’s job.”
