Owning a cat is known to have positive health effects, such as reducing stress and improving mood. But it can have downsides, too.
Allergy UK estimates that half of all asthmatic children are allergic to cats, for example.
Now scientists are looking at whether Britain’s ten million cats are putting their owners at risk of another condition: anxiety.
The focus of their investigation is Toxoplasma gondii, a tiny, single-cell parasite commonly found in cat droppings, which causes the disease toxoplasmosis.
Pregnant women have long been advised to avoid emptying cat litter trays, as ingesting the parasite accidentally from unwashed hands in early pregnancy can cause miscarriage or stillbirth. In adults, more rarely it can also lead to blindness by causing scarring at the back of the eye. Other sources of infection include undercooked meat and unwashed fruit and vegetables.
Now scientists think exposure to Toxoplasma gondii could be to blame for many cases of anxiety.
It’s a theory backed by some of Britain’s leading experts on parasitic diseases.
In a study at the University of Michigan, blood samples from 450 adults were checked for antibodies to Toxoplasma gondii.
The presence of these antibodies is a sign the person has been infected. Researchers then identified the study participants who had been diagnosed with generalised anxiety disorder, or GAD.
Anxiety is a normal but temporary response to stress or danger. But with GAD, the anxiety is excessive – a constant worrying when there is no danger present. Up to 7 per cent of the population – nearly five million people in Britain – are thought to be affected with symptoms, ranging from a racing heartbeat and shortness of breath to agitation and constant dread. The University of Michigan research, published in the journal Brain, Behaviour and Immunity, found that people with antibodies to the parasite were twice as likely to have been diagnosed with GAD.
And those with the highest levels of antibodies, suggesting greater exposure to the parasite, were three times more likely to be affected by GAD. It’s thought that while antibodies might destroy some parasites, others evade detection and head for the brain.
The researchers said: ‘Our study is the first to examine the association between T. Gondii infection and diagnosed anxiety disorder.’
This is not the first time the parasite has been linked to brain-related problems. Previous research has linked infection with this parasite to an increased risk of schizophrenia and learning disabilities in children.
‘There are even some fascinating studies showing Toxoplasma can cause decreased reaction times and a greater chance of being involved in road accidents,’ says Professor Joanne Webster, from the Centre for Emerging, Endemic and Exotic Diseases at the Royal Veterinary College in London, whose research in the Nineties led to an extraordinary discovery about the effect Toxoplasma had on rats’ behaviour.
The parasite exits the body via droppings – and, if these are then eaten by a rat, the parasite heads straight for the brain, where it somehow switches off the innate fear the rodent has of cats, and makes it crave the smell of feline urine.
All the rat’s survival instincts disappear and it effectively serves itself up as dinner for a cat. ‘It’s basically manipulating the rat’s mind,’ says Professor Webster.
It’s this same ‘manipulation’ which scientists think might trigger anxiety in humans.
To be on the safe side, wear gloves when cleaning a litter tray
Studies suggest Toxoplasma produces its own supply of dopamine, a brain chemical that sends signals between cells.
The parasite’s dopamine might disrupt the normal chemical balance in the human brain.
Although too little dopamine has been linked with anxiety, so has too much, and research suggests that the parasite can trigger over-activity in the amygdala, the area of the brain that is involved in expressing anxiety.
But should cat owners be worried? There are only 350 confirmed human cases of toxoplasmosis in England and Wales a year.
But according to NHS Choices, up to a third of us will be infected at some point, yet not even notice any symptoms (typically it causes flu-like symptoms that last for a couple of weeks).
Maggie Roberts, director of veterinary services at Cats Protection, says: ‘I have lots of friends who are vets and have handled hundreds of cats and were tested during pregnancy for Toxoplasma – and I don’t know one who has tested positive.’
She says cats are infectious for only about ten days in their whole life. Afterwards, they are resistant. ‘To be on the safe side, wear gloves when cleaning a litter tray and wash your hands thoroughly afterwards.’
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