A parasite spread by cats is infecting 1000 new people every day in Britain - about 350,000 a year - according to an official assessment of the risks posed by toxoplasma, which can cause serious illness and has been tentatively linked with schizophrenia and other psychotic disturbances.
In news that will challenge public perceptions about the country's most popular pet, official figures to be published later this week will reveal the shocking levels of infection within the UK human population of Toxoplasma gondii, a microscopic parasite that forms cysts in the human brain and other vital organs of the body.
Toxoplasma infections come either through direct contact with cats or from eating contaminated meat or vegetables, tests on British blood donors have revealed.
Although the clinical signs can be mild, risk groups, such as pregnant women and patients with compromised immune systems, can suffer very serious side-effects, leading to congenital birth deformities, blindness, dementia and even death.
The true scale of the hidden problem has shocked experts who believe not enough is being done to warn the public of the known risks posed by toxoplasma, which they judge to be one of the worst food-borne illnesses because of the severity of its effects.
Some experts say the condition should be made a notifiable disease in England and Wales bringing the two countries on a par with Scotland, where infections must be reported on a national database. Others question whether families with young children should have pet cats, while some say advice on cooking lamb and preparing vegetables should be changed.
In addition to infections caused by direct contact with cats, people can pick up the parasite by eating the meat of infected animals or from raw vegetables that have not been washed properly to rid them of any toxoplasma eggs contaminating the soil.
About 80 per cent of infected people show no obvious symptoms of toxoplasma and are completely unaware that they are harbouring the parasite. However, new estimates suggest that up to 70,000 people a year in the UK develop some kind of symptoms. Experts are especially concerned about the emerging scientific evidence suggesting that apparently healthy people with toxoplasma may still be affected unwittingly by the parasite, even when they show no obvious symptoms.
A number of small-scale studies suggest that toxoplasma infection may alter people's personality, making them more prone to risk-taking or delayed reaction times. Studies have also linked toxoplasma infection to psychotic disturbances such as self-harm and suicide, and to serious psychiatric illnesses such as schizophrenia.
This week the Food Standards Agency (FSA) will publish a "risk profile" of toxoplasma in the food chain. The group of experts commissioned to write the report estimates 350,000 new toxoplasma infections occur each year in the UK, most of them probably from eating contaminated food.
Experts have urged the FSA to review its advice to pregnant women and immune-compromised patients, and strongly advised it to change its policy stating it is safe for people to eat rare lamb.
Sheep are thought to pick up the parasite by eating s grass or feed that is contaminated with cat faeces. Preliminary studies indicate that nearly 70 per cent of British sheep have been exposed to the feline parasite.
Pregnant women and patients with compromised immune systems are already warned to avoid pink meat.
Dr Fuller Torrey, an expert on schizophrenia and toxoplasma at America's Stanley Medical Research Institute in Maryland, said all meat should be cooked thoroughly to kill parasitic cysts lying dormant within muscle tissue because of the severity of the potential risks posed by the parasite.
"I would not advise anyone to eat undercooked meat given what we know and don't know about this organism."
Richard Holliman, a consultant medical microbiologist at St George's Hospital in London, who chaired the FSA's working group on toxoplasma, said that, based on existing scientific evidence, it is not yet justified to change the official advice on the safety of eating rare lamb for the general public.
"Toxoplasma is more important, or as important, as salmonella and campylobacter, which affect a lot of people. Toxoplasma affects a few people but when it does affect them it can be devastating. A child born with congenital toxoplasma is damaged for life."
Q&A: Cats, sheep and raw veges all pose threat
Is it safe to have a cat?
Cats are the ultimate source of toxoplasma and are the only species in which the parasite completes its lifecycle. It is not clear how the parasite is transmitted to people. Only 1 per cent of cats are infected at any one time, and the highest risk animals are kittens that have just learned to kill wild mice and birds. A survey of 51 cats on 22 sheep farms in the south west of England found nearly half the felines carried antibodies to toxoplasma, indicating past exposure to the parasite. Sensible hygiene, and avoiding cat litter if you are pregnant, should be enough to limit the risk.
I have small children. Should I complain to my neighbours about their cats coming into my garden?
Cover children's sandpits at night or when people are not around. Studies show they are rich in toxoplasma eggs if left unprotected. Anti-cat devices may help to reduce the risk of cats defecating in your garden.
Are some people more at risk than others?
Women who are pregnant for the first time and get infected are at high risk of passing on the infection to their unborn baby. This can cause serious, life-long problems for the child and can lead to miscarriages. Likewise, people with compromised immune systems cannot easily fend off infection and may be vulnerable to a latent infection acquired years earlier. The late dementia of some Aids patients may be the result of toxoplasma.
Can the parasite be caught by stroking cats? Or only by touching their faeces?
Touching an infected cat carries a risk of the parasite being ingested. Accidentally touching its infected faeces carries an even higher risk if hands are not washed thoroughly.
What steps can I take to reduce the risks of catching the parasite from food?
Wash any vegetables thoroughly if they are to be eaten raw. A high proportion of vegetarians are known to be infected with toxoplasma. Meat can also harbour toxoplasma tissue cysts. Lamb meat has been shown from limited testing to carry the greatest risk of toxoplasma by various farm animals. Sheep acquire the infection by eating cysts on pasture grass or from concentrated feed contaminated with cat faeces. A survey of 3539 blood samples taken from breeding ewes in Britain revealed 68.6 per cent are positive for toxoplasma antibodies, indicating past exposure to the parasite. The official advice to pregnant women and immune-compromised patients is to cook all meat well and not to eat it rare.
What are the main symptoms of toxoplasmosis?
If symptoms develop they usually occur within about one or two weeks after initial contact. The disease can affect the brain, lungs, heart, eyes or liver. Healthy people can show flu-like symptoms, such as sore throat, headaches and fever.
What should I do if I suspect I am infected?
Go to a doctor.
Can toxoplasmosis in humans be treated?
Toxoplasma can be treated with a range of drugs, including antimalarial medicines and antibiotics.
How do I find out if my cat is infected?
You can't as no commercial test is available.