Cat parasite linked with schizophrenia and suicide infects 350,000 Britons a year
Published 04/09/2012 | 04:31
A parasite spread by cats is infecting 1,000 new people every day in the UK – 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 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 call in The Independent today for the condition to be made a notifiable disease in England and Wales – meaning that medical staff must be put on alert – 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 clinical 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 Independent has learnt.
The group of experts commissioned to write the report estimates that 350,000 new toxoplasma infections occur each year in the UK, most of them probably from eating contaminated food.
Experts contacted by The Independent have urged the FSA to review its advice to pregnant women and immune-compromised patients and have strongly advised it to change its policy stating that it is safe for people to eat rare lamb.
Sheep are thought to pick up the parasite by eating pasture grass or concentrated feed that is contaminated with cat faeces and preliminary studies indicate that nearly 70 per cent of British sheep have been exposed to the feline parasite.
Although pregnant women and patients with compromised immune systems are warned to avoid pink meat, the FSA's chief scientist, Andrew Wadge, said that it is safe for people to serve lamb rare, even though one study found that two thirds of lamb samples from one Manchester butcher tested positive for toxoplasma.
"People traditionally eat and enjoy lamb cooked rare, the same as beef. That's how people enjoy it and for most people that is perfectly safe," Dr Wadge said.
"Our advice is always on the basis of what we know and the science changes. I'm not going to tell you about the safety of lamb based on another five or ten years of research, I'm going to tell you what I know now, and there is a long history of people eating rare lamb without any adverse consequences," he said.
However, other experts warned that much of the lamb sold in British supermarkets is likely to be contaminated with toxoplasma cysts in the muscle tissue, which can survive cooking when meat is served pink.
"I would steer very well clear of rare lamb. I would certainly not recommend eating rare lamb," said Barbara Lund, a microbiologist at the Institute of Food Research in Norwich.
"Regarding the comment that it is safe to serve whole cuts of beef and lamb rare as long as they have been properly cooked on the outside, it is not clear to me that we can be confident of this advice for sheep meat," Dr Lund said.
Fuller Torrey, an expert on schizophrenia and toxoplasma at America's Stanley Medical Research Institute in Maryland, said that the seriousness of the potential risks posed by the parasite to the general public means that all meat should be cooked thoroughly to kill parasitic cysts lying dormant within muscle tissue.
"Eating any meat that is rare or undercooked is not safe. I would not advise anyone to eat undercooked meat given what we know and don't know about this organism," Dr Torrey said.
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.
"Certainly for pregnant women, the advice is to eat meat that has been thoroughly cooked through, but it's difficult to advise the wider population because you have to balance the risk against people's personal tastes," Dr Holliman said
"Some people enjoy food if the meat it not well done. To them it would be a disbenefit to cook meat until it is well done," he said.
Dr Holliman pointed out that vegetarians also suffer from high levels of toxoplasma infection which indicates that meat is not the only source of food-borne contamination.
"Toxoplasma infects a lot of people but only has an impact in terms of lifestyle on a small proportion of them," Dr Holliman said.
"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," he said.
How toxoplasma spreads... and spreads
The microscopic parasite Toxoplasma gondii is small enough to get inside the cells of the animals it infects. A single-celled organism known as a protozoan, it has a complicated life cycle involving rodents and cats. Toxoplasma can also infect humans and farm animals, but these species are accidental hosts.
The parasite can only reproduce sexually and form eggs within cats, which can shed millions of toxoplasma “oocysts” in their faeces. Cats become infected if they eat contaminated raw meat, or catch and eat infected mice or rats.
Cats can shed up to 10 million oocysts a day for up to 14 days after they initially become infected. Given that there are 8 million pet cats in Britain and 1 per cent of them are shedding toxoplasma oocysts at any one time, this means there are in the region of 800 billion toxoplasma eggs being released each day in Britain by domestic cats.
Toxoplasma oocysts in the soil remain viable for several years where they can be ingested by other animals which then become infected with tissue cysts in their vital organs, including brain and muscle tissue.
A survey of 51 cats on 22 sheep farms the South West of England found that nearly half the felines carried antibodies to toxoplasma, indicating past exposure to the parasite.
Lamb meat has been shown from limited testing to carry the greatest risk of toxoplasma based on past exposure to the parasite by various farm animals.
Fuller Torrey, director of the Stanley Medical Research Institute in Maryland, carried out studies of playground sandpits that are left uncovered at night and found that they are a favourite haunt of cats, which prefer to defecate in loose sand and soil.
“I’ve just been looking at the data on levels of oocysts secreted by cats and I’m frankly appalled at the level of contamination of areas where they go to the bathroom,” Dr Torrey said.
“I estimated in some of these public sandboxes [sandpits] there are about a million viable oocysts per square foot of sand. I would not let any children of mine play in a sandbox that had not been covered at all times when not in use, or in a sheltered place that cats could not enter,” he said.
Dr Torrey said that he would go as far as to recommend against having cats as pets if there are young children in a household.
“I would certainly advise families not to get a cat if they have small children. I gave this advice to my own daughter and granddaughter,” he said.
Toxoplama infection can result from the ingestion of oocysts or from tissues cysts that form in the muscle of infected farm animals. Most people show no symptoms but it is estimated that between 10 and 20 per cent of infected individuals develop flu-like symptoms in the early, acute stages of the infection.
The latent stage of toxoplasma is normally without obvious clinical symptoms but it can develop into serious illness when people become immune-compromised, for instance during Aids or when undergoing certain types of cancer treatment. One notable manifestation in immune-compromised patients is toxoplasmic encephalitis of the brain, which can be lethal.
If women become infected with toxoplasma while pregnant, the parasite can seriously damage the development of their baby in the womb, leading to miscarriage or congenital birth problems.
Toxoplasma is the second most common cause of abortion in sheep, resulting in the loss of more than 500,000 lambs a year at a cost of up to £24m. Sheep acquire the infection by eating oocysts on pasture grass or from concentrated feed contaminated with cat faeces.
A survey of 3,539 blood samples taken from breeding ewes in Britain revealed that 68.6 per cent are positive for toxoplasma antibodies, indicating past exposure to the parasite.
Q&A: Toxoplasma - what can be done?
Should I get rid of my cat?
Cats are the ultimate source of toxoplasma and are the only species in which the parasite completes its lifecycle. However, 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 risks animals are kittens that have just learned to kill wild mice and birds. 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?
It would be a good idea to 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 with toxoplasma at that moment 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 sometimes lead to miscarriages. Likewise, people with compromised immune systems cannot easily fend off an infection and may be vulnerable to a latent infection acquired many years previously. The late dementia of some Aids patients may be the result of latent toxoplasma infection.
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. But not all cats are infected.
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, so vegetables must be considered to be a source of infection. Meat can also harbour toxoplasma tissue cysts, and lamb in particular is believed to be a particular risk. The official advice to pregnant women and immune-compromised patients is to cook all meat well and not to eat it rare. Some experts suggest this should be standard advice for the wider public.
What are the main symptoms of toxoplasmosis?
Most people show no obvious symptoms. 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 and can in healthy people result in flu-like symptoms, such as sore throat, headaches and fever.
What should I do if I suspect that 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. Most people with a healthy immune system recover, but the disease may return in later life due to latent infection.
How do I find out if my cat is infected?
You cannot, at the moment, as no commercial test is available.
Scientist who uncovered 'fatal feline attraction'
The key piece of evidence that has convinced many scientists to take the link between toxoplasma and schizophrenia seriously came out of the Oxford laboratory of Professor Joanne Webster, now at Imperial College London, who demonstrated that the parasite is responsible for what she has called “fatal feline attraction”.
In a series of pioneering experiments that began in 1994, Professor Webster revealed how toxoplasma infection can cause changes to the behaviour of rats that would in the wild make them easy prey for cats, the only species in which toxoplasma can complete is complex life cycle.
If a tiny parasite can change the behaviour of a rodent through subtle alterations to its brain chemistry, then something similar may happen when the parasite infects the human brain, triggering schizophrenia in extreme situations.
The studies, which have been replicated by other scientists, proved beyond doubt that this tiny single-celled organism can trigger behavioural changes in the rat and mouse. Instead of freezing at the first whiff of cat’s urine, the infected rodents go exploring – a fatal change of behaviour.
It made evolutionary sense for toxoplasma to have this effect on rodents, which are a secondary host, because it needs an infected rat or mouse to be eaten by a cat for it to complete the all-important sexual stage of its life cycle.
Three years ago, Glenn McConkey of Leeds University found further evidence to explain this behaviour when he identified two genes in the toxoplasma genome that together would allow the parasite to make L-dopa, the molecular precursor of the dopamine neurotransmitter.
It seemed logical to suppose that the parasite’s genes may be involved in bringing about changes to the behaviour of an infected rodent – dopamine is a key chemical modulator in the mammalian brain.
Professor Webster had earlier showed that treating infected rats with haloperidol, an antipsychotic drug known to block dopamine, reverses the behavioural changes brought about by toxoplasma.
It all pointed to the possibility that toxoplasma infections in mice, rats and possibly humans could interfere with the normal dopamine processes of the brain. Dopamine is also known to be involved in a host of psychological conditions in humans, so the hypothesis that toxoplasma could cause changes to human behaviour suddenly seemed less crazy.
“I think what we are going to see in humans is going to be similar to what we see in rats,” Professor Webster said.
“There’s going to be these very subtle changes such as slightly decreased reaction times and gender effects between males and females. Most of it will be so subtle we won’t see it unless we look very carefully, which goes with it being dismissed as an asymptomatic parasite for so long,” she said.
The idea that toxoplasma may prove to be one of several triggers for serious mental disturbances in humans, such as schizophrenia, is looking increasingly likely, although far from proven, Professor Webster said.
“A lot of it is association but it’s getting more and more convincing. It’s a correlation but a correlation amongst an ever-gathering body of evidence,” Professor Webster said.
“There were some nice studies in Germany with schizophrenia patients with high levels of antibodies for toxoplasma. If they were treated for schizophrenia in the past, their antibodies went down, if they were under current treatment the levels went down even further suggesting the drugs were impacting the toxoplasma,” she said.
“We have a parasite that appears to have evolved to change the behaviour of its host in order to increase transmission. Pretty much as a by-product of that we are maybe going to see quite severe behavioural deficits in humans,” she added.
Dr McConkey said that although the link between toxoplasma infection and schizophrenia falls well short of cause and effect, the dopamine genes clearly indicate a plausible mechanism where the parasite can bring about subtle changes to the chemistry of the human brain, albeit as an unintended consequence of the intended effect on rodents.
“It’s still inconclusive whether there’s a severe effect on human behaviour , which is why there is a need for larger studies. There is no question from animals studies that there’s some neurological effect of being infected,” Dr McConkey.
As for the suggestions of a link with schizophrenia, he believes these are not as weak as some people have suggested. “The links with schizophrenia are stronger than the link between the illness and genes that have been labelled ‘schizophrenia genes’,” he said.