The facts about...

Toxoplasmosis

What is toxoplasmosis?

Toxoplasmosis is a disease which occurs world-wide in both humans and a wide range of animals. It is caused by infection with a single celled parasite different from bacteria and viruses called Toxoplasma gondii which was discovered in 1908 in the gondi, a small rat-like animal from North Africa. Although many people are infected, most of them never show any signs of disease. Infection is most hazardous in persons whose resistance to infection is low such as organ transplant recipients, AIDS patients and unborn children. The parasite spreads to humans in undercooked or raw meat and in soil or vegetables contaminated with infected cat faeces or cat litter. Although Toxoplasma gondii is commonly found in many animals, as far as is known, it is only in the cat gut that the male and female parasites come together to produce one of the infective forms. If these are swallowed by a suitable host then infection may follow.

How common is toxoplasmosis?

Between a quarter and a half of the world's population is infected, and infection is most common in places with warm moist climates. In the United Kingdom approximately 0.5% to 1% of the population acquire the infection each year, so that about 40% of people aged 50 years have been infected. The precise number of newborn babies infected in Britain is uncertain. Currently, about twelve severely affected children are reported each year, but this is almost certainly an underestimate.

The Disease

Most infections in adults either cause no symptoms or only mild illness with various aches and pains, a slightly raised temperature and enlarged lymph glands. Sometimes the disease may be more severe. In the majority of infections a doctor is not consulted, a laboratory confirmed diagnosis is not made and patients get better in a few months. Sometimes eye infection, discovered by chance when the eyes are being examined, brings the condition to the doctor's notice. Rarely an apparently healthy person may develop serious disease which affects the heart, lungs and brain.

In patients with poor immunity, including those being treated for illnesses such as Hodgkin's disease or leukaemia, organ transplant recipients or patients with AIDS, the consequences of infection with toxoplasma are more likely to be serious and the disease is often severe and may be fatal. Infection in such patients is usually a re-awakening of an old infection but can be newly acquired.

Infection in the unborn child, called congenital toxoplasmosis, is the result of an acute usually asymptomatic infection acquired by the mother in pregnancy and passed on to the baby in the womb. If the infection occurs in the first three months of pregnancy there is an increased risk of miscarriage, severe disease in the newborn child or even stillbirth, but if it occurs in the last three months, signs of disease in the newborn child are rare. Some of the affected children will eventually suffer eye damage but the proportion is uncertain.

Severely affected babies may suffer infection of the brain tissue causing mental retardation, accumulation of fluid inside the brain and blindness. Signs of the disease may not be present at birth, but may appear later.

Can the disease be prevented?

There is no vaccine suitable for humans and so there is no foolproof way of preventing infection. To minimise the risk, pregnant women should not eat undercooked meat. They should thoroughly wash their hands before eating and before and after preparing food. Rubber gloves should be worn when handling dirty cat litter. If possible, get others to do the job. They should always wear rubber gloves when gardening and wash their hands afterwards.

How is toxoplasmosis diagnosed

The infection is usually diagnosed by blood tests which measure antibodies to the parasite. Tests which detect the parasite itself are used in patients with poor immunity.

How is toxoplasmosis treated?

Adults with normal immune defences need not be treated unless the illness is unusually severe or persistent, but immunodeficient patients with active disease should always be treated. If the diagnosis of infection is made in a woman during pregnancy she may be offered the choice of termination or drug treatment to reduce the severity of any damage to her child.

Should pregnant women be tested for toxoplasmosis?

This is a controversial subject. If infection is diagnosed in pregnancy, treatment can be given to the mother to try to prevent the baby becoming infected. If the baby is already infected, treatment is given to try to stop further damage to the child. As most pregnant women who become infected have no symptoms, the only way to minimise the damage to babies would be to test all pregnant women repeatedly, possibly do tests on blood taken from the unborn babies of those mothers found to be acutely affected, and to offer treatment or termination of pregnancy to those mothers with acute infection and those whose babies were infected.

Whether or not an antenatal screening programme should be instituted in Britain is arguable and would depend on the number of women and unborn children infected and the cost, safety and efficiency of the procedures used to detect and treat infection. The cost of any programme would have to be balanced against the current costs both financial and in human terms of infection in babies. More research is required before a conclusion can be reached, but on the basis of current knowledge, a recent harm/benefit assessment found that routine antenatal screening should not be undertaken in the United Kingdom.