TOXOPLASMOSIS An Introduction

What is Toxoplasmosis?

Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. It is not a worm or a larva, and cannot be seen by the naked eye. It is a microscopic single cell organism which can be found in meat, cat faeces and the foil where cats defecate.

It can affect almost all animals and humans. Most animals are infected by toxoplasmosis at some time in their life. Once infected the animal never rids itself entirely of toxoplasmosis, although the parasite usually remains dormant for the natural life of the animal.

What are the effects of Toxoplasmosis on humans?

Toxoplasmosis is not dangerous to the normal healthy adult or child. Many people do not even know that they have the infection as they might only have mild 'flu-like symptoms or no symptoms at all. Occasionally a person may suffer from a prolonged and debilitating glandular-fever type illness. This lack of any distinct symptoms is the reason why toxoplasmosis is so rarely diagnosed in adults.

Toxoplasmosis is only dangerous to humans if their immune system is undeveloped, as in the unborn child, or not healthy (eg. people with HIV and AIDS or on immuno-suppressant drugs). In such cases the immune system is unable to restrict the spread of the parasite which can then cause the damage described below. Once a person has had the disease they are protected for life, unless they should suffer an impairment to their immune system.

How does it affect the unborn child?

If a woman catches the disease during pregnancy, then there is a substantial risk that her child will become infected. If the disease is caught early in pregnancy, it is less likely to cross the placenta to the foetus, but if it does, the effects are more serious. If the pregnant woman catches the infection later, it is more likely to cross the placenta, but the effects on the foetus are less severe. On average 40 per cent of babies of affected mothers are infected themselves. Of these, 10 per cent are likely to be seriously affected.

Babies that are infected in the womb early on in pregnancy may be miscarried or stillborn. Babies that are born with toxoplasmosis (usually when infected between the third and sixth month) may develop severe symptoms. These include hydrocephalus (an excess of fluid on the brain), brain lesions (scarring of the brain tissue), both of which can cause sever mental retardation and epilepsy, and retinochoroiditis (damage to the retina at the back of the eye) which causes partial sight and occasionally blindness. A child may not suffer from all these symptoms, but the classic triad is retinochoroiditis, hydrocephalus and brain lesions.

Some infected children are born apparently normal but may develop symptoms weeks, months or even years later. Most commonly this will manifest itself as retinochoroiditis and may affect previously apparently healthy children in their teens, twenties or even later in life.

How does it affect adults?

Symptoms of toxoplasmosis may include swollen glands, fatigue, headache, sore throat, aching muscles and occasionally fever. Sometimes these symptoms will last weeks or even months. However many sufferers show no symptoms at all.

How common is it?

Toxoplasmosis is common in that 30 per cent of 30 year olds and 50 per cent of 70 year olds show signs of having had the disease at some time in their lives.

The results of a survey carried out by Dr Joynson, Director of the Public Health Laboratory in Swansea in 1988† involving 30,000 pregnant women, indicate that the overall rate of infection in pregnant women is 2 per 1,000. If this rate is applied to the 700,000 (approx) births in Britain every year, then on average every year 1,4000 women become infected by toxoplasmosis during their pregnancy. 40 per cent of these women will pass on the infection to their babies. Therefore every year up to 560 babies may be born infected by toxoplasmosis.

Furthermore in France, where, as in Britain, 2 per 1,000 women acquire an infection during pregnancy, toxoplasmosis is routinely screened for monthly during pregnancy. It is not at present routinely tested for in the UK.

†Joynson DHM, Payne R Screening for toxoplasma in pregnancy, the Lancet 1988, ii, 795-6

How is toxoplasmosis caught?

A person contracts toxoplasmosis by eating, often accidentally, anything infected with the parasite. Undercooked meat is thought to be the principal source of human infection, although exactly how any individual case is caught is difficult to pinpoint.

The significance of the cat is that it is the only known host in which the reproductive cycle of Toxoplasma gondii takes place. The domestic cat catches toxoplasmosis from birds or mice when hunting, or from other raw meat. For 14 days after having become infected the cat sheds infectious organisms in its faeces. Thereafter a healthy cat will not normally be a source of infection again. Sick cats may re-shed infected faeces.

In the earth cats' faeces may remain infective for up to 18 months. Humans are at risk because they can catch the infection from contaminated soil, when during gardening for instance they accidentally put an earthy hand to their mouth. Vegetables and fruit which are not properly washed can also be a source of infection.

Although only cats shed infected faeces, humans can catch the infection by eating undercooked or raw meat from other animals. Cats on farms may defecate in grain stores and hay used for animal feed and this is one way that farm animals and therefore meat becomes infected. Grazing land may also be contaminated.

There may also be a risk of catching toxoplasmosis from handling sheep at lambing time. In sheep, which are particularly susceptible to the infection, it is a significant cause of abortion.