Toxoplasmosis is a common disease that occurs worldwide in most birds and warm-blooded mammals, including humans. It's caused by infection with Toxoplasma gondii (T. gondii), one of the world's most common parasites, which is contained in undercooked meat, raw meat, cat faeces and any soil or cat litter contaminated with infected cat faeces. It can't be passed on through person-to-person contact.

Around a quarter to half of the world's population is thought to be infected. Around 1% of people in the UK catch toxoplasmosis each year. In most cases, the person affected doesn't have any symptoms. This is because a healthy immune system is usually able to defend the body from the parasite.

There may be slight flu-like symptoms, such as a high temperature or body aches, but these generally pass without treatment after a few weeks.

However, toxoplasmosis can cause serious health problems, and can be fatal, for babies born to infected mothers and for people with immune deficiencies (weakened immune systems caused by illnesses including AIDS and cancer).

Babies may develop jaundice (yellowing of the skin) and have seizures or brain defects. People with immune deficiencies may develop damage to the eyes, brain and other organs.

You cannot catch toxoplasmosis from contact with another infected person, but you are at risk from infection if you come into contact with any of the following.

  • Cat faeces. The parasite breeds in the bowels of cats, and then is released in their faeces. Cat faeces have been known to go on to contaminate (infect) soil, fruits and vegetables, water, and other animals.
  • Undercooked or raw, infected meat either by eating it or touching your mouth after handling it. Grazing animals can contract the infection by eating contaminated grass and animal feed. Venison, lamb and pork are more likely to be infected with T. gondii parasite.
  • Contaminated knives, cutting boards and other utensils that have been in contact with contaminated undercooked or raw meat.

In extremely rare cases, toxoplasmosis has been caught via an infected organ transplant or blood transfusion, and from un-pasteurised goats milk.

When you are infected, your immune system will start to produce special antibodies to fight the T. gondii parasite. If you or your GP suspect you have toxoplasmosis, you will need to have blood tests to check for these antibodies.

Occasionally, early testing can produce a negative result even if you are infected (false negative). This is because your body has not had a chance to start producing antibodies to the parasite. So your GP may want to retest your blood 2-3 weeks later. In most cases, however, a negative toxoplasmosis test result means you have not been infected.

On the other hand, a positive result does not necessarily mean you are actively infected. In many cases, it is a sign that you have been infected at some time in your life and are now immune to the parasite. So further blood tests can be performed to help determine when the infection occurred based on the types and levels of antibodies in your blood. Further testing is important if you are pregnant or have HIV/AIDS.

Pregnant women

If you are pregnant and have toxoplasmosis, you will need tests to determine if your unborn baby is infected. The most effective and common test performed is an amniocentesis. This can be done after 16 weeks of pregnancy. A fine needle is inserted through the mother's abdomen and into the fluid surrounding the foetus. The sample is then tested for evidence of toxoplasmosis. The procedure usually takes 5-10 minutes. Having an amniocentesis does carry a slight risk of causing a miscarriage – around 1 in every 200 amniocentesis procedures result in miscarriage.

People with immune deficiencies

If you have a weak immune system, maybe due to illness, routine blood tests for antibodies can produce a false negative. This is because it is very likely your immune system will not be producing antibodies to fight off the infection.

Therefore, your doctor may want to perform the following tests to create images of your brain and check for evidence of toxoplasmosis.

  • Computerised Tomography (CT or CAT) scan instead of a single X-ray, several beams are sent from different angles at the same time.
  • Magnetic Resonance Imaging (MRI) scan this test uses a magnetic field and radio waves to produce detailed, two-dimensional images.

Both of these procedures are non-evasive (performed outside of the body) and will not put any further risk on your health.

Toxoplasmosis symptoms vary. In about 80% of cases, the infection causes no symptoms and you are not even aware that you infected. The immune system in healthy adults and children is usually strong enough to keep the T. gondii parasite in check – you become immune to it and it will live harmlessly in your body for life.

The infection can however cause the lymph nodes (part of your immune system) to swell, especially in the throat or the armpits. This can cause symptoms similar to that of flu, such as:

  • body aches,
  • fever,
  • tiredness,
  • feeling sick, and
  • a sore throat.

It is very rare for otherwise healthy people to suffer any serious symptoms of toxoplasmosis.

Pregnant women

Toxoplasmosis can be serious if a woman catches the infection during her pregnancy because, although she will probably have no symptoms, the infection may be passed on via the placenta to her unborn child (known as congenital toxoplasmosis). This is because a foetus' immune system is not properly developed and is unable to defend against infection. This can lead to complications such as miscarriage, stillbirth or birth defects.

Babies whose mothers are infected during the last 28 weeks of pregnancy very rarely experience any problems or symptoms at birth. Complications of the infection can however develop in their 20s or 30s (see complications).

If you are pregnant and have toxoplasmosis, you will need special tests to determine if your unborn baby is infected (see diagnosis).

People with immune deficiencies

Toxoplasmosis can prove fatal to those with weakened immune systems caused by illness, such as AIDS and cancer, or people who are having chemotherapy or have recently had an organ transplant. There is also a risk of developing permanent eye or brain damage, such as encephalitis (brain inflammation). In fact, around 10% of people with AIDS develop severe encephalitis from toxoplasmosis. Other symptoms include:

  • headaches,
  • confusion,
  • poor coordination,
  • seizures,
  • chest pains,
  • coughing up blood, and
  • trouble breathing.

As well as this, toxoplasmosis of the eye can sometimes develop as a symptom. This is a problem especially for unborn babies of infected mothers and people with immune deficiencies (it is less common in healthy adults). This is because antibodies (special proteins which help destroy disease-causing germs, such as bacteria and viruses) are less effective in the inner tissues of the eye area.

Symptoms include:

  • blurred vision,and
  • floaters – dark spots that pass across your field of vision and move as you move your eye, they may look like clouds, spots, wavy lines or other shapes.

In most cases, healthy people do not require any treatment for toxoplasmosis. Either no symptoms develop or a full recovery is made with no complications.

Those who experience more severe symptoms of the infection (especially people with immune deficiencies) are usually prescribed the following medications to treat toxoplasmosis:

  • pyrimethamine, and
  • sulfadiazine.

This drug combination blocks the parasites production of a chemical called folinic acid, which is essential for the parasite to reproduce. Therefore stopping the parasite from multiplying in the body.

This treatment is usually given for 3-6 weeks. If further courses of treatment are required, there will be a rest period of two weeks between. Although this is just a general guideline – your treatment plan will depend on your health, what symptoms you have, and how severe they are. People living with HIV or AIDS may need to take these medications for life to keep the parasite at bay.

Pregnant women and babies

If you are pregnant and infected with toxoplasmosis, but your baby is not, you may be given the antibiotic spiramycin. This can reduce the chance of your baby becoming infected, without being a risk to either of you.

If tests have shown that your unborn child has toxoplasmosis (congenital toxoplasmosis), your doctor may suggest treatment with pyrimethamine and sulfadiazine, which can reduce the severity of congenital toxoplasmosis, but will not undo any damage that has already been done.