Toxoplasmosis is an infection caused by a common parasite called Toxoplasma gondii (T. gondii).
Most warm-blooded animals, including sheep, cattle, dogs and humans, can be infected with this tiny single-celled parasite.
However, the parasite can only be passed on if it enters the environment or food chain, or if it passes from an infected mother to her unborn baby (known as congenital toxoplasmosis). Rarely, the parasite can also be passed from human to human through organ transplantation.
Although toxoplasmosis is common worldwide, it's not reported that often in the UK. This may be because the symptoms of toxoplasmosis tend to be mild and general in otherwise healthy people, which may lead to a large proportion of cases going unnoticed.
Most people who get toxoplasmosis don't have symptoms. Around 10-15% of people develop symptoms similar to mild flu or glandular fever, such as a temperature, sore throat and muscle aches.
Toxoplasmosis is more serious in people with weakened immune systems, such as those who have had an organ transplant, those with HIV and AIDS, and those receiving certain types of chemotherapy treatment.
Ocular toxoplasmosis is a possible and serious complication of toxoplasmosis. The infection spreads to the eye, where ocular lesions (wounds) develop, which can lead to a partial or complete loss of vision in the affected eye.
Congenital toxoplasmosis is also serious. It occurs when a woman becomes infected during pregnancy and passes the infection on to her unborn baby. This can result in the baby developing serious health problems, such as brain damage and partial blindness.
How is toxoplasmosis spread?
The T. gondii parasite that causes toxoplasmosis is often found in the faeces of infected cats. Cats don't usually show any symptoms of toxoplasmosis, so you may not know whether your cat is infected.
Infected cats usually only excrete the parasite for a short period of time, usually two to three weeks after they're first infected.
If the T. gondii parasite gets into the environment or food chain, it can be ingested by humans. Infection can occur by:
consuming food, water or soil that's contaminated with infected cat's faeces
eating or handling raw, undercooked infected meat, such as pork, lamb or venison, or infected cured meat, such as salami
using knives and other utensils that have been in contact with undercooked or raw infected meat
drinking unpasteurised goats' milk or eating products made from it, such as cheese
Toxoplasmosis can't be passed from person to person, other than in rare cases, such as receiving an infected organ or blood products during an organ transplant, or if a newly infected mother passes the infection on to her unborn baby.
If you're infected with the T. gondii parasite, your immune system will start to produce antibodies (infection-fighting proteins). If toxoplasmosis is suspected, you'll have a blood test to check for antibodies.
If you're pregnant and tests confirm that you've had a recent toxoplasmosis infection, you'll need a further test to determine whether your unborn baby is also infected. Amniocentesis is the test most commonly used.
In otherwise healthy people, toxoplasmosis doesn't usually require treatment.
Medication is usually only prescribed to treat severe or prolonged cases of toxoplasmosis, particularly in people with a weakened immune system. Pyrimethamine plus sulfadiazine or azithromycin are the medications often prescribed.
Pregnant women infected with toxoplasmosis for the first time may be prescribed antibiotics. This aims to reduce the risk of the unborn baby becoming infected, and limit the severity of congenital toxoplasmosis if the baby does become infected.
There are a number of measures you can take to reduce your risk of developing toxoplasmosis, including:
wearing gloves while gardening, particularly when handling soil
not eating raw or undercooked meat
washing utensils and other kitchenware thoroughly after preparing raw meat
washing fruit and vegetables thoroughly before cooking and eating them
Coloured transmission electron micrograph (TEM) of Toxoplasma gondii parasites inside a human cell
Who is affected by toxoplasmosis?
Estimates suggest that up to a third of the UK population will acquire a toxoplasmosis infection at some point in their life. However, most people won't notice any symptoms.
Severe congenital toxoplasmosis – where a mother passes the infection on to her unborn baby, who then goes on to develop significant complications – is rare, with about 3 in every 100,000 babies born with the condition in the UK.
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Symptoms of toxoplasmosis
In most cases, toxoplasmosis doesn't cause any symptoms and a person is not aware they are infected.
In healthy children and adults, the immune system is usually strong enough to prevent the Toxoplasma gondii (T. gondii) parasite causing serious illness.
After a toxoplasmosis infection, most people are immune to further infection for the rest of their life. This means that if a woman who has previously been infected becomes pregnant, there will be no risk to her baby.
However, a blood test would be needed to check for immunity. You should also still take precautions if you're pregnant, such as wearing gloves while gardening or cleaning out your cat's litter tray.
In some cases, toxoplasmosis can cause the lymph nodes (glands that are part of your immune system) to swell, particularly in the throat or armpits. This can lead to flu-like symptoms, such as:
a high temperature (fever) of 38°C (100.4°F) or over
People who are otherwise healthy rarely experience any serious symptoms of toxoplasmosis.
The risk of getting toxoplasmosis when you're pregnant is low. For example, a 2008 study showed that in non-immune pregnant women (those who haven't had the infection before), about 5 in 1,000 may get a toxoplasmosis infection.
However, if you develop toxoplasmosis when you're pregnant or shortly before conceiving, there's a risk that you'll pass it on to your baby. If a baby gets toxoplasmosis from their mother, it's known as congenital toxoplasmosis.
Although you probably won't experience any symptoms of the infection yourself, an infection that develops during the early stages of pregnancy also increases the risk of:
miscarriage – the loss of a pregnancy during the first 23 weeks
stillbirth – where a baby is born after 24 weeks of pregnancy without any signs of life
The symptoms of congenital toxoplasmosis vary depending on when the mother becomes infected. The baby's symptoms will usually be more severe if the mother is infected around the time she became pregnant or during the first or second trimester (up to week 27 of the pregnancy).
Symptoms of toxoplasmosis in babies can include:
hydrocephalus – water on the brain
epilepsy – a condition that affects the brain and causes repeated seizures (fits)
jaundice – yellowing of the skin and the whites of the eyes
eye infections and reduced vision
an enlarged liver or spleen (the organ that filters impurities from your blood)
cerebral palsy – a brain and nervous system condition that affects a child's movement and co-ordination
If a mother becomes infected during the third trimester of pregnancy (from week 27 to birth) and the infection is passed on to her baby, the baby may not have any symptoms at birth.
However, complications may develop later in life. For example, months or years later a child born with congenital toxoplasmosis may develop reduced vision, hearing loss or learning difficulties..
People with immune deficiencies
Toxoplasmosis can be fatal for someone with a weakened immune system. This is because their body may not be able to fight off the infection.
Your immune system can be weakened if you:
have an illness that affects your immune system, such as HIV and AIDS or cancer
are receiving certain types of medication, such as chemotherapy (powerful cancer-killing medication)
are taking immunosuppressant medication – for example, after having an organ transplant
There's also a risk of permanent eye or brain damage when toxoplasmosis infects someone with a weakened immune system.
If toxoplasmosis begins to affect the brain, it can cause encephalitis. This is sometimes called toxoplasmosis encephalitis (TE) and it can be fatal in people with AIDS.
Signs and symptoms of TE and toxoplasmosis in people with immune deficiency include:
coughing up blood
a high temperature (fever) of 38°C (100.4°F) or over
vision problems, such as blurred vision and floaters (small pieces of debris that can cloud your vision)
multiple lesions (wounds) in the brain
Causes of toxoplasmosis
The toxoplasmosis infection is caused by the Toxoplasma gondii (T. gondii) parasite.
The T. gondii parasite can be found in the faeces of infected cats and the meat of infected animals.
T. gondii can reproduce inside a cat's bowel. An infected cat can pass the parasite in its faeces for the following few weeks. The cat won't usually have any symptoms, so you may not know it's infected.
Infection via the food chain and environment
There are four ways the T. gondii parasite can enter the food chain and cause a toxoplasmosis infection. You can become infected by:
ingesting food, such as fruit and vegetables, water or soil contaminated with infected cat's faeces
eating or handling raw or undercooked infected meat, usually pork, lamb or venison (grazing animals can become infected after eating contaminated grass or animal feed)
using knives, cutting boards and other utensils that have been in contact with contaminated, undercooked or raw meat
eating or drinking infected unpasteurised goats' milk or products made from it, such as cheese
There's also a small risk of the toxoplasmosis infection being passed from sheep during the lambing season. This is because the T. gondii parasite is sometimes found in the afterbirth and on newborn lambs after an infected sheep has given birth.
Toxoplasmosis can't be passed on through person-to-person contact. This means that:
you can't catch toxoplasmosis from coming into contact with an infected person
you can't pass toxoplasmosis on to your children if you have previously had the infection
it's not possible to pass the infection on through breastfeeding
However, in rare cases, people have developed toxoplasmosis from an infected organ transplant or blood transfusion.
Congenital toxoplasmosis is where a baby is born with toxoplasmosis. The mother passes the infection to her baby through the placenta (the organ that links the mother's blood supply to her unborn baby's).
However, congenital toxoplasmosis can only occur if the mother becomes infected for the first time, either while she's pregnant or shortly before conceiving.
Congenital toxoplasmosis is more likely to occur if a woman becomes infected later in her pregnancy. For example, if you become infected around the time of conception, there's a less than 5% chance that your baby will also develop the infection.
However, if you become infected during the third trimester of your pregnancy (from week 27 until the birth), there's about a 65% chance your baby will also be infected.
One study calculated that for every week further along the pregnancy that the mother was infected, the likelihood of the toxoplasmosis infection being passed on to her baby increased by 12%.
Coming into contact with the faeces of a cat that has been infected with the T. gondii parasite can cause toxoplasmosis
If you're infected with the Toxoplasma gondii (T. gondii) parasite, your immune system will start producing antibodies to fight it.
Antibodies are proteins that neutralise or destroy invaders such as bacteria or parasites. If toxoplasmosis is suspected, you'll have a blood test to check for these antibodies.
Early testing can sometimes produce a negative result when a person is actually infected. Known as a false-negative, it occurs when the body hasn't had time to start producing antibodies to the parasite, which usually takes up to 23 days after the initial infection. However, a person who has developed symptoms of toxoplasmosis will nearly always have a positive blood test.
If you're symptom-free but concerned, your GP may recommend another blood test two to three weeks later. If the result is still negative, it's unlikely you have the toxoplasmosis infection.
A positive result doesn't necessarily mean you have an active toxoplasmosis infection. After having toxoplasmosis, the antibodies will remain in your blood for the rest of your life, protecting you from being infected again by the parasite. A positive result usually indicates you've been infected at some point in the past.
You will need more blood tests to find out whether you have a new infection. Checking the levels of antibodies in your blood a second time can help determine when the infection occurred. For example, if the levels of antibodies are:
increasing – suggests a new active infection (perhaps a few weeks old)
still the same – suggests an old infection and that you are now immune
decreasing – suggests a recent infection that is no longer active (perhaps a few months old)
Specialised testing is important if you're pregnant or have a weakened immune system.
If you are pregnant and tests confirm you have a recent toxoplasmosis infection, you'll need a further test to determine whether your unborn baby is also infected.
Amniocentesis is the most effective and commonly used test and can be carried out any time from 15 weeks of pregnancy onwards. It involves inserting a fine needle through the mother's abdomen to collect a sample of amniotic fluid (the fluid surrounding the foetus in the womb). The sample will be tested for toxoplasmosis.
Amniocentesis carries around a 1% risk of causing a miscarriage. The procedure usually takes 10 to 30 minutes, and you may find it slightly uncomfortable.
Amniocentesis can confirm whether your baby has congenital toxoplasmosis. However, it can't determine whether the infection has caused any damage to your unborn baby or, if it has, how much.
If your unborn baby has congenital toxoplasmosis, treatment with medication will be started as soon as possible. After your baby is born, he or she will be examined for any signs of damage from the infection and treatment may continue.
If necessary, your baby will continue to have blood tests for up to a year, or possibly longer, until the test results for toxoplasmosis antibodies are negative. When results indicate there are no antibodies, your baby has been shown to be infection free.
If you have a weakened immune system (because of chemotherapy, for example), routine blood tests for antibodies can produce a false-negative result. This is because it's possible your immune system won't produce antibodies to fight the infection.
Your doctor may request the following tests:
computerised tomography (CT) scan – where several X-rays are taken at slightly different angles and assembled by a computer to produce a clear, three-dimensional image of the inside of your brain
magnetic resonance imaging (MRI) scan – where a strong magnetic field and radio waves are used to produce detailed images of the inside of your brain
The test results will reveal if you have any lesions (wounds) in your brain, which occur in some cases of toxoplasmosis.
New research is looking at other ways of diagnosing toxoplasmosis – for example, by identifying the DNA of the T. gondii parasite in samples of cerebrospinal fluid (the fluid that surrounds your brain and spinal cord).
It's hoped that this may be a quicker and more reliable method of diagnosis, and will help confirm whether the damage to the brain has been caused by toxoplasmosis rather than by another condition.
Most cases of toxoplasmosis are mild and don't require treatment. Either no symptoms develop, or a full recovery is made without complications.
If you're diagnosed with toxoplasmosis, your GP will recommend the most appropriate treatment for you. This will depend on your health and symptoms.
If you have severe symptoms, you may be prescribed a combination of pyrimethamine and sulfadiazine, or a medication called azithromycin, which is given on its own. Folinic acid supplements may also be recommended.
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 in between.
If you have HIV or AIDS, you may need medication until anti-HIV therapy has restored your immune function.
If you're pregnant and develop toxoplasmosis for the first time, you may be treated with an antibiotic called spiramycin.
Spiramycin may reduce the risk of your unborn baby becoming infected and limit the severity of congenital toxoplasmosis if your baby does become infected.
There's some evidence to suggest that the earlier treatment starts, the lower the risk of congenital toxoplasmosis. However, in most cases, whether or not your baby becomes infected will depend on when you were infected.
Overall, about 30-40% of mothers who have a toxoplasmosis infection during pregnancy give birth to a baby with congenital toxoplasmosis.
Antibiotic treatments aren't always effective at preventing the damage caused by congenital toxoplasmosis in the unborn baby. Sulfadiazine plus pyrimethamine is usually prescribed in cases where tests show the baby has become infected in the womb (congenital infection).
Other antibiotics are being investigated, but until more research has been carried out, sulfadiazine and pyrimethamine remain the most effective treatment for toxoplasmosis.
If your baby is born with congenital toxoplasmosis, he or she will be examined to see whether the infection has caused any damage. Your baby will have the following tests:
blood test – blood will be taken either from a vein in your baby's arm or by pricking the skin of their heel, and the sample will be tested for toxoplasmosis antibodies
eye examination – an ophthalmologist (an eye specialist) will examine your baby's eyes to check for any lesions (wounds) at the back of the eye affecting the retina choroid
an X-ray or scan – may be taken of your baby's head to check for any brain damage
Congenital toxoplasmosis is usually treated with antibiotics. These will probably be a combination of pyrimethamine and sulfadiazine. These drugs have been shown to be effective for treating moderately and severely affected babies. One study found 72% of babies with moderate or severe congenital toxoplasmosis had normal intelligence and motor function by their early teenage years.
Treatment with these medications may continue for up to a year. Unfortunately, some babies with congenital toxoplasmosis develop long-term disabilities because it's not possible for antibiotics to undo the damage already sustained. It's also possible for eye infections to recur at a later stage of childhood.
Complications of toxoplasmosis
A toxoplasmosis infection can sometimes spread to the eyes (ocular toxoplasmosis).
The Toxoplasma gondii (T. gondii) parasite, which causes toxoplasmosis, can lie dormant (inactive) in the retina for many years. But if the parasite becomes active again, it can start a new infection.
Ocular toxoplasmosis causes ocular lesions, which are wounds in the eyes caused by inflammation and scarring. These can appear in the:
retina – the light-sensitive layer of tissue at the back of the eye
choroid – the layer behind the retina that contains major blood vessels
This damage to the eyes is called retinochoroiditis and can cause eye problems, such as:
a partial loss of eyesight in one eye
squint – where one eye looks in a slightly different direction to the other one
clouding of the eye's lens (cataracts)
eye shrinking (microphthalmia)
loss of cells and tissue from the optic nerve, which connects the eye to the brain, resulting in poor vision (optic atrophy)
Antibiotics and steroids are often used to treat the lesions. The scarring caused by toxoplasmosis will not clear up, but treatment may prevent it from getting worse.
If the infection keeps returning, antibiotics may be prescribed on a long-term basis. While this may help prevent the infection recurring, the long-term side effects of these medications are not yet known.
In most cases, babies born with congenital toxoplasmosis develop normally after treatment with antibiotics.
However, in up to 4% of cases, serious complications can develop within the first years of life. These include:
permanent visual impairment (partial or, very rarely, complete sight loss)
permanent brain damage
Retinochoroiditis is a common complication of congenital toxoplasmosis.
One study found that 18% of children with congenital toxoplasmosis had at least one ocular lesion as a result of retinochoroiditis. Of these, 42% developed a second ocular lesion later in life.
Cases of ocular toxoplasmosis can also occur years after infection. One study found the average age at which it appeared was nine years old.
It's also possible for someone to develop complications in their 20s or 30s. These may include:
Further research is needed to follow-up cases of congenital toxoplasmosis. Regular eye tests may help detect any abnormalities as they develop, and treatment with antibiotics can limit damage toxoplasmosis causes. However, at present it's not possible to reverse damage already done.
There are a number of measures you can take to help reduce your risk of developing a toxoplasmosis infection.
For example, you should:
wear gloves when gardening, particularly when handling soil – wash your hands thoroughly afterwards with soap and hot water
avoid eating raw or undercooked meat, particularly lamb, pork and venison, including any ready-prepared chilled meals
wash all kitchenware thoroughly after preparing raw meat
always wash fruit and vegetables before cooking and eating them, unless they are labelled as 'ready to eat'
avoid drinking unpasteurised goats' milk or eating products made from it
avoid handling or adopting stray cats
avoid cat faeces in cat litter or soil – wear gloves when changing a cat's litter tray and wash your hands thoroughly afterwards; if you're pregnant or immune deficient, ask someone else to change it for you
give your cat dried or canned cat food rather than raw meat
It's very important that pregnant women and those with a weakened immune system follow this advice to avoid becoming infected.
If you're pregnant, you should also avoid coming into contact with sheep and newborn lambs during the lambing season as there's a small risk that an infected sheep or lamb could pass the infection on at this time.