Toxoplasmosis
Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii (T. gondii). It occurs worldwide and affects a wide range of warm-blooded animals, including humans.
Key facts
About
Why it matters
Toxoplasmosis is one of the most common causes of abortion in sheep and is responsible for reproductive and production losses in sheep. Toxoplasmosis can also affect other livestock such as goats and pigs, as well as humans. In sheep, toxoplasmosis causes around a quarter of diagnosed abortions each year.
Abortions and barren ewes can cause many issues for farmers. Farm profitability can be impacted, as a lower lamb crop means fewer lambs to sell or to choose replacements from.
Species affected
- Sheep
- Goats
- Pigs
- Other livestock
Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii (T. gondii). It occurs worldwide and affects a wide range of warm blooded animals, including humans. Most people infected with toxoplasmosis do not develop symptoms, but the infection can cause serious health problems in certain groups, particularly during pregnancy or in people with weakened immune systems.
Sexual reproduction of the parasite only occurs in the definitive host (felids) while asexual reproduction can occur in all warm-blooded intermediate hosts. Following ingestion of tissue cysts by the cat the cyst wall is broken down in the gut releasing bradyzoites. The bradyzoites replicate asexually within the epithelial cells of the small intestine and form gametes. Two days after ingestion by the felid host sexual reproduction begins. The female macrogamete is fertilized by the male microgamete forming a zygote which develops into an oocyst containing two sporoblasts.
The unsporulated oocyst is released from the epithelial cell and shed in the faeces along with thousands of other oocysts. Once in the environment the sporoblasts within the oocyst sporulate producing four sporozoites each, oocysts are only infective once they have successfully sporulated. This process takes around 1 – 5 days but is dependent on the right environmental conditions (11-25°C, aerobic conditions with adequate humidity). Sporulated oocysts are resistant to environmental conditions and in a mild moist climate they can survive many months in soil or water.
Intermediate hosts can become infected by either ingesting oocysts (from the environment) or tissue cysts (in undercooked or raw meat). Sporozoites are released from the oocysts following ingestion, the sporozoites invade the epithelial cells and transform into rapidly multiplying tachyzoites. In a similar way when tissue cysts are ingested the cyst walls are broken down to release bradyzoites which invade the host cells and transform into tachyzoites.
In both circumstances the tachyzoites replicate asexually within the host cells, the number of tachyzoites eventually causes the host cells to burst releasing the tachyzoites which then go on to infect neighbouring cells. Tachyzoites can also be disseminated via the bloodstream and invade other tissues developing into bradyzoites. The bradyzoites are enveloped by a cyst wall forming a tissue cyst which remains within the tissues throughout the lifetime of the host.
Lifecycle summary
- Horizontal transmission: ingestion of oocysts consumed from the environment
- Horizontal transmission: ingestion of tissue cysts (bradyzoites) from raw/undercooked infected meat
- Vertical transmission: tachyzoites passed to the unborn foetus via the placenta
Transmission
Transmission
Cats
Shed millions of tough-shelled oocysts in their faeces, durable in temperate, moist conditions.
Feed contamination
Cat faeces can contaminate feed, fodder, and bedding materials.
Water
Cat faeces can contaminate feed, fodder, and bedding materials.
Environmental survival
Oocysts survive for long periods once shed into the environment.
Cat Contact
Cleaning litter trays without washing hands afterwards.
Soil & produce
Gardening in contaminated soil, or eating unwashed fruit and vegetables.
Food & drink
Undercooked meat, cured meat, undercooked shellfish, raw goat's milk.
Congenital
Passed from mother to unborn baby during pregnancy.
Clinical Signs
Clinical Signs
Early pregnancy
Infection is likely to result in resorption of the foetus, with the ewe returning to service or remaining empty
Mid-pregnancy
Foetuses are often mummified or aborted a few days before the expected lambing date
Late pregnancy
Infection may not be fatal to the foetus, and lambs that survive are born live but may be weak, many dying shortly after birth
Non-pregnant or male sheep
Infection is usually not noticed. Some sheep may have a mild temperature or lack of appetite for a short time.
Healthy people
If you are otherwise healthy, it is unlikely that you will know you have become infected. Most healthy people experience no symptoms; some experience mild flu-like symptoms such as fatigue, muscular aches, enlarged lymph nodes, headache, or high temperature.
People with weakened immune systems
More serious symptoms can occur in people with weakened immune systems, such as those with HIV infection or undergoing chemotherapy. If infected before becoming immunocompromised, the parasite can reactivate and cause disease.
During pregnancy:
If you are already infected before becoming pregnant, there is no risk to your unborn child. If infection occurs shortly before or during early pregnancy, it can be passed to the unborn child. The earlier in pregnancy infection occurs, the more severe the outcome, potentially miscarriage, stillbirth, or a child born with an unusually large or small head. Babies born with the infection may show no symptoms at birth but can go on to develop cognitive disabilities, vision or hearing problems, or seizures.
Other countries:
In some parts of the world the strain of T. gondii is more harmful than the types usually found in the UK and Europe, and can cause eye disease with damage to the retina and vision loss.
Diagnosis
Diagnosis
What farmers might notice
You cannot diagnose toxoplasmosis from clinical signs alone, but you may notice placentas with small white, chalky areas in the cotyledons (buttons). These are caused by necrosis and calcification in areas of the placenta due to damage from the parasite multiplying.
Lab confirmation
Confirmation must be done by a vet. Collect freshly aborted lambs and placentas, if possible into separate bags, and speak with your vet to arrange testing.
When to call a vet
If more than 2% of your ewes are barren or abort in the lambing season, it’s advisable to call your vet. They can screen blood samples from a small group of ewes to determine whether toxoplasmosis, or another infectious disease such as enzootic abortion (EAE), has been involved.
Clinical Signs of Toxoplasmosis Abortion
One clinical sign that is characteristic of Toxoplasmosis abortion is the development of small, white chalky areas, 1–3 mm in diameter, in the cotyledons of the placenta. These are caused by the necrosis of the placenta during the parasite multiplication.
Diagnosis
The clinical signs of Toxoplasmosis are often indicative of infection, but it is important to obtain a definitive diagnosis.
Testing Foetal Fluid
Foetal fluid (from the thorax, pericardium or unclotted blood is best) can be tested for T. gondii antibodies. Testing serum from ewes is not recommended as a diagnostic as T. gondii is ubiquitous in the environment and a positive result does not indicate the cause of abortion. The absence of T. gondii antibodies in foetal fluids does not exclude T. gondii as the causative agent because the development of foetal antibodies is dependant on the age of the foetus at the time of infection and the time between infection and examination.
Examination of the Foetus
Examination of the foetus itself is not useful for diagnosis of T. gondii but may be required to identify other causative agents. Lesions are found more frequently in the brain of affected foetuses than in other tissues, occasionally T. gondii can be recognised from impression smears from placenta, or foetal tissues such as lungs and brain.
Prevention
Prevention and Control
Vaccination
There is no way to cure sheep once infected with T. gondii, but it's worth keeping sheep that have aborted due to toxoplasmosis, as they are now immune and will not abort future pregnancies due to Toxoplasma. The gold standard for prevention and control is vaccination, though difficulties in obtaining the vaccine can make this hard. The vaccine available in the UK provides life-long immunity (only one vaccination per ewe needed), so each year it's only necessary to vaccinate replacements. Non-pregnant females should be vaccinated up to three weeks prior to tupping. It is a live vaccine; pregnant women and immune-compromised people should not administer it.
Feed storage & cat management
Keep feed bins and stores covered to prevent cats defecating in them, and keep cats away from fodder and bedding. Cats are the definitive host of the parasite, and older, neutered cats are preferable to kittens for rodent control on farm.
Biosecurity
Although T. gondii cannot pass from one sheep to another, isolate any aborted ewes and dispose of the placenta, dead lambs, and soiled bedding appropriately. Disinfect the lambing pen. At the point of abortion the cause cannot be determined, so each case should be treated as though it could spread throughout the flock.
Food safety
You can reduce the risk of becoming infected by freezing meat before cooking (freezing kills the parasite, even in rare meat), cooking meat thoroughly until it reaches an internal temperature of 75°C or above, not eating raw shellfish, and washing fruit and vegetables, including pre-washed salad, before eating.
Reduce risk from cats
Avoid cleaning litter trays if pregnant, or wear gloves and wash hands thoroughly if you must; clean litter trays daily, since the parasite is not immediately infectious; feed cats commercially available cat food rather than raw meat; and consider keeping cats indoors to prevent hunting.
ToxoVax
A live vaccine ToxoVax is available commercially in some regions (Europe and New Zealand) for reducing the effect of infections by T. gondii and is known to protect for at least two lambing seasons from a single vaccination.
Protective Immunity
The vaccine consists of a modified strain of T. gondii known as S48 which has lost its ability to form tissue cysts or oocysts following several years of passage in mice. Following inoculation of sheep the S48 tachyzoites multiply, the host immune system produces CD4 and CD8 T cells along with IFN-γ
Public Health
Am I at risk?
Toxoplasmosis does not usually cause serious illness, and most people show no signs of infection. If you are pregnant or have a weakened immune system, you are at greater risk of more severe infection than if you were healthy. In pregnancy, it is not the pregnant woman at risk but the unborn child. Speak to your GP if you’re worried about your risk.
History
Timeline
Downloads & links
Related Resources
Video
Tackling Toxoplasmosis Animation
Key resource
Factsheets
Control of Toxoplasma Abortion in Sheep
Published 31/05/2019Who's involved
Our Experts
Frequently asked questions
FAQ
A common parasitic infection caused by the protozoan parasite Toxoplasma gondii. Many people have no symptoms when they become infected, but it can cause serious illness during pregnancy or in people with weakened immune systems.
People can become infected by:
- Eating undercooked or raw meat
- Handling contaminated soil (e.g. gardening)
- Eating unwashed fruit or vegetables
- Contact with cat faeces (for example, cleaning litter trays)
- Infection passed from mother to unborn baby
Many people have been exposed to Toxoplasma gondii at some point in their lives, symptoms are often mild so people are not aware that they have been exposed. Studies looking at blood samples estimate that around 18% of the population have been exposed to Toxoplasma (Eurosurveillance | Systematic review and modelling of Toxoplasma gondii seroprevalence in humans, Europe, 2000 to 2021). The seroprevalence (number of people in a population who test positive for a specific disease) in the UK and Northern Europe is much lower than in other European countries where the seroprevalence is around 40 – 50%.
Most healthy people have no symptoms. When symptoms do occur, they may include:
- Mild flu-like illness
- Swollen lymph nodes
- Muscle aches
- Fatigue
More severe illness can occur in pregnancy or in people with weakened immune systems. In other part of the world, such as South America, Toxoplasma gondii can cause more serious illness and can affect the eyes.
If you have severe symptoms such as blurred vison then you should contact NHS24 or your GP for advice.
For most healthy people, toxoplasmosis is not dangerous. However, it can cause serious health problems:
- During pregnancy, it can harm the unborn baby
- In people with weakened immune systems, it can affect the brain, eyes, or other organs
Yes, but the risk is often misunderstood. Cats can shed the parasite in their faeces for a short period, usually after first becoming infected as a young kitten. Good hygiene and simple precautions greatly reduce the risk. It is important to understand that you cannot become infected with Toxoplasmosis from touching or stroking a cat or having a cat in your house.
You can reduce your risk by:
- Cooking meat thoroughly (especially lamb, pork and venison)
- Avoiding eating raw/undercooked meat or cured meats
- Washing hands after handling raw meat or soil
- Washing fruit and vegetables
- Wearing gloves when gardening
- Avoiding handling cat litter where possible, especially during pregnancy. If unavoidable then wear gloves and washing your hands afterwards
If a woman becomes infected for the first time during pregnancy, the infection can be passed to the unborn baby and may cause serious health problems. If you are infected before you get pregnant then there is no risk to your baby. If you are infected in early pregnancy, it can increase your risk of miscarriage and stillbirth, later in pregnancy becoming infected may cause serious health problems for your baby.
The chances of becoming infected for the first time during pregnancy is low.
No. Pregnant women do not need to avoid cats, but they should avoid handling cat litter and take extra hygiene precautions. You cannot become infected with Toxoplasmosis from touching or stroking a cat or having a cat in your house.
Routine screening for toxoplasmosis is not currently offered as part of NHS antenatal care in the UK. Testing may be carried out if there is a specific clinical concern. If you think you have been exposed to Toxoplasmosis during pregnancy you should speak to your GP or midwife for advice.
