Toxoplasmosis is a common parasitic infection caused by Toxoplasma gondii, a single-celled parasite that mainly affects felines as its definitive hosts, but can also infect all warm-blooded vertebrates, including humans. Often benign and asymptomatic, this infection can nevertheless lead to serious complications in immunocompromised individuals and pregnant women.
What infectious agent is responsible?
Toxoplasma gondii (T. gondii) is an obligate intracellular parasite of the class Coccidia and the phylum Apicomplexa. This unicellular haploid eukaryotic parasite has a distinctive bow-shaped morphology. It measures around 8 micrometres long and 3 micrometres wide. It has specific organelles such as the apical complex, which is crucial for host cell invasion.
T. gondii modulates its host’s cellular mechanisms to promote survival and proliferation. Autonomous in the synthesis and transport of proteins, lipids and ATP, it depends on the host cell for certain essential nutrients that it cannot synthesise itself. The exact mechanisms of this import remain partially elucidated, but are vital to its survival.
The life cycle of T. gondii is complex and comprises several stages:
- Tachyzoite: Proliferative and infectious form in the intermediate host, developing rapidly in transitory vacuoles.
- Bradyzoite: Slow cystic form, contained in resistant intracellular cysts that can measure around 100 micrometres in diameter.
- Merozoite: Stage of sexual reproduction in the definitive host (felines), unique to this cycle.
- Sporozoite: Infectious form released into the environment via feline faeces, contained in oocysts 10 to 15 micrometres in diameter.
The oocysts are resistant to adverse environmental conditions. They can survive for several months in soil or water. They are also resistant to variations in temperature, pH and common disinfectants.
The T. gondii cycle alternates between intermediate and definitive hosts. Felines, the definitive hosts, play a crucial role in dissemination. Cats ingest infected prey, releasing bradyzoites that infect intestinal cells. These transform into merozoites and undergo sexual reproduction to form oocysts excreted in the faeces. These become infectious after sporulation. These oocysts can then infect other animals and humans.
How does this disease manifest itself in animals?
Toxoplasmosis in animals varies depending on the species and the immune status of the host. Felines, particularly domestic cats, play a crucial role as the definitive hosts of Toxoplasma gondii. They become infected by ingesting infected prey such as rodents, birds or other small animals. They then excrete the oocysts in their faeces, contaminating the environment.
In cats, the symptoms of toxoplasmosis are often absent or very discreet. However, some cats may show clinical signs such as swollen glands, fever, temporary fatigue and diarrhoea. A distinctive sign of feline toxoplasmosis isinflammation of the eyes. This can take the form of uveitis or retinitis.
The period during which a cat is likely to contaminate its environment is relatively short, around three weeks after the initial infection. Once this period has passed, the cat develops immunity to the parasite and no longer becomes an active source of contamination.
Young cats and those with weakened immune systems are at greater risk of developing serious symptoms. In other animals, toxoplasmosis can cause various symptoms depending on the organ affected. For example, farm animals such as sheep and goats can suffer abortions if primo-infected during gestation. In young animals, symptoms such as fever, nervous disorders, pneumonia and digestive problems can occur.
The T. gondii parasite cycle is spread worldwide. The parasite persists in the form of cysts in the tissues of many vertebrate animals, mainly in the muscles and brain. Felines ingest these cysts by eating infected prey, thus completing the parasite’s life cycle.
How is it transmitted?
Toxoplasmosis is mainly transmitted orally. Toxoplasma gondii can infect a wide variety of intermediate hosts, including humans, via ingestion of tissue cysts present in infected meat or sporulated oocysts present in the environment. In France, around 30% of the population is seropositive for toxoplasmosis, indicating previous exposure to the parasite.
The main sources of contamination are as follows:
- Ingestion of contaminated food: Oocysts excreted by felines can contaminate fruit, vegetables and other foods that come into contact with soil orcontaminated water. These oocysts become infectious after sporulation in the environment.
- Consumption of raw or undercooked meat or offal: Gondii tissue cysts are present in the muscles of many farm animals, particularly pigs and sheep. Undercooking meat can lead to infection.
- Handling contaminated products: The parasite can be transmitted by dirty hands after handling soil, vegetables, raw meat or objects contaminated with cat excrement.
Certain occupations present an increased risk of transmitting toxoplasmosis. For example, vets, farmers, slaughterhouse workers and gardeners are more likely to be exposed to the parasite because of their regular contact with infected animals or potentially contaminated products.
Immunocompromised people and pregnant women are particularly vulnerable to serious complications from toxoplasmosis. In pregnant women, the parasite can cross the placental barrier and infect the foetus. This can have serious consequences, such as congenital malformations or spontaneous abortions. It is therefore crucial to follow strict hygiene measures and take specific precautions to avoid infection during pregnancy.
What are the symptoms of this infection in humans?
In humans, toxoplasmosis is often asymptomatic, particularly in healthy people. However, when symptoms do appear, they may vary in intensity and nature. The incubation period for toxoplasmosis is generally five to ten days after infection with the parasite.
In over 80% of cases, toxoplasmosis goes unnoticed. When it does appear, symptoms may include :
- Moderate fever (below 38°C)
- Prolonged fatigue (lasting several weeks or months)
- Swollen lymph nodes (especially in the neck and at the base of the skull)
- Skin rash (small pinkish pimples all over the body)
- Headaches
- Pain in joints and muscles
In immunocompromised people, toxoplasmosis can lead to serious complications, including brain abscesses. These patients may present specific symptoms such as severe and persistent headaches, epileptic seizures, high fever (over 38°C), and motor difficulties or partial paralysis.
Congenital toxoplasmosis is a particularly worrying form of the infection. Transmitted from mother to foetus, it can lead to serious malformations, spontaneous abortions and neurological complications in the newborn. The earlier the infection occurs during pregnancy, the more severe the consequences can be.
Symptoms in newborns can include :
- Jaundice
- Enlarged spleen and liver
- Convulsions
- Chorioretinitis (inflammation of the choroid and retina, which can lead to reduced visual acuity)
After the initial infection, Toxoplasma gondii can remain in the body in the form of cysts, mainly in nerve and muscle tissue. These cysts can reactivate in the event of immunosuppression, leading to relapses of the disease.
How is the disease diagnosed?
Diagnosis of toxoplasmosis is based mainly on serological tests. These detect the presence of specific antibodies to Toxoplasma gondii in the blood. The main serological tests used include indirect immunofluorescence (IFI) and enzyme-linked immunosorbent assays for IgG and IgM antibodies.
IgM antibodies generally appear within the first two weeks following acute infection. They peak in 4 to 8 weeks and then become undetectable. However, they can persist for up to 18 months after the acute infection. IgG antibodies appear more slowly. They reach a peak titre in 1 to 2 months and may remain elevated for months or years.
For pregnant women, an IgG avidity test can be carried out. High IgG avidity during the first 12 to 16 weeks of pregnancy essentially rules out an infection contracted during gestation. Low avidity, on the other hand, may indicate a recent infection, but requires further testing for confirmation.
If central nervous system toxoplasmosis is suspected, doctors use imaging tests such as CT (computed tomography ) orMRI (magnetic resonance imaging ) to detect characteristic brain lesions. Specialists may also perform a lumbar puncture to analyse the cerebrospinal fluid.
Tests based on Polymerase Chain Reaction (PCR) can detect T. gondii DNA in blood, cerebrospinal fluid, tissue or amniotic fluid during pregnancy. PCR analysis of amniotic fluid is the method of choice for diagnosing toxoplasmosis during pregnancy.
Detection of specific IgM antibodies in newborns suggests congenital infection. Detection of specific IgA antibodies, which is more sensitive, is less commonly used. If you suspect congenital infection, consult a toxoplasmosis expert.
What is the appropriate treatment?
Treatment of toxoplasmosis depends on the severity of the infection and the patient’s immune status. Asymptomatic immunocompetent individuals or those with a mild infection generally do not require treatment. However, serious or persistent infections require medical intervention.
Common treatments include the use of combinations of antiparasitics and antibiotics:
- Pyrimethamine and sulphadiazine: This combination is often combined with folic acid (leucovorin) to prevent side effects on bone marrow. The typical dosage for adults is 50 mg of pyrimethamine twice a day for two days, then once a day, and 1 g of sulphadiazine four times a day. Folinic acid is administered simultaneously to protect against bone marrow suppression.
- Spiramycin: Used mainly in pregnant women to reduce the risk of transmission to the foetus. Spiramycin is less active than pyrimethamine but is preferred during the first trimesters of pregnancy.
- Clindamycin or atovaquone: Alternatives in cases of allergy to sulfa drugs. Clindamycin is administered at a dose of 600 mg three times a day, combined with pyrimethamine and leucovorin.
For immunocompromised patients, higher doses of pyrimethamine and sulphadiazine are used, and maintenance treatment is necessary to prevent relapses. Patients with cerebral or ocular toxoplasmosis require specific management and often prolonged treatment.
In infants with congenital toxoplasmosis, treatment with pyrimethamine and sulphonamide is prescribed for one year. This is accompanied by regular medical monitoring to detect and treat any complications. The typical dosage for infants is 1 mg/kg of pyrimethamine twice a day for two days, then once a day, combined with 50 mg/kg of sulphadiazine twice a day.
What preventive measures are available?
The prevention of toxoplasmosis relies on strict hygiene measures and safe eating practices to reduce the risk of contamination by Toxoplasma gondii. Here are a few recommendations for avoiding infection:
Individual prevention
- Wash vegetables and fruit thoroughly before eating to eliminate any oocysts that may be present on their surface.
- Cook meat to an internal temperature of at least 67°C to kill T. gondii cysts present in muscle tissue.
- Wash your hands regularly with soap and water after handling soil, vegetables or raw meat, and before preparing or eating food.
- Avoid handling cat litter if you are pregnant or immunocompromised, or wear gloves and wash your hands after handling litter.
- Do not eat raw or undercookedmeat. Freezing meat at -12°C for three days can also eliminate the parasite.
Action at farm level
- Report and investigate suspected abortions on cattle, sheep and goat farms to identify and control toxoplasmosis.
- Isolate aborted females and destroy abortion products to prevent the parasite from spreading.
- Store foodstuffs away from cats and pests to avoid contamination by oocysts.
Collective measures
- Clean surfaces in contact with animals or raw meatdaily with soapy water, as oocysts are resistant to many disinfectants, including bleach.
- Autoclave or heat to 70°C any utensils used to clean feline cages or pens.
- Provide appropriate means of hygiene (drinking water, soap, single-use wipes) in workplaces at risk.
Prophylaxis
- Wear protective gloves and safety glasses when handling products likely to be contaminated.
- Train and inform workers about the risks of toxoplasmosis and preventive measures, particularly for pregnant women.
Preventing toxoplasmosis involves a combination of personal hygiene practices, farm management and collective measures to minimise exposure to the parasite. Particular attention must be paid to people at risk, such as pregnant women and the immunocompromised, to prevent the serious complications associated with this infection.
Some epidemiological data…
Toxoplasmosis is one of the most widespread parasitic diseases in the world. In France, around 30% of the population is seropositive for Toxoplasma gondii. Prevalence is higher among older women and people living in rural areas. Seroprevalence has fallen over the years, from 54% in 1995 to 37% in 2010. This is attributed in part to improved food hygiene practices and awareness-raising.
A specific system for monitoring congenital toxoplasmosis has been in place in France since 2007. In 2018, there were 151 cases of congenital toxoplasmosis, including 108 babies born without symptoms, 12 with symptoms, and 3 deaths in utero. Surveillance is used to monitor the impact of prevention and screening measures.
Congenital toxoplasmosis is rare but serious, with around 3 cases per 10,000 pregnancies in France. Infections are more frequent at the end of pregnancy, but the consequences are more severe at the beginning. Serological screening of pregnant women is compulsory in France, with around 2,700 infections acquired each year.
Worldwide, up to a third of the population is thought to be infected with Toxoplasma gondii, with prevalence rates varying from region to region. In Western Europe, prevalence varies from 50% to 70%, while it is less than 30% in Scandinavian countries and the UK. In Asia and America, prevalence is generally lower. However, it can reach high levels in certain humid regions of Africa.
Toxoplasmosis is usually benign and goes unnoticed. However, it poses a serious threat to immunocompromised individuals and pregnant women. Severe forms of the disease are mainly observed in HIV/AIDS patients and in newborns infected through maternal-foetal transmission.
