Toxoplasma gondii (T. gondii) is a single-celled (protozoan) parasite that causes the infection toxoplasmosis. In most individuals, the infection doesn't produce symptoms because a healthy person's immune system usually keeps the parasite from causing illness. However, when symptoms do occur they can be serious and sometimes fatal. Foetuses and individuals who have compromised immune systems are particularly at risk.
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Life Cycle
 Background Information
Domestic and wild cats are the only definitive (primary) hosts for T. gondii, making them the main source for human toxoplasmosis. However, virtually all vertebrate animals (i.e. those having a backbone) can be infected by the oocysts (eggs) passed by cats in their faeces, and thereby act as intermediate hosts. This can include sheep, pigs and cattle.
Life Cycle
T. gondii has three stages in its life cycle - the oocyst, the tachyzoite, and the tissue cyst, as follows:
- 1. The parasite reproduces in the cat's intestine to form oocysts that are excreted in faeces. Oocysts become infective 1 - 5 days later and may survive for over a year in soil or water.
- 2. Infection in intermediate hosts (arising primarily from exposure through ingestion) is followed by the development of tachyzoites. When freed in the intestines, they pass through intestinal walls and multiply rapidly in many tissues and organs of the body, giving rise to clinical symptoms.
- 3. The host's immune responses lead to the formation of tissue cysts that contain viable organisms and lead to (for the most part) permanent host immunity. However, the tissue cysts may reactivate if the immune system becomes compromised e.g. individuals with HIV/AIDS, those taking certain types of chemotherapy, and those who have recently received an organ transplant.
Humans can become infected with T. gondii by four routes:
- Accidentally swallowing oocysts from soil or water contaminated with cat faeces from a Toxoplasma-infected cat that is shedding the organism in its faeces. Infection by this route may arise, for example, from accidentally touch your hands to your mouth after gardening, cleaning a cat's litter box, or touching anything that has come into contact with cat faeces.
- Ingesting viable tissue cysts in raw or undercooked meat (especially pork, lamb, or venison) or tachyzoites in milk from infected intermediate hosts. Note that infection from this route can include touching your hands to your mouth after handling undercooked meat, or contaminating food with knives, utensils, cutting boards and other foods that have had contact with contaminated raw meat.
- From an acutely infected mother transmitting the parasite across the placenta. NB. Foetuses and infants born to mothers who became infected with T. gondii for the first time during or just before pregnancy are at particular risk of developing severe toxoplasmosis.
- Receiving an infected organ transplant or blood transfusion, though this is rare.
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Symptoms
Most people who become infected with T. gondii are not aware of it. When symptoms do develop (10 - 13 days after ingestion), they are usually mild, self-limiting and mainly flu-like i.e.
- swollen lymph glands (most commonly)
- fatigue
- fever
- sore throat
- muscle aches and pains
- aching joints
- rashes
- enlarged liver and spleen
These symptoms can last for a month or more, and in exceptional cases persist for many years due to chronic active infection.
Severe toxoplasmosis, causing damage to the brain, eyes, or other organs, can develop from an acute T. gondiiinfection or one that had occurred earlier in life and is now reactivated. Severe cases are more likely in individuals who have weak immune systems, though occasionally even persons with healthy immune systems may experience eye damage from toxoplasmosis.
Infection of women during pregnancy may result in transplacental infection of the foetus. This can lead to stillbirth or perinatal death (occurs around the time of the birth). Although most infants who are infected while still in the womb have no symptoms at birth, they can go on to develop toxoplasmosis later in life. A small percentage of infected newborns have serious eye or brain damage at birth.
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Control
There are several hygiene and food safety steps you can take to reduce your chances of becoming infected with T. gondii.
- Wear gloves when you are gardening or do anything outdoors that involves handling soil or sand. This is because cats, which may pass the parasite in their faeces, often use gardens and sand-pits to defaecate in.
- Wash your hands well with soap and water after outdoor activities, especially before you eat or prepare any food.
- When preparing raw meat, wash any cutting boards, sinks, knives, and other utensils that might have touched the raw meat thoroughly with detergent and hot water to avoid cross-contaminating other foods.
- Wash your hands well with soap and water after handling raw meat.
- Cook all meat thoroughly until it is no longer pink in the centre or until the juices become colourless.
- Do not taste meat before it is fully cooked.
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Additional Information
- On average, there are about 750 confirmed cases of toxoplasmosis in England and Wales every year, with female cases tending to outnumber male cases.
- The infection is regarded widespread - it is estimated that 50% of Americans, for example, show evidence of having been infected by the time of adulthood.
- As few as 100 oocysts can produce toxoplasmosis in humans.
- If a woman acquires toxoplasmosis during pregnancy, infection of the foetus through the placenta is estimated to occur in about 45% of cases. The effect of infection on the foetus can range from subclinical infection (no clinical symptoms) to the death of the foetus in the womb. Only 10 - 20% of congenitally infected infants have clinically apparent disease at birth.
- The number of cases of toxoplasmosis that are acquired from foodstuffs is unknown, although as early as 1954, undercooked meat was suspected to be the source of human toxoplasmosis. Fresh meats may contain T. gondii oocysts, and these appear to be the main source of foodborne infection when raw or undercooked. T. gondii is more readily isolated from sheep than from other meat animals. Examples of cases of toxoplasmosis that have been proved or suspected to have involved foods include:
- undercooked mutton (France, early 1960s)
- hamburger cooked rare (New York, 1968)
- unpasteurised goat's milk (1974 & 1978)
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