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Toxoplasmosis is a parasitic disease caused by the parasite Toxoplasma gondii. It infects most animals and causes human parasitic diseases, but the primary host is the felid (cat) family. People usually get infected by eating raw or undercooked meat, or more rarely, by contact with cat faeces.

Toxoplasma gondii is a species of parasitic protozoa belonging to the Apicomplexa. The life cycle of Toxoplasma gondii has two phases. The sexual part of the lifecycle takes place only in cats, the definitive host of Toxoplasma gondii. The asexual life cycle can take place in any warm-blooded animal, like other mammals and birds. In these intermediate hosts, the parasite invades cells forming a so-called parasitophorous vacuole. Within this vacuole T. gondii propagates by a series of binary fissions until the infected cell bursts. This fast replicating form of the asexually reproducing T. gondii is called tachyzoite. Additionally, a slow replicating form exists, the bradyzoite. Bradyzoite vacuoles can form tissue cysts in the infected animals/humans. Resistance to antibiotics varies, but the cysts are very difficult to erradicate entirely. The immune system of the host does not detect these cysts, while free tachyzoites are efficiently cleared by the immune response.

Tissue cysts can be ingested by a cat, e.g. by feeding on an infected mouse. The cysts survive passage through the digestive system of the cat and the parasites infect epithelial cells of the small intestine where they undergo sexual reproduction and oocyst formation. Oocysts are shed with the feces. Animals and humans that ingest oocysts (e.g. by eating unwashed vegetables etc.) become infected. The parasite enters macrophages in the intestinal lining and is distributed via the blood stream throughout the body.

Latent infection is normally asymptomatic; but often gives flu-like symptoms in the early acute stages. However, in immune compromised patients toxoplasmosis can develop. During pregnancy Toxoplasma can lead to abortion or hydrocephalus.

In the U.S. NHANES III national probability sample, 22.5% of 17,658 persons >12 years of age had Toxoplasma-specific IgG antibodies, indicating that they had been infected with the organism. There is some research to suggest that infection in humans can lead to slower reactions and an increase in risk-taking behaviors.

At least one third of the world population may have contracted a toxoplasmosis infection in their lifetime, after the acute infection has passed the parasite rarely causes any symptoms in otherwise healthy adults. However, people with a weakened immune system are particularly susceptible, such as people infected with HIV. The parasite can cause encephalitis (inflammation of the brain) and neurologic diseases and can affect the heart, liver, and eyes.

Transmission may occur through:

· Ingestion of raw or partly cooked meat, especially pork, lamb, or venison, or by hand to mouth contact after handling undercooked meat. Infection prevalence is higher in countries that traditionally eat undercooked meat, such as France. This seems to be by far the most common route of infection.

· Accidental ingestion of contaminated cat faeces. This can occur through hand to mouth contact following gardening, cleaning a cat's litter box, children's sandpits, or touching anything that has come into contact with cat faeces.

· Contamination of knives, utensils, cutting boards and other foods that have had contact with raw meat.

· Drinking water contaminated with Toxoplasma.

· The reception of an infected organ transplant or blood transfusion, although this is extremely rare.

The cyst form of the parasite is extremely hardy, capable of surviving exposure to cooling down to subzero temperatures and chemical disinfectants such as bleach and can survive in the environment for over a year. It is however susceptible to high temperatures, and is killed by cooking. Cats excrete the pathogen for a number of weeks or months after contracting the disease, generally by eating an infected rodent. Even then, cat faeces are not generally contagious for the first day or two after excretion, after which the cyst 'ripens' and becomes potentially pathogenic.

Although the pathogen has been detected on the fur of cats, the pathogen has not been found in a 'ripe' form, and direct infection from handling cats is generally believed to be very rare.

Congenital toxoplasmosis is a special form in which an unborn child is infected via the placenta. This is the reason that pregnant women should be checked to see if they have a titer to toxoplasmosis. A titer indicates previous exposure and largely ensures the unborn baby's safety. If a woman receives her first exposure to Toxoplasma while pregnant then the baby is at particular risk. A woman with no previous exposure should avoid handling raw meat, exposure to cat faeces, and gardening (a common place to find cat faeces). Most cats are not actively shedding oocysts and so are not a danger, but the risk may be reduced further by having the litterbox emptied daily (oocysts require longer than a single day to become infective), and/or by having someone else empty the litterbox.

Treatment is very important for recently infected pregnant women to prevent infection of the fetus. Since a baby's immune system does not develop fully for the first year of life, and the resilient cysts that form throughout the body are very difficult to eradicate with antiprotozoans, an infection can be very serious in the very young.

Infection has two stages. During acute toxoplasmosis, symptoms are often flu-like: swollen lymph nodes, or muscle aches and pains that last for a month or more. Rarely, a patient with a fully functioning immune system may develop eye damage from toxoplasmosis. However, most patients who become infected with toxoplasmosis do not know it. In most non-immunodeficient patients, the infection enters a latent phase, during which only bradyzoites are present, forming cysts in nervous and muscle tissue. Young children and immunocomprimised patients, such as those with HIV/AIDS, those taking certain types of chemotherapy, or persons who have recently received an organ transplant, may develop severe toxoplasmosis. This can cause damage to the brain or the eyes. Most infants who are infected while in the womb have no symptoms at birth but may develop symptoms later in life. Only a small percentage of infected newborns have serious eye or brain damage at birth.

It is possible that the latent phase of the infection previously considered asymptomatic may cause behavioral changes in infected patients, as it has previously been shown to do in mice and rats. Toxoplasma is one of the number of parasites, which require alteration of host's behaviour for their life cycle. The changes observed are likely due to the presence of cysts in the brain, which produce or induce production of a neurotransmitter, possibly dopamine, therefore acting similarly to dopamine reuptake inhibitor type antidepressants.

A slightly increased car accident rate, and reaction time slowed by a few percent have been observed (specifically, the infected lose concentration more quickly than the controls in the second and third minute). "If our data are true then about a million people a year die just because they are infected with toxoplasma," the researcher Jaroslav Flegr told The Guardian. The data shows that the risk decreases with time after infection, however all older drivers are generally able to compensate for longer reaction time. Ruth Gilbert, medical coordinator of the European Multicentre Study on Congenital Toxoplasmosis, told BBC News Online these findings could be due to chance, or due to social and cultural factors associated with toxoplasma infection.