Toxoplasmosis

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. Cats get infected by eating infected rats. Rats get infected by accidentally absorbing cat faeces. The parasite subtly alters the rat's behavior. Whereas healthy rats avoid areas that smell like cat urine, infected rats may actually seek these areas out.

People usually get infected by eating raw or undercooked meat, or more rarely, by contact with cat faeces.

At least one third of the world population may have contracted a toxoplasmosis infection in their lifetime but, 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 (chorioretinitis).

Transmission
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.
 * Ingestion, accidental or otherwise, 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.
 * Ingestion of raw or unpasteurized milk and milk products, particularly those containing goat's milk.
 * 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.

Pregnancy precautions
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 feces). 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 foetus. 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.

Symptoms
Infection has two stages:

Acute toxoplasmosis
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. Young children and immunocompromised 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. Only a small percentage of infected newborns have serious eye or brain damage at birth.

Treatment
A combination of sulfadiazine and pyrimethamine should be given.

Latent toxoplasmosis
Most patients who become infected with Toxoplasma gondii and develop 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. Most infants who are infected while in the womb have no symptoms at birth but may develop symptoms later in life.

Treatment
The cysts are immune to the standard acute treatments.

The antibiotic atovaquone has been used to kill Toxoplasma cysts in situ in AIDS patients. In mice, a combination of atovaquone with clindamycin seemed to optimally kill cysts.

Risk factors

 * Infants born to mothers who became infected with Toxoplasma for the first time during or just before pregnancy.
 * Persons with severely weakened immune systems, such as persons with AIDS. This results from an acute Toxoplasma infection or an infection that occurred earlier in life that reactivates and causes damage to the brain, eyes, or other organs.

Possible effects on human behavior
The findings of behavioral alteration in rats and mice have lead some scientists to speculate that toxoplasma may have similar effects in humans, even in the latent phase that had previously been considered asymptomatic. Toxoplasma is one of a 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.

The evidence for behavioral effects on humans, although intriguing, is relatively weak. There have been no randomized clinical trials studying the effects of toxoplasma on human behavior. Although some researchers have found potentially important associations with toxoplasma, it is possible that these associations merely reflect factors that predispose certain types of people to infection.

Studies have found that toxoplasmosis is associated with an increased auto accident rate, roughly doubling or tripling the chance of an accident relative to uninfected people. This may be due to the decreased reaction times that are associated with infection. "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, but is not due to age. 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.

Other studies suggest that the parasite may influence personality. There are claims of toxoplasma causing antisocial attitudes in men and promiscuity (or even "signs of higher intelligence") in women, and greater susceptibility to schizophrenia and manic depression in all infected persons. A review of research focused on the schizophrenia connection confirms an association but does not confirm a causal relationship.

Human prevalence
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. It is thought that between 30% and 60% of the world's population are infected. However, there is large variation countries: in France, for example, about 85% of the population are carriers, probably due to a high consumption of raw and lightly cooked meat.

Animal prevalence
A University of California Davis study of dead Sea Otters collected from 1998 to 2004 found that toxoplasmosis was the cause of death for 13% of the animals. Proximity to freshwater outflows into the ocean were a major risk factor. Ingestion of oocysts from cat faeces is considered to be the most likely ultimate source.