toxoplasmosis

Wikipedia: toxoplasmosis
Toxoplasmosis
Classification & external resources
Toxoplasma_gondii_tachy.jpg
T. gondii tachyzoites
ICD-10 B58.
ICD-9 130
DiseasesDB 13208
MedlinePlus 000637
eMedicine med/2294

Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii.[1] The parasite infects most warm-blooded animals, including humans, but the primary host is the felid (cat) family. Animals are infected by eating infected meat, by contact with cat feces, or by transmission from mother to fetus. Cats have been shown as a major reservoir of this infection. [2] While this is true, contact with infected undercooked meat seems to be a more important cause of human infection in many countries.

Up to one third of the world's population is estimated to carry a Toxoplasma infection.[3] The Centers for Disease Control and Prevention notes that overall seroprevalence in the United States as determined with specimens collected by the third National Health and Nutritional Assessment Survey (NHANES III) between 1988 and 1994 was found to be 22.5%, with seroprevalence among women of childbearing age (15 to 44 years) of 15%.[4] During the first few weeks, the infection typically causes a mild flu-like illness or no illness. After the first few weeks of infection have passed, the parasite rarely causes any symptoms in otherwise healthy adults. However, people with a weakened immune system, such as those infected with HIV, may become seriously ill, and it can occasionally be fatal. The parasite can cause encephalitis (inflammation of the brain) and neurologic diseases and can affect the heart, liver, and eyes (chorioretinitis).

Transmission
Life cycle of Toxoplasma gondii.
Enlarge
Life cycle of Toxoplasma gondii.

Transmission may occur through:

* Ingestion of raw or partly cooked meat, especially pork, lamb, or venison containing Toxoplasma cysts. Infection prevalence in countries where undercooked meat is traditionally eaten, such as France, has been related to this transmission method. Oocysts may also be ingested during hand-to-mouth contact after handling undercooked meat, or from using knives, utensils, or cutting boards contaminated by raw meat.[5]

* Ingestion of contaminated cat feces. This can occur through hand-to-mouth contact following gardening, cleaning a cat's litter box, contact with children's sandpits, or touching anything that has come into contact with cat feces.
* Drinking water contaminated with Toxoplasma.
* Transplacental infection in utero.
* Receiving an infected organ transplant or blood transfusion, although this is extremely rare.[5]

The cyst form of the parasite is extremely hardy, capable of surviving exposure to freezing down to −12 degrees Celsius, moderate temperatures and chemical disinfectants such as bleach, and can survive in the environment for over a year. It is, however, susceptible to high temperatures—above 66 degrees Celsius, and is thus killed by thorough cooking, and would be killed by 24 hours in a typical domestic freezer.[6]

Cats excrete the pathogen in their feces for a number of weeks after contracting the disease, generally by eating an infected rodent. Even then, cat feces are not generally contagious for the first day or two after excretion, after which the cyst 'ripens' and becomes potentially pathogenic. Studies have shown that only about 2% of cats are shedding oocysts at any one time, and that oocyst shedding does not recur even after repeated exposure to the parasite. Although the pathogen has been detected on the fur of cats, it has not been found in an infectious 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 for Toxoplasma antibodies. A positive titer indicates previous exposure and immunity and largely ensures the unborn baby's safety. If a woman receives her first exposure to Toxoplasma while pregnant, the baby is at particular risk. A woman with no previous exposure should avoid handling raw meat, exposure to cat feces, and gardening (cat feces are common in garden soil). 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 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 young.

Transplacental transmission:(a) infection in 1st trimester - incidence of transplacental infection is low (15%) but disease in neonate is most severe. (b) infection in 3rd trimester - incidence of transplacental infection is high (65%) but infant is usually asymptomatic at birth.

Clinical manifestations

Infection has two stages:

Acute toxoplasmosis

During acute toxoplasmosis, symptoms are often influenza-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 those 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 newborn babies have serious eye or brain damage at birth.

Latent toxoplasmosis

Most patients who become infected with Toxoplasma gondii and develop toxoplasmosis do not know it. In most immunocompetent 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.[7]

Treatment

Treatment is often only recommended for people with serious health problems, because the disease is most serious when one's immune system is weak.

Medications that are prescribed for acute Toxoplasmosis are:

* Pyrimethamine - an antimalarial medication.
* Sulfadiazine - an antibiotic used in combination with pyrimethamine to treat toxoplasmosis.
* clindamycin — an antibiotic. This is used most often for people with HIV/AIDS.
* spiramycin - another antibiotic. This is used most often for pregnant women to prevent the infection of their child.

In people with latent toxoplasmosis, the cysts are immune to these treatments, as the antibiotics do not reach the bradyzoites in sufficient concentration.

Medications that are prescribed for latent Toxoplasmosis are:

* atovaquone - an antibiotic that has been used to kill Toxoplasma cysts in situ in AIDS patients. [7]
* clindamycin - an antibiotic which, in combination with atovaquone, seemed to optimally kill cysts in mice.[8]

Biological modifications of the host

The parasite itself can cause various effects on the host body, some of which are not fully understood.

Reproductive changes

A recent study [9] has indicated Toxoplasmosis correlates strongly with an increase in boy births in humans. According to the researchers, depending on the antibody concentration, the probability of the birth of a boy can increase up to a value of 0.72 ... which means that for every 260 boys born, 100 girls are born. The study also notes a mean rate of 0.60 to 0.65 (as opposed to the normal 0.51) for Toxoplasma positive mothers.

Behavioral changes

It has been found that the parasite has the ability to change the behavior of its host: infected rats and mice are less fearful of cats - in fact, some of the infected rats seek out cat-urine-marked areas. This effect is advantageous to the parasite, which will be able to sexually reproduce if its host is eaten by a cat [10]. The mechanism for this change is not completely understood, but there is evidence that toxoplasmosis infection raises dopamine levels in infected mice.

The findings of behavioral alteration in rats and mice have led 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 that may alter their host's behaviour as a part of their life cycle. [11] The behaviors observed, if caused by the parasite, are likely due to infection and low-grade encephalitis, which is marked by the presence of cysts in the brain, which may produce or induce production of a neurotransmitter, possibly dopamine, [12] therefore acting similarly to dopamine reuptake inhibitor type antidepressants and stimulants.

"In populations where this parasite is very common, mass personality modification could result in cultural change. [Variations in the prevalence of Toxoplasma gondii] may explain a substantial proportion of human population differences we see in cultural aspects that relate to ego, money, material possessions, work and rules." — Kevin Lafferty [13]

Correlations have been found between latent Toxoplasma infections and various characteristics: [14]

* Increased risk taking behavior
* Slower reactions
* Feelings of insecurity and self-doubt
* Neuroticism (one of the Big Five personality traits)

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 (e.g., people who exhibit risk-taking behaviors may be more likely to take the risk of eating undercooked meat).

Studies have found that toxoplasmosis is associated with an increased car accident rate, roughly doubling or tripling the chance of an accident relative to uninfected people.[12] [15] This may be due to the slowed reaction times that are associated with infection.[15] "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. [16] The data shows that the risk decreases with time after infection, but is not due to age.[12] 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. [17]However there is also evidence of a delayed effect which increases reaction times. [18]

Other studies suggest that the parasite may influence personality. There are claims of toxoplasma causing antisocial attitudes in men and promiscuity [19] (or even "signs of higher intelligence" [20] ) in women, and greater susceptibility to schizophrenia and manic depression in all infected persons.[19] A 2004 study found that toxoplasma "probably induce[s] a decrease of novelty seeking." [21]

According to Sydney University of Technology infectious disease researcher Nicky Boulter in an article that appeared in the January/February 2007 edition of Australasian Science magazine, Toxoplasma infections lead to changes depending on the sex of the infected person. [22]

The study suggests that male carriers have lower IQs, a tendency to achieve a lower level of education and have shorter attention spans, a greater likelihood of breaking rules and taking risks, and are more independent, anti-social, suspicious, jealous and morose. It also suggests that these men are deemed less attractive to women. Women carriers are suggested to be more outgoing, friendly, more promiscuous, and are considered more attractive to men compared with non-infected controls.

Toxoplasma's role in schizophrenia

The possibility that toxoplasmosis is one cause of schizophrenia has been studied by scientists since at least 1953. [23] These studies had attracted little attention from U.S. researchers until they were publicized through the work of prominent psychiatrist and advocate E. Fuller Torrey. In 2003, Torrey published a review of this literature, reporting that almost all the studies had found that schizophrenics have elevated rates of toxoplasma infection.[23] A 2006 paper has even suggested that prevalence of toxoplasmosis has large-scale effects on national culture. [24] These types of studies are suggestive but cannot confirm a causal relationship (because of the possibility, for example, that schizophrenia increases the likelihood of toxoplasma infection rather than the other way around).[23]

* Acute Toxoplasma infection sometimes leads to psychotic symptoms not unlike schizophrenia.
* Some anti-psychotic medications that are used to treat schizophrenia, such as Haloperidol, also stop the growth of Toxoplasma in cell cultures.
* Several studies have found significantly higher levels of Toxoplasma antibodies in schizophrenia patients compared to the general population.[25]
* Toxoplasma infection causes damage to astrocytes in the brain, and such damage is also seen in schizophrenia.

Human prevalence

The U.S. NHANES (2004-2005) national probability sample found that 33.1% of U.S. persons above 12 years of age had Toxoplasma-specific IgG antibodies, indicating that they had been infected with the organism. This prevalence has significantly increased from the 1999-2000 data.[26]

It is estimated that between 30% and 65% of all people worldwide are infected with Toxoplasmosis. However, there is large variation countries: in France, for example, around 88% of the population are carriers, probably due to a high consumption of raw and lightly cooked meat. [27] Germany, the Netherlands and Brazil also have high prevalences of around 80%, over 80% [28] and 67% respectively. In Britain, about 22% are carriers, and South Korea's rate is only 4.3%.[14]

Two risk factors for contracting toxoplasmosis are:

* 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 those with AIDS. Illness may result from an acute Toxoplasma infection or reactivation of an infection that occurred earlier in life.

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.[29] Proximity to freshwater outflows into the ocean were a major risk factor. Ingestion of oocysts from cat feces is considered to be the most likely ultimate source.[30]

Notable people with toxoplasmosis

* Martina Navrátilová (tennis player) retired from a competition in 1982 with symptoms of a mystery 'virus' that were later found to be due to toxoplasmosis[31]
* Arthur Ashe developed neurological problems from toxoplasmosis (and was later found to be HIV-positive)[32]
* Leslie Ash contracted toxoplasmosis in the second month of pregnancy[33]
* François, comte de Clermont, Dauphin of France and Orléans pretender to the French throne. Both he and his younger sister Blanche are mentally disabled due to congenital toxoplasmosis.
* Louis Wain was a prominent cat artist who later developed schizophrenia, which some believe was due to toxoplasmosis resulting from his prolonged exposure to cats.[34]