Toxoplasmosis

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Toxoplasmosis
Classifications and external resources
ICD-10 B58.
ICD-9 130
DiseasesDB 13208
MedlinePlus 000637
eMedicine med/2294 

Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. 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 faeces, or by transmission from mother to fetus. The most common means of transmission to humans is raw or undercooked meat. The illness is usually minor and self-limiting.

Between 30 and 60 percent of the world population is estimated to carry a Toxoplasma infection. After the first few weeks of infection (where it typically causes mild or no illness, or a flu-like illness) 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).

Contents

[edit] 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.[1]
  • Ingestion, accidental or otherwise, 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.[1]

The cyst form of the parasite is extremely hardy, capable of surviving exposure to freezing down to -12 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 Centrigrade, and is thus killed by thorough cooking, and would be killed by 24 hours in a typical domestic freezer.[2]

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. Cats excrete the pathogen in their faeces for a number of weeks 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. Studies have shown that only about 2% of cats are shedding at any one time, and that shedding does not recur even after repeated exposure to the parasite.

[edit] 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 faeces, and gardening (cat faeces 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.

[edit] Clinical manifestations

Infection has two stages:

[edit] 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 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 newborns have serious eye or brain damage at birth.

[edit] Treatment

The vast majority of patients with a normal immune system require no treatment.

Patients with HIV/AIDS or patients who are immunosuppressed should be given sulfadiazine and pyrimethamine. Pregnant women who become ill with acute toxoplasmosis should be treated with spiramycin. Folinic acid (leucovorin) is administered in all cases to reduce the bone marrow suppression caused by pyrimethamine (ref: Montoya JG, Liesenfeld O. Toxoplasmosis. Lancet. 2004 Jun 12;363(9425):1965-76. )

[edit] 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.

[edit] Treatment

The cysts are immune to the standard acute treatments as the antibiotics do not reach the bradyzoites in sufficient concentration.

The antibiotic atovaquone has been used to kill Toxoplasma cysts in situ in AIDS patients.[3] In mice, a combination of atovaquone with clindamycin seemed to optimally kill cysts.[4]

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

[edit] Possible effects on human behavior

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.[5] The behaviors observed, if caused by the parasite, are likely due to the presence of cysts in the brain, which may produce or induce production of a neurotransmitter, possibly dopamine,[6] therefore acting similarly to dopamine reuptake inhibitor type antidepressants and stimulants.

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.[6][7] This may be due to the decreased reaction times that are associated with infection.[7] "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.[8] The data shows that the risk decreases with time after infection, but is not due to age.[6] 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.[9]

Other studies suggest that the parasite may influence personality. There are claims of toxoplasma causing antisocial attitudes in men and promiscuity[10] (or even "signs of higher intelligence")[11] in women, and greater susceptibility to schizophrenia and manic depression[10] in all infected persons. A 2004 study found that toxoplasma "probably induce[s] a decrease of novelty seeking."[12]

The possibility that toxoplasmosis is one cause of schizophrenia has been studied by scientists at least since 1953. 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. A 2006 paper has even suggested that prevalence of toxoplasmosis has large-scale effects on national culture.[13] These types of studies are suggestive but cannot confirm a causal relationship (because of the possibility that schizophrenia increases the likelihood of toxoplasma infection, for ex., rather than the other way around).[14]

[edit] Human prevalence

In the U.S. NHANES III national probability sample, 22.5% of 17,658 persons over the age of 12 years 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.

[edit] 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.[15]

[edit] Famous people with toxoplasmosis

  • Martina Navratilova retired from a competition in 1982 with mystery 'virus' later found to be due to the illness
  • Arthur Ashe developed neurological problems from toxoplasmosis (later found to be HIV-positive)
  • Miriam Pires (Brazilian actress) died of toxoplasmosis at the age of 77
  • Leslie Ash contracted toxoplasmosis in the second month of pregnancy
  • François, comte de Clermont, is Dauphin of France, son and heir of the current pretender to the French Throne. Both he and his younger sister Blanche are mentally handicapped due to toxoplasmosis-related ailments.
  • Louis Wain was a prominent cat artist who later developed schizophrenia which some believe to be due to his constant exposure to cats and thus Toxoplasmosis.[16]

[edit] Footnotes

  1. ^ a b Toxoplasmosis. Centers of Disease Control and Prevention (2004-11-22).
  2. ^ [1]
  3. ^ Toxoplasmosis - treatment key research. NAM & aidsmap (2005-11-02).
  4. ^ Djurković-Djaković O, Milenković V, Nikolić A, Bobić B, Grujić J (2002). "Efficacy of atovaquone combined with clindamycin against murine infection with a cystogenic (Me49) strain of Toxoplasma gondii." (PDF). J Antimicrob Chemother 50 (6): 981-7. PMID 12461021 DOI:10.1093/jac/dkf251.
  5. ^ "'Cat Box Disease' May Change Human Personality And Lower IQ", The Daily Telegraph, April 8, 2000.
  6. ^ a b c Flegr J, Havlícek J, Kodym P, Malý M, Smahel Z (2002). "Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study.". BMC Infect Dis 2: 11. PMID 12095427.
  7. ^ a b Yereli K, Balcioglu IC, Ozbilgin A. (Dec 2 2005). "Is Toxoplasma gondii a potential risk for traffic accidents in Turkey?". Forensic Sci Int. PMID 16332418.
  8. ^ "Can a parasite carried by cats change your personality?", The Guardian, September 25, 2003.
  9. ^ "Dirt infection link to car crashes", BBC News, August 10, 2002.
  10. ^ a b "Dangerrrr: cats could alter your personality", Times Online, June 23, 2005.
  11. ^ "Can a parasite carried by cats change your personality?", The Guardian, September 25, 2003.
  12. ^ Novotná M, Hanusova J, Klose J, Preiss M, Havlicek J, Roubalová K, Flegr J (Jul 6 2004). "Probable neuroimmunological link between Toxoplasma and cytomegalovirus infections and personality changes in the human host.". BMC Infect Dis 5: 54. PMID 16000166.
  13. ^ Lafferty, Kevin D. (2006). "Can the common brain parasite, Toxoplasma gondii, influence human culture?". Proceedings of the Royal Society B: Biological Sciences (FirstCite Early Online Publishing). DOI:10.1098/rspb.2006.3641. ISSN 0962-8452 (Paper) 1471-2954 (Online).
  14. ^ E. Fuller Torrey and Robert H. Yolken (November 2003). Toxoplasma gondii and Schizophrenia. Emergent Infectious Diseases.
  15. ^ 17:30-22:00 Treating Disease in the Developing World.. Talk of the Nation Science Friday. National Public Radio (December 16, 2005).
  16. ^ Topic 33. Coccidia and Cryptosporidium spp.. Biology 625: Animal Parasitology. Kent State Parasitology Lab (October 24, 2005). Retrieved on 2006-10-14. Includes a list of famous victims.

[edit] External links

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