Toxoplasmosis

Toxoplasmosis is a parasitic disease caused by the protozoan Toxoplasma gondii. The parasite infects most genera of warm-bloodedanimals, including humans, but the primary host is the felid (cat) family. Animals are infected by eating infected meat, by ingestion of fecesof a cat that has itself recently been infected, or by transmission from mother to fetus. Although cats are often blamed for spreading toxoplasmosis, contact with raw meat is a more significant source of human infections in many countries, and fecal contamination of hands is a greater risk factor.

Up to one third of the world's human population is estimated to carry a Toxoplasma infection. The Centers for Disease Control and Prevention notes that overall seroprevalence in the United States as determined with specimens collected by the National Health and Nutritional Examination Survey (NHANES) between 1999 and 2004 was found to be 10.8%, with seroprevalence among women of childbearing age (15 to 44 years) 11%.

During the first few weeks post-exposure, the infection typically causes a mild flu-like illness or no illness. Thereafter, the parasite rarely causes any symptoms in otherwise healthy adults. However, those with a weakened immune system, such as AIDS patients or pregnantwomen, may become seriously ill, and it can occasionally be fatal. The parasite can cause encephalitis (inflammation of the brain) andneurologic diseases, and can affect the heart, liver, inner ears, and eyes (chorioretinitis).

Signs and symptoms

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 functioningimmune system may develop eye damage from toxoplasmosis. Young children and immunocompromised patients, such as those with HIV/AIDS, those taking certain types ofchemotherapy, or those who have recently received an organ transplant, may develop severe toxoplasmosis. This can cause damage to the brain (encephalitis) or the eyes (necrotizing retinochoroiditis). Infants infected via placental transmission may be born with either of these problems, or with nasal malformations, although these complications are rare in newborns.

Swollen lymph nodes are more commonly found in the neck followed by axillae and then groin. Swelling may occur at different times after the initial infection, persist, and/or recur for various times independently of antiparasitic treatment. It is usually found at single sites in adults, but in children multiple sites may be more common. Enlarged lymph nodes will resolve within one to two months in 60% of patients. However, a quarter of patients take 2–4 months to return to normal and 8% take 4–6 months. A substantial number of patients (6%) do not return to normal until much later.

Latent toxoplasmosis

It is easy for a host to become infected with Toxoplasma gondii and develop toxoplasmosis without knowing 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.

Cutaneous toxoplasmosis

While rare, skin lesions may occur in the acquired form of the disease, including roseola and erythema multiforme-like eruptions, prurigo-like nodules, urticaria, and maculopapular lesions. Newborns may have punctate macules, ecchymoses, or “blueberry muffin” lesions. Diagnosis of cutaneous toxoplasmosis is based on the tachyzoite form of T. gondii being found in the epidermis. It is found in all levels of the epidermis, is about 6 μm by 2 μm , bow-shaped, the nucleus being one-third of its size. It can be identified by electron microscopy or by Giemsa staining tissue where the cytoplasm shows blue, the nucleus red.

Possible link to psychiatric disorders

Studies have been conducted that show the toxoplasmosis parasite may affect behavior and may present as or be a causative or contributory factor in various psychiatric disorders such as depression, anxiety and schizophrenia. In 11 of 19 scientific studies, T. gondii antibody levels were found to be significantly higher in individuals affected by first-incidence schizophrenia than in unaffected persons. Individuals with schizophrenia are also more likely to report a clinical history of toxoplasmosis than those in the general population. Recent work at the University of Leeds has found that the parasite produces an enzyme with tyrosine hydroxylase and phenylalanine hydroxylase activity. This enzyme may contribute to the behavioral changes observed in toxoplasmosis by altering the production of dopamine, a neurotransmitter involved in mood, sociability, attention, motivation and sleep patterns. Schizophrenia has long been linked to dopamine dysregulation.

Diagnosis

Detection of Toxoplasma gondii in human blood samples may be achieved by using the polymerase chain reaction (PCR). Inactive cysts may exist in a host which would evade detection.

Toxoplasmosis can't be detected with immunostaining. Lymph nodes affected by toxoplasma have characteristic changes, including poorly demarcated reactive germinal centers, clusters of monocytoid B cells and scattered epithelioid histiocytes.

Transmission

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 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.
• 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 leach, and can survive in the environment for over a year. It is, however, susceptible to high temperatures—above 66 degrees Celsius (150 degrees Fahrenheit), and is thus killed by thorough cooking, and would be killed by 24 hours in a typical domestic freezer.

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 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. A positive antibody titer indicates previous exposure and immunity and largely ensures the unborn baby's safety. A simple blood draw at the first pre-natal doctor visit can determine whether or not the woman has had previous exposure and therefore whether or not she is at risk. If a woman receives her first exposure to toxoplasmosis 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. However, while risks can be minimized, they cannot be eliminated. For pregnant women with negative antibody titer, indicating no previous exposure to T. gondii, as frequent as monthly serology testing is advisable as treatment during pregnancy for those women exposed to T. gondii for the first time decreases dramatically the risk of passing the parasite to the fetus.

Despite these risks, pregnant women are not routinely screened for toxoplasmosis in most countries (Portugal, France, Austria,Uruguay, and Italy being the exceptions) for reasons of cost-effectiveness and the high number of false positives generated. As invasiveprenatal testing incurs some risk to the fetus (18.5 pregnancy losses per toxoplasmosis case prevented), postnatal or neonatal screening is preferred. The exceptions are cases where fetal abnormalities are noted, and thus screening can be targeted.

Some regional screening programmes operate in Germany, Switzerland and Belgium.

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 anti-protozoans, an infection can be very serious in the young.

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.

Orphaned kittens

Kittens require a high-calorie diet that contains more protein than the diet of adult cats. Young orphaned kittens require milk every two to four hours, and they need physical stimulation to defecate and urinate. Cat milk replacement is manufactured to feed to young kittens, because cow's milk does not provide all of their necessary nutrients.

Hand-reared kittens tend to be affectionate to humans as adults and more dependent on them than those reared by their mothers, but they can also show volatile mood swings and aggression.

Cat play and toys

Domestic cats, especially young kittens, are known for their love of play. This behavior mimics hunting and is important in helping kittens learn to stalk, capture, and kill prey. Cats will also engage in play fighting, with each other and with humans. This behavior may be a way for cats to practice the skills needed for real combat, and might also reduce any fear they associate with launching attacks on other animals.

Owing to the close similarity between play and hunting, cats prefer to play with objects that resemble prey, such as small furry toys that move rapidly, but rapidly lose interest (they become habituated) in a toy they have played with before. Cats also tend to play with toys more when they are hungry. String is often used as a toy, but if it is eaten it can become caught at the base of the cat’s tongue and then move into the intestines, a medical emergency which can cause serious illness and death. Owing to the risks posed by cats eating string, it is sometimes replaced with a laser pointer's dot, which cats may chase. While concerns have been raised about the safety of these lasers, Professor John Marshall, an ophthalmologist at St Thomas' Hospital, has stated that it would be "virtually impossible" to blind a cat with a laser pointer.