The only known definitive hosts for Toxoplasma gondii are members of family Felidae (domestic cats and their relatives). Unsporulated oocysts are shed in the cat’s feces . Although oocysts are usually only shed for 1–3 weeks, large numbers may be shed. Oocysts take 1–5 days to sporulate in the environment and become infective. Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water or plant material contaminated with oocysts . Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites . Cats become infected after consuming intermediate hosts harboring tissue cysts . Cats may also become infected directly by ingestion of sporulated oocysts. Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment .
Humans can become infected by any of several routes:
1-Eating undercooked meat of animals harboring tissue cysts . Consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) . 2-Blood transfusion or organ transplantation . 3-Transplacentally from mother to fetus.
In the human host, the parasites form tissue cysts, most commonly in skeletal muscle,
myocardium, brain, and eyes; these cysts may remain throughout the life of the host
Toxoplasmosis
Epidemiology & Risk Factors
Toxoplasmosis is caused by the protozoan parasite Toxoplasma gondii. In the United States it is estimated that 11% of the population 6 years and older have been infected with Toxoplasma. In various places throughout the world, it has been shown that more than 60% of some populations have been infected with Toxoplasma. Infection is often highest in areas of the world that have hot, humid climates and lower altitudes, because the oocysts survive better in these types of environments.
Toxoplasmosis is not passed from person-to- person, except in instances of mother-to-child (congenital) transmission and blood transfusion or organ transplantation. People typically become infected by three principal routes of
transmission:
Foodborne
Animal-to-human (zoonotic) Mother-to-child (congenital) Rare instances
Foodborne transmission
1-The tissue form of the parasite (a microscopic •
cyst consisting of bradyzoites) can be transmitted to humans by food. People become infected by:
2-Eating undercooked, contaminated meat •
(especially pork, lamb, and venison) Accidentally ingesting undercooked, contaminated meat or after handling it and not washing hands thoroughly (Toxoplasma cannot be absorbed through intact skin); and
3-Eating food that was contaminated by knives,• utensils, cutting boards or other foods that had contact with raw, contaminated meat.
4-Drinking unpasteurized goat’s milk (tachyzoites). •
Animal-to-human (zoonotic) transmission
Kittens and cats can shed millions of oocysts • in their feces for as long as 3 weeks after infection. Mature cats are less likely to
shed Toxoplasma if they have been previously infected. A Toxoplasma-infected cat that is shedding the parasite in its feces contaminates the litter box. If the cat is allowed outside, it can contaminate the soil or water in the environment as well.
People can be infected by: Accidental ingestion of oocysts after cleaning a • cat’s litter box when the cat has
shed Toxoplasma in its feces
Accidental ingestion of oocysts after touching or • ingesting anything that has come into contact
with a cat’s feces that contain Toxoplasma
Accidental ingestion of oocysts in contaminated • soil (e.g., not washing hands after gardening or eating unwashed fruits or vegetables from a garden)
Drinking water contaminated with • the Toxoplasma parasite
Mother-to-child (congenital) transmission
A woman who is newly infected •with Toxoplasma during or just before pregnancy can pass the infection to her unborn child (congenital infection).
The woman may not have symptoms, but • there can be severe consequences for the unborn child, such as diseases of the nervous system and eyes.
Rare instances of transmission
Organ transplant recipients can become • infected by receiving an organ from
a Toxoplasma-positive donor.
Rarely, people can also become infected •
by receiving infected blood via transfusion. Laboratory workers who handle infected blood can also acquire infection through accidental inoculation.
Disease
Healthy people (non-pregnant) •
• Healthy people who become infected with Toxoplasma gondii often do not have symptoms because their immune system usually keeps the parasite from causing illness. When illness occurs, it is usually mild with “flu-like” symptoms (e.g., tender lymph nodes, muscle aches, etc.) that last for weeks to months and then go away. However, the parasite remains in the person’s body in an inactive state. It can become reactivated if the person becomes immunosuppressed.
Mother-to-child (congenital)
Generally, if a woman has been infected before becoming • pregnant, the unborn child will be protected because the mother has developed immunity. If a woman becomes newly infected with Toxoplasma during or just before pregnancy, she can pass the infection to her unborn baby (congenital transmission). The damage to the unborn
child is often more severe the earlier in pregnancy the
transmission occurs. Potential results can be
A miscarriage • A stillborn child •
A child born with signs of congenital toxoplasmosis (e.g., • abnormal enlargement or smallness of the head)
Infants infected before birth often show no symptoms at • birth but may develop them later in life with potential
vision loss, mental disability, and seizures.
Persons with ocular disease
Eye disease (most frequently retinochoroiditis) •
from Toxoplasma infection can result from congenital infection or infection after birth by any of the modes of transmission. Eye lesions from congenital infection are often not identified at birth but occur in 20-80% of congenitally-infected persons by adulthood. However, in the U.S. <2% of persons infected after birth develop eye lesions. Eye infection leads to an acute inflammatory lesion of the retina, which resolves leaving retinochoroidal scarring. Symptoms of ocular disease include
Eye pain •
Sensitivity to light (photophobia) •
Tearing of the eyes •
Blurred vision •
The eye disease can reactivate months or years later, each time causing •
more damage to the retina. If the central structures of the retina are involved there will be a progressive loss of vision that can lead to blindness.
Persons with compromised immune systems
Persons with compromised immune systems may • experience severe symptoms if they are infected
with Toxoplasma while immune suppressed. Persons who acquire HIV infection and were not infected previously with Toxoplasma are more likely to develop a severe primary infection.
Immunocompromised persons who were infected •
with Toxoplasma at some point before they become immunosuppressed are at risk for developing a relapse (reactivation) of toxoplasmosis. For example, a person who is HIV-infected and who has
reactivated Toxoplasma infection can have symptoms that include fever, confusion, headache, seizures, nausea, and
poor coordination.
Toxoplasma infection can reactivate in •
immunocompromised pregnant women who were infected with Toxoplasma before their pregnancy, and this can lead to congenital infection.
Diagnosis
The diagnosis of toxoplasmosis is typically made •
by serologic testing. A test that measures immunoglobulin G (IgG) is used to determine if a person has been infected. If it is necessary to try to estimate the time of infection, which is of particular importance for pregnant women, a test which measures immunoglobulin M (IgM) is also used along with other tests such as an avidity test.
Diagnosis can also be made by direct observation of the parasite in • stained tissue sections, cerebrospinal fluid (CSF), or other biopsy material. These techniques are used less frequently because of the difficulty of obtaining these specimens.
Parasites can also be isolated from blood or other body fluids (for • example, CSF) but this process can be difficult and requires considerable time.
Molecular techniques that can detect the parasite’s DNA in the • amniotic fluid can be useful in cases of possible mother-to-child (congenital) transmission.
Ocular disease is diagnosed based on the appearance of the • lesions in the eye, symptoms, course of disease, and often serologic testing.
A Toxoplasma-positive reaction, stained by
.immunofluroescence (IFA). (CDC Photo- tachyzoites and bradyzoites)
Prevention & Control
• Reduce Risk from Food
• To prevent risk of toxoplasmosis and other infections from food:Cook food to safe temperatures. A food thermometer should be used to measure the internal temperature of cooked meat. Color is not a reliable indicator that meat has been cooked to a temperature high enough to kill harmful pathogens like Toxoplasma. Do not sample meat until it is cooked. USDA recommends the following for meat preparation:
• For Whole Cuts of Meat (excluding poultry)
Cook to at least 145° F (63° C) as measured with a food thermometer placed in the thickest part of the meat, then allow the meat to rest for three minutes before carving or consuming. *According to USDA, “A ‘rest time’ is the amount of time the product remains at the final temperature, after it has been removed from a grill, oven, or other heat source. During the three minutes after meat is removed from the heat source, its temperature remains constant or continues to rise, which destroys pathogens.”
For Ground Meat (excluding poultry) •
Cook to at least 160° F (71° C); ground meats do not require a rest time.
• For All Poultry (whole cuts and ground)
Cook to at least 165° F (74° C). The internal temperature should be checked in the innermost part of the thigh, innermost part of the wing, and the thickest part of the breast. Poultry do not require a rest time.