Eye disease (most frequently retinochoroiditis) from Toxoplasma infection can result from congenital infection or infection after birth by any of the modes of transmission discussed on the epidemiology and risk factors page. 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.

Causal Agent:

Toxoplasma gondii is a protozoan parasite that infects most species of warm-blooded animals, including humans, and causes the disease toxoplasmosis.

Life Cycle:

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 (1). 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 (2). Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites (3). Cats become infected after consuming intermediate hosts harboring tissue cysts (4). 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 (5). Humans can become infected by any of several routes:

• Eating undercooked meat of animals harboring tissue cysts (6).
• 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) (7).
• Blood transfusion or organ transplantation (8).
• Transplacentally from mother to fetus (9).

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. Diagnosis is usually achieved by serology, although tissue cysts may be observed in stained biopsy specimens (10). Diagnosis of congenital infections can be achieved by detecting T. gondii DNA in amniotic fluid using molecular methods such as PCR (11).

Life cycle image and information courtesy of DPDx.

Healthy people (nonpregnant)

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 discussed on the epidemiology and risk factors page. 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.

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.

People who are healthy should follow the guidelines below to reduce risk of toxoplasmosis.

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.

• Freeze meat for several days at sub-zero (0° F) temperatures before cooking to greatly reduce chance of infection. *Freezing does not reliably kill other parasites that may be found in meat (like certain species of Trichinella) or harmful bacteria. Cooking meat to USDA recommended internal temperatures is the safest method to destroy all parasites and other pathogens.
• Peel or wash fruits and vegetables thoroughly before eating.

• Wash cutting boards, dishes, counters, utensils, and hands with soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables.
• Do not drink unpasteurized goat’s milk.
• Do not eat raw or undercooked oysters, mussels, or clams (these may be contaminated with Toxoplasma that has washed into seawater).

The U.S. Government and the meat industry continue their efforts to reduce T. gondii in meat.

Reduce Risk from the Environment

To reduce risk of toxoplasmosis from the environment:

• Avoid drinking untreated water.
• Wear gloves when gardening and during any contact with soil or sand because it might be contaminated with cat feces that contain Toxoplasma. Wash hands with soap and water after gardening or contact with soil or sand.
• Teach children the importance of washing hands to prevent infection.
• Keep outdoor sandboxes covered.

• Feed cats only canned or dried commercial food or well-cooked table food, not raw or undercooked meats.
• Ensure that the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s feces.
• If you are pregnant or immunocompromised:
  1. Avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and water afterwards.
  2. Keep cats indoors to prevent them from hunting and reduce the chances they will become infected with Toxoplasma.
  3. Do not adopt or handle stray cats, especially kittens. Do not get a new cat while you are pregnant or immunocompromised.

What is toxoplasmosis?

Toxoplasmosis is an infection caused by a single-celled parasite called Toxoplasma gondii. While the parasite is found throughout the world, more than 40 million people in the United States may be infected with the Toxoplasma parasite. The Toxoplasma parasite can persist for long periods of time in the bodies of humans (and other animals), possibly even for a lifetime. Of those who are infected however, very few have symptoms because a healthy person’s immune system usually keeps the parasite from causing illness. However, pregnant women and individuals who have compromised immune systems should be cautious; for them, a Toxoplasma infection could cause serious health problems.

How do people get toxoplasmosis?

A Toxoplasma infection occurs by one of the following:

• Eating undercooked, contaminated meat (especially pork, lamb, and venison) or shellfish (for example, oysters, clams or mussels).
• Accidental ingestion of undercooked, contaminated meat or shellfish after handling them and not washing hands thoroughly (Toxoplasma cannot be absorbed through intact skin).
• Eating food that was contaminated by knives, utensils, cutting boards and other foods that have had contact with raw, contaminated meat or shellfish.
• Drinking water contaminated with Toxoplasma gondii.
• Accidentally swallowing the parasite through contact with cat feces that contain Toxoplasma. This might happen by

1. 1. Cleaning a cat’s litter box when the cat has shed Toxoplasma in its feces;
   2. Touching or ingesting anything that has come into contact with cat feces that contain Toxoplasma; or
   3. Accidentally ingesting contaminated soil (e.g., not washing hands after gardening or eating unwashed fruits or vegetables from a garden).

• Mother-to-child (congenital) transmission.
• Receiving an infected organ transplant or infected blood via transfusion, though this is rare.

What are the signs and symptoms of toxoplasmosis?

Symptoms of the infection vary.

• Most people who become infected with Toxoplasma gondii are not aware of it because they have no symptoms at all.
• Some people who have toxoplasmosis may feel as if they have the “flu” with swollen lymph glands or muscle aches and pains that may last for a month or more.
• Severe toxoplasmosis, causing damage to the brain, eyes, or other organs, can develop from an acute Toxoplasma infection or one that had occurred earlier in life and is now reactivated. Severe toxoplasmosis is more likely in individuals who have weak immune systems, though occasionally, even persons with healthy immune systems may experience eye damage from toxoplasmosis.
• Signs and symptoms of ocular toxoplasmosis can include reduced vision, blurred vision, pain (often with bright light), redness of the eye, and sometimes tearing. Ophthalmologists sometimes prescribe medicine to treat active disease. Whether or not medication is recommended depends on the size of the eye lesion, the location, and the characteristics of the lesion (acute active, versus chronic not progressing). An ophthalmologist will provide the best care for ocular toxoplasmosis.
• Most infants who are infected while still in the womb have no symptoms at birth, but they may develop symptoms later in life. A small percentage of infected newborns have serious eye or brain damage at birth.

Who is at risk for developing severe toxoplasmosis?

People who are most likely to develop severe toxoplasmosis include:

• Infants born to mothers who are newly infected with Toxoplasma gondii during or just before pregnancy.
• Persons with severely weakened immune systems, such as individuals with AIDS, those taking certain types of chemotherapy, and those who have recently received an organ transplant.

What should I do if I think I am at risk for severe toxoplasmosis?

If you are planning to become pregnant, your health care provider may test you for Toxoplasma gondii. If the test is positive it means you have already been infected sometime in your life. There usually is little need to worry about passing the infection to your baby. If the test is negative, take necessary precautions to avoid infection (See below).

If you are already pregnant, you and your health care provider should discuss your risk for toxoplasmosis. Your health care provider may order a blood sample for testing.

If you have a weakened immune system, ask your doctor about having your blood tested for Toxoplasma. If your test is positive, your doctor can tell you if and when you need to take medicine to prevent the infection from reactivating. If your test is negative, it means you need to take precautions to avoid infection. (See below).

What should I do if I think I may have toxoplasmosis?

If you suspect that you may have toxoplasmosis, talk to your health care provider. Your provider may order one or more varieties of blood tests specific for toxoplasmosis. The results from the different tests can help your provider determine if you have a Toxoplasma gondii infection and whether it is a recent (acute) infection.

What is the treatment for toxoplasmosis?

Once a diagnosis of toxoplasmosis is confirmed, you and your health care provider can discuss whether treatment is necessary. In an otherwise healthy person who is not pregnant, treatment usually is not needed. If symptoms occur, they typically go away within a few weeks to months. For pregnant women or persons who have weakened immune systems, medications are available to treat toxoplasmosis.

How can I prevent toxoplasmosis?

There are several steps you can take to reduce your chances of becoming infected with Toxoplasma gondii.

• 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.
• More on Safe Food Handling: Fight BAC: Safe Food Handling

• Freeze meat* for several days at sub-zero (0° F) temperatures before cooking to greatly reduce chance of infection. *Freezing does not reliably kill other parasites that may be found in meat (like certain species of Trichinella) or harmful bacteria. Cooking meat to USDA recommended internal temperatures is the safest method to destroy all parasites and other pathogens.
• Peel or wash fruits and vegetables thoroughly before eating.
• Do not eat raw or undercooked oysters, mussels, or clams (these may be contaminated with Toxoplasma that has washed into seawater).
• Do not drink unpasteurized goat’s milk.
• Wash cutting boards, dishes, counters, utensils, and hands with soapy water after contact with raw meat, poultry, seafood, or unwashed fruits or vegetables.
• Wear gloves when gardening and during any contact with soil or sand because it might be contaminated with cat feces that contain Toxoplasma. Wash hands with soap and water after gardening or contact with soil or sand.
• Ensure that the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s feces.
• Wash hands with soap and water after cleaning out a cat’s litter box.
• Teach children the importance of washing hands to prevent infection.

If I am at risk, can I keep my cat?

Yes, you may keep your cat if you are a person at risk for a severe infection (e.g., you have a weakened immune system or are pregnant); however, there are several safety precautions you should take to avoid being exposed to Toxoplasma gondii, including the following:

• Ensure the cat litter box is changed daily. The Toxoplasma parasite does not become infectious until 1 to 5 days after it is shed in a cat’s feces.
• If you are pregnant or immunocompromised:

1. 1. Avoid changing cat litter if possible. If no one else can perform the task, wear disposable gloves and wash your hands with soap and water afterwards.
   2. Keep cats indoors. This is because cats become infected with Toxoplasma through hunting and eating rodents, birds, or other small animals that are infected with the parasite.
   3. Do not adopt or handle stray cats, especially kittens. Do not get a new cat while you are pregnant or immunocompromised.

• Feed cats only canned or dried commercial food or well-cooked table food, not raw or undercooked meats.
• Keep your outdoor sandboxes covered.

Your veterinarian can answer any other questions you may have regarding your cat and risk for toxoplasmosis.

Once infected with Toxoplasma is my cat always able to spread the infection to me?

No, cats only spread Toxoplasma in their feces for 1-3 weeks following infection with the parasite. Like humans, cats rarely have symptoms when infected, so most people do not know if their cat has been infected. Your veterinarian can answer any other questions you may have regarding your cat and risk for toxoplasmosis.