Note: All links within content go to MayoClinic.com

Toxoplasma gondii is one of the world's most common parasites. In the United States alone, it's estimated that more than 60 million people have toxoplasmosis, the infection the parasite causes.

Most people affected never develop signs and symptoms because their bodies effectively keep T. gondii in check. But for infants born to infected mothers and for people with compromised immune systems — especially those living with HIV/AIDS — toxoplasmosis can be extremely serious.

If you're generally healthy, you probably won't need any treatment for toxoplasmosis. And if you are pregnant or have lowered immunity, certain medications can help reduce the severity of the infection. The best approach, though, is prevention. A few precautions can protect you from the disease.

Most often, you won't know that you've contracted toxoplasmosis, although some people may develop signs and symptoms similar to those of the flu or mononucleosis, such as:

• Body aches
• Swollen lymph nodes
• Fever
• Fatigue
• Occasionally, a sore throat

If you are living with HIV/AIDS, are receiving chemotherapy or have recently had an organ transplant, you're more likely to develop signs and symptoms of severe toxoplasmosis infection, including:

• Headache
• Confusion
• Poor coordination
• Seizures
• Lung problems that may resemble tuberculosis or pneumocystis carinii pneumonia, the most common opportunistic infection that occurs in people with AIDS in the United States
• Severe inflammation of your retina (ocular toxoplasmosis)

Signs in babies
Most pregnant women with toxoplasmosis don't have signs or symptoms of the disease, but if you become infected for the first time just before or during your pregnancy, you have nearly an even chance of passing the infection to your baby (congenital toxoplasmosis), even if you aren't sick yourself.

The risk and severity of your baby's infection often depends on when in your pregnancy you were infected. Your baby is most at risk of toxoplasmosis if you become infected in the third trimester and least at risk if you become infected during the first trimester. On the other hand, the earlier in your pregnancy infection occurs, the more serious the outcome for your baby. Many early infections end in stillbirth or miscarriage, and children who do survive are likely to be born with serious problems, such as:

• An unusually large head caused by excess accumulation in the brain of the clear fluid surrounding the brain and spinal cord (cerebrospinal fluid), a condition known medically as hydrocephalus
• Seizures
• An enlarged liver and spleen
• Yellowing of the skin and whites of the eyes (jaundice)
• Severe eye infections

Children born to mothers infected during the second trimester, when the possibility of transmission is greater, also may be born with serious problems, although only a small number of babies who have toxoplasmosis show signs of the disease at birth. Instead, the majority of infected newborns — mainly those whose mothers were infected during the last three months of pregnancy (third trimester) — don't develop signs and symptoms of the disease until they're in their 20s or 30s. Those signs and symptoms include:

• Hearing loss
• Mental retardation
• Serious eye infections that may lead to blindness

T. gondii is a single-celled parasitic organism that can infect most animals and birds. But because it reproduces sexually only in cats, wild and domestic felines are the parasite's ultimate host. T. gondii's complex life cycle begins when a cat eats infected prey, usually a mouse or bird. Cats can also become infected if they are fed raw, contaminated meat or eat infected soil. Once ingested, T. gondii burrows into the walls of the cat's small intestine, forming early-stage cells called oocysts that the cat eliminates in its feces, usually for a period of two to three weeks. A single stool may contain millions of oocysts. Most healthy cats won't shed oocysts after this initial acute stage.

Within a few days, the oocysts develop into mature, highly infectious cells that under certain conditions can survive in the soil for up to a year. If they're ingested by another animal, they multiply rapidly inside the host, eventually forming inactive eggs (cysts) that lodge mainly in the brain or muscles. Although the new host animal usually remains symptom-free and won't excrete oocysts, it can still transmit the parasite to any predator that eats it.

In many respects, the pattern is similar in humans. After you're infected with T. gondii, the parasite forms cysts that can affect almost any organ but that most often affect your brain, skeleton and heart muscle. If you're generally healthy, your immune system keeps the parasites at bay, and they remain in your body in an inactive state for life. This provides immunity so that you can't become infected with the parasite again. But if your resistance is weakened by disease or certain medications, the infection can be reactivated, leading to serious complications.

Although you can't "catch" toxoplasmosis from an infected child or adult, you can become infected if you come in contact with:

• Cat feces that contain the parasite. You may accidentally ingest the parasites if you touch your mouth after gardening, cleaning a litter box or touching anything that's come in contact with infected cat feces. Cats who hunt or who are fed raw meat are most likely to harbor T. gondii. But even if you're at high risk of the disease, you don't have to give up your cat. A few simple precautions can protect you from infection.
• Undercooked, infected meat. Venison, lamb and pork are especially likely to be infected with T. gondii — in many parts of the world, up to one-fourth of all consumed pork contains toxoplasma cysts. Occasionally, unpasteurized goat's milk also may contain the cysts.
• Contaminated knives, cutting boards or other utensils. Kitchen utensils that come in contact with raw meat can harbor the parasites unless they're washed carefully in plenty of hot, soapy water.
• Contaminated, unwashed fruits and vegetables. The surface of fruits and vegetables may contain traces of the parasite. To be safe, thoroughly wash all produce, especially any you eat raw.
• Contaminated water. Although not common in the United States, the drinking water in many parts of the world is contaminated with T. gondii. And even the most sophisticated treatment methods don't reliably eliminate the parasite from municipal water supplies.
• An infected organ transplant or transfused blood. In rare cases, toxoplasmosis can be transmitted through an organ transplant or blood transfusion.

Anyone can become infected with toxoplasmosis. The parasite is found throughout the world, although it's less common at high altitudes and in regions with extreme temperatures. In most cases, if you do contract toxoplasmosis, you'll have few, if any, signs and symptoms. But you're at risk of serious health problems if you:

• Are living with HIV/AIDS. Roughly 40 percent of people living with HIV/AIDS have toxoplasmosis. In some cases, the infection is recent (acute), and in others, an old infection has turned active. If you have HIV/AIDS, it's important to have a toxoplasmosis test. If the test is positive, your doctor can monitor the infection, which is most likely to become active if your CD4 lymphocyte count — the measure of your body's immunity — falls below 100. A negative test means you can take measures to prevent future infection.
• Are undergoing chemotherapy. Chemotherapy affects your immune system, making it difficult for your body to fight even minor infections.
• Are taking steroids or other immunosuppressant drugs. Medications used to treat certain nonmalignant conditions suppress your immune system and make you more likely to develop complications of toxoplasmosis.
• Are pregnant. Nearly half the children born to infected women develop toxoplasmosis, although these children may not experience signs and symptoms for years. If you're pregnant or planning to become pregnant, ask your doctor whether it would be appropriate for you to be tested. If you have active toxoplasmosis, treatment can greatly reduce the risk to your baby.

See your doctor if you develop any of the signs or symptoms of severe toxoplasmosis — confusion, seizures or serious eye infections — especially if you're at high risk of the disease. If you are living with HIV or AIDS or are pregnant or thinking of becoming pregnant, talk to your doctor about being tested. You can be tested at any time, but it takes about three weeks for your body to develop detectable antibodies after an initial infection.

Most pregnant women in the United States aren't routinely screened for toxoplasmosis, and only two states — New Hampshire and Connecticut — screen infants for the infection. Without specific screening, toxoplasmosis is often difficult to diagnose because signs and symptoms, when they occur, are similar to those of more common illnesses such as the flu and mononucleosis.

Testing in pregnancy
If your doctor suspects you have the infection, you may have several blood tests that check for antibodies to the parasite. Antibodies are proteins produced by your immune system in response to the presence of foreign substances, including viruses, bacteria, parasites, drugs and toxins. Because these antibody tests can be difficult to interpret, the Centers for Disease Control and Prevention (CDC) recommends that all positive results be confirmed by a laboratory that specializes in diagnosing toxoplasmosis. Ordinarily, you'll receive the test results in about a week, although expert confirmation will likely take longer.

What the test results mean
Sometimes you may be tested early in the course of the disease before your body has a chance to produce antibodies. In that case, you may have a negative result, even though you're infected. If your doctor is still suspicious, you'll need to be retested in two to three weeks. In most cases, though, a negative toxoplasmosis test result means you've never been infected and therefore aren't immune to the disease. If you're at high risk, you can take certain precautions so that you don't become infected in the future.

A positive result, on the other hand, doesn't necessarily mean you're actively infected. In many cases, it's a sign that you were infected at some time in your life and are now immune to the disease. Further tests can help determine when the infection occurred, based on the types of antibodies in your blood, and whether the levels of these antibodies are rising or falling. This is especially important if you're pregnant or living with HIV/AIDS.

Testing your baby
If you are pregnant and have a current toxoplasmosis infection, the next step is to determine whether your baby also is infected. Tests your doctor may recommend include:

• Amniocentesis. In this procedure, which can be done safely after 15 weeks of pregnancy, your doctor uses a fine needle to remove a small amount of fluid from the fluid-filled sac that lines your uterus (amniotic sac). Tests are then performed on the fluid to check for evidence of toxoplasmosis. The test carries a slight risk of miscarriage, ranging from one in 200 to one in 400. You may also experience minor complications, such as cramping, leaking fluid or irritation where the needle was inserted.
• Ultrasound scan. This test uses sound waves to produce images of your baby in the womb. A detailed ultrasound can't diagnose toxoplasmosis, although it can show whether your baby has certain signs, such as hydrocephalus. But because most infants don't show signs of toxoplasmosis at birth, a negative ultrasound doesn't rule out the possibility of infection. For that reason, your newborn will need a thorough examination after birth and follow-up blood tests during the first year of life.

Testing in severe cases
If you've developed a life-threatening illness such as toxoplasmic encephalitis, you may need one or more imaging tests to check for lesions or cysts in your brain. These include:

• Magnetic resonance imaging (MRI). This test uses a magnetic field and radio waves to create cross-sectional images of your head and brain. During the procedure, you lie inside a large, doughnut-shaped machine that contains a magnet with coils that send and receive radio waves. In response to these radio waves, your body produces faint signals that are picked up by the coils and processed into images by a computer. MRI is noninvasive and poses no risks to your health.
• Brain biopsy. In rare cases, especially if you don't respond to treatment, a neurosurgeon may take a small sample of tissue from your brain. The sample is then analyzed in a laboratory to check for the presence of toxoplasmic cysts.

If you have a strong immune system, you're not likely to experience any complications of toxoplasmosis, although otherwise healthy people sometimes develop eye infections. But if your immune system is compromised, especially as a result of HIV/AIDS, toxoplasmosis can lead to seizures and life-threatening illnesses such as encephalitis — a serious brain infection. In people living with AIDS, untreated encephalitis resulting from toxoplasmosis is always fatal. Relapse also is a constant concern for immunocompromised people with toxoplasmosis.

Children with congenital toxoplasmosis may develop disabling complications, including hearing loss, blindness and mental retardation.

Most healthy people don't require any treatment for toxoplasmosis. But if you're otherwise healthy and have signs and symptoms of acute toxoplasmosis, your doctor may prescribe the following drugs:

• Pyrimethamine (Daraprim). This antimalarial medication is also used to treat toxoplasmosis. It's a folic acid antagonist, which means it may prevent your body from absorbing the important B vitamin folate (folic acid, vitamin B-9), especially when you take high doses over a long period of time. For that reason, your doctor may recommend taking additional folic acid. Other potential side effects of pyrimethamine include bone marrow suppression and liver toxicity.
• Sulfadiazine. This antibiotic is used in combination with pyrimethamine to treat toxoplasmosis. Possible side effects include nausea, vomiting and diarrhea.

Treating people with HIV/AIDS
If you are living with HIV/AIDS and have toxoplasmosis, the treatment of choice is also pyrimethamine and sulfadiazine, along with folic acid. An alternative is pyrimethamine in conjunction with clindamycin (Cleocin) — an antibiotic that can sometimes cause severe diarrhea. Normally, you'll need to take these medications for life, although in some cases, your doctor may consider stopping toxoplasmosis therapy if your CD4 count remains very high for at least three to six months. Side effects of most drugs can be more severe in people with HIV/AIDS.

Treating pregnant women and babies
If you're pregnant and currently infected with toxoplasmosis but your baby isn't affected, you may be given the antibiotic spiramycin. Use of this drug can reduce the likelihood that your baby will become infected, without posing a risk to you or your child. Although routinely used to treat toxoplasmosis in Europe, spiramycin is still considered an experimental drug in the United States. Your doctor can obtain it from the Food and Drug Administration.

When tests indicate that your unborn child has toxoplasmosis, your doctor may suggest treatment with pyrimethamine and sulfadiazine. Because these drugs can have serious side effects for both women and babies, they're normally not used during pregnancy, but doctors sometimes prescribe them in extreme circumstances. Drug treatment can lessen the severity of congenital toxoplasmosis, but it won't undo any damage that's already been done.

Although effective therapy is available for toxoplasmosis, all treatments have side effects and may not protect an unborn child. That's why the best approach is prevention. These precautions can help keep you safe:

• Wear gloves when you garden or handle soil. Gardening can be relaxing and fulfilling, but it can also expose you to toxoplasmosis. Wear gloves whenever you work outdoors, and then wash your hands thoroughly with soap and water, especially before you eat or prepare food.
• Don't eat raw or undercooked meat. Meat, especially lamb, pork and venison, can harbor toxoplasma organisms. For that reason, cook all red meat until it reaches an internal temperature of 160 F or until no trace of pinkness remains and juices run clear. Don't taste meat before it's fully cooked. Avoid raw cured meat such as Parma ham.
• Wash kitchen utensils thoroughly. After preparing raw meat, thoroughly wash cutting boards, knives and other kitchen utensils in hot, soapy water to prevent cross-contamination of other foods. Wash your hands carefully after handling raw meat.
• Wash all fruits and vegetables. If possible, use a vegetable soap to wash fruits and vegetables, especially if you're eating them raw. Otherwise, scrub them carefully.
• Avoid unpasteurized goat's milk. Raw goat's milk and goat's milk products may contain toxoplasma parasites.

For cat lovers
If you are living with HIV/AIDS, or are pregnant or planning to become pregnant, you're right to be concerned about toxoplasmosis. But you don't have to give up your cat. Here are a few simple steps that can keep both you and your animal companion healthy:

• Help your cat stay healthy. Keep your cat indoors and feed it dry or canned cat food, not raw meat. Cats can become infected from eating infected prey or undercooked meat that contains the parasite.
• Don't adopt stray cats or kittens. Although all stray animals need good homes, it's best to let someone else adopt them. Most cats don't show signs of toxoplasma infection, and although they can be tested for toxoplasmosis, it may take up to a month to get the results.
• Have someone else clean your cat's litter box. If that's not possible, always wear gloves to change the litter and then wash your hands well with soap and hot water. Change the litter box every day so that any excreted oocysts don't have time to become infectious. Disinfect the litter box with scalding water — chemical disinfectants aren't effective against T. gondii — but don't set the box on the kitchen counter or allow your cat on the kitchen counter.