Toxoplasmosis, infection of tissue cells of the central nervous system, spleen, liver, and other organs by a parasite, Toxoplasma gondii. Infection occurs in domestic and wild animals, birds, and humans and is worldwide in distribution. It is estimated that 30 to 50 percent of the world’s human population carries demonstrable antibodies, but overt symptoms are rare in adults. Swollen glands and fever are the most common findings in those who have any symptoms.
Organisms of the genus Toxoplasma reproduce by fission or internal budding. They move by a gliding motion, lacking either flagella or pseudopodia. Of uncertain taxonomic position, they are considered to be related to the Sporozoa and possibly to the fungi. They are generally placed in the sporozoan class Toxoplasmea, although they may more properly be classed as coccidian parasites under the order Eucoccidia.
The mode of transmission of the organisms is unknown, although they can be passed from mother to fetus through the placenta. Congenital toxoplasmosis may result in stillbirth or abortion; infected infants may show various symptoms including jaundice, encephalitis, mental defects, and eye disease. Siblings of an infected infant are usually normal.
Toxoplasmosis life cycle
- A cat becomes infected with T gondii by eating contaminated meat containing bradyzoites
- Macro and micro-gametocytes fuze to form zygotes in the GI tract
- They undergo change and are shed a oocysts
- Within these, zygotes divide into sporozoites which become infectious about 24 hrs after being shed in cat faeces (and can remain so for up to 1 year in humid environments)
- Humans ingest the T.gondii
- Sporozoites are released from oocysts and the organisms enter GI tract cells by a process similar to phagocytosis
- Tachyzoites multiply within the cells
- Cells rupture releasing tachyzoites infect adjoining cells and are spread via the lymphatic and bloodstream to other tissues.
- As the tachyzoites proliferate they produce necrotic foci surrounded by an inflammatory reaction.
- Tissue cysts with bradyzoites form in the tissues and remain for the whole lifespan of the host.
- The normal immune response eventually clears tachyzoites from tissues.
- The cysts produce little.no inflammatory response but can reactivate in immune-compromised patients.
Risk factors of toxoplasmosis
Anyone can become infected with toxoplasmosis. The parasite is found throughout the world.
You’re at risk of serious health problems from toxoplasmosis infection if:
- You have HIV/AIDS. Many people with HIV/AIDS also have toxoplasmosis, either a recent infection or an old infection that has reactivated.
- You’re undergoing chemotherapy. Chemotherapy affects your immune system, making it difficult for your body to fight even minor infections.
- You take steroids or other immunosuppressant drugs. Medications used to treat certain non-malignant conditions suppress your immune system and make you more likely to develop complications of toxoplasmosis.
Toxoplasmosis is caused by the parasitic protozoa Toxoplasma gondii. Cats are the only definitive hosts (a host in which the parasite can sexually reproduce) and so both wild and domestic cats serve as the main reservoir of infection. There are three infectious stages in the lifecycle of this protozoa. Stage one is within the tissue of the host, stage two is when the protozoan is excreted by the host in the feces and stage three is when the protozoan transferred across the placenta in mammals to their offspring. Transmission can, therefore, occur by eating infected tissue, contact with infective feces or transfer from an infected mother to a developing fetus (in mammals only).
People get toxoplasmosis when they come into contact with the parasite. This can happen in several ways.
You can get it through food if you:
- Eat raw or undercooked meat, especially pork, lamb, or wild game.
- Touch something, such as a cutting board or dish, that has been in contact with raw or undercooked meat and then touch your mouth, eyes, or face.
- Eat unwashed raw fruits and vegetables.
- Drink water with the parasite in it.
Cats can carry the parasite, especially if they hunt animals or eat raw meat. You can expose yourself to the parasite through your cat’s feces, either in its litter or in dirt that has cat droppings in it. You also can get it from touching anything that has been in contact with cat droppings.
People who have an organ transplant or a blood transfusion can get toxoplasmosis if the organ or blood is infected. This is very rare.
In most cases of animal and human infection, toxoplasmosis does not cause any symptoms. The only evidence of infection is the detection of antibodies in the blood against the toxoplasmosis parasite.
Symptoms, if they do occur, include:
- Swollen lymph glands, especially around the neck
- Muscle aches and pains
- Generally feeling unwell
- Inflammation of the lungs
- Inflammation of the heart muscle
- Inflammation of the eye, for example, the retina (at the back of the eye).
Complications of toxoplasmosis
The reason that pregnant women should take special precautions to avoid toxoplasmosis is that it can be very serious, even fatal, for a baby infected in the uterus. For those who survive, toxoplasmosis can have lasting consequences on the:
They may also have mental and physical developmental delays and recurring seizures.
In general, babies who are infected early during pregnancy suffer from more severe issues than those infected later in the pregnancy. Babies born with toxoplasmosis may have a higher risk of hearing and vision losses. Some children may be affected by learning disabilities
Diagnosis and tests for toxoplasmosis
Most pregnant women in the United States aren’t routinely screened for toxoplasmosis, and most states don’t 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 during pregnancy
If your doctor suspects you have the infection, you may have 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, such as parasites. Because these antibody tests can be difficult to interpret, the Centers for Disease Control and Prevention recommends that all positive results be confirmed by a laboratory that specializes in diagnosing toxoplasmosis.
What 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. To be certain, your doctor may recommend retesting several weeks later.
In most cases 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.
A positive result could mean that you have an active infection, or it could mean that you were once infected and are immune to the disease. Additional tests can pinpoint when the infection occurred, based on the types of antibodies in your blood. This is especially important if you’re pregnant or you have 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 may 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 surrounds the fetus (amniotic sac). Tests are then performed on the fluid to check for evidence of toxoplasmosis. Amniocentesis carries a slight risk of miscarriage and 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. It can, however, show whether your baby has certain signs, such as fluid buildup in the brain (hydrocephalus). However, a negative ultrasound doesn’t rule out the possibility of infection. For that reason, your newborn will need an examination 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 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 (electromagnetic) 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 surrounded by coils that send and receive radio waves. In response to the 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 minimal 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 brain tissue. The sample is then analyzed in a laboratory to check for toxoplasmosis cysts.
Treatment and medications
In acute toxoplasmosis in generally healthy people, pyrimethamine (anti-malaria medication) may be prescribed in conjunction with the antibiotic, sulfadiazine.
For those with HIV/AIDS, the same combination is recommended with additional folic acid. Special considerations need to be given to pregnant women who are infected, and treatment options should be discussed with your doctor.
To treat an acute (active) infection, doctors will prescribe antibiotics and other medications able to clear the infection and prevent the disease from progressing. The most commonly prescribed medications include:
- Pyrimethamine, an antimalarial drug considered the most effective agent in treating an acute toxo infection2
- Sulfadiazine, an antibiotic drug used in combination with pyrimethamine
- Clindamycin, an alternative antibiotic used alongside pyrimethamine
- Minocycline, an antibiotic used only when a person is resistant to both sulfadiazine and clindamycin
- Folinic acid used as a supplement to reduce the risk of thrombocytopenia (low platelets)
To prevent a recurrence, the antibiotic drug trimethoprim and sulfamethoxazole (TMP-SMX) would be taken once daily for four weeks.
The transmission of T. gondii from mother to child is rare unless the mother is severely immune-compromised among these mothers, treatment is commonly prescribed to prevent fetal infection.
Scientists remain less certain about mothers with normal immune function.
To this end, special care should be sought if you are pregnant and are at increased risk of toxoplasmosis. Treatment is typically prescribed as follows:
- If an acute infection occurs during the first trimester, spiramycin would be prescribed from the time of diagnosis to the time of delivery.
- After the first trimester (or if a fetal infection has been confirmed), pyrimethamine, sulfadiazine, and folinic acid would be used from diagnosis to delivery.
Prevention of Toxoplasmosis
You can help prevent toxoplasmosis by taking the following precautions:
- Do not eat meat that is raw or rare. If you have a meat thermometer, cook meat to an internal temperature of at least 140° Fahrenheit.
- If you are pregnant or have a weakened immune system, do not handle raw meat or change a cat’s litter box. If you cannot avoid doing these things, use gloves.
- To own a cat, keep it indoors and feed it store-bought canned or dry cat food.
- Wash your hands thoroughly after you handle raw meat after you work in your garden and after you change a cat’s litter box.
- If you have HIV, you will be tested to see if you have antibodies against toxoplasmosis in your blood, which would indicate that you were infected in the past. If the antibody test is positive, and your immune system is severely weakened, you will be treated with medications, such as the antibiotic trimethoprim-sulfamethoxazole (Proloprim, Trimpex) to prevent the disease from reactivating. If the antibody test is negative, you will be counseled to avoid infection by using the techniques described above.