Toxoplasmosis: causes and risk factors

Toxoplasmosis (also known as "toxo") is caused by a single-celled parasite known as Toxoplasma gondii. This is most often due to ingestion of contaminated food or accidental hand-to-mouth contact with cat feces. The parasite can also be passed from mother to child during pregnancy and, less frequently, during organ or stem cell transplants.

According to statistics from the Centers for Disease Control and Prevention, about 11 percent of the US population over the age of five is infected with T. gondii (or approximately 39 million people).

Although the disease generally causes few or no symptoms , it can be fatal for people with weakened immune systems or infected babies during pregnancy.

By understanding the causes and risks of toxoplasmosis , you can take steps to prevent infection at any stage in your life.

© Get Information on Medicines, 2018

Transmission routes

The T. gondii parasite is found throughout the world and in virtually all warm-blooded animals. Transmission of T. gondii is unique in that it can occur in two ways: by eating infected meat or by accidentally ingesting cat feces.

Contaminated meat

When infected, the immune system of the host (whether animal or human) can usually control the infection. However, the parasite does not go away. Rather, it goes into a dormant state, forming tiny tissue cysts (called bradyzoites) throughout the body.

If a person eats an infected animal, these tissue cysts can reactivate into fully formed parasites (known as tachyzoites) and cause infection.

Cat feces

Cats, domestic or wild, are unique in that T. gondii can survive and reproduce in the intestinal lining of the animal. Within these tissues, the parasite can produce tiny cysts called oocysts, which are shed by the millions in the cat's feces.

These oocysts are ready to replicate and can survive for many months at high or low temperatures due to their thick-walled structure. They can even survive and reproduce in water sources.

When oocysts enter the body, they undergo a process known as excitation, in which the parasite is released and can infect cells in the digestive tract, lungs, and other organ systems.

Common causes

Toxoplasmosis is most often caused by accidental ingestion of oocysts or tissue cysts of T. gondii . This usually happens when :

  • Eat contaminated, raw, or undercooked meat (especially pork, lamb, or venison).
  • Touch infected meat or touch surfaces or utensils contaminated with raw meat.
  • Accidentally swallowed cat feces while cleaning the toilet or gardening on contaminated soil.
  • Eat unwashed fruits and vegetables that have been in contact with soil contaminated with cat feces.
  • You drink water contaminated with cat feces.
  • You are consuming contaminated unpasteurized dairy products.
  • You are eating contaminated raw shellfish.

During pregnancy

Congenital toxoplasmosis occurs when T. gondii is transmitted from mother to child during pregnancy. This usually occurs when the mother becomes infected during pregnancy or within three months of conception.

Infection does not necessarily mean that your child is infected. In fact, at the beginning of the first quarter, the risk will be relatively low (less than six percent). However, as the pregnancy progresses, the risk steadily increases .

Less frequently, transmission can occur from mothers previously infected with T. gondii. This is seen mainly in women with HIV . Among this group of women, bradyzoites can sometimes reactivate and become infectious. The risk tends to increase due to decreased immune function.

Who is in danger

Although the risk during pregnancy is roughly the same as in the general population, research from the Centers for Disease Control and Prevention has identified 11 characteristics that increase the risk of a pregnant woman contracting T. gondii :

  • Cat property
  • Clean the litter box
  • Eating raw or undercooked pork, lamb, lamb, beef, or meat products.
  • Gardening
  • Eating raw or unwashed vegetables or fruits.
  • Eating raw vegetables outside the home
  • Contact with the ground
  • Clean kitchen knives infrequently
  • Poor hand hygiene
  • Traveling outside of Europe, Canada or the United States
  • Drinking untreated water from a contaminated source.

In people with HIV

Toxoplasmosis is considered an opportunistic infection (OI) in people with HIV because it causes disease only when the immune system is severely weakened. We can measure this by the number of CD4 T cells in the blood. In healthy people, a blood sample contains between 800 and 1500 of these cells. Those under 200 are at risk for a growing range of serious and life-threatening opportunistic infections.

For most people with HIV, T. gondii infection is not new, but rather a reactivation of a previous infection. When a person 's CD4 count falls below 50, the immune system will no longer be able to control the inactive bradyzoites .

The bradyzoites, taking advantage of this opportunity, will again become tachyzoites and will cause damage to the tissues and organs in which they were embedded. It most often affects the brain and central nervous system (CNS toxoplasmosis), the eyes (ocular toxoplasmosis), and the lungs (pulmonary toxoplasmosis).

Fortunately, antiretroviral therapy used to treat HIV infection can inhibit the virus's ability to replicate. In this way, the viral population can be suppressed to an undetectable level , allowing the immune system to repair itself and regain control of T. gondii .

In organ transplant recipients

Transplantation of organs infected with T. gondii can also lead to infection of the recipient organ. This is most commonly seen in heart, kidney, and liver transplants, as well as hematopoietic and allogeneic stem cell transplants .

While it would be reasonable to assume that this would be dangerous, given that the recipient would not have protection against reactivation of T. gondii , the research to date is largely controversial.

A 2013 study from the Netherlands found that transmission of T. gondii during heart transplantation did not affect the survival time of 577 patients who underwent transplant surgery between 1984 and 2011.

Of these, 324 were positive for T. gondii.

In contrast, a smaller 2017 study in Mexico looked at 20 cases of T. gondii transmission that occurred as a result of a liver transplant. According to the researchers, 14 patients (or 70 percent) were required to receive treatment for reactivation of T. gondii after transplantation. Of these, eight (or 40 percent) died as a result of the infection.

Despite conflicting data, the Organ Procurement and Transplantation Network (OPTN) , created by the US Congress in 1984, required that all donor organs be periodically screened for T. gondii . Those who test positive are not removed from the supply chain, but are paired with donors who also test positive.

Frequently asked questions

  • Toxoplasmosis is caused by the parasite Toxoplasma gondii. It is relatively common and affects one in ten people in the United States.

  • Although toxoplasmosis can affect almost any warm-blooded creature, the parasite can only reproduce in the intestines of cats. Because of this, an infected cat can shed millions of T. gondii cysts in its stool for up to two weeks. After decomposition, cysts can remain infectious for more than a year in a warm, humid environment, including soil.

  • The infection is transmitted primarily by ingesting food containing T. gondii cysts or by accidentally ingesting feline feces through hand-to-mouth contact. It can also be passed from mother to child during pregnancy. Less commonly, toxoplasmosis can be passed from a donor to an organ or stem cell transplant recipient.

  • Hand-to-mouth practice helps alleviate T. gondii infestation. Common routes of oral transmission include:

    • Eating raw or undercooked meats that contain T. gondii cysts (especially lamb, pork, and venison)
    • Eating raw fruits or vegetables contaminated with soil that contains cat feces.
    • Touching your mouth after contact with a contaminated litter box or soil
    • Drink unpasteurized milk, especially goat milk.
    • Eat raw seafood

  • People with weakened immune systems, especially those with advanced HIV , are at risk because their immune systems are less able to control the infection. This can lead to a cascade of life-threatening infections in the brain or lungs, as well as serious vision damage.

  • Congenital toxoplasmosis occurs when the mother's infection spreads to the fetus, increasing the risk of preterm delivery, miscarriage, and stillbirth. Babies born with congenital toxoplasmosis often have brain defects, neurological problems, vision or hearing loss, and movement disorders.

  • If left untreated, the risk of mother-to-child transmission of T. gondii increases with each trimester, from 25% in the first trimester to 54% in the second trimester and to 65% in the third trimester.

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