Causes and Risk Factors of Toxoplasmosis

Toxoplasmosis (also known as "toxo") is caused by a single-celled parasite known as Toxoplasma gondii. It is most commonly caused by eating contaminated food or by accidental hand-to-mouth contact with cat feces. The parasite can also be passed from mother to child during pregnancy and, less commonly, during an organ or stem cell transplant.

While the disease usually causes few, if any, symptoms, it can turn deadly in people with compromised immune systems or in babies infected during pregnancy.

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

Routes of Transmission

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

Infected Meat

When infected, the immune system of the host (whether it be animal or human) will usually be able to control the infection. However, the parasite doesn’t disappear. Rather, it goes into a state of dormancy, forming tiny cysts in tissues (called bradyzoites) throughout tissues of the body.

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

Cat Feces

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

These oocysts are replication-ready and able to survive for many months in hot or cold temperatures because of their thick-walled structure. They can even survive and proliferate in water supplies.

Once ingested, the oocysts undergo a process known as excystation in which the parasite is released and is able to infect cells of the digestive tract, lungs, and other organ systems.

Common Causes

Toxoplasmosis most often occurs when T. gondii oocysts or tissue cysts are accidentally eaten. This usually occurs when:

During Pregnancy

Congenital toxoplasmosis occurs when T. gondii is passed from the mother to the child during pregnancy. this usually happens when the mother is infected during the pregnancy itself or in the three months leading up to conception.

Getting infected doesn’t necessarily mean that your baby will be infected. In fact, during the early part of the first trimester, the risk will be relatively low (less than six percent). However, as the pregnancy progresses, the risk will steadily increase.

Less commonly, transmission can occur in mothers previously infected with T. gondii. We see this mostly in women with HIV. Among this population of women, bradyzoites can sometimes reactivate and become infectious. The risk tends to increase in association with the decline in immune function.

Who Is at Risk?

While the risk during pregnancy is more or less the same as those of the general population, research from the Centers for Disease Control and Prevention identified 11 characteristics that place a pregnant woman at an increased risk of T. gondii infection:

• Owning a cat
• Cleaning a litter box
• Eating raw or undercooked pork, mutton, lamb, beef, or minced-meat products
• Gardening
• Eating raw or unwashed vegetables or fruits
• Eating raw vegetables outside of the home
• Coming into contact with soil
• Washing kitchen knives infrequently
• Having poor hand hygiene
• Traveling outside of Europe, Canada, or the United States
• Drinking unpurified water from a contaminated source

In People With HIV

Toxoplasmosis is considered an opportunistic infection (OI) in people with HIV in that it only causes illness when the immune system has been severely depleted. We can measure this by the number of CD4 T-cells in our blood. Healthy people will have anywhere from 800 to 1,500 of these cells in a sample of blood. Those with less than 200 are at risk of an ever-widening range of serious and potentially deadly OIs.

For most people with HIV, a T. gondii infection is not newly acquired but rather the reactivation of a past infection. When a person's CD4 count dips below 50, the immune system will no longer be able to keep the dormant bradyzoites in check.

The bradyzoites, seizing the opportunity, will convert back into tachyzoites and reap havoc on the tissues and organs in which they were embedded. These would most commonly involve the brain and central nervous system (CNS toxoplasmosis), the eyes (ocular toxoplasmosis), and the lungs (pulmonary toxoplasmosis).

Fortunately, antiretroviral therapy used to treat an HIV infection can inhibit the virus' ability to replicate. By doing so, the viral population can be suppressed to undetectable levels, allowing the immune system to reconstitute itself and place T. gondii back into check.

In Organ Transplant Recipients

The transplantation of organs infected with T. gondii can also lead to infection in the organ recipient. This is most often seen with 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 have no defense against T. gondii reactivation, the research to date has been largely conflicted.

Of these, 324 tested positive for T. gondii.

By contrast, a smaller study from Mexico in 2017 looked at 20 cases of T. gondii transmission that occurred as a result of a liver transplant. According to the investigators, 14 patients (or 70 percent) had to be treated for T. gondii reactivation after the transplant. Of these, eight (or 40 percent) died as a result of the infection.

Despite the conflicting evidence, the Organ Procurement and Transplantation Network (OPTN) established by the U.S. Congress in 1984, has dictated that all donated organs be routinely screened for T. gondii. Those that test positive are not removed from the supply chain but are rather matched with donors who also test positive.