Toxoplasmosis is an infectious disease caused by a single-celled protozoan parasite called Toxoplasma gondii. This microscopic organism has a complex life cycle that involves cats, other animals, and humans.

The key to understanding the disease is to know the parasite’s life cycle and the central role of the feline family.

• Cats as the Definitive Host: The Toxoplasma parasite can only complete its full sexual reproductive cycle inside the intestines of a member of the cat family (both domestic and wild cats).
• Oocysts in Feces: An infected cat sheds millions of microscopic, egg-like structures called oocysts in its feces. These oocysts are not immediately infectious.
• Maturation in the Environment: The oocysts must “ripen” or sporulate in the soil, sand, or litter box for one to five days before they can cause infection.
• Intermediate Hosts: Other animals, such as mice, birds, sheep, and pigs, can become infected by ingesting these sporulated oocysts from the environment. Once inside an intermediate host, the parasite forms dormant tissue cysts, typically in the brain and muscle tissue.
• Humans as Accidental Hosts: Humans become infected either by ingesting the sporulated oocysts from the environment or by eating undercooked meat containing the dormant tissue cysts.

After a person is infected, a healthy immune system quickly controls the parasite, forcing it into a dormant state within tissue cysts. The person then has a lifelong latent infection and is generally immune to reinfection. The problem arises when this process occurs for the first time during pregnancy or when the dormant cysts reactivate in someone whose immune system can no longer keep them in check.

In my experience, toxoplasmosis often goes unnoticed in healthy individuals, but it can become life-threatening in immunocompromised patients or during pregnancy.

The sole cause of toxoplasmosis is infection with the parasite Toxoplasma gondii. It is not caused by a bacterium or a virus. The parasite exists worldwide, and humans become infected through several well-defined routes of transmission.

Clinically, the infection is caused by ingesting oocysts from contaminated soil, water, or undercooked meat or through vertical transmission from mother to fetus.

Common conditions include: Ocular Toxoplasmosis

A person can become infected with Toxoplasma gondii in one of several ways. The infection is primarily acquired from the environment or food.

The most common modes of transmission include:

• Foodborne Transmission (from Undercooked Meat): This is considered the leading cause of infection in many parts of the world. A person can become infected by eating raw or undercooked meat, particularly pork, lamb, or venison, that contains the dormant Toxoplasma tissue cysts.
• Zoonotic Transmission (from Cat Feces): This happens through the accidental ingestion of the parasite’s oocysts.
  • Cleaning a cat’s litter box: If an infected cat has shed oocysts in its feces, a person can become contaminated while cleaning the litter box and can then infect themselves if they do not wash their hands thoroughly afterward.
  • Gardening: Cats often use gardens or sandboxes as a litter box. A person can get oocysts on their hands while gardening in contaminated soil.
• Environmental Contamination: Drinking water from a source that has been contaminated with oocysts.
• Congenital Transmission (Mother to Child): This is a critical route of transmission. A woman who gets a new, primary infection with Toxoplasma just before or during her pregnancy can pass the infection across the placenta to her developing fetus.
• Rare Routes: In very rare cases, the infection can be transmitted by organ transplantation from an infected donor or blood transfusion.

It is important to know that you cannot get toxoplasmosis from petting a cat. The infection is only spread by coming into contact with the cat’s feces.

Clinically, transplacental transmission is a major concern especially if a woman acquires the infection during the first trimester of pregnancy, potentially affecting fetal development.

The clinical presentation of toxoplasmosis depends entirely on the immune status of the infected person.

In Immunocompetent Individuals (People with a Healthy Immune System)

More than 80% of healthy children and adults who get infected with Toxoplasma gondii have no symptoms at all and are completely unaware they have been infected.

• When symptoms do occur, they are typically mild, non-specific, and flu-like. They can include:
  • Swollen lymph nodes, especially in the neck.
  • Muscle aches and pains.
  • Headache.
  • Fever.
  • Sore throat.
• In healthy individuals, the immune system brings the infection under control within a few weeks, and the parasite enters its dormant state.

In Pregnant Women and Congenital Toxoplasmosis

This is where toxoplasmosis becomes a major concern.

• An expectant mother who gets infected for the first time during pregnancy is usually asymptomatic herself.
• However, the parasite can cross the placenta and infect the fetus, leading to congenital toxoplasmosis, which can have devastating consequences.
• The risk of transmission to the fetus and the severity of the outcome depend on when the mother gets infected. Infection during the first trimester is less likely to be transmitted but can cause the most severe damage to the baby.
• Potential consequences for the baby include miscarriage, stillbirth, or a child born with serious health problems, including the “classic triad” of:
  • Chorioretinitis: Inflammation of the retina and choroid of the eye, which can lead to vision loss and blindness.
  • Intracranial Calcifications: Small areas of calcium deposits in the brain.
  • Hydrocephalus: A buildup of fluid in the brain, causing an enlarged head.
• Many infected infants show no symptoms at birth but can develop vision problems, hearing loss, or learning disabilities years later.

In Immunocompromised Individuals

This is the other high-risk scenario where toxoplasmosis can be life-threatening.

• For a person with a severely weakened immune system, such as a patient with advanced HIV/AIDS or an organ transplant recipient taking immunosuppressive drugs, a latent Toxoplasma infection can reactivate.
• The dormant cysts can “wake up” and begin to multiply uncontrollably.
• The most common presentation of reactivated toxoplasmosis is toxoplasmic encephalitis, a brain infection. Symptoms can include:
  • Severe headache.
  • Confusion, disorientation.
  • Fever.
  • Seizures.
  • Focal neurological deficits, like weakness on one side of the body.
  • Poor coordination.

Most healthy individuals are asymptomatic or experience mild flu-like symptoms. But I watch for severe neurological or ocular symptoms in immunocompromised patients.

Toxoplasmosis diagnosis is usually made with serological blood tests. These tests do not look for the parasite itself, but for the antibodies that the body’s immune system produces in response to the infection.

• IgG Antibodies: The presence of IgG antibodies indicates that a person has been infected with Toxoplasma at some point in the past. A positive IgG test in a healthy person means they have a latent infection and are likely immune to getting a new infection.
• IgM Antibodies: The presence of IgM antibodies usually indicates a recent or acute infection.
• Interpreting these tests, especially during pregnancy, can be complex and requires an expert. Additional specialized tests are often needed to confirm the timing of the infection.

Diagnosing in other settings:

• Congenital Toxoplasmosis: The diagnosis can be made during pregnancy by testing the amniotic fluid for the parasite’s DNA using a PCR test. After birth, the baby’s blood and spinal fluid can be tested.
• Toxoplasmic Encephalitis: In an immunocompromised patient, the diagnosis is often suspected based on a brain MRI or CT scan, which will typically show one or more characteristic “ring-enhancing” lesions. A brain biopsy may be needed to definitively confirm the diagnosis.

In my experience, brain imaging may show ring-enhancing lesions in immunocompromised patients, and ophthalmologic exams are crucial in those with visual symptoms.

The decision to treat toxoplasmosis depends entirely on the clinical situation and the immune status of the patient.

• For Healthy, Non-Pregnant Individuals: Because the infection is mild and self-limiting, no treatment is necessary.
• For Pregnant Women: If an acute infection is diagnosed during pregnancy, a maternal-fetal medicine specialist will be consulted. Treatment with a specific antiparasitic drug, such as spiramycin, may be started to try to reduce the risk of the parasite being transmitted to the fetus.
• For Congenital Toxoplasmosis: All infants diagnosed with congenital toxoplasmosis must be treated, even if they have no symptoms at birth. The standard treatment is a year-long course of a combination of medications, typically pyrimethamine, sulfadiazine, and folinic acid.
• For Ocular Toxoplasmosis: Inflammation in the eye is also treated with the same combination of medications.
• For Immunocompromised Patients: Individuals with toxoplasmic encephalitis require urgent, aggressive treatment with pyrimethamine and sulfadiazine. After the initial infection is controlled, patients with persistent immunosuppression (like AIDS) will often be placed on lifelong, lower-dose maintenance therapy to prevent the infection from relapsing.

Prevention: The Key to Safety

Since treatment is often complex and the consequences can be severe for high-risk groups, the most important strategy is prevention.

• Cook Meat Thoroughly: Cook all meat to safe internal temperatures to kill any potential tissue cysts.
• Practice Good Hand Hygiene: Wash hands thoroughly with soap and water after handling raw meat, gardening, or cleaning a cat’s litter box.
• Wash Fruits and Vegetables: Wash all produce thoroughly before eating it raw.
• Precautions for Pregnant Women: Pregnant women who are not already immune to toxoplasmosis should take extra precautions. They should avoid changing cat litter if possible, or wear gloves and wash hands thoroughly afterward. They should also avoid eating undercooked meat.

Clinically, I stress the importance of early treatment in congenital cases and immunocompromised individuals to prevent long-term neurological and visual damage.

Toxoplasmosis is a ubiquitous parasitic infection that exists in a silent, harmless state in a vast portion of the human population. For the average healthy person, it poses no threat. However, its potential to cause devastating damage to a developing fetus and to wreak havoc in a person with a weakened immune system makes it a disease of significant public health importance. The power to avoid the severe consequences of this disease lies in prevention. Clinically, I find that preventive measures like proper food handling and avoiding cat litter exposure during pregnancy can significantly reduce transmission risk.

Centers for Disease Control and Prevention (CDC). (2023). Parasites – Toxoplasmosis. Retrieved from https://www.cdc.gov/parasites/toxoplasmosis/index.html

The Mayo Clinic. (2022). Toxoplasmosis. Retrieved from https://www.mayoclinic.org/diseases-conditions/toxoplasmosis/symptoms-causes/syc-20356249

American College of Obstetricians and Gynecologists (ACOG). (2023). Cytomegalovirus, Parvovirus B19, Varicella Zoster, and Toxoplasmosis in Pregnancy. Retrieved from https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2015/06/cytomegalovirus-parvovirus-b19-varicella-zoster-and-toxoplasmosis-in-pregnancy