Toxoplasmosis.. A Parasitic Disease

Toxoplasmosis is a parasitic disease caused by Toxoplasma gondii, a single-celled protozoan that infects warm-blooded animals, including humans. It is one of the most widespread parasitic infections globally, with an estimated 30–50% of the world’s population having been exposed to the parasite.

Toxoplasma gondii can cross the placenta and infect the fetus, causing miscarriage, stillbirth, or severe lifelong complications in the child.

Transmission

Toxoplasmosis is a unique health disease that significantly affects the health of humans, domestic animals, wildlife and is present in ecosystems, including water, soil and food.

Transmission of Toxoplasma gondii occurs through several pathways. The most common is the ingestion of undercooked or raw meat that contains tissue cysts, especially pork, lamb, or venison. The parasite can also be acquired by consuming food or water contaminated with oocysts shed in the feces of infected cats—the definitive host of Toxoplasma gondii.

Another significant route of transmission is congenital: when a woman acquires the infection during pregnancy, the parasite can cross the placenta and infect the fetus.

Rarely, the parasite may be transmitted through organ transplantation or blood transfusion.

Animal Toxoplasmosis

Toxoplasma gondii is found in all habitats and regions, from the Arctic to the tropics in terrestrial, aquatic and marine environments, affecting a wide range of hosts. Theoretically all warm-blooded animals can be infected and at least 350 host species have been described to date.

Most cats infected with Toxoplasma gondii do not exhibit symptoms, but some may experience mild gastrointestinal distress or fever. Infected cats shed oocysts in their feces. Cats can become infected by eating raw meat from intermediate hosts like rodents or birds that have cysts in their tissues.

Infected livestock (e.g., sheep and goats) may experience mild symptoms such as fever, loss of appetite, and general malaise. However, Toxoplasma gondii is often subclinical in adult animals. The greatest concern arises when pregnant animals become infected. Pregnant sheep and goats may experience spontaneous abortion or birth defects if they are infected during pregnancy. Infected newborns may exhibit neurological symptoms, such as tremors, paralysis, or developmental defects.

Rodents, especially rats and mice, are often intermediate hosts. Infection can alter their behavior, making them less fearful of predators like cats. This behavior makes them more likely to be preyed upon, thereby facilitating the parasite's life cycle.

Transmission to the Fetus

Primary infection during pregnancy poses the greatest risk to the fetus. If the mother was infected before pregnancy, she usually has protective antibodies and is not at risk of transmitting the parasite.

Transmission to the fetus depends on gestational age. Transmission in the first trimester is low (~10–15%) but more severe fetal damage. Transmission in the third trimester is high (up to 60–80%) but damage is often milder or delayed.

Incubation Period

When humans become infected by consuming oocysts (eggs) from contaminated food, water, or soil, or by handling cat litter, the incubation period is typically 5 to 23 days after exposure. However, it can range from as short as 1 week to several months.

If the infection is acquired through the consumption of undercooked or contaminated meat containing tissue cysts, the incubation period can also be 5 to 23 days.

In the case of a pregnant woman transmitting the infection to her unborn child (congenital toxoplasmosis), the incubation period is variable and depends on the stage of pregnancy at the time of infection. The parasite may cross the placenta at different stages, with varying impacts on the fetus.

In immunocompromised individuals (e.g., people with HIV/AIDS or those undergoing immunosuppressive treatment), the incubation period may be shorter, and the disease may progress more rapidly.

Symptoms of Toxoplasmosis

The parasite can invade different organs (like the brain, eyes, or muscles), and the site of infection may also affect how soon symptoms manifest.

In most healthy adults, toxoplasmosis is usually asymptomatic or causes only mild flu-like symptoms. When symptoms do occur, they can include flu-like symptoms such as fever, fatigue, headaches, muscle aches, sore throat, swollen lymph nodes (particularly around the neck or underarms). Occasionally, a rash may appear on the skin. Mild gastrointestinal symptoms such as nausea or diarrhea may also occur but are less common. These symptoms typically last for a few weeks and are often mistaken for other common viral infections, making the condition difficult to diagnose without specific tests. In most cases, the immune system controls the infection, and individuals recover without medical treatment.

In individuals with weakened immune systems, toxoplasmosis can cause more severe symptoms due to the inability of the immune system to keep the parasite under control. Encephalitis is one of the most common and serious complications in immunocompromised individuals. It can cause headaches, seizures, confusion or altered mental state, motor weakness or paralysis, and loss of coordination. Toxoplasmosis can cause eye inflammation, leading to blurry vision, eye pain, redness in the eye, sensitivity to light, loss of vision.

In rare cases, toxoplasmosis can lead to pneumonia, causing symptoms like cough, difficulty breathing, and chest pain.

Congenital Toxoplasmosis

If a pregnant woman becomes infected with Toxoplasma gondii, it can be passed to the fetus, causing congenital toxoplasmosis. The symptoms of congenital toxoplasmosis can vary depending on the stage of pregnancy at the time of infection.

If the infection occurs during early pregnancy, it can lead to miscarriage, stillbirth, and severe developmental abnormalities in the fetus (e.g., brain damage, hydrocephalus).

If the infection happens later in pregnancy, the baby may be born with milder symptoms or may appear asymptomatic at birth. However, the child may develop symptoms later in life, such as vision problems (e.g., retinitis), mental disabilities, seizures, and hearing loss.

In newborns with congenital toxoplasmosis, symptoms may not appear immediately after birth. Some possible symptoms that can develop over time include jaundice, enlarged liver or spleen, mental and developmental delays, severe eye infections leading to vision loss, and seizures.

Complications

Complications of toxoplasmosis can be life-threatening. In immunocompromised patients, reactivation of latent infection can result in fatal encephalitis. In the eyes, inflammation of the retina can impair vision or lead to blindness. Congenital toxoplasmosis may result in long-term neurological deficits, developmental delays, or death. Because symptoms can be delayed, especially in infants, long-term monitoring and follow-up are essential.

Prevention

Preventing toxoplasmosis relies on food safety, personal hygiene, and environmental precautions.

Cooking meat thoroughly, washing fruits and vegetables, and avoiding raw or unpasteurized dairy products can significantly reduce the risk of infection.

Pregnant women should avoid cleaning cat litter or use gloves and wash hands thoroughly afterward. Litter boxes should be cleaned daily before oocysts become infective. People should also wear gloves while gardening or handling soil and avoid drinking potentially contaminated water.

Public education, especially for at-risk populations such as pregnant women and immunocompromised individuals, remains essential in controlling the disease’s impact. Early detection and proper treatment can significantly reduce fetal damage.