Toxoplasma gondii is a fascinating and complex parasite that infects a wide range of warm-blooded animals, including humans. It is estimated that over one-third of the world’s population has been exposed to this parasite, making it one of the most successful and widespread parasites on the planet. Despite its prevalence, many people are unaware of the potential health risks associated with Toxoplasma gondii and the various ways in which it can be transmitted. Therefore, it is crucial to delve into the biology, transmission methods, and potential consequences of this parasite to better understand its impact on both human and animal health. Toxoplasma gondii has a complex life cycle that involves both intermediate and definitive hosts. Cats are the definitive host, as they are the only animals in which the parasite can sexually reproduce. Intermediate hosts, such as rodents and birds, become infected by ingesting oocysts shed by cats in their feces. Humans can also become infected by ingesting oocysts from contaminated soil or water or by consuming undercooked meat from infected animals. While most healthy individuals experience no symptoms or only mild flu-like symptoms, Toxoplasma gondii can cause severe illness in people with weakened immune systems, such as those with HIV/AIDS or undergoing chemotherapy. Pregnant women can also transmit the parasite to their unborn child, leading to serious birth defects or developmental delays. It is important for individuals with weakened immune systems and pregnant women to take precautions to avoid exposure to Toxoplasma gondii. , such as avoiding handling cat litter or consuming raw or undercooked meat. Additionally, practicing good hygiene, such as washing hands thoroughly after handling soil or raw meat, can also help reduce the risk of infection.  Toxoplasma gondii is a parasite that can be found in the feces of infected cats, which is why it is important for individuals with weakened immune systems and pregnant women to avoid handling cat litter. Furthermore, consuming raw or undercooked meat, especially pork, lamb, and venison, can also increase the risk of infection. Therefore, it is advisable for these individuals to ensure that all meat is cooked thoroughly before consumption.

Through this article, you’ll be able to answer the following questions:

• What does Toxoplasma gondii do to humans?
• Where is Toxoplasma gondii found?
• What are the characteristics of Toxoplasma gondii?
• What kills Toxoplasma gondii in humans?
• What disease does toxoplasmosis cause?
• What are 3 signs or symptoms of being infected with toxoplasmosis?
• What causes Toxoplasma gondii?
• How do humans get Toxoplasma?
• What is the main host of Toxoplasma gondii?
• What is the scientific name for toxoplasmosis?
• What diseases can cats give humans?
• Can Toxoplasma be cured?
• What is the best treatment for Toxoplasma gondii?
• How can you prevent Toxoplasma gondii?

Toxoplasmosis is a common and widespread parasitic infection caused by the parasite Toxoplasma gondii. It is usually self-limiting and often recursive, with no apparent symptoms. However, it can be a serious cause of mortality and morbidity for fetuses and immunodeficient individuals. The parasite was discovered by French parasitologists Charles Nicolle and Louis Herbert Manceaux while searching for a reservoir host for Leishmania in the North African rodent, the gundi. Gundis, found in rocky outcroppings in desert-like environments, were first discovered by Western naturalists in Tripoli in 1774 and were given the name gundi mice (Cox FE, 2002).

Toxoplasmosis is a parasitic disease that can infect various warm-blooded animals, including humans, but is primarily found in cats. Infection occurs through eating infected meat, consuming recently infected cat fecal matter, or transplacental transmission from the mother to her fetus. However, cats are a major reservoir of this infection.

Toxoplasmosis is a widespread disease worldwide, with up to one-third of the world’s population carrying evidence of infection. In the United States, the infective rate was 10.8% for the general population and 11% for women of childbearing age (Jones JL et al, 2006). The infection usually causes mild flu-like symptoms in the first few weeks, but after that, it rarely causes harm in healthy adults. However, those with weakened immune systems, HIV infection, or pregnant women may become seriously ill, with sometimes fatal results. The parasite may also cause encephalitis, inflammation of the brain’s lining, including sensory organs like the eyes, ears, heart, and liver.

Classification of Toxoplasma gondii

• Domain: Eukaryota
• Kingdom: Chromalveolata
• Phylum: Apicomplexa
• Class: Conoidasida
• Order: Eucoccidiorida
• Family: Sarcocystidae
• Genus: Toxoplasma
• Species: T. gondii

Morphology of Toxoplasma gondii

Human trophozoites exist in two morphologic forms: tachyzoites and bradyzoites, with the oocyst being the most infectious form, often encountered in veterinary parasitologic procedures.

Oocyst

Toxoplasma gondii’s infective form, the oocyst, is similar to Isospora belli but smaller. It measures 10-15 µm long by 8-12 µm wide. The transparent oocyst contains two sporocysts with four sporozoites and is bordered by a clear, colorless, two-layered cell wall.

Tachyzoites

• Actively multiplying, crescent-shaped tachyzoites range from 3 to 7 µm by 2 to 4 µm.
• One end often appears more rounded.
• Each tachyzoite has a single central nucleus surrounded by a cell membrane.
• Other organelles include mitochondrion and Golgi apparatus.

Bradyzoite Physical Characteristics

• Bradyzoites have similar physical appearance to tachyzoites, but smaller.
• Slow-growing, viable forms form cysts in host tissues and muscles outside the intestinal tract.
• Cysts can contain 50 to several thousand bradyzoites.
• Cysts range from 12 to 100 µm in diameter.

Life Cycle of Toxoplasma gondii

• Definitive host is the cat or other animals.
• Ingestion of T. gondii cysts in contaminated mice or rats releases enclosed bradyzoites, transforming into tachyzoites.
• Cats undergo both sexual and asexual reproduction in their gut.
• Sexual cycle results in production of immature oocysts, shed in stool, maturing in the outside environment.
• Intermediate hosts, mice and rats, consume infected mature oocysts.
• Sporozoites emerge from mature oocysts, transforming into tachyzoites.
• Tachyzoites migrate into the brain or muscle of the intermediate host, forming cysts filled with bradyzoites.
• Cat becomes infected on ingestion of a contaminated rodent, repeating the cycle.

T. gondii Human Infection

• Initiation via contact with infected cat feces that is, fecal oral transmission.
• Primary sources include cat litter boxes and children’s sandboxes.
• Human ingestion of contaminated undercooked meat from cattle, pigs, or sheep.
• Infective sporozoites released following ingestion, following the same cycle as natural intermediate hosts.
• Resulting cysts form in animal muscle, with parasites remaining viable for years.

Transplacental T. gondii Transfer

• Unknowingly transmitted infection from mother to unborn fetus.
• Mother produces IgG, which crosses placenta and appears in fetus/newborn’s circulation.
• Mother produces IgM, which doesn’t cross placenta.
• Infant may demonstrate anti–T. gondii IgM from birth to several months old.

Human Infection: T. gondii Tachyzoites and Their Impact

This route of toxoplasmosis is rare and involves contaminated blood being transfused into an uninfected person.

• T. gondii tachyzoites emerge from ingested cyst.
• They rapidly grow and divide, causing tissue damage and infection.
• Tachyzoites migrate to tissues and organs, including brain, where cysts filled with bradyzoites form.

Clinical Disease (Signs and symptoms)

Asymptomatic Toxoplasmosis Overview

• Many T. gondii patients remain asymptomatic, especially in children.
• Disease in humans occurs when a virulent strain enters the body, the host is susceptible (e.g., AIDS patients), and the parasite’s specific site in the body is likely to cause tissue destruction.

Toxoplasmosis Symptoms Overview

• Mild symptoms mimic those of infectious mononucleosis.
• Acute form: fatigue, lymphadenitis, chills, fever, headache, myalgia.
• Chronic disease: maculopapular rash, encephalomyelitis, myocarditis, hepatitis.
• Rare: Retinochoroiditis with subsequent blindness.

Congenital Toxoplasmosis Overview

• Severe and often fatal condition in one to five out of 1000 pregnancies.
• Transmission occurs when fetus is unknowingly infected by an asymptomatic mother.
• Severity depends on mother’s antibody protection and fetus’s age at infection time.
• Mild infections can result in complete recovery but may lead to retinochoroiditis years later.
• Symptoms include hydrocephaly, microcephaly, intracerebral calcification, chorioretinitis, convulsions, and psychomotor disturbances.
• Most infections lead to mental retardation, severe visual impairment, or blindness.
• In T. Gondii Infant Symptoms 5% to 15% of infected infants die.
• 10% to 13% develop moderate to severe handicaps.
• 8% to 10% of infected infants experience severe eye and brain damage.
• 58% to 72% of infected infants are asymptomatic at birth.
• A small percentage of infected infants may develop mental retardation or retinochoroiditis later in life.

Toxoplasmosis in Immunocompromised Patients

• Patient’s immunosuppressed due to organ transplantation or neoplastic diseases like Hodgkin’s lymphoma contract toxoplasmosis.
• Screening potential donor units for toxoplasmosis is crucial, especially for blood transfusions.

Cerebral Toxoplasmosis in AIDS Patients

• T. gondii is associated with AIDS patients, with toxoplasmic encephalitis being a significant complication since the 1980s.
• Early symptoms include headache, fever, altered mental status, and lethargy.
• Subsequent focal neurologic deficits, brain lesions, and convulsions usually develop.
• T. gondii organisms stay confined within the CNS, with a rise in spinal fluid IgG antibody levels and tachyzoites in the cerebrospinal fluid (CSF) being diagnostic.
• Most infected patients do not have serum levels of IgM antibodies, suggesting infections occurred due to reactivation of a chronic latent infection, not an acquired primary infection.

Toxoplasma gondii Infection Diagnosis

• Blood samples analyzed using serologic test methods.
• Recommended test for determining IgM antibodies: double-sandwich ELISA.
• IgM and IgG levels determined using indirect fluorescent antibody (IFA) test.
• Additional tests for IgG antibody: indirect hemagglutination (IHA) test and ELISA.
• Demonstrating T. gondii trophozoites and cysts requires microscopic examination of infected human tissue samples or inoculation of laboratory animals.

Toxoplasma gondii Infection Treatment

• Combination of trisulfapyrimidines and pyrimethamine (Daraprim) is the preferred treatment.
• Pregnant women should not be given pyrimethamine.
• Spiramycin is an alternative drug, used in Europe, Canada, Mexico, and the US.
• Corticosteroids as anti-inflammatory agents may be beneficial.
• Folinic acid (leucovorin) can counteract bone marrow suppression caused by pyrimethamine.
• Atovaquone is an effective drug for toxoplasmic encephalitis in AIDS patients.

Prevention and Control of T. gondii Infections

• Avoid contact with cat feces: Wear protective gloves, disinfect litter boxes, and wash hands after handling contaminated meat.
• Avoid ingesting contaminated meat: Hand wash after handling, taste raw meat, and cook all meat before human consumption.
• Keep cats away from infective rodents: Feed cats only dry or cooked canned cat food, or not have cats at all.
• Pregnant women should be cautious around cat feces and contaminated meat due to potential toxoplasmosis transmission to unborn children.

Conclusion

Toxoplasma gondii is a fascinating parasite that has a complex life cycle involving both intermediate hosts, such as rodents and birds, and definitive hosts, such as cats. The parasite can be transmitted to humans through the ingestion of contaminated food or water, or through contact with infected cat feces. In humans, Toxoplasma gondii can cause flu-like symptoms, but it is most concerning in pregnant women, as it can lead to serious birth defects in the unborn child. Diagnosis of the infection can be challenging, as many infected individuals may not exhibit any symptoms. Treatment typically involves a combination of antibiotics, but prevention is key, especially for pregnant women. Control measures, such as proper hygiene practices and avoiding contact with cats, can help reduce the risk of infection. Overall, understanding the epidemiology, morphology, and lifecycle of Toxoplasma gondii is crucial in order to effectively prevent and manage this potentially harmful parasite.

Reference:

Elizabeth A. Gockel-Blessing (formerly Zeibig), Clinical Parasitology: A PRACTICAL APPROACH. Second Edition182 CHAPTER 7 Miscellaneous Protozoa

For Further Reading

• History of Human Parasitology
• Recently acquired Toxoplasma gondii infection, Brazil