Toxoplasma: its Life Cycle,Pathogenesis & Epidemiology, Clinical Findings, Laboratory Diagnosis, Treatment, Symptoms, and Prevention

 

Definition

Toxoplasma gondii (T. gondii) is a parasitic protozoan that causes toxoplasmosis, a disease transmitted through the ingestion of cysts in uncooked meat or food contaminated by cat feces.

Life cycle

The life cycle of Toxoplasma gondii involves various hosts and environments. Cats, as definitive hosts, play a crucial role in completing the parasite's sexual cycle. Oocysts shed in the feces of infected animals can be ingested by other hosts. Once ingested, oocysts transform into tachyzoites, which spread to different tissues via the bloodstream. Toxoplasmosis, caused by T. gondii, can also be transmitted through various means such as ingestion of raw or undercooked meat, congenital transfer, blood transfusion, or tissue transplant.

Pathogenesis & Epidemiology

Toxoplasma gondii is primarily acquired through the ingestion of cysts present in uncooked meat or food contaminated with cat feces. Transplacental transmission from an infected mother to the fetus is also possible. Human-to-human transmission, apart from transplacental transmission, is rare. Once inside the human body, the organisms spread to various organs, especially the brain, lungs, liver, and eyes.

Progression of the infection is typically limited by a competent immune system, with cell-mediated immunity playing a pivotal role. Most initial infections are asymptomatic, with the organisms persisting as cysts within tissues. However, immunosuppression can lead to the activation of organisms within the cysts, resulting in life-threatening disseminated disease, particularly in patients with conditions such as AIDS.

Congenital infection occurs when the mother is infected during pregnancy, with the risk of transmission to the fetus. However, maternal immunity acquired from a previous infection can prevent transmission. Despite the global prevalence of T. gondii infection, outbreaks are sporadic, often associated with the ingestion of raw meat or contaminated water.

Clinical Findings

In immunocompetent adults, most primary infections are asymptomatic or may resemble infectious mononucleosis. However, congenital infection poses significant risks, including abortion, stillbirth, or neonatal diseases such as encephalitis, chorioretinitis, and hepatosplenomegaly. Symptoms of congenital infection may include fever, jaundice, and intracranial calcifications.

Infected newborns may remain asymptomatic initially, but complications such as chorioretinitis or mental retardation may develop later in life. Congenital toxoplasmosis is a leading cause of childhood blindness. In immunocompromised patients, particularly those with AIDS, disseminated disease, primarily encephalitis, can be life-threatening.

Laboratory Diagnosis

For the diagnosis of acute and congenital infections, immunofluorescence assays for IgM antibody are commonly used. IgG antibody tests can diagnose acute infections by observing a significant rise in antibody titer in paired sera. Microscopic examination of Giemsa-stained preparations may reveal crescent-shaped trophozoites during acute infections, along with the presence of cysts in tissues.

Treatment

Congenital toxoplasmosis requires treatment with a combination of sulfadiazine and pyrimethamine, also used for the treatment of disseminated disease in immunocompromised patients. Acute toxoplasmosis in immunocompetent individuals typically resolves spontaneously, but patients with complications such as chorioretinitis should receive treatment.

Prevention

Thoroughly cooking meat is the most effective preventive measure against toxoplasmosis. Pregnant women should avoid undercooked meat and contact with cat feces, including refraining from emptying cat litter boxes. Cats should not be fed raw meat. Trimethoprim-sulfamethoxazole is recommended for preventing Toxoplasma encephalitis in HIV-infected patients.