*Pathogenesis

Most cases of toxoplasmosis in humans are acquired by ingestion of infected meat containing tissue cysts  with bradyzoites  or  food contaminated with cat feces containing oocysts. Bradyzoites or sporozoites penetrate intestinal cells and then spread locally to the mesenteric lymph nodes and then to distant organs via the lymphatics and blood. Focal areas of necrosis may develop in a variety of organs and the clinical manifestations reflect injury to specific tissues. Tissue death is not the result of a  Toxoplasma toxin, but is a consequence of the egress of the tachyzoites which destroys the host cell. Only 10-30% of toxoplasma infections are symptomatic and the most common clinical manifestation in immunocompetent adults is lymphadenitis and lymphadenopathy.
 

The most common symptom is swollen lymph nodes which may be associated with fever, headache, muscle pain, anemia and sometimes lung complications. Any lymph node can be infected but commonly  the deep cervical nodes of the neck are involved. There is malaise, fever, and lymphocytosis which mimics infectious mononucleosis. The infection usually resolves on its own in weeks or months. 

 

Although toxoplasmosis in humans is usually of little consequence there are two conditions where it can be  devastating and lethal:

 

 *Toxoplasmosis during Pregnancy

 In the child of a woman who acquires the infection for the first time during pregnancy. About 0.8% of pregnant females are expected to sero-convert. If T. gondii is acquired during the first pregnancy congenital toxoplasmosis will occur 45% of the time. Most severe effects are acquired during the first trimester, but if a chronically infected female becomes pregnant then the child will not suffer from toxoplasmosis.

 

Transplacental infections (by tachyzoites) results in:

 

subclinical !!                             60%

 

encephalitis                              

hydrocephalus                         30%

microcephalus

retinochoroiditis

 

abortion                                    10%

 

 

 Hydrocephaly in a baby (left) and retinochoroiditis (right)

 

Newborns with toxoplasmosis may have fever, convulsions, mental retardation, blindness, pneumonitis, and hepatosplenomegaly. Recovery from congenital toxoplasmosis is rare and treatment is usually useless.

 

 *An Opportunistc Disease

In the 1980's toxoplasmosis became one of the opportunistic diseases associated with immunocompromised patients, such as AIDS patients, transplantation patients and cancer patients treated with immunosuppressive drugs.  In AIDS patients 25-50% develop encephalitis due to reactivation of chronic or latent infections, and if left untreated 90% will die from toxoplasma induced encephalitis due to necrosis and multiple abscesses. Symptoms include headache, fever, lethargy, altered mental status and can progress to focal neurological defects and convulsions. Lesions can be seen in CT scans and MRI imaging. Ocular involvement can also occur. Reactivation probably results from rupture of the cyst and renewed multiplication of bradyzoites into tachyzoites. The cause of cyst rupture is unknown. Myocarditis and pneumonitis may also occur.