Toxoplasmosis



T. gondii tachyzoites, stained with fluorescent antibodies
Courtesy of DPDx

Diagnosis

The diagnosis of toxoplasmosis can be done using a variety of methods. The difficulty lies in determining whether the infection is acute or chronic. Acute infection can best be verified by isolating T. gondii or T. gondii DNA from the patient's blood or finding tachyzoites in tissue or bodily fluids. Congenital infection of fetuses can be identified by the presence of cysts in the placenta or fetus.

The isolation of T. gondii tissue cysts is not sufficient to determine whether the infection is still active or has entered the latent phase. To differentiate the two, patients are subjected to several serological exams, the specific combination of which depends on clinical category of the patient. These exams include the Sabin-Feldman dye test, which tests for IgG antibodies; ELISAs targeted at IgM, IgA and IgE; differential agglutination tests and IgG avidity tests.

Of particular interest is determining acute infection in pregnant women, due to the risk of congenital toxoplasmosis. This is complicated by the fact that many women have existing IgG and IgM antibodies to T. gondii from infection in the past. There are effective diagnostic techniques that monitor changes in the mother's antibody expression over time, but quick diagnosis is greatly preferred because fetuses often rapidly become infected.

Driven by this need, Cynthia Press, Jose Montoya and Jack Remington at the Palo Alto Medical Foundation identified a series of tests that are effective at determining very early infection from a single sample of maternal serum. This is based on the finding that VIDAS IgG avidity tests usually fail during early infection because IgG titers are too low. The recommended diagnostic procedure consists of looking for positive results from the Sabin-Feldman dye test and an IgM ELISA, in conjunction with a negative VIDAS IgG avidity test. This was determined to be an effective set of criteria for diagnosing early acute infection, which can then be followed up with early treatment to minimize the risk of congenital infection.

Mandell, Bennett and Dolin.
Press, Montoya and Remington.

 

 

Last Update: 24 May 2006