LABORATORY DIAGNOSIS OF TOXOPLASMOSIS

Toxoplasmosis is a zoonotic parasitic disease caused by Toxoplasma gondii, which is transmitted to humans from infected cats. Infection can be acquired by ingestion of oocysts from cat feces or by eating raw or undercooked meat containing tissue cysts. This obligate intracellular parasite produces asymptomatic infection in healthy but causes serious disease in immunosuppressed patients. Toxoplasmosis can be transmitted transplacentally from infected mother during pregnancy.

Laboratory diagnosis:
Specimens collected are serum, CSF, tissue biopsy, bronchoalveolar lavage and amniotic fluid. Specific diagnosis in patients with AIDS and CNS symptoms requires a brain biopsy.

Serology: Detection of IgM or IgG antibodies by indirect immunofluorescence (IFA) or Sabin-Feldman dye test. Other serologic tests including the indirect hemagglutination test, the latex agglutination test, modified agglutination test, and the enzyme-linked immunoabsorbent assay (ELISA) are also available. Antenatal screening of pregnant woman for antibodies against Toxoplasma, Rubella, Cytomegalovirus and Herpes Simplex is performed under "TORCH" tests.

Significance of serological tests:
o Specific IgM antibodies appear during the first two weeks of illness, peak within four to eight weeks, and then typically become undetectable within several months. It can be detected by IgM capture ELISA.
o IgG antibodies arise more slowly, peak in one to two months and may remain high and stable for months to years.
o Specific IgM antibodies or a fourfold rise in one of the IgG titre usually indicates acute disease.
o Most immunocompetant persons with acute toxoplasmosis have IgG levels of titre >1024 (by IFA)
o Titers exceeding 1:1000 must be considered significant in the presence of lymphadenopathy in a pregnant woman or encephalitis in an immunocompromised host.
o Detection of specific IgM antibody in neonatal disease suggests congenital infection. If the infant's IgG level is four-fold higher than the mother, it may be considered infected.
o Serology is not useful for diagnosis of toxoplasmosis in patients with AIDS. IgM antibodies are not present during reactivation, and IgG antibodies to T.gondii do not distinguish between latent and reactivated infection.
o False positive reaction in IFA occurs in patients with Rheumatoid factor or with anti-nuclear antibodies. 

Animal inoculation: The parasite can be isolated during the acute phase of disease by inoculating mice or tissue cultures (MRC-5) with biopsy materials or body fluids, but this requires up to six weeks. Antibodies can be demonstrated in the animal.

Histology: Tachyzoites, which are present during acute infection, can be demonstrated by staining with Giemsa or Wright's stain. Tachyzoites may also be demonstrated in pneumocytes in bronchoalveolar lavage by Giemsa stain or immunofluorescence.
Molecular techniques: Parasitic DNA can be detected in blood, CSF or amniotic fluid by techniques such as PCR.