Toxoplasma Gondii Serology

Diagnostic Use

Toxoplasmosis results from infection by the protozoan parasite Toxoplasma gondii . This organism is found throughout the world and can infect essentially all mammals. Most human populations show an incidence of past or present infection of 20-50% although higher figures have been recorded.

Toxoplasma gondii is an obligate intracellular parasite capable of infecting most warm blooded animals including man. Cats (all felines) are the definitive hosts. Oocytes shed in cat faeces are ingested by intermediate hosts where they multiply rapidly as tracyzoites. These also encyst as the slower growing but infectious bradyzoites. Therefore, horizontal transmission to man is by either eating poorly cooked meat or by accidental ingestion of  sporulating  oocytes from soil. Vertical transmission also occurs, with the incidence of congenital toxoplasmosis increasing with gestational age however, severity is greatest when maternal infection is acquired in the first trimester.

Incubation of Toxoplasma infection appears to be in the order of  7 days with the infection of the normal healthy adult being usually asymptomatic. Where symptoms occur, lymphadenopathy is common. The disease can be fatal in the severely immunocompromised host (patients undergoing chemotherapy, immunosuppressive therapy and those with AIDS), thought to be due to reactivation of latent infections.

In pregnancy, primary infection is associated with a significant risk of foetal infection. The incidence of foetal infection rises from 10-15% in the first trimester to 30% in the second and 60% in the third. The majority (70-90%) of congenitally infected infants are asymptomatic at birth, although visual impairment, learning disabilities or mental retardation will become apparent in a large percentage of children months to years later. Symptoms at birth include a maculopapular rash, generalised lymphadenopathy, hepatosplenomegaly, jaundice and thrombocytopenia. As a consequence of intrauterine meningoencephalitis, the infant can develop CSF abnormalities, hydrocephalus, microcephaly, chorioretinitis and convulsions. Cerebral calcifications may be present. Severely affected infants die in utero or within a few days of birth.

Serology (IgM testing) is the primary means of diagnosis but results must be carefully interpreted. If IgM is positive and the patient is pregnant it is important to liaise with the O&G service to advise re treatment.

NOTE: Toxoplasma IgM may persist for many months, it is not a reliable marker of recent primary infection.

It is sensible to test the mother’s blood for toxoplasma antibodies before bleeding the baby. If the mother is antibody negative, then congenital toxoplasmosis can be excluded.

For cases of congenitally acquired infection in the neonate detection of Toxoplasma DNA by PCR is the method of choice. Amniotic fluid should be assayed for Toxoplasma DNA to exclude (or identify) transplacental infection.

References

1.      Abbott Laboratories Axsym TOXO IgG product insert.

2.  1997 Red Book. Report of the Committee on Infectious Diseases. 24 ^th Edition. American Association of Paediatrics p  i-xxvi, 1-764. 1997.

Reference Intervals

IgM antibodies may persist for many months following infection.

* Changing IgG levels are useful, especially in patients monitored serially (transplant recipients).