Infection: Toxoplasmosis; Toxoplasma gondii infection

Brief description:
  • A type of protozoa that causes a systemic infection, which is most commonly asymptomatic.
  • If symptoms occur, the commonest pattern is a short-lived systemic illness followed by cervical lymphadenopathy, which is bilateral and non-tender, and can persist for weeks.
  • After primary (acute) infection, the organism persists in the body life-long in a dormant state. May reactivate if a person subsequently becomes severely immunocompromised.
  • Can cause congenital infection if first acquired during pregnancy. Seek specialist advice.
Did you know?

Is acquired either by ingestion of eggs, which are present in the faeces of infected cats, or by ingesting undercooked meat from infected animals e.g. rare beef.

Who should I test?
  • Consider testing in those with multiple enlarged lymph nodes, or nodes that are not reducing in size at 2 weeks. Keep in mind other more serious causes of lymphadenopathy.
  • Testing in pregnant women with cat exposure but no symptoms is not recommended.
  • Test if abnormalities consistent with congenital infection seen on antenatal USS.
Test of choice:

Request Toxoplasma serology

Toxoplasma IgM is a marker of primary (acute) infection and usually appears within 1 week of symptoms. May persist for months or years.

  • Good sensitivity – a negative resultmakes the diagnosis very unlikely if >1 week of symptoms.
  • Moderate specificity – a positive result in someone with compatible symptoms is supportive, but cannot confirm toxoplasmosis.

Toxoplasma IgG indicates infection at some point in time (can be acute or past infection). Usually appears within 2 weeks of symptoms and persists life-long.

  • Excellent sensitivity – a negative result excludes toxoplasma infection, unless within 2 weeks of infection
  • Excellent specificity – a positive result confirms infection at some stage
  • Conversion from negative to positive IgG confirms acute toxoplasma infection during the intervening period.

IgG avidity testing can also be performed (discuss with microbiologist):

  • High avidity suggests infection >4 months prior
  • Low avidity is less specific and can mean recent or old infection
Tests to avoid/specialist tests:
  • Toxoplasma PCR is reserved for specialist use, e.g.:
    • To diagnose reactivation infection in severely immunocompromised people.
    • To confirm congenital infection via amniocentesis or placental testing.
Other considerations:

In the setting of possible congenital toxoplasmosis and a mother with a positive IgG, testing of the booking blood sample can sometimes be useful – discuss with a microbiologist.