Toxoplasma gondii

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Toxoplasma gondii / (Redirected from Toxoplasmosis)
  • Protozoan parasite associated with cats and raw beef mostly known for causing opportunistic infections and congenital infections



  • Protozoan parasite
  • Organized into twelve haplotypes


  • Zoonotic disease with worldwide distribution
  • Modes of transmission
    • Ingesting tissue cysts in meat, or oocytes in food or water
    • Solid organ transplantation, especially heart
    • Vertical or transplacental transmission
    • Case reports of lab-acquired needlestick transmission
    • Theoretical risk with blood transfusion
  • Seroprevalence around 10-18% in Canada 12
    • As high as 60% in Nunavut, however 3
  • There are large parts of South and Central America, as well as Pacific Islands, that have very high seroprevalence 4

Life Cycle

  • The only definitive hosts are in the Felidae family, essentially housecats and their relatives
  • Intermediate hosts are many, and include birds and rodents
  • An infected cat sheds oocytes into the environment (for 1 to 3 weeks), where they spend 1 to 5 days sporulating
    • Each sporulated oocyst contains two sporocysts, and each sporocyst contains four sporozoites
  • Intermediate hosts ingest the sporozoites, where they mature into tachyzoites
  • Tachyzoites migrate to brain and muscle, where they encyst and become bradyzoites
  • Bradyzoites are ingested by a cat, completing the life cycle


  • Following ingestion, bradyzoites and sporozoites invade the small intestinal mucosa and develop into tachyzoites within the gut epithelium
  • There, they insert themselves into monocytes and other nucleated cells
  • Infected cells travel throughout the body, carrying the tachyzoite with them
  • Infection triggers a Th-1 response

Clinical Manifestations


  • Asymptomatic in 80% of primary infections
  • Symptoms, when they occur, can involve fever, cervical lymphadenopathy (painless and rubbery), myalgias, and weakness/fatigue
  • Can also cause chorioretinitis
  • Severity of illness depends in part on genotype, with strain II in North America and Europe being less severe
    • Rarely, unusual strains may cause pneumonitis, myocarditis, meningoencephalitis, or polymyositis, and can lead to death


  • May be from primary infection or, more commonly, reactivation
  • Unlike in immunocompetent people, it is always a serious infection in the immunocompromised
  • Major risk factor is cellular immunodeficiency, as in HIV and some immunosuppressive medications
    • In HIV, beware with CD4 < 100
  • Typically presents with CNS involvement as encephalitis
    • Symptoms include fever, headache, lethargy, incoordination, ataxia, hemiparesis, loss of memory, dementia, or seizures
  • Can also present with pneumonitis (especially with bone marrow transplant), chorioretinitis, or myocarditis, and rarely involves essentially any other organ




  • Immunocompetent or pregnant women with primary infection: IgG/IgM serology, possibly with avidity testing for pregnant women
  • Fetus, to rule out congenital infection following maternal primary infection: PCR of amniotic fluid
  • Newborn, to rule out congenital infection: PCR of placenta or cord, or serology
  • Immunocompromised patient, to diagnose cerebral or disseminated disease: PCR of blood, CSF, BAL, or tissue
  • Patient with chorioretinitis: Parallel serologies from aqueous humour and serum, or PCR of aqueous humour


  • Serology is the mainstay of diagnosis5
  • IgM antibodies
    • Detectable within 1 week, and titres plateau within 1 month and start decreasing after 1 to 6 months
    • IgM is still detectable for months or years after infection
      • Rarely, lost within 3 months
      • 25% lost within 7 months
      • Often detectable for more than a year
  • IgG antibodies
    • Detectable 2 to 4 weeks after infection, and plateaus within 2 to 3 months
    • Declines but persists lifelong
  • IgG avidity testing can help to assess how recently the infection was acquired
    • Provides a measure of how tightly the antibodies bind, which is highest in early infection
    • A high avidity ratio (weak binding) suggests that the infection was acquired at least 4 months prior
    • Treatment delays avidity
    • Most useful during the first trimester of pregnancy, when high avidity effectively rules out acquisition during pregnancy


  • Not routinely done
  • May be helpful from CSF, vitreous humour, or amniotic fluid
  • Not helpful on brain biopsy tissue


Toxoplasmosis Encephalitis

  • In general, in the setting of known HIV and one or more suspicious lesions, treat empirically for CNS toxoplasmosis and reassess with repeat imaging at around 10 days, at which time there should be some response
  • First-line is a combination of pyrimethamine and sulfadiazine, though that may be changing (see TMP-SMX, below)
  • Alternatives
    • TMP-SMX
      • Dose unclear; 40-120 mg/kg/day or 5 mg/kg (TMP component) p.o./IV every 8 hours
      • Likely safer than and as effective as the pyramethamine-based regimens[1]
      • Given these new data, in the context of the difficulty and expense of obtainined pyramethamine, TMP-SMX may be considered first-line
    • Atovaquone 1500 mg PO bid + pyrimethamine
    • Atovaquone 1500 mg PO bid ± sulfadiazine
    • Azithromycin 900-1200 mg PO daily + pyrimethamine

Patients with HIV



  • Cats: hand hygiene after handling cat, use gloves and wash hands when handling litter, wash litter tray with hot >60ºC water, keep litter out of kitchen
  • Soil: use gloves for gardening, wash hands after soil contact
  • Water: avoid tap water in highly endemic countries, avoid ingestion of lake and river water
  • Food: avoid raw oysters/clams/mussels, wash all vegetables/fruits/herbs, cook meat well down

Further Reading

  • Epidemiology of and Diagnostic Strategies for Toxoplasmosis. Clin Microbiol Rev. 2012;25(2):264. doi: [10.1128/CMR.05013-11]
  1. Prosty C, Hanula R, Levin Y, Bogoch II, McDonald EG, Lee TC. Revisiting the Evidence Base for Modern Day Practice of the Treatment of Toxoplasmic Encephalitis: A Systematic Review and Meta-Analysis. Clin Infect Dis. 2022 Aug 9:ciac645. doi: 10.1093/cid/ciac645. Epub ahead of print. PMID: 35944134.


  1. ^  Samar Shuhaiber, Gideon Koren, Rada Boskovic, Thomas R Einarson, Offie Porat Soldin, Adrienne Einarson. Seroprevalence of Toxoplasma gondiiinfection among veterinary staff in Ontario, Canada (2002): Implications for teratogenic risk. BMC Infectious Diseases. 2003;3(1). doi:10.1186/1471-2334-3-8.
  2. ^  EL Ford-Jones, I Kitai, M Corey, R Notenboom, N Hollander, E Kelly, H Akoury, G Ryan, I Kyle, R Gold. Seroprevalence of Toxoplasma Antibody in a Toronto Population. Canadian Journal of Infectious Diseases. 1996;7(5):326-328. doi:10.1155/1996/172651.
  3. ^  V. Messier, B. Lévesque, J.-F. Proulx, L. Rochette, M. D. Libman, B. J. Ward, B. Serhir, M. Couillard, N. H. Ogden, É. Dewailly, B. Hubert, S. Déry, C. Barthe, D. Murphy, B. Dixon. Seroprevalence of Toxoplasma gondii Among Nunavik Inuit (Canada). Zoonoses and Public Health. 2009;56(4):188-197. doi:10.1111/j.1863-2378.2008.01177.x.
  4. ^  Georgios Pappas, Nikos Roussos, Matthew E. Falagas. Toxoplasmosis snapshots: Global status of Toxoplasma gondii seroprevalence and implications for pregnancy and congenital toxoplasmosis. International Journal for Parasitology. 2009;39(12):1385-1394. doi:10.1016/j.ijpara.2009.04.003.
  5. ^ gangneux2012ep