Toxoplasmosis is caused by infection with the protozoan parasite Toxoplasma gondii, which infects up to one-third of the world’s population. Although acute infections may be asymptomatic in pregnant women, the infection can be transmitted to the fetus and cause severe complications, including cognitive impairment, blindness, and epilepsy. As many as 4000 new cases of congenital toxoplasmosis occur each year in the United States. Of the nearly 750 US deaths attributed to toxoplasmosis each year, approximately half are believed to be caused by eating contaminated undercooked or raw meat. Toxoplasma gondii can also be transmitted to humans by ingestion of oocysts, an environmentally resistant form of the organism, through exposure to cat feces, water, or soil containing the parasite or from eating unwashed contaminated fruits or vegetables.
Infection can be prevented in large part by cooking meat to a safe temperature, peeling or thoroughly washing fruits and vegetables before eating, and cleaning cooking surfaces and utensils after they have come into contact with raw meat. Pregnant women should avoid changing cat litter and handling raw or undercooked meat. Also, pet owners should keep cats indoors, where they are less likely to eat infected prey and subsequently acquire T gondii.
Primary infection is usually subclinical, but cervical lymphadenopathy or ocular disease can be present in some patients. The ocular manifestations include uveitis and chorioretinitis with macular scarring. The clinical picture and histopathology of toxoplasmosis reflect the immune response. In immunocompromised patients, reactivation of latent disease can cause life-threatening encephalitis.
Diagnosis of toxoplasmosis can be established by direct detection of the parasite or by serologic techniques. Real-time PCR is a very sensitive technique for diagnosing infection caused by T gondii and for determining the precise genotype of the organism. In the past, the most commonly used therapeutic regimen was pyrimethamine combined with sulfadiazine and folinic acid. Recently, this regimen has been largely replaced by trimethoprim-sulfamethoxazole (TMP-SMX), which is more readily available and less expensive. Other drugs with activity against T gondii include azithromycin, atovaquone, and clindamycin. See BCSC Section 12, Retina and Vitreous, for more details on the treatment of ocular toxoplasmosis.
Villard O, Breit L, Cimon B, et al; French National Reference Center for Toxoplasmosis Network. Comparison of four commercially available avidity tests for Toxoplasma gondii-specific IgG antibodies. Clin Vaccine Immunol. 2013;20(2):197–204.
Excerpted from BCSC 2020-2021 series: Section 1 - Update on General Medicine. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.