What is toxoplasmosis?

What is Toxoplasmosis?

Toxoplasmosis is a zoonotic parasitic infection caused by Toxoplasma gondii, an apicomplexan protozoan that infects approximately one-third of the world's population and can infect virtually any nucleated cell in warm-blooded animals. 1, 2

The Causative Organism

• Toxoplasma gondii is an obligate intracellular parasite with a complex life cycle involving definitive feline hosts and intermediate hosts (including humans, livestock, and other warm-blooded animals). 2, 3
• The parasite exists in three infectious forms: tachyzoites (rapidly dividing form during acute infection), bradyzoites (slow-growing form within tissue cysts), and sporozoites (contained within oocysts shed by cats). 3

Transmission Routes

• The CDC emphasizes that older children, adolescents, and adults typically acquire infection through consuming undercooked meat containing parasitic cysts or by ingesting sporulated oocysts from contaminated soil, food, or water. 1
• Congenital transmission occurs transplacentally when a pregnant woman acquires primary infection, with maternal-fetal transmission risk averaging 29% overall but varying dramatically by gestational timing. 1
• The American Academy of Pediatrics notes transmission risk increases from 2-6% in the first trimester to as high as 81% when maternal infection occurs in the final weeks of pregnancy, though paradoxically early infection causes more severe fetal disease. 1
• Less common routes include organ transplantation, blood transfusion, and laboratory accidents. 4

Clinical Manifestations by Population

Immunocompetent Adults

• Most infections are asymptomatic or cause mild, self-limited flu-like illness with lymphadenopathy. 5, 4
• The parasite establishes lifelong chronic infection by forming tissue cysts primarily in brain and muscle tissue, which are resistant to current medications. 5, 3

Congenital Toxoplasmosis

• The CDC states that 70-90% of infected infants are asymptomatic at birth, but the majority develop late sequelae including retinitis, visual impairment, intellectual or neurologic impairment with onset ranging from months to years. 1, 6
• Symptomatic newborns may present with the classic triad of chorioretinitis, hydrocephalus, and intracranial calcifications, along with hepatosplenomegaly, jaundice, thrombocytopenia, and microcephaly. 6
• The incidence in the United States is estimated at 1 per 1,000-12,000 live births, with substantial decline over the past 20 years. 1

Immunocompromised Patients

• The Infectious Diseases Society of America recommends that toxoplasmic encephalitis should be considered in all HIV-infected patients presenting with new neurologic findings, regardless of whether focal or diffuse. 1
• Focal neurological deficits are the most common presentation, though diffuse CNS disease can occur. 1
• CT typically shows multiple, bilateral, ring-enhancing lesions especially in the basal ganglia and cerebral corticomedullary junction, though MRI is more sensitive. 7, 1
• Reactivation of latent infection is the primary mechanism in immunocompromised hosts, causing devastating disseminated disease affecting virtually any organ system. 5, 3

Ocular Toxoplasmosis

• Can cause serious ocular disease even in immunocompetent individuals, presenting as chorioretinitis that may lead to blindness. 5, 3

Epidemiology and Global Burden

• Toxoplasmosis affects approximately 2 billion people worldwide, making it one of the most damaging zoonotic diseases despite only a small percentage developing serious disease. 2
• The disease has the broadest host range among apicomplexan parasites, with variable clinical presentation among species—asymptomatic in most hosts but fatal to marsupials, neotropical primates, and some marine mammals. 5

Key Clinical Pitfall

• A critical caveat: the majority of infected pregnant women show no symptoms, making screening and high clinical suspicion essential for preventing devastating congenital outcomes. 6 This asymptomatic nature in both immunocompetent adults and pregnant women means the infection often goes unrecognized until severe complications develop in vulnerable populations.