Brain

This section will cover the brain-invading parasites Toxoplasma, Naegleria, and Acanthamoeba – protozoa that can cause serious neurological problems. We’ll explore their etiology, epidemiology, and transmission routes

Toxoplasma gondii (Toxoplasmosis)

• Etiology: Toxoplasma gondii is an obligate intracellular protozoan parasite that can infect virtually all warm-blooded animals, including humans
• Epidemiology
  • Global Distribution: Worldwide, with seroprevalence (evidence of past infection) varying widely depending on geographic location and dietary habits
  • Risk Factors
    • Consumption of raw or undercooked meat, especially pork, lamb, and venison, containing tissue cysts
    • Ingestion of oocysts from contaminated soil, water, or food (e.g., unwashed fruits and vegetables)
    • Contact with cat feces containing oocysts (e.g., cleaning litter boxes)
    • Vertical transmission (mother to fetus) during pregnancy
    • Organ transplantation or blood transfusion
  • Immunocompromised Individuals: Individuals with weakened immune systems (e.g., HIV/AIDS patients, organ transplant recipients) are at higher risk of developing severe toxoplasmosis, including encephalitis
  • Congenital Toxoplasmosis: Infection during pregnancy can lead to congenital toxoplasmosis in the fetus, causing severe neurological damage, vision loss, and other complications
• Transmission
  • Ingestion of Tissue Cysts: Consumption of raw or undercooked meat containing tissue cysts
  • Ingestion of Oocysts: Ingestion of oocysts from contaminated soil, water, or food
  • Vertical Transmission: Transmission from mother to fetus during pregnancy
  • Organ Transplantation or Blood Transfusion: Transmission via infected organs or blood products
  • Life Cycle
    • Cats are the definitive hosts, where Toxoplasma undergoes sexual reproduction, producing oocysts that are shed in their feces
    • Humans and other warm-blooded animals become infected by ingesting tissue cysts or oocysts
    • In the intermediate host, Toxoplasma transforms into tachyzoites (rapidly dividing form) that disseminate throughout the body and invade various tissues
    • Tachyzoites eventually convert into bradyzoites (slowly dividing form) and form tissue cysts, primarily in the brain and muscle tissue

Naegleria fowleri (Primary Amebic Meningoencephalitis - PAM)

• Etiology: Naegleria fowleri is a free-living ameba found in warm freshwater environments that can cause primary amebic meningoencephalitis (PAM), a rare but almost always fatal brain infection
• Epidemiology
  • Geographic Distribution: Worldwide, but more common in warm freshwater environments, such as lakes, rivers, hot springs, and poorly maintained swimming pools
  • Risk Factors
    • Nasal exposure to contaminated water, such as during swimming, diving, or nasal irrigation
    • Warm water temperatures promote the growth of Naegleria fowleri
    • Children and young adults are more frequently affected
  • Rare Disease: PAM is a rare disease, but it is almost always fatal, with a very low survival rate
• Transmission
  • Nasal Exposure: Naegleria fowleri enters the body through the nose when contaminated water is forcefully inhaled, such as during swimming or diving
  • Olfactory Nerve: The ameba travels along the olfactory nerve to the brain, where it causes severe inflammation and destruction of brain tissue
  • Life Cycle
    • Naegleria fowleri exists in three forms: cysts, trophozoites (amoeboid form), and flagellates
    • Only the trophozoite form can invade human tissue
    • Trophozoites enter the nasal passages and migrate to the brain, causing PAM

Acanthamoeba spp. (Granulomatous Amebic Encephalitis - GAE and Amebic Keratitis)

• Etiology: Acanthamoeba is a genus of free-living amebas that can cause several infections in humans, including granulomatous amebic encephalitis (GAE) and amebic keratitis (corneal infection)
• Epidemiology
  • Global Distribution: Worldwide, as Acanthamoeba is commonly found in soil, dust, freshwater, and tap water
  • Risk Factors
    • GAE: Immunocompromised individuals (e.g., HIV/AIDS patients, organ transplant recipients) are at higher risk of developing GAE
    • Amebic Keratitis: Contact lens wearers, especially those who do not properly disinfect their lenses or use tap water to rinse or store their lenses, are at higher risk of developing amebic keratitis
  • Chronic Infection: GAE is a chronic and often fatal infection of the brain and spinal cord
• Transmission
  • GAE: Acanthamoeba can enter the body through the skin, lungs, or nasal passages and spread to the brain via the bloodstream
  • Amebic Keratitis: Acanthamoeba can enter the eye through contaminated contact lenses, corneal injuries, or exposure to contaminated water
  • Life Cycle
    • Acanthamoeba exists in two forms: trophozoites and cysts
    • Both trophozoites and cysts can invade human tissue
    • In GAE, Acanthamoeba trophozoites and cysts cause inflammation and granuloma formation in the brain
    • In amebic keratitis, Acanthamoeba trophozoites and cysts invade the cornea, causing severe pain, inflammation, and vision loss

Key Takeaways

• Environmental Sources: Naegleria and Acanthamoeba are free-living amebas found in the environment (water and soil, respectively), highlighting the importance of avoiding exposure to contaminated water and practicing good hygiene
• Immunocompromised Individuals: Immunocompromised individuals are at higher risk of developing severe infections with Toxoplasma and Acanthamoeba
• Routes of Transmission: Toxoplasma is primarily transmitted through ingestion of tissue cysts or oocysts, while Naegleria enters the body through the nose, and Acanthamoeba can enter through the skin, lungs, nasal passages, or eyes
• Neurological Damage: All three parasites can cause significant neurological damage, leading to severe illness and potentially fatal outcomes

Key Terms

• Obligate Intracellular Parasite: A parasite that can only replicate inside host cells
• Seroprevalence: The proportion of individuals in a population who have antibodies against a specific pathogen, indicating past exposure or infection
• Tissue Cyst: A dormant form of a parasite enclosed in a protective wall within host tissues
• Oocyst: A resistant, thick-walled stage of certain protozoan parasites that contains sporozoites and is shed in feces
• Tachyzoite: A rapidly dividing form of certain protozoan parasites that disseminates throughout the body
• Bradyzoite: A slowly dividing form of certain protozoan parasites found within tissue cysts
• Encephalitis: Inflammation of the brain
• Free-Living Ameba: An ameba that can survive and reproduce in the environment without requiring a host
• Primary Amebic Meningoencephalitis (PAM): A rare and almost always fatal brain infection caused by Naegleria fowleri
• Granulomatous Amebic Encephalitis (GAE): A chronic and often fatal infection of the brain and spinal cord caused by Acanthamoeba
• Amebic Keratitis: A corneal infection caused by Acanthamoeba, often associated with contact lens use
• Trophozoite: The active, feeding, and motile stage of a protozoan parasite
• Cyst: The dormant, non-motile, and infective stage of a protozoan parasite
• Olfactory Nerve: The nerve responsible for the sense of smell