Protozoan Parasites

Protozoan Parasites

Entamoeba histolytica

• Taxonomy: Belongs to Sarcomastigophora (Sarcodina)
• Cyst:
  • Size: 10-20 μm
  • Number of nuclei: 4
  • Features: 2 chromatoidal bars
• Trophozoite:
  • Size: 12-50 μm
  • Number of nuclei: 1
  • Mobility: Actively mobile
• Reproduction:
  • Method: Binary fission of trophozoite
  • Cyst development: Up to 4 trophozoites can develop within a mature multinucleated cyst.

Life Cycle of Entamoeba histolytica

• Stages:
  • Ingestion of mature cysts (Infective Stage)
  • Diagnostic Stage: Cysts passed in feces
  • Trophozoites form from excystation
  • Leads to noninvasive colonization, intestinal disease, and extraintestinal disease.
• Multiplication: Trophozoites can multiply after excystation.

Epidemiology of Entamoeba histolytica

• Prevalence:
  • Developed countries: 0.5%
  • Developing countries: 10-15% (can be as high as 50-80%)
• Infection:
  • Most infections are asymptomatic, but individuals shed large numbers of cysts in feces.
• Incubation period: 1-4 weeks
• Symptoms:
  • Mild gastrointestinal symptoms: abdominal pain, cramps, colitis, diarrhea
  • Severe: bloody diarrhea (amoebic dysentery)
• High Risk Groups:
  • Travelers, recent immigrants, male homosexuals, institutionalized populations
• Reservoir: Human (only host)
• Transmission:
  • Direct (sexual transmission), fecal-oral route, waterborne, foodborne
• Prevention:
  • Adequate sanitation and excreta disposal
  • Provision of uncontaminated water and food

Giardia lamblia

• Taxonomy: Belongs to Sarcomastigophora (Mastigophora)
• Cyst:
  • Size: 8-14 μm
  • Number of nuclei: 2-4
  • Features: Thick cyst wall (0.3 μm)
• Trophozoite:
  • Shape: Heart-shaped, symmetrical
  • Size: 10-18 μm long, 6-8 μm wide
  • Number of nuclei: 2
  • Features: 8 flagella
• Reproduction:
  • Method: Binary fission of trophozoites

Life Cycle of Giardia lamblia

• Infection Routes:
  • Consumption of contaminated water or fecal-oral transmission is common.
• Reservoirs: Many animals, especially beavers.
• Life Cycle Details:
  • Cysts can pass through the stomach and excyst in the duodenum.
  • Cysts can encyst again in the colon as feces dehydrate.
  • Trophozoites can attach to epithelial cells via an adhesive disc.

Epidemiology of Giardia lamblia

• Prevalence: 7.2% in USA
• Infectious Dose: Approximately 10 cysts
• Incubation Period: 1-14 days
• Duration of Illness: 1-3 weeks
• Symptoms:
  • Abdominal cramps, nausea, vomiting, anorexia, low-grade fever, flu-like headache, malaise, weakness, weight loss, profuse greasy diarrhea
• Fecal Shedding:
  • Humans: 10^3 - 10^8 cysts/gram
  • Calves: 10^7 - 10^8 cysts/gram
• Sewage Levels: 3,000 - 30,000 cysts/liter
• High Risk Groups:
  • Children in day-care centers, immunosuppressed individuals, institutionalized populations
• Reservoirs: Humans and various animals (e.g., dogs, beavers, elk)
• Transmission:
  • Fomites, waterborne, foodborne
• Prevention: Personal hygiene, surface disinfection, water treatment, food safety programs

Cryptosporidium parvum

• Taxonomy: Belongs to Apicomplexa (Coccidia)
• Oocyst:
  • Size: 4-6 μm
  • Number of sporozoites: 4
  • Features: Thick oocyst wall
• Sporozoite:
  • No locomotive structure
• Reproduction:
  • Both asexual and sexual processes occur.

Life Cycle of Cryptosporidium parvum

• Infection Routes:
  • Consumption of contaminated water or fecal-oral transmission is common.
• Life Cycle Details:
  • Oocysts can sporulate in intestines and reinfect hosts.
  • Microgamete and macrogamete join to form a zygote which develops into oocysts.

Epidemiology of Cryptosporidium parvum

• Prevalence:
  • General population: 0.3%
  • Day-care center children: 6-54%
• Infectious Dose: Less than 10 oocysts
• Incubation Period: 7 days
• Duration of Illness: 1-4 weeks
• Symptoms:
  • Immunocompetent: Similar to giardiasis
  • Immunocompromised: Life-threatening conditions (e.g., respiratory cryptosporidiosis)
• Fluid Loss: 2-6 liters/day; can reach 17 liters/day
• Sewage Levels: 10^4 oocysts/liter
• High Risk Groups: Children, immunocompromised individuals, institutionalized populations
• Reservoirs: Humans and various animals (e.g., calves, goats, and deer)
• Transmission:
  • Fomites, waterborne, foodborne
• Prevention: Personal hygiene, surface disinfection, water treatment, food safety programs

Balantidium coli

• Taxonomy: Belongs to Ciliophora
• Cyst:
  • Size: 50-70 μm
  • Number of nuclei: 2
  • Features: Various vacuoles
• Trophozoite:
  • Size: 50-100 μm long, 40-70 μm wide
  • Number of nuclei: 2
  • Features: Multiple vacuoles, locomotion via cilia
• Reproduction:
  • Method: Binary fission of trophozoites

Life Cycle of Balantidium coli

• Infective Stage: The cyst is ingested through contaminated food or water.
• Pathogenesis: Some trophozoites invade the wall of the colon.

Epidemiology of Balantidium coli

• Prevalence: Worldwide
• Symptoms: Mostly asymptomatic or resemble amebiasis.
• Reservoirs: Humans and pigs, among others (e.g., chimpanzees)
• Transmission:
  • Waterborne, foodborne routes
• Prevention: Personal hygiene, water treatment, food safety programs

Emerging Protozoan Parasites

• Toxoplasma gondii
• Microsporidia spp.
  • Now may be classified as fungi?

Toxoplasma gondii

• Taxonomy: Belongs to Apicomplexa (Coccidia)
• Oocyst:
  • Two phases: unsporulated and sporulated
  • Size: 10-13 μm
  • Features: Two sporocysts, four sporozoites, distinctive cell walls with multiple layers
  • Environmental Persistence: Scatters UV light; can survive in soil for months and moist conditions for years

Other Forms of Toxoplasma gondii

• Tachyzoite:
  • Shape: Crescent-shaped (2 x 6 μm)
  • Features: Rapid multiplication, can be transmitted through the placenta
• Bradyzoite:
  • Features: Slowly multiplying, found in tissues of intermediate hosts
• Reproduction:
  • Both asexual (in intermediate hosts) and sexual (in definitive hosts, i.e., cats)

Transmission of Toxoplasma gondii

• Key Routes:
  • Tachyzoites transmitted through placenta
  • Oocysts passed in feces, ingested by cats or intermediate hosts via contaminated food, water, or undercooked meat

Epidemiology of Toxoplasma gondii

• Prevalence: 22.5% in general U.S. population (1988-1994)
  • Unusually high in France (65-85% due to raw/undercooked meat)
  • Central America has a high prevalence due to stray cats
• Symptoms:
  • Immunocompetent: Generally asymptomatic or mild flu-like symptoms
  • Immunocompromised: Life-threatening (encephalitis, blindness, myocarditis, pneumonia)
  • Congenital infections can lead to impaired vision and mental retardation in infected children.
• Fecal Shedding: Approximately 10^5 oocysts/gram in cats
• High Risk Groups: Infants born to infected mothers and immunosuppressed individuals

Microsporidia spp.

• Classification: New phylum with 143 genera and over 1200 species, 14 identified human pathogens
• Spores: Very resistant and usually small (1-4 μm)
• Polar Tubule:
  • A unique organelle coiled inside spores used for injecting infective contents into host cells

Life Cycle of Microsporidia spp.

• Life Cycle Stages:
  • Involves intracellular development of spores in hosts like E. bieneusi and E. intestinalis

Epidemiology of Microsporidia spp.

• Prevalence: Seen worldwide in both developed and developing countries
• Symptoms:
  • Immunocompetent individuals: Asymptomatic or self-limiting diarrhea
  • Immunocompromised individuals: Can exhibit chronic diarrhea, disseminated diseases (e.g., keratoconjunctivitis, bronchitis, pneumonia)
• Reservoirs: Humans and various animals (e.g., rabbits, mice, dogs)
• Transmission:
  • Uncertain; possibly includes airborne, waterborne, and transplacental routes (notably for Encephalitozoon spp.)

Other Protozoa

• Trypanosomes:
  • Causes sleeping sickness (African with tsetse flies; American with kissing bugs)
• Acanthamoeba spp. (GAE)
• Balamuthia mandrillaris (GAE)
• Naegleria fowlerii (PAM)
• Pneumocystis carinii (now termed P. jiroveci, reclassified as a fungus)