L 29 Intestinal Protozoans and Associated Diarrheal Diseases

Intestinal Protozoans (Afebrile Watery Diarrheas) - Giardia lamblia and Cryptosporidium

Introduction to Protozoans

  • Protozoans: Unicellular eukaryotic organisms, microscopic in size, can be motile, and are capable of replicating through asexual or sexual cycles.

  • Transmission: Acquired by ingestion of fecally contaminated food or water, leading to zoonosis, where humans and animals serve as reservoirs.

  • Infective Dose: Low dose infectious agents, ranging from 10 to 1000 cysts or oocysts.

  • Specific Pathogens:

    • Giardia lamblia contains cysts that cause intestinal issues.

    • Cryptosporidium parvum forms oocysts, which are the infective stages that can develop following oral ingestion.

Giardia lamblia

  • Characteristics: An extracellular, non-invasive flagellated protozoan.

  • Life Forms: Exists in two forms:

    1. Trophozoite:

    • Shape: Tear-shaped, contains 2 nuclei and 2 pairs of flagella.

    • Movement known as “falling leaf motility”.

    1. Cysts:

    • Infectious unit found in stool, can survive for up to 3 months in the environment.

Epidemiology of Giardia lamblia

  • Most common intestinal parasite in the U.S.

  • Transmission: Occurs mainly via the fecal/oral route through contaminated food and water or among travelers to endemic areas. Vulnerable populations include hikers and campers who drink untreated water from natural sources.

Pathology of Giardia lamblia

  • Incubation Period: Ranges from 9 to 15 days.

  • Life Cycle: Ingestion of cysts leads to excystation in the small intestine, where trophozoites adhere to the surface of villi, destroying microvilli and resulting in malabsorption of fats and proteins. Trophozoites are eventually passed in feces.

Clinical Manifestations of Giardia lamblia

  • Asymptomatic Carriers: Individuals may pass cysts without showing symptoms.

  • Acute Infections:

    • Characterized by afebrile watery diarrhea.

    • Associated symptoms include:

    • Steatorrhea: Foul-smelling stool due to fat malabsorption.

    • Flatulence and abdominal distension.

    • Symptoms typically last 3 to 4 days.

  • Chronic Infections:

    • Symptoms include acute infection signs, with added malabsorption and potential weight loss, which can persist for years.

Diagnosis and Treatment of Giardia lamblia

  • Diagnosis:

    • Ova & Parasites (O&P) test to confirm the presence of cysts and trophozoites.

    • Antigen testing and PCR can be used for confirmation.

    • Fecal fat testing can indicate malabsorption issues.

  • Treatment:

    • Utilizes antimicrobials to eliminate the infection.

Cryptosporidium

  • Nature of the Parasite: Obligate intracellular parasite affecting mammalian intestinal tracts.

  • Key Species: Two main species responsible for cryptosporidiosis:

    1. Cryptosporidium hominis: Primarily infects humans.

    2. Cryptosporidium parvum: Zoonotic species infecting various animals (cattle, sheep, goats, deer).

  • Life Cycle: Undergoes sexual reproduction which culminates in the production of oocysts, all developmental stages occur intracellularly.

Epidemiology of Cryptosporidium

  • Transmission: Utilizes both humans and animals as reservoirs.

  • Contamination Sources: Majorly from fecal contamination of water and food. C. hominis spreads through direct person-to-person contact or through contaminated water sources (e.g., pools).

  • Prevalence: Recognized as the leading cause of water-associated gastroenteritis in the U.S.

  • High-Risk Groups: Individuals traveling to endemic areas and those who are immunocompromised are at heightened risk.

Pathology of Cryptosporidium

  • Incubation Period: Approximately 1 week.

  • Infective Stage: Oocyst is the infective form, which transforms into motile sporozoites that adhere and invade the small intestine mucosa, leading to gastrointestinal symptoms.

Clinical Manifestations of Cryptosporidium

  • Typically afebrile presenting with:

    • Profuse, watery diarrhea.

    • Often self-limiting, but may last 5–10 days, which is longer than viral or bacterial infections.

  • Immunocompromised Hosts: More susceptible to severe, prolonged watery diarrhea, significant weight loss, and extra-intestinal involvement affecting the liver, pancreas, and lungs.

Diagnosis and Treatment of Cryptosporidium

  • Diagnosis:

    • Employs O&P testing, PCR, DFA (Direct Fluorescent Antibody), and EIA (Enzyme Immunoassays) to identify the presence of the parasite in stool samples.

  • Treatment: Primarily supportive for healthy individuals; antimicrobials are provided to immunocompromised patients to manage symptoms.

Additional Protozoans and Related Diseases

  • Cyclospora spp.:

    • Obligate intracellular parasite causing a diarrheal disease similar to Cryptosporidium.

    • Higher incidence reported in Asia and South America, often associated with contaminated food (berries, water).

    • Increased risk for immunocompromised individuals.

  • Pathology: Oocysts require incubation prior to passing in feces and are not transmitted directly between individuals. Once ingested, oocysts develop into sporozoites in the small intestine and invade intestinal epithelial cells, reproducing asexually.

  • Clinical Manifestations: Incubation of approximately 1 week leading to profuse watery diarrhea in most cases.

  • Diagnosis: O&P testing.

  • Treatment: Utilizes antimicrobials.

  • Microsporidia spp.:

    • Obligate unicellular intracellular parasites closely related to fungi, encompassing over 100 genera and typically zoonotic.

    • Known as the most prevalent cause of chronic diarrhea in HIV patients, particularly Enterocytozoan bieneusi, which seldom affects immunocompetent individuals.

  • Pathology: Infectious stage involves spores that possess polar tubes for injecting content into mucosal cells, where replication occurs.

  • Clinical Manifestations: Can range from asymptomatic to self-limiting watery diarrhea; in immunocompromised patients, severe watery diarrhea may occur alongside acalculous cholecystitis.

  • Diagnosis: Standard O&P tests or transmission electron microscopy.

  • Treatment: Managed with antimicrobials.

Diarrheas With Or Without Fever - Shiga-like toxin-producing E. coli (STEC)

  • Introduction to STEC:

    • Also referred to as Enterohemorrhagic E. coli (EHEC), specifically type O157:H7, which produces virulence factors including Shiga-like toxin, contributing to GI bleeding and acute hemorrhagic colitis.

  • Pathology:

    • Non-invasive organism adheres to colonic mucosa utilizing two main virulence factors:

    1. Pili for adherence paired with the Locus of Enterocyte Effacement (LEE), critical for attachment and lesion formation.

    2. Shiga Toxins (Stx-1 and Stx-2), which target endothelial cells in the gut, kidneys, and brain causing ensuing gastrointestinal bleeding.

    • Antibiotic treatments may increase toxin production and risk for severe outcome.

  • Hemolytic Uremic Syndrome (HUS):

    • Potentially fatal complication developing roughly a week after the onset of diarrhea, characterized by:

    • Toxemia rather than bacteremia, leading to high mortality rates.

    • Microangiopathic hemolytic anemia, thrombocytopenia, and acute renal failure due to microvascular thrombosis.

    • Possible CNS involvement resulting in encephalopathy.

  • Clinical Manifestations:

    • Incubation Period: 3–4 days before diarrhea onset.

    • Initial watery diarrhea transitioning to bloody stools within a week along with abdominal cramps, nausea, and vomiting; febrile response seen in fewer than half the cases.

    • Fecal leukocytes present in about 1/3 of cases, alongside potential bleeding and severe complications such as HUS.

  • Diagnosis:

    • MacConkey agar with sorbitol for culture, Stx ELISA on feces, and screening tests via PCR.

  • Treatment: Supportive care; antibiotics contraindicated due to increased risk of adverse outcomes.

Amoebic Dysentery - Entamoeba histolytica

  • Characteristics: Invasive eukaryotic protozoan and anaerobe with humans as the definitive host and primary reservoir.

  • Epidemiology: Common in developing countries due to low infectious dose (1-100 cysts), often a risk factor for travelers to endemic regions such as Southeast Asia.

  • Pathology:

    • Infectious cysts survive outside the host for over a week. Upon ingestion, cysts develop into trophozoites that invade the colonic epithelium, causing cell destruction and flask-shaped ulcers.

    • E. histolytica is capable of consuming red blood cells (RBCs) for energy, and virulent strains produce toxins leading to minimal inflammation while forming cysts. Hematogenous dissemination can lead to abscess formation in various organs.

  • Clinical Manifestations: Present with bloody, loose, watery diarrhea; abdominal pain in the lower right quadrant, urgency to defecate, and potential fever with systemic signs.

    • Extraintestinal disease includes amebic liver abscess, presenting with upper right quadrant pain, fever, and pleuritic pain, possibly leading to liver failure.

  • Diagnosis: Confirmed through O&P testing, antigen testing, serology, and PCR as needed.

  • Treatment: Managed effectively with antimicrobials.

Diarrheas Presenting With Fever - Clostridium difficile (C. difficile)

  • Characteristics: Anaerobic, spore-forming, gram-positive rod, a component of normal intestinal flora found in 3% of the general population, with 20–40% carriage rate in hospitalized and nursing home patients.

  • Pathogenesis: C. difficile is largely toxigenic, producing toxins A and/or B leading to antibiotic-associated colitis (inflammation of large intestine).

  • Epidemiology:

    • Transmission occurs via person-to-person contact and fomites in healthcare settings. Risk factors for infections include hospitalization and previous antibiotic therapies lasting longer than three days, with increased transmission risk correlating to acid suppression medications (e.g., antacids, PPIs).

  • Clinical Manifestations:

    • Various clinical presentations including asymptomatic carriers, watery diarrhea associated with colitis, as well as tenesmus, abdominal cramping, malaise, and low-grade fever.

    • Pseudomembranous colitis manifests similarly to diarrhea with colitis but includes a visible pseudomembrane on mucosa. Recurrent C. difficile infection is observed within 60 days of initial treatment; severe cases may lead to fulminant colitis and risks of complications such as bradycardia, hypotension, shock, and toxic megacolon.

Diagnosis and Treatment of C. difficile

  • Diagnosis: Suspected in any patient with diarrhea occurring after antibiotic use within the previous two months or after 72 hours of hospital admission. Diagnostic methods include PCR and assays for toxin detection or C. difficile glutamate dehydrogenase in feces by EIA and cell culture.

  • Treatment: Requires cessation of broad-spectrum antibiotics with further treatment involving:

    • Administering Vancomycin or Fidaxomicin while ensuring supportive care through fluid and electrolyte replacement, probiotics, immunoglobulins, and fecal transplants.

  • Prevention: Emphasis on barrier precautions, patient isolation, thorough cleaning with sporicides, hand hygiene, and restraint in utilizing implicated antimicrobial agents along with avoiding gastric suppression methods.