Protozoa

Protozoa

Protozoa: Intestinal Ciliate

B. coli (Balantidium coli)

• Largest protozoan and only species in the Ciliophora class that infects humans
• Characteristics:
  • Oval shaped with uniform cilia
  • Boring motility
  • Distinctive kidney-shaped macronucleus and spherical micronucleus

Infection

• Causes a diarrheal disease known as balantidiasis.
• Transmission:
  • Ingestion of cysts from fecally contaminated water or food
  • Swine serve as the primary hosts, while humans and other mammals act as accidental hosts or reservoirs.
• Life Cycle:
  • Cysts excyst in the small intestine and colonize the large intestine.
  • Multiply via binary fission and encyst.
  • Shed in stool.
• Pathogenic Effects:
  • Can cause ulcers in the colon or bowel wall, increasing the risk of disease transmission, especially in areas where pigs are free-roaming.

B. coli Life Cycle

• Cyst Structure:

  • Size: 50-70 μm
  • Contains a cyst wall, granules, and contractile vacuoles
  • Has two forms: old (cyst) and young (trophozoite)

• Trophozoite Details:

  • Size: 50-100 μm long by 40-70 μm wide
  • Features include macronucleus, micronucleus, contractile vacuole, cytostome, and peristome

Protozoa: Intestinal Coccidia

• Characteristics:
  • Intracellular parasites belonging to the Apicomplexan group, previously known as Sporozoa
  • Possess an apical complex and proliferate through sporulation and cyst formation
  • Exhibit both sexual and asexual life cycles
  • Typically require 1-2 hosts
  • Exhibit gliding motility via adhesion and small myosin motors.

Coccidia: Cryptosporidium

Cryptosporidiosis

• Different species include C. parvum, C. hominis, C. canis, and C. muris
• Causes enterocolitis of varying severity
• Incidence: Approximately 748,000 cases/year in the U.S.
• Severity:
  • In immunocompetent individuals, it is self-limiting, lasting 2-3 weeks.
  • In immunocompromised patients, can cause cholera-like infections and infrequently infiltrate tissues (e.g., liver, lungs).

Transmission & Diagnosis

• Transmission:

  • Ingestion of contaminated food or water
  • Direct contact with infected water
  • Oocysts are resistant to chlorine

• Diagnosis:

  • Identification of small 4-5 μm oval oocysts in stool containing 4 sporozoites
  • Oocysts stain pink-red/magenta using a modified acid-fast technique or immunofluorescent (IF)/enzyme immunoassay (EIA)
  • PCR may be used to identify species.

Life Cycle

1. Ingested cysts incubate in the host for 7-10 days
2. Excystation occurs, with sporozoites invading epithelial cells
3. Asexual Reproduction (Schizogony):
   • Occurs in host cells, producing merozoites
4. Sexual Reproduction (Gametogony):
   • Microgametes fertilize macrogametes, resulting in zygotes that produce two types of oocysts:
     • Thin-walled: Cause autoinfection
     • Thick-walled: Excreted, undergo sporogony, and produce 4 sporozoites

Coccidia: Cyclospora cayetanensis

Cyclosporiasis

• A food and waterborne disease with no direct human-to-human transmission
• Requires incubation at temperatures between 22-32°C
• Characterized by oocysts containing 2 sporocysts, each with 2 sporozoites
• In the host, a 1-week incubation is observed
• Clinical Presentation:
  • Symptoms include watery diarrhea, nausea/vomiting, and abdominal pain, varying based on immune status.
• Identification:
  • Oocysts are recognized when collapsed on one side and stain poorly using modified safranin stain, requiring heat treatment to improve visibility.

Coccidia: Cystoisospora belli (Isospora belli)

Isosporiasis

• Considered the least common human coccidia, more prevalent in tropical and subtropical areas
• Symptoms:
  • Causes acute, non-bloody diarrhea with cramps/pain that can last several weeks.
  • Particularly severe in immunocompromised individuals with an eosinophilia response.
• Life Cycle:
  • Occurs upon ingestion of infective oocysts.
  • Involves multiple divisions of sporoblasts, leading to the release of sporozoites that multiply asexually.

Diagnosis of C. belli

• Diagnosis is facilitated by the presence of immature oocysts excreted in stool, which mature post-excretion (20-30 × 10-20 μm, ellipsoid shape).
• The morphological differences between immature (single granular mass) and mature forms (two sporoblasts with cyst walls) are important for diagnosis.
• Stains utilized for detection include iodine or modified acid-fast stains, alongside molecular diagnostics such as stool panels.

Coccidia: Toxoplasma gondii

Toxoplasmosis

• Hosts:
  • Definitive hosts include cats while humans serve as accidental hosts.
• Transmission:
  • Toxoplasma can be ingested through contaminated meat or food (tissue cysts) and oocysts from contaminated water or soil through direct contact with litter boxes.

Symptoms & Risks

• General Symptoms:

  • Often asymptomatic or mild

• Severe Risks:

  • Immunocompromised individuals may experience severe complications like encephalitis, myocarditis, and hepatosplenomegaly.
  • The infection can spread through blood transfusions or organ transplants.
  • Pregnant women are at risk due to transplacental passage, with heightened effects in the third trimester leading to CNS or ocular damage if not previously immunized.

Life Cycle

1. Sexual (Enteric) Cycle:

   • Tissue cysts are ingested by definitive hosts, allowing reproduction in intestinal cells which leads to sporulation and oocyst shedding in feces.

2. Asexual (Extraintestinal) Cycle:

   • In intermediate hosts, trophozoites multiply in various tissues: brain, eye, lymph nodes, heart.
   • They transform into tachyzoites (crescent-shaped), spread through blood, and may encyst as bradyzoites which remain dormant unless the host is immunocompromised.

Diagnosis of Toxoplasma gondii

• Identification relies on pear or crescent-shaped tachyzoites in tissue sections or lymph node biopsies, using various stains such as Wright-Giemsa, PAS, or HE.
• Serological testing (EIA or IFA) is prevalent, with half the adult population showing positive serology results and careful distinctions between active infection and recovery vital.
• Molecular diagnostics may be necessary using PCR techniques.

Microsporidia: Diverse Intracellular Parasites

• Description:

  • Unicellular, intracellular parasites related to fungi, characterized by the formation of spores (1-4 μm) with polar filaments.

• Species Identification:

  • 15 species identified, with Enterocytozoon being the most common.

• Associated with opportunistic infections, particularly in immunocompromised patients.

Diagnosis of Microsporidia

• Diagnosis techniques include Giemsa or electron microscopy on tissue sections, trichrome stain, chemifluorescent stains, IFA, and PCR, particularly significant in immunocompromised patients.

Blastomyces: Misc. Protozoa

Blastomyces hominis

• Reclassified multiple times, currently categorized as Stramenopile (which includes yeasts, fungi, amoeboids, flagellates, and sporozoans).
• Diagnosed through gastrointestinal disease with transmission mainly occurring via contaminated food or water.
• Diagnosis:
  • Challenging to detect in stool samples, as cysts are 3-5 μm and amoeboid forms 3-8 μm.
  • Staining methods reveal a grey-green appearance with large central vacuoles and red inclusions in the cytoplasm.

Sporozoa: Blood & Tissue

Blood Parasites: Malaria

• Major causative agents of malaria include species such as P. falciparum, P. malariae, P. vivax, P. ovale, and P. knowlesi which are zoonotic.

- Transmission occurs through Anopheles mosquito vectors acting as the definitive host, with humans as the intermediate hosts.

Reproduction & Life Cycle

• Initial Infection:

  1. Following a mosquito bite, infected salivary fluids introduce sporozoites into the bloodstream.
  2. These motile, spindle-shaped sporozoites (10-15 µm) travel to the liver.

• Exoerythrocytic Schizogony:

  • The nucleus divides multiple times, maturing into schizonts that fragment into merozoites.

Intraerythrocytic Cycle

• Erythrocytic Schizogony:
  • Asexual reproduction where merozoites develop ring forms using hemoglobin, stained as blue-black granules referred to as hemozoin.
  • Mature schizonts rupture, releasing merozoites back into the blood causing periodic fever associated with malaria.

Symptoms of Malaria

• Classic symptoms include fever and chills.

  • A prodromal period characterized by headache, anorexia, low-grade fever precedes shaking chills that coincide with the cyclical rupture of red blood cells.
  • Symptoms follow a periodic cycle:
  • tertian (every 48 hours) and quartan (every 72 hours).

• Further complications:

  • Anemia, black water fever (release of hemozoin into urine), and chronic splenomegaly.

Diagnosis & Identification of Malaria

• Diagnosis relies on clinical symptoms, patient history, and examination of thick blood smears to identify Plasmodium species, notably ruling in or out P. falciparum, which is linked to severe malaria.
  • Distinctions made based on various characteristics such as ring forms, dots, and the morphology of infected red blood cells.

P. falciparum: Malignant Tertian Malaria

• Characterized by a 36-48 hour cycle, with a significant prevalence in regions such as Africa, Southeast Asia, and the Eastern Mediterranean.
• Notably severe infections, leading to symptoms such as acute respiratory distress syndrome, severe anemia, and CNS involvement.
  • Infection often results in multiple small rings per RBC and Maurer's clefts in infected cells.

P. malariae: Quartian Malaria

• Exhibits a 72-hour cycle, predominantly found in Sub-Saharan Africa and the Southwest Pacific.
• Notable for its long exoerythrocytic schizogony period, leading to lower infectious rates compared to other strains.

P. vivax: Benign Tertian Malaria

• Exhibits a 48-hour cycle, prevalent in the Americas, Southeast Asia, and the Western Pacific.
• Features a dormant hypnozoite stage, allowing for relapse weeks or even years later.

P. ovale: Ovale Malaria

• Shares similar characteristics with P. vivax but has distinct morphological features such as fimbriated edges and larger merozoite counts during schizont formation.

P. knowlesi: Zoonotic Infections

• Primarily found in Malaysia, affecting old world monkeys and presenting similar characteristics to P. falciparum but with varying degrees of morbidity based on the host.

Key Characteristics of Plasmodium Species

Babesia species

Overview

• Tick-borne sporozoan parasites where humans act as accidental hosts, with a higher risk of severe disease in splenectomized or immunocompromised patients.
• Symptoms resembling malaria include hemolytic anemia and hepatosplenomegaly.

Human Pathogens

• Notably include B. microti, B. divergens, and B. duncani, among others.
• Diagnosis:
  • Identification of pleomorphic ring forms and tetrad arrangements resembling rabbit ears or Maltese cross appearances are indicative of infection.

Hemoflagellates: Trypanosomes & Leishmania

• Infect various organ systems including blood and lymph nodes, characterized by their distinct life cycle stages: Amastigote, Promastigote, Epimastigote, and Trypomastigote.

Trypanosomes

Trypanosoma brucei

• Two subspecies causing African sleeping sickness:
  • T. b. rhodesiense
  • T. b. gambiense
• Vector: Tsetse flies (genus Glossina).
• Diagnosis:
  • Identification of trypomastigotes in peripheral blood smears.

Trypanosoma cruzi

• Causes American trypanosomiasis (Chagas disease)
• Vector: Triatomid or reduviid bugs, also known as 'kissing bugs.'
• Diagnosis:
  • Identification of C-shaped trypomastigotes in blood smears or bodily fluids.

Life Cycle of T. cruzi

• Phases:
  • Metacyclic trypomastigotes are present in the hindgut of the bug; they multiply intracellularly in the host’s reticuloendothelial cells and may spread to lymphatic, heart, and CNS regions.

Leishmania

Disease and Geographic Locale

Transmission & Diagnosis of Leishmania

• Transmission:
  • Occurs via the bite of infected sand flies, where promastigotes enter through the bite and develop while multiplying in macrophages.
• Diagnosis:
  • Clinical signs vary based on the form (cutaneous, mucocutaneous, or visceral) and include skin lesions or signs of organ involvement, with amastigote detection from skin lesions, spleen, lymph nodes, or other tissues.