L 16

Introduction to Parasitology

• Definition of Parasite: Any organism that lives on or within a host organism and derives nutrients at the host's expense.

• Relationships:

  • Many parasitic organisms have a commensal relationship with the host, meaning they do not cause disease.

  • Mutualism: Both organism and host benefit.

  • Commensalism: One organism benefits without harming the other.

  • Parasitism: One organism benefits at the expense of the other.

• Medical Microbiology Focus:

  • Parasitology specifically pertains to eukaryotic pathogens, such as protozoa and helminths, though many organisms can have parasitic relationships.

  • Distinction:

  • Bacterial cells: Prokaryotic parasites.

  • Eukaryotes: Contain a nucleus, chiefly pathogenic protists and helminths.

Significance of Parasitology

• Global Impact:

  • Approximately 4.5 billion people harbor parasites, meaning about 25% of the world’s population suffers from parasitic infections.

• Factors Contributing to Parasitic Infections:

  • Predominantly found in underdeveloped, tropical, and subtropical countries due to:

  • Greater diversity of organisms.

  • Limited access to marginal water sources.

  • Poor sanitation and public health conditions.

  • High population density.

  • Climate change which affects the geographic regions of vectors.

  • Increased international trade leading to travel to endemic areas.

Diseases Caused by Parasites Endemic in the U.S.

1. Cystericerosis (Helminth):

   • Adult tapeworms acquired through undercooked meat (e.g., pork) may cause minor or no symptoms.

   • Tapeworm cysts in the brain result in cystericerosis and lead to seizures, with approximately 1,000 hospitalizations per year.

2. Toxocara (Helminth):

   • A roundworm that can cause blindness, primarily in children due to exposure to dog and cat feces.

3. Enterobiasis (Helminth):

   • Often asymptomatic, this pinworm infestation is common in children.

4. Toxoplasmosis (Protozoan):

   • Over 60 million in the U.S. are chronically infected.

   • In pregnant women, it can lead to birth defects and chronic health issues.

5. Trichomoniasis (Protozoan):

   • An STI affecting 3.7 million in the U.S.

   • Often asymptomatic but increases the risk of developing other STIs.

6. Giardiasis (Protozoan):

   • Causes intestinal infection characterized by cramps, bloating, and severe diarrhea; often found in campers or hikers consuming untreated water.

Types of Parasites

• Obligate Parasite:

  • A parasite that is dependent on the host for part of its life cycle (e.g., Plasmodium spp.).

• Facultative and Opportunistic Parasite:

  • Can exhibit both parasitic and non-parasitic modes of living.

  • Do not require a host to survive but can become parasitic when the opportunity arises (e.g., Naegleria fowleri).

• Accidental Parasite:

  • Infects a host that is not typically associated with it (e.g., Toxocara spp.).

Types of Hosts

• Definitive Host:

  • A host that harbors sexually mature parasites and/or where the parasite undergoes sexual reproduction.

  • This is the primary focus for managing disease.

• Intermediate Host:

  • A host that is infected with asexual stages (larval) of the parasite.

  • In some cases, larval development occurs in multiple intermediate hosts, referred to as the first and second intermediate hosts.

Parasite Life Cycles

• Complex Life Cycles:

  • Many parasites require more than one host during their life cycles, showcasing varied designs and strategies for infection.

• Forms of Parasites:

  • Different forms can be infectious or non-infectious and may cause various disease symptoms.

Review Question

• For the effective eradication of toxoplasmosis (caused by Toxoplasma gondii), it is crucial to mitigate the definitive host (cat), as opposed to the intermediates (mouse and human).

Classification of Parasites

• Protists (Protozoa):

  • Single-celled eukaryotic organisms, ranging from 2–100µm in size.

  • Most are non-pathogenic, consisting of free-living organisms like algae, amoebas, and flagellates.

  • Some can infect humans as frank or opportunistic pathogens; they often have complex life cycles involving non-human hosts or environmental periods of maturation.

• Helminths:

  • Multicellular worms, variable in size from millimeters to meters, infecting many regions of the body, particularly the gastrointestinal system.

  • Most cannot fully reproduce within their host, often requiring additional definitive or intermediate hosts, with life cycles dependent on soil or environmental incubation.

Helminth Life Cycle Overview

• Stages:

  • Eggs (Ova): Infectious stage.

  • Larva: Immature stage prior to reaching adulthood.

  • Cyst: Stage where the parasite can be dormant and resistant.

  • Adult Worm: Mature form capable of reproduction.

  • Disease caused by infestation can depend on the infectious stage, as cysts may affect the brain.

• Baylisacaris (Raccoon Roundworm):

  • Transmission via fecal-oral routes (e.g., through sandboxes or pica behavior).

  • Larvae can migrate throughout the body after infection via egg, potentially leading to severe damage, including asymptomatic cases or death.

  • Larvae or larval tracts can sometimes be observed through ocular exams (ocular larva migrans).

Parasitic Pathogenesis

• Variability and Specificity:

  • Factors contributing to pathogenesis include:

  • Size and load of the parasite.

  • Type (protozoa vs. helminth).

  • Reproductive potential (many helminths cannot reproduce within a host).

  • Nutritional requirements.

  • Location within the host’s body.

  • Immune response variability to the parasite.

• Example: D. latum (a type of salmon) may grow up to 3m in the GI tract but only cause pernicious anemia in fewer than 1% of infected individuals.

• Statistics: Approximately 445,000 deaths occur annually due to malaria caused by Plasmodium spp.

Progression of Infection to Pathogenesis

1. Exposure and Entry:

   • Entry methods vary based on the agent and tissue affinity (tropism):

     • Fecal-oral route (most common):

     • Includes direct person-to-person transmission and auto-infection (e.g., Enterobius vermicularis).

     • Indirect transmission through contaminated food and water.

     • Direct penetration of skin:

     • Ex: Arthropod bites and hookworms (penetrating the skin of the feet).

     • Sexually transmitted infections:

     • e.g., Trichomonas.

     • Transplacental transmission:

     • e.g., Toxoplasma from kitty litter.

2. Adherence and Replication:

   • Relies on the agent and tropism; examples include:

     • Circulation in host bloodstream (e.g., Plasmodium - malaria, Trypanosoma - Chagas disease).

     • Attachment to the intestinal lining or lung.

     • Adult helminths lay eggs while residing in the intestine or other organs.

3. Cell and Tissue Damage:

   • Helminths can induce tissue inflammation, causing damage due to larvae migration in the intestine, lung, heart, or brain.

   • Causes conditions such as glomerulonephritis from immune complexes.

   • May lead to Vitamin B12 deficiency and anemia.

   • Expected intestinal blockage can occur due to rapid replication.

4. Interference with Immune Response:

   • Antigenic Variation:

     • E.g., Plasmodium changes its surface antigens to evade immune response, resulting in cyclical dysregulation of immune responses.

Immune Response to Parasites

• Due to the larger size of helminths, the immune response differs from that elicited against viruses or bacteria; phagocytosis is generally ineffective.

• Innate Immune Response:

  • Parasites stimulate an innate response involving complement and natural killer (NK) cells, which are usually insufficient for clearing the infection.

• Adaptive Immune Response:

  • Critical for effective clearance; involves generating anti-parasite antibodies.

  • CD8+ T Cells: Respond by releasing cytokines such as TNF-α and IFN-γ which activate macrophages.

  • Macrophages: Engage and engulf smaller parasites (such as protists).

  • Eosinophils: Attack helminths by binding to IgE antibodies, followed by the release of toxic chemicals.

  • IgM and IgE: IgM neutralizes parasites, while IgE activates mast cells and basophils, which helps in expelling helminths through histamine release and other mediators.

Intestinal Parasite Diagnosis

• Patient History:

  • Critical to inquire about travel history, as many parasites are endemic in specific geographic locations.

  • Ask about food and liquid intake, as well as exposure to insects.

• Specimen Types: Commonly utilized specimens include:

  • Stool

  • Blood

  • Other types: corneal scrapings, duodenal material, skin snips, sputum, and vaginal specimens.

Gastrointestinal Parasites

1. Type of Specimen: Fresh stool is required for a pinworm paddle test.

2. Type of Test: Ova and Parasite evaluation aimed at detecting ova (eggs), larvae, or worms.

3. Considerations:

   • Unacceptable specimens are those insufficient in quantity or contaminated with urine/toilet water.

   • It is advisable to collect 3 stool specimens over a 2-3 day interval, as protozoa may be passed intermittently.

   • Helminth eggs usually pass at regular intervals, so one specimen is typically sufficient to diagnose helminth infections.

Ova and Parasite (O & P) Stool Evaluation

• Macroscopic Examination:

  • Visually assess for large parasites (e.g., Ascaris lumbricoides).

• Stool Consistency:

  • Formed: likely to contain the cyst stage of protozoa.

  • Liquid: loose and watery stools are more likely to contain the trophozoite stage of protozoa.

• Wet Preparations:

  • Iodine may be used to enhance the visibility of protozoan cyst nuclei.

• Microscopic Examination:

  • Permanent stains for assessment:

  • Trichrome Stain:

    • Cytoplasm of trophozoites appears blue-green, cysts are purple, and nuclei or inclusions are red, with a green background.

  • Iron Hematoxylin Stain: Used for stool analysis.

Blood Parasites (e.g., Plasmodium)

• Type of Specimen: Peripheral blood samples.

• Type of Test: Serology along with blood smears including both thick and thin smears for analysis.

• Common Stains:

  • Iodine, Trichrome, and Iron hematoxylin for stool analysis.

  • Giemsa stain for blood samples.

Ova and Parasite Evaluation in Blood

• Thick Smear:

  • Screening slide to check for the presence of parasites.

• Thin Smear:

  • Used to differentiate between parasite strains for accurate diagnosis.

Immunologic Diagnosis of Parasites

• Methods:

  • ELISA (for detection), direct and indirect immunofluorescence.

  • PCR: While it is not an immunological diagnosis, it serves as a direct test for parasite DNA.

Treatment Options for Eukaryotic Diseases

• Diagnosis Challenges:

  • Diagnosis can be complicated given the non-specific symptoms presented by many parasitic infections.

• Importance of Clinical Suspicion:

  • A thorough patient history is essential for accurate diagnostics.

• Drugs Available:

  • Many antiparasitic drugs can be toxic to patients due to their eukaryotic nature, leading to low selective toxicity of these treatments.

• Prevention:

  • Education on disease prevention is typically more effective than treatment itself.

  • Key strategies include ensuring clean water, proper food preparation, travel health tips, and avoiding vectors such as mosquitoes, while reducing unnecessary antibacterial drug use.