Microbiology - Parasitic Protozoa, Helminths, and Arthropod Vectors

Microbiology with Diseases by Taxonomy: Chapter 23 - Parasitic Protozoa, Helminths, and Arthropod Vectors

Parasitology Overview

• Protozoan and helminthic parasites exist worldwide and are found primarily among populations in:

  • Rural, undeveloped, or overcrowded places

  • Emerging as serious threats even in developed nations

• Transmission of parasitic infections often involves several hosts:

  • Definitive host: The host in which the parasite reaches maturity.

  • Intermediate host: The host that harbors the juvenile or larval stages of the parasite.

• Parasites can infect humans in one of three ways:

  • Ingestion: Consuming infected food or water.

  • Vectorborne transmission: Through arthropods that carry parasites.

  • Direct contact: Skin contact with contaminated surfaces or bodily fluids.

Arthropod Vectors

• Vectors are defined as animals that carry microbial pathogens.

• Arthropods are the most common vectors, including:

  • Biological vectors: Serve as hosts for the pathogens they transmit.

  • Mechanical vectors: Carry pathogens but do not serve as hosts.

• Classes of arthropod disease vectors:

  • Arachnida (e.g., ticks, mites)

  • Insecta (e.g., mosquitoes, flies)

Protozoan Parasites of Humans

• General characteristics:

  • Protozoa: Unicellular eukaryotes.

  • Morphological forms upon entering the body via ingestion:

  • Trophozoite: The feeding and reproducing stage living within the host.

  • Cyst: The infective form that survives in the environment and undergoes excystment once ingested, developing into trophozoites.

  • Trophozoites can undergo encystment before leaving the host in feces.

• Protozoan parasites can be classified based on their locomotive structures:

  • Ciliates

  • Amoebae

  • Flagellates

  • Apicomplexans

Ciliates

• Protozoan that utilize cilia for movement in their trophozoite stage.

• Example:

  • Balantidium coli:

  • Only ciliate known to cause disease in humans.

  • Found in animal intestinal tracts and transmitted to humans via contaminated food or water.

  • Trophozoites attach to intestinal mucosal epithelium.

  • Symptoms typically include:

    • Asymptomatic in healthy adults.

    • Symptoms in compromised health:

    • Persistent diarrhea

    • Abdominal pain and weight loss

    • Severe infections may cause dysentery and intestinal mucosal ulceration.

  • Diagnosis: Presence of trophozoites in stool is diagnostic.

  • Treatment: Antibacterial tetracycline alters normal intestinal microbiota.

  • Prevention: Good personal hygiene and water sanitation.

Amoebae

• Characteristics:

  • Lack a defined shape, move and feed using pseudopods.

  • Found globally in natural water sources.

  • Few species cause disease.

• Amoeba example:

  • Entamoeba histolytica:

  • Carried asymptomatically in human digestive tracts, predominantly in less-developed countries.

  • Infection commonly through drinking contaminated water.

  • Upon ingestion, excysts in the small intestine and migrates to the large intestine.

  • Types of amebiasis:

    • Luminal amebiasis: Asymptomatic.

    • Invasive amebic dysentery: Severe symptoms, including diarrhea and ulceration.

    • Invasive extraintestinal amebiasis: Trophozoites enter the bloodstream, potentially fatal.

  • Diagnosis: Identification of cysts/trophozoites in stool samples.

  • Treatment for asymptomatic: Paromomycin; symptomatic cases treated with idoquinol and oral rehydration therapy.

  • Prevention: Improving water quality and sanitation.

• Other related amoebae:

  • Acanthamoeba and Naegleria:

  • Cause rare, often fatal brain infections; enter through cuts or by inhalation.

  • Symptoms: Keratitis and amebic meningoencephalitis.

Flagellates

• Defined by presence of at least one flagellum.

• Example:

  • Trypanosoma cruzi:

  • Causes Chagas’ disease, endemic in Central and South America.

  • Main reservoirs: Opossums and armadillos; transmitted through the bite of “kissing bugs” (Triatoma).

  • Disease progression through four stages:

    • Acute stage with chagomas.

    • Generalized stage.

    • Asymptomatic chronic stage.

    • Symptomatic stage marked by congestive heart failure due to pseudocyst formation.

  • Diagnosis: Identification of trypomastigotes in clinical samples.

  • Treatment: Early stages treated with benznidazole or nifurtimox, late stages are not treatable.

  • Prevention: Reducing exposure to kissing bugs.

• Another flagellate example:

  • Trypanosoma brucei:

  • Causative agent of African sleeping sickness; transmitted by the tsetse fly.

  • Three stages if left untreated:

    • Lesion at bite site

    • Symptoms from parasites in the blood

    • Meningoencephalitis leading to death.

  • Treatment for early stages: Pentamidine or suramin; central nervous system infections treated with melarsoprol.

• Leishmania:

  • Causes leishmaniasis with three forms: cutaneous, mucocutaneous, and visceral.

  • Diagnosis involves identifying Leishmania in patient samples; treatment varies based on form.

• Giardia intestinalis:

  • Common cause of giardiasis, a gastrointestinal disease; transmission through contaminated water/food.

Apicomplexans

• Defined as aveolate protozoa characterized by a complex of organelles at the apical end.

• Plasmodium: Causative agent of malaria; several species (P. falciparum, P. vivax, P. ovale, P. malariae) and a life cycle with distinct stages involving mosquitoes (Anopheles).

  • General life cycle overview:

  • Sporozoites invade liver cells, undergo schizogony, and release merozoites causing erythrocyte lysis every 48-72 hours, leading to symptoms such as fever and chills.

  • Genetic factors can confer resistance in populations, and severe disease such as blackwater fever exists.

• Toxoplasma gondii: Causative agent of toxoplasmosis, commonly asymptomatic but severe in immunocompromised individuals.

  • Diagnosis: Typically through serology; symptomatic infections require treatment.

• Cryptosporidium parvum: Causes Cryptosporidium enteritis, primarily through contaminated water; severe diarrhea occurs especially in immunocompromised patients.

• Cyclospora cayetanensis: Emerging pathogen causing cyclosporiasis, often linked to contaminated food; symptoms include cramping and watery diarrhea.

Helminthic Parasites of Humans

• General characteristics: Helminths are macroscopic, multicellular worms; life cycles typically involve intermediate hosts and can be classified into:

  • Cestodes (tapeworms)

  • Trematodes (flukes)

  • Nematodes (roundworms)

Cestodes (Tapeworms)

• Example:

  • Taenia saginata (beef tapeworm) and Taenia solium (pork tapeworm):

  • Diagnosis is based on the identification of proglottids in fecal samples; treatment with praziquantel.

• Echinococcus granulosus:

  • Accidental human hosts causing hydatid disease; diagnose with immunoassays and imaging.

Trematodes (Flukes)

• Commonly lack complete digestive tracts; attach to tissues via a sucker.

• Example includes Fasciola hepatica, infecting sheep and cattle; chronic infection causes symptoms related to bile duct obstruction.

• Blood flukes: Schistosoma species responsible for schistosomiasis; diagnosis via eggs in stool or urine.

Nematodes (Roundworms)

• Cylindrical worms with a complete digestive tract. Most have complex life cycles, including direct skin penetration or ingestion of eggs.

• Ascaris lumbricoides: Endemic in the rural southeastern United States; diagnosis by eggs or larvae in stool; treatment with albendazole or mebendazole.

• Hookworms (Ancylostoma duodenale, Necator americanus): Cause anemia and intestinal symptoms.

• Enterobius vermicularis: The pinworm common in the U.S., treatable with pyrantel pamoate.

• Wuchereria bancrofti: Filariasis-causing nematodes transmitted by certain mosquitoes, leading to eventual lymphatic complications.

Key Features of Protozoan Parasites of Humans

Key Features of Helminthic Parasites of Humans