Parasitology 101

Introduction

Parasites are a diverse group of organisms that include bacteria, fungi, protozoa, helminths (worms), and even some arthropods. They rely on a host for survival and reproduction, causing a range of health impacts on their hosts, from mild symptoms to severe diseases.

Symbiotic Relationships

• Mutualism: Both organisms benefit from the relationship. Example: Termites and protozoa in their gut. Termites provide food for the protozoa, and the protozoa help the termites digest cellulose.

• Commensalism: One organism benefits, while the other is neither harmed nor benefited. Example: Crabs and sea anemones. The sea anemone attaches to the crab’s back, benefiting from transportation for food while the crab isn’t affected.

• Phoresis: A smaller organism “hitchhikes” on a larger one, with no physiological dependence between the two. Example: Protozoa or fungi may attach to the surface of a larger animal for transportation.

• Parasitism: One organism (the parasite) lives at the expense of another organism (the host), causing harm to the host. Example: Plasmodium (malaria parasite) lives inside human red blood cells.

Parasite Classification

• By Location:

• Endoparasite: Lives inside the host’s body, e.g., roundworms in the human gut.

• Ectoparasite: Lives on the host’s surface, e.g., mosquitoes feeding on blood.

• By Relationship with the Host:

• Permanent parasite: Lives for an extended period or for life, e.g., roundworms.

• Temporary parasite: Lives for a short period or only when seeking food, e.g., mosquitoes.

• Based on Host Specificity:

• Monoxenous: Parasites a single host species, e.g., *Ascaris lumbricoides* (roundworm).

• Heteroxenous: Parasites multiple host species, e.g., liver fluke, *Toxoplasma gondii*.

• Aberrant host: A parasite that cannot mature and reproduce in a host it has accidentally entered, e.g., liver fluke.

• Incidental/ Accidental: Parasites that enter a host by chance, e.g., the *Naegleria* amoeba.

Hosts

• Definitive host: Harbors the adult stage of the parasite and where reproduction occurs.

• Intermediate host: Harbors immature stages of the parasite, often essential for development but not for reproduction.

• Paratenic host: A temporary environment that is necessary for the development of the parasite, and where reproduction may occur.

• Transport host: Provides temporary shelter, a necessary step for finding the definitive host, e.g., flies can carry single-celled parasites.

• Vector: An arthropod or vertebrate that transmits disease, e.g., mosquitoes.

Disease Transmission Vectors

• Mechanical: Parasites are carried on the surface or through the digestive tract of the vector but do not reproduce or develop within it, e.g., flies transmitting amoebas (causing intestinal illness).

• Biological: Parasites undergo developmental stages and may reproduce within the vector, e.g., *Plasmodium falciparum* (malaria parasite) transmitted by *Anopheles* mosquitoes.

Parasite Life Cycles

• Type 1: Entirely external: The life cycle of a mosquito.

• Type 2: Entirely within the host: *Trichomonas vaginalis* (causes vaginitis).

• Type 3: Host-external: Intestinal worms.

• Type 4: Host-external-intermediate host: Flukes.

• Type 5: Host-intermediate host: *Plasmodium* species (malaria).

Ecological Characteristics

• Higher parasite numbers than host organisms.

• Factors impacting the host indirectly affect the parasite.

Morphology

• Round, oval, tubular

• Unicellular: Round and oval.

• Roundworms: Tubular.

• Tapeworms: Long, ribbon-like body.

Reproduction

• Asexual: Unicellular organisms, fungi.

• Sexual: Worms, segmented fungi, some unicellular organisms.

• Sporogony: In larval stages, common in flukes.

Harmful Effects of Parasites

• Tissue damage.

• Altered tissue structure.

Tissue Damage:

• Albumin/fat degeneration/ tissue necrosis: *Trichinella spiralis* causes degeneration in surrounding tissue.

• Example: *Trichinella spiralis* causes degeneration in surrounding tissue.

Altered Tissue Structure:

• Hyperplasia: *Clonorchis sinensis* causes thickening of bile ducts.

• Hypertrophy: *Plasmodium vivax* causes red blood cells to enlarge.

• Neoplasia: *Paragonimus westermani* (formation of a different type of tissue).

• Metaplasia (tumors):* *Fasciola hepatica* causes tumors in the liver. *Schistosoma haematobium* causes malignant tumors in the bladder.

Nomenclature

• Latin

• Family ending in -IDEA, subfamily ending in -INAE.

• Genus (capitalized) + species (lowercase).

The Six Kingdoms

• Bacteria

• Animals

• Plantae

• Fungi

• Chromista (multicellular)

• Protista (unicellular)

Kingdom Animalia

• Phylum Nematoda (roundworms)

• Phylum Platyhelminthes (flatworms): Divided into four classes, commonly encountered:

• Cestoda (tapeworms)

• Trematoda (flukes)

• Phylum Arthropoda (arthropods)

Kingdom Protozoa

• 7 Phyla

Notes

• Parasites can cause a range of illnesses, from mild to severe.

• Prevention is crucial for controlling parasite-borne diseases.

• Follow health guidelines and practice good personal hygiene to avoid parasite infections.

Adding information about disease-causing fungi

• Aspergillus: Primarily affects the lungs.

• Aspergillus flavus: Causes liver cancer.

• Aspergillus oryzae: Used in food fermentation.

• Trichophyton spp., Microsporum, Epidermophyton: Cause skin diseases.

• Candida: Causes skin, gut, genital, and internal organ infections.

• Coccidioides immitis: Causes severe pneumonia.

• Blastomyces dermatitidis: Causes acute respiratory infection.

• Cryptococcus neoformans: Causes central nervous system disease.

• Enterocystozoon bienusi: Causes diarrhea and cholecystitis (gallbladder inflammation).

Conclusion

Parasites are a vital part of the ecosystem, but they can also cause serious diseases in humans and animals. Understanding the biology, life cycles, and harmful effects of parasites is essential for prevention and treatment.