An Overview of Parasites

Parasites are organisms that live in or on another organism (host) for survival and reproduction, causing harm to the host.

1. Medical Parasitology

Medical parasitology focuses on parasites that can cause diseases in humans. It investigates the intricate relationships between parasites and their hosts, aiming to develop strategies for preventing and treating parasitic diseases.

2. Interrelationships Between Organisms

In nature, organisms exhibit various interactions, including:

• Mutualism: Both organisms benefit from the relationship. Example: Nitrogen-fixing bacteria in legume roots provide nitrogen to the plant while receiving nourishment and a habitat in return.

• Commensalism: One organism benefits, while the other neither benefits nor suffers harm. Example: Bacteria in the human gut aid in digestion without negatively affecting the host’s health.

• Phoresis: A smaller organism attaches to a larger one for transportation, without feeding on it. Example: Ticks hitch a ride on bird feathers for dispersal.

• Parasitism: The parasite depends on the host for survival, causing harm. Parasites are often smaller than their hosts and rely on the host for physiological support.

3. Types of Parasites

• Endoparasites: Parasites that live inside the host’s body. Example: Roundworms (Ascaris lumbricoides) residing in the human small intestine.

• Ectoparasites: Parasites that live on the host’s surface. Example: Female mosquitos (Anopheles spp.) feeding on blood from the skin.

4. Classifying Parasites

• Based on Host Relationship:

• Permanent parasites: Remain on the host’s body for a long time or for their entire lifespan.

• Temporary parasites: Only reside on the host for a short duration, typically for feeding.

• Based on Host Specificity:

• Monoxenous parasites: Only parasitize a single species of host.

• Polyxenous parasites: Can parasitize multiple species of hosts.

• Heteroxenous parasites: Prefer one host species but can invade other hosts, although they may not develop fully or cause disease.

• Accidental parasites: Free-living organisms that become parasites under favorable conditions. Example: *Naegleria fowleri*, an amoeba.

5. Hosts

Hosts are living organisms that are parasitized and have their nutrients exploited.

• Host Roles:

• Definitive host (Permanent host): Harbors the adult, sexually mature stage of the parasite. Example: Humans are the definitive host for roundworms.

• Intermediate host: Provides a temporary environment essential for the parasite’s development. The parasite may reproduce sexually or asexually in the intermediate host. Example: Snails and fish are intermediate hosts for the liver fluke.

• Paratenic host: A temporary reservoir for the parasite, not essential for its life cycle but provides shelter. Example: Insects can act as paratenic hosts for certain parasites.

• Vector: Typically arthropods or invertebrates that transmit parasites. Two modes of transmission occur:

• Biological vector: The parasite undergoes developmental stages (reproduction, etc.) within the vector. The vector is also an intermediate host. Example: *Plasmodium falciparum*, transmitted by *Anopheles* mosquitos.

• Mechanical vector: The parasite merely attaches to the vector’s surface or passes through its digestive system without undergoing any reproductive or developmental stages. Example: Houseflies can mechanically transmit *Entamoeba histolytica* by carrying amoebas on their bodies or through their digestive system.

6. Source of Infection

The source of infection is where mature parasites reside and release infectious agents. Example: Individuals infected with *Ascaris lumbricoides* are a source of infection.

7. Environment

The environment provides a suitable habitat for parasite development. It can also act as a source of infection. Example: Contaminated water and moist soil are environments for roundworm development.

8. Parasite Life Cycle

The complete developmental sequence of a parasite from its initial infectious stage to adulthood, capable of producing new infectious stages.

9. Ecological Characteristics

• Parasite survival and reproduction rely on the presence of hosts, so parasite population is directly influenced by host abundance.

• The definitive host provides a living environment for the parasite for a period of time or its entire lifespan.

10. Morphological Features

• Diverse shapes: round, oval, tubular.

• A single parasite species can exhibit significant size variations across different life cycle stages.

11. Reproduction

• Asexual: Single-celled organisms, fungi.

• Sexual: Worms, tapeworms, fungi, some single-celled organisms.

• Sporogony: Reproduction during the larval stage. Example: Flukes.

12. Harmful Effects of Parasites

• Damage to tissues and cells.

• Alterations in tissue structure.

13. Nomenclature of Parasites

• Written in Latin.

• Each parasite has a scientific classification: phylum, class, order, family, genus, and species.

• Family names typically end in IDEA (subfamily names end in INEA).

• Parasite name = Genus + Species.

• Written in italics or underlined. Example: *Ancylostoma duodenale*.

14. Classification of Parasites

• Kingdom Animalia (Animals):

• Phylum Nematoda (Roundworms): *Trichinella spiralis*.

• Phylum Platyhelminths (Flatworms):

• Class Cestoda (Tapeworms): *Taenia saginata*, *Taenia solium*.

• Class Trematoda (Flukes): *Clonorchis sinensis*, *Schistosoma haematobium*.

• Phylum Arthropoda (Arthropods):

• Subphylum Chelicerata (Ticks): *Ixodes scapularis*.

• Subphylum Uniramia (Insects): *Anopheles spp.*, *Aedes aegypti*.

• Kingdom Protozoa (Single-celled organisms):

• Phylum Euglenista: *Leishmania spp.*, *Trypanosoma spp*.

• Phylum Metamonada: *Giardia spp*.

• Phylum Parabasala: *Trichomonas spp*.

• Phylum Ciliophora: *Balantidium coli*.

• Phylum Rhizopoda: *Acanthamoeba spp.*, *Entamoeba histolytica*.

• Phylum Apicomplexa: *Plasmodium spp.*, *Toxoplasma gondii*.

• Phylum Percolozoa: *Naegleria fowleri*.

• Kingdom Fungi (Fungi):

• Class Ascomycota: *Aspergillus spp.*, *Trichophyton spp.*, *Microsporum spp.*, *Epidermophyton spp.*, *Candida spp.*, *Coccidioides immitis*, *Blastomyces dermatitidis*.

• Class Basidiomycota: *Cryptococcus neoformans*.

• Class Microsporidia: *Encephalitozoon cuniculi*.

• Class Zygomycota: *Rhizopus spp*.

Note:

• This overview provides a general understanding of parasites.

• Numerous parasite species exist, each with diverse biological characteristics and life cycles.

• Studying parasites is a complex field requiring in-depth exploration.

• Comprehending parasites empowers us to effectively prevent and treat parasitic diseases.