Medical Parasitology: Concepts and Types

General Concepts in Medical Parasitology

• Medical Parasitology studies parasites infecting humans and the diseases they cause.
• A parasite lives on/in another organism (host), benefiting at the host's expense by deriving nutrients and protection.
• The host is the organism a parasite lives on or in.

Types of Parasites

• Obligatory parasites can only live in or on a host to complete their life cycle.
• Facultative parasites live freely in the environment but can live as parasites in hosts under unfavorable conditions.
• Ectoparasites live on the host.
• Endoparasites live within the host's body.
• Temporary parasites visit the host for feeding.
• Permanent parasites infect the host and remain there.
• Host-specific parasites affect only one host species.
• Accidental parasites infect a human by mistake, but cannot be maintained normally.
• Coprozoic (Spurious) parasites are foreign parasitic species that pass through the host's alimentary canal without causing infection.

Types of Hosts

• Definitive host: Parasite's adult stage is present, sexual reproduction occurs.
• Intermediate (amplifier) host: Immature (larval) stages are present; asexual reproduction occurs.
• Reservoir host: An animal that harbors the parasite's adult stage and acts as a continuous source of human infection.
• Vector: Usually an arthropod that transmits the parasite from one host to another.
• Transport (Paratenic) host: Parasite is in an arrested state of development but remains alive and ready to infect a suitable host.
• Dead-end (Final) host: A host from which infection cannot be transmitted.

Host-Parasite Relationships

• Symbiosis is a relationship between two organisms that live closely together.
• Parasitism means one organism benefits (nutrition and shelter) at the expense of the other, which suffers.
• Commensalism is when the organism benefits while the other is neither harmed nor benefits.
• Mutualism is when both organisms benefit to an extent where neither can survive without the other.
• Phoresis happens when the phoront (smaller organism) is carried mechanically by a larger organism.

Pathological Effects of Parasites

• Lytic necrosis: Parasites produce enzymes that cause lysis of tissue; Entamoeba histolytica causes intestinal ulcers.
• Trauma: Attachment of some parasites causes tear and damage to the tissue; Hookworm (Ancylostoma) causes bleeding and anemia.
• Physical obstruction: Masses of collected worms obstruct luminal organs; Ascaris causes intestinal obstruction.
• Inflammatory reaction: Clinical illness results from inflammatory changes & fibrosis; Schistosoma mansoni causes liver fibrosis, Entamoeba histolytica causes intestinal granulomas (amoeboma).
• Allergic manifestation: Host immune response to the parasite causes allergic manifestations like insect bites causing allergic rash.
• Neoplasia: Parasitic infections lead to malignancy; Schistosoma haematobium causes cancer bladder.
• Space-occupying lesions: Parasites cause cystic lesions that compress surroundings; Echinococcus causes hydatid cysts.

Classification Based on Pathogenicity

• Pathogenic parasites cause pathological lesions in the host; Schistosoma and Ancylostoma fit this classification.
• Opportunistic parasites cause no or mild disease in immunocompetent hosts and severe disease in immunocompromised hosts; Toxoplasma and Cryptosporidium fit this classification.
• Non-pathogenic (commensal) parasites derive food and protection from the host but do not cause definite pathological lesions; Entamoeba coli fits this classification.

Major Groups of Parasites

• Helminths
• Protozoa
• Arthropods

Case Scenario

• A 10-year-old boy exhibits repeated diarrhea and abdominal pain
• The microscopic stool examination revealed the presence of Giardia intestinalis
• The relation being parasitism, where one organism is benefiting and the other is being harmed.