Medical Parasitology: Study Notes

• Study notes on medical parasitology

Key Definitions

• Parasitology is the scientific study of parasites
• Medical parasitology focuses on parasites causing diseases in humans
• A parasite is an organism living in or on a host, depending on it for physical or physiological needs

Nature and Features of Parasites

• Parasites can be unicellular organisms, worms, or arthropods
• Compared to their hosts, parasites are typically:
  • Smaller
  • More numerous
  • Have a shorter lifespan
  • Possess a greater reproductive potential

Associations Between Organisms

• Mutualism: Both organisms benefit from the association
• Symbiosis: A permanent association where two different organisms depend on each other for survival
• Commensalism: One organism (the parasite) benefits, while the host is neither harmed nor benefits
• Parasitism: One organism (the parasite) benefits at the expense of the other (the host), causing harm to the host

Classification of Parasites by Habitat

• Ectoparasites live on the host's skin surface e.g., lice and ticks
• Endoparasites live within the host's body e.g., Leishmania species and Ascaris lumbricoides

Classification of Parasites by Dependence on Host

• Permanent (obligate) parasites rely entirely on their host for survival e.g., Plasmodium species and Trichomonas vaginalis
• Temporary (facultative) parasites can live independently and lead a parasitic life e.g., Strongyloides stercoralis and Naegleria fowleri

Classification of Parasites by Pathogenicity

• Pathogenic parasites cause disease in the host e.g., E. histolytica
• Non-pathogenic (commensal) parasites derive food and protection without harming the host e.g., Entamoeba coli
• Opportunistic parasites cause mild disease in healthy individuals but severe disease in immunocompromised hosts e.g., Pneumocystis carnii, Toxoplasma gondii, and Isospora belli

Types of Hosts

• Hosts are organisms that harbor parasites
• Definitive host:
  • Harbors the adult stage of a parasite or where the most developed form occurs
  • Where the sexual maturity and fertilization of a parasite take place
• Intermediate host:
  • Host harboring sexually immature or larval stage of a parasite
  • No fertilization takes place
  • Example: Cow is an intermediate host for Taenia saginata
• Amplifier host: Intermediate hosts in which parasites undergo multiplication
• Reservoir host: Wild or domestic animal that harbors a parasite and acts as a source of infection for humans
• Carrier host: Hosts harboring and disseminating a parasite without showing clinical signs
• Accidental (incidental) host: Host infected that isn't the parasite's normal host, and full parasite development may or may not occur

Stages of Parasites and Infection Types

• Diagnostic stage: A developmental stage of a pathogenic organism detectable in stool, blood, urine, sputum, CSF, or other body secretions
• Infective stage: The stage at which a parasite can enter a host and continue its development
• Infection: Invasion of the body by any pathogenic organism (except arthropods)
  • The reaction of the host's tissue to the parasite's presence or related toxins
• Biological incubation (prepatent) period: The time between initial infection and demonstration of the parasite or its stages in excreta, blood, or other diagnostic material
• Clinical incubation period: The interval between exposure and the earliest manifestation of infestation
• Autoinfection: An infected individual serves as a source for hyperinfection to themselves
• Superinfection (hyperinfection): An individual with a parasite is reinfected by the same parasite
• Retroinfection: Newly hatched larvae of E. vermicularis cause retrograde infection from the perianal region to the colon, where the adolescent parasite develops

Sources of Exposure to Parasitic Infections

• Contaminated soil: Soils polluted with human excreta are commonly responsible for infections with Ascaris lumbricoides, S. stercoralis, Trichuris trichiura, and hookworms
• Contaminated water: may contain
  • Viable cysts of Amoeba, flagellates, and T. solium eggs
• Insufficiently cooked meat of pork and beef: Contains infective stages of parasites such as Trichenilla spiralis and Taenia species
• Blood-sucking arthropods transmit parasites such as malaria parasites by female anopheles mosquitoes
• Animals (domestic or wild) harboring parasites - dogs can directly cause human infection with the cutaneous larva migrans caused by Toxocara canis
• Humans - another person his clothing, bedding, or the immediate environment contaminated can be directly responsible e.g.,E. histolytica and E. vermicularis
• Sexual intercourse - e.g., Trichomonas vaginalis
• Autoinfection - e.g., S. stercoralis, E. vermicularis, and T. solium

Mode of Transmission

• Direct Mode: no biological vectors and/or intermediate hosts are required; a single host is sufficient to complete the parasite's life cycle
  • Horizontal Direct Mode: transmitted through
    • Feco-oral route: Many intestinal parasites
    • Sexual intercourse
    • Blood transfusion
    • Direct skin penetration (example: soil-transmitted helminths)
  • Vertical Direct Mode: transmission from mother to child
    • Congenital/transplacental
    • Transmammary (breast milk)
• Indirect Mode: requires biological vectors and/or one or more intermediate hosts for transmission

Routes of Transmission of parasites

• By ingesting infective stage of parasites:
  • Food, water, or hands contaminated with faeces, e.g., E. histolytica, E. vermicularis
  • Raw or undercooked meat, e.g., T. saginata, T. solium, T. spiralis
  • Raw or undercooked fish, crab, or water vegetation e.g., intestinal flukes
  • Water containing Cyclopes e.g., D. medinensis
• Penetration of Skin When in Contact with
  • Soil faecally polluted, e.g., S. stercoralis & Hook worms
  • Water containing infective stages of the parasite E.g., Cercaria of Schistosome species.
• Through Insect Bite: filarial worms, Trypanosoma species, Plasmodium species, Leishmania species
• Sexual Contact, e.g., Trichomonas vaginalis
• Transmammary, e.g., S. stercoralis
• Inhalation of contaminated air, e.g., E. vermicularis, P. carnii
• Transplacental, e.g., T. gondii
• Kissing, e.g., Trichomonas gingivalis, Trichomonas tenax

Host Parasite Relationship and Effects of Parasites

• Parasites can affect hosts in various ways:
  • Consumption of nutritive elements e.g., Hookworm
  • Obstruction of passages e.g., heavy infection with adult Ascaris may cause intestinal obstruction
  • Bleeding e.g., Schistosomes eggs
  • Destruction of tissues e.g., Trophozoites of E. histolytica causes necrosis of liver
  • Compression of organs e.g., Hydatid cysts in liver, brain cause pressure
  • Release of toxic substances e.g., Rupture of E. granulosus cyst result anaphylactic shock
  • Opening path way to secondary infections
  • Allergy development, e.g., Bite of arthropode
  • Transmission of pathogens, e.g., lice transmit Rickettsia
  • Predisposition to malignancy-e.g., Infection with bilharziasis predisposes to malignancy
  • Chronic immune stimulation leading to unresponsiveness to infections.

Host Susceptibility Factors

• Host Factors:
• Genetic constitution
• Age
• Sex
• Level of immunity: natural and acquired immunity.
• Nutrition (malnutrition or under nutrition)
• Intensity and frequency of infections
• Presence of co-existing disease or conditions which reduces immune response. e.g. Pregnancy, HIV
• Life style and occupation

Parasite factors

• Strain of the parasite and adaptation to human host
• Parasite load ( number of parasite )
• Site (s) occupied in the body
• Metabolic process of the parasite, particularly the nature of any waste products or toxins produced by the parasite during its growth and reproduction.

Escape mechanism of parasite from the immune system

• Parasites have the capacity to escape or render ineffective the host internal defense mechanisms as Parasites can evade the host immune responses by variety mechanisms

General Life Cycles of Parasites

• Direct Life Cycle: Parasites can complete its life cycle in a single host e.g., S. stercoralis, Hook worms, G. lamblia, E,. histolytica, etc.
• Indirect Life Cycle: Parasite require an intermediate host or vector to complete its development; e.g., Plasmodium species, Leishmania species, Taenia species. etc.

Types of Specimen Used For Parasitological

• Stool: intestinal nematodes, cestodes, trematodes and protozoa
• Blood: Haemoparasites
• Urine: S. hematobium, T. vaginalis
• Sputum: P. westermani.
• Skin: O. volvulus, E. vermiculari
• Cerebro-Spinal fluid: Trypanosoma rhodisense and Naegleria fowleri.