Parasitology: Definitions and Features

Definition of Terms in Parasitology

• Parasitology is the study of parasites
• Medical parasitology specifically studies parasites causing disease in humans
• A parasite is an organism that lives temporarily or permanently on or in another organism (host)
• The parasite is physically or physiologically dependent on the host
• Parasites can be unicellular, worms, or arthropods

Features of Parasites

• Parasites are typically smaller than their hosts
• Parasites outnumber their hosts
• They have a shorter lifespan than their hosts
• Parasites possess a greater reproductive potential than their hosts

Association of Organisms

• Mutualism involves a mutual benefit derived from the association
• Symbiosis describes a permanent association between two different organisms entirely dependent on each other
• Commensalism happens when the parasite benefits from the host, without the host being affected
• Parasitism is when an organism lives at the expense of another, usually causing the other to suffer

Classification of Parasites Based on Habitat

• Ectoparasites live on the skin surface of the host; examples include lice and ticks
• Endoparasites live within the body of the host; examples include Leishmania and Ascaris lumbricoides

Classification of Parasites Based on Dependence

• Permanent (obligate) parasites depend completely on the host for metabolites, shelter, and transportation and cannot live outside of it
• Temporary (facultative) parasites are capable of independent existence in addition to parasitic life; an example is Strongyloides stercolaris

Classification of Parasites Based on Pathogenicity

• Pathogenic parasites cause diseases in the host; an example is E. histolytica
• Non-pathogenic (commensal) parasites derive food and protection from the host without causing harm; an example is Entamoeba coli
• Opportunistic parasites cause mild disease in healthy individuals but severe disease in immunocompromised hosts; examples include Pneumocystis carnii, Toxoplasma gondii, and Isospora belli

Host Types

• A host is defined as an organism that harbors a parasite
• A definitive/primary host is the one harboring the adult stage or where the sexual reproduction of the parasite occurs
• Intermediate host harbors the sexually immature or larval stage of the parasite, where no fertilization occurs
• An amplifier host refers to intermediate hosts in which parasites undergo multiplication
• A reservoir host is a wild or domestic animal that harbors a parasite and serves as a source infection for humans and is often asymptomatic
• Carrier hosts harbor and disseminate parasites without exhibiting clinical signs
• Accidental (incidental) hosts are infected with a parasite that normally infects other species, and in which the parasite may or may not continue full development
• Paratenic or transport hosts do not experience parasite development but host a parasite that remains alive and may cause damage

Vectors

• Vectors are arthropods or living carriers transporting pathogenic organisms from infected to non-infected hosts
• Biological vectors complete the parasite's life cycle
• Mechanical (parathenic or transport) vectors are passive carriers and are not essential to the parasite's life cycle

Parasitology Terms

• Diagnostic stage is the stage of a pathogenic organism that can be detected in stool, blood, urine, sputum, CSF, or other secretions
• Infective stage is when the parasite is capable of entering the host and continuing development
• Infection refers to the invasion of the body by pathogenic organisms (except arthropods)
• Infestation refers to establishing arthropods on or within a host
• Zoonosis refers to diseases of animals transmittable to man
• Biological incubation (prepatent) period refers to the time elapsing between initial infection and demonstration of the parasite
• Clinical incubation period is the time between exposure and the earliest manifestation or infestation
• Autoinfection is when an infected individual acts as a source for hyperinfection to himself
• Superinfection (hyperinfection) occurs when an individual harboring the parasite is reinfected by the same parasite
• Retroinfection is a retrograde infection caused by newly hatched larva reaching the colon from the perianal region

Sources of Exposure to Parasitic Infections

• Contaminated soil, especially when polluted with human excreta, can contain Ascaris lumbricoides, S. stercolaris, Trichuris trichiura, and hookworms
• Contaminated water may contain cysts, cercarial stages, or the Cyclops larva of Dracunculus medinesis
• Undercooked meat, like pork and beef, can contain Trichinella spiralis or Taenia species
• Blood-sucking arthropods such as the anopheles mosquito, phlebotomus, tsetse fly, and culicine mosquito
• Animals like dogs or herbivores can harbor parasites like E. granulosus, Toxocara canis, or Trichostrongylus species
• Direct contact with contaminated clothing or bedding for pathogenic amoeba, such as E. histolytica, E. vermicularis, and H. nana
• Sexual intercourse in the transmission of Trichomonas vaginalis
• Autoinfection can occur with S. stercoralis, E. vermicularis, and T. solium

Mode of Transmission

• Horizontal direct transmission occurs through the fecal-oral route, sexual intercourse, blood transfusion, or direct skin penetration
• Vertical direct transmission occurs from mother to child congenitally, transplacentally, or transmammary (breast milk)
• Indirect transmission involves a complex life cycle that requires biological vectors and/or one or more intermediate hosts

Route of Transmission

• Ingesting infective stages in food, water, or from hands after contamination
• Penetration of skin in contact with fecally polluted soil or infective parasite stages in water
• Through insect bites, sexual contact, or transmammary routes
• Inhalation of contaminated air
• Transplacental transmission
• Kissing

Effects of Parasites on Their Hosts

• Consumption of the host's nutritive elements; for example, hookworms suck blood, and D. latum removes V B12
• Obstruction of passages occurs in heavy infections with adult Ascaris, causing intestinal obstruction
• Bleeding happens because of Schistosomes eggs
• Destruction of tissues occurs; trophozoites of E. histolytica cause necrosis of the liver, and Leishmania donovani can cause destruction of marrow elements
• Compression of organs can arise because of Hydatid cysts in the liver and brain
• Release of toxic substances can result in anaphylactic shock
• Secondary infections arise from exposure to bacterial and viral infections.
• Allergy development can occur from arthropod bites

Host Susceptibility Factors

• Genetic constitution
• Age
• Sex
• Level of immunity: natural and acquired
• Nutrition (malnutrition or undernutrition)
• Intensity and frequency of infections
• Co-existing diseases or conditions that reduce immune response (e.g., pregnancy, HIV)
• Lifestyle and occupation

Types of Specimens Used for Parasitological Examination

• Stool for intestinal nematodes, cestodes, trematodes, and protozoa
• Blood for haemoparasites
• Urine for S. hematobium and T. vaginalis
• Sputum for P. westermani
• Skin for L. aethopica, O. volvulus, D. medinensis, and E. vermiculari
• Cerebrospinal fluid for Trypanosoma rhodisense and Naegleria fowleri
• Bone marrow for L. donovani and T. gondii
• Lymph gland aspirates for Trypanosoma rhodisense, L. donovani, and T. gondii
• Liver aspirate for E. histolytica, L. donovani, and T. gondii
• Spleen aspirate for L. donovani and T. gondii
• Muscle biopsy for T. spiralis

Differences Between Parasitic Protozoa and Metazoa

• Protozoa are unicellular, while metazoa are multicellular
• Protozoa multiply asexually (with some exceptions) while metazoa multiply sexually (with some exceptions)
• Protozoa multiply, while metazoa accumulate
• Protozoa have a fast multiplication rate, while metazoa have a slow rate
• Protozoa have short longevity, while metazoa are long

Lytic Necrosis and Trauma

• Enzymes can cause lytic necrosis
• Entamoeba histolytica lyses intestinal cells, producing amoebic ulcers
• Hookworm attachment leads to traumatic damage and bleeding

Parasite Virulence

• Parasites can passively evade the immune system
• Parasites can become ‘invisible’ to the immune system
• Parasites can change their surface identity to remain undetected