Parasitology

• The earliest agents of human infection to have been observed were helminthic parasites
• Intestinal worms and their empirical remedies were apparently known from early antiquity in different parts of the world
• Whipworm eggs were identified in the colonic contents of a well-preserved body of a young man who died on the snow-clad Alps mountain


  5300 years ago
• Parasitism


  Parasites are organisms that infect other living beings, live in or on the body of another living being (the host), and obtain shelter and nourishment from it
• Parasitism arose early in the course of biological evolution
• Ways in which organisms obtain nourishment instead of remaining as free-living forms

  • Predation (larger animals prey on smaller ones)
  • Saprophytism (feeding on dead and decaying bodies of animals, plants and other organic matter)
  • Parasitism (more durable and intimate association)
• Medical parasitology deals with the parasites which cause human infections and the diseases they produce
• Parasites


  • They multiply or undergo development in the host
• By mid-twentieth century, with dramatic advances in antibiotics and chemotherapy, insecticides and antiparasitic drugs, and increased affluence and improved lifestyles, all infectious diseases seemed amenable to control
• Great dreams of eradicating infectious diseases were entertained and when global eradication of the great scourge smallpox became a reality, euphoria prevailed
• Then came nemesis, with microbes rebounding. Antibiotics and antipesticides lost their efficacy, faced with microbial and vector resistance. New emerging diseases became a serious threat
• The HIV pandemic provided a fertile field for old and new pathogens to spread. This applies equally to parasitic infections as to bacterial, viral or mycotic infections
• In this context a new enhanced interest attaches to the study of human parasites
• Parasites


  Organisms that infect other living beings, live in or on the body of another living being (the host), and obtain shelter and nourishment from it
• Parasitism


  A durable and intimate association in which the parasite establishes itself in or on the living body of the host, being physically and physiologically dependent on it for at least part of its life cycle
• Commensals


  Parasites which live in complete harmony with the host, without causing any damage to it
• Pathogens


  Parasites which cause disease
• The distinction between commensals and pathogens is not absolute, as many commensals can act as facultative or opportunist pathogens when the host resistance is lowered
• Rarely, even free-living organisms may become pathogenic under special circumstances
• Parasitology deals only with parasites belonging to the animal kingdom, excluding bacteria, fungi and viruses
• Human parasites


  • Unicellular microbes (protozoa)
  • Larger organisms (metazoa)
• Ectoparasites


  Parasites that inhabit the body surface only, without penetrating into the tissues
• Ectoparasites


  • Lice
  • Ticks
  • Mites
  • Other haematophagous arthropods
• Infestation


  Term often employed for parasitisation with ectoparasites, in place of the term infection used with reference to endoparasites
• Endoparasites


  Parasites that live within the body of the host
• All protozoan and helminthic parasites of humans are endoparasites
• Definitive host


  The host in which the adult stage lives or the sexual mode of reproduction takes place
• Intermediate host


  The species in which the larval stage of the parasite lives or the asexual multiplication takes place
• Man is the definitive host for most human parasitic infections (e.g. filaria, roundworm, hookworm), but is the intermediate host in some instances (e.g. malaria, hydatid disease)
• Paratenic host


  A vertebrate host in which a parasite merely remains viable without development or multiplication, and may serve to pass on the infection to another
• Proliferous parasites


  Parasites that proliferate in the human body so that the parasite originally introduced multiplies many fold to cause high intensity of infection
• Nonproliferous parasites


  Most adult helminths that do not multiply in the human body
• High intensity of infection with nonproliferous parasites results from repeated infection as in roundworm, or from high multiplicity of initial infection as in trichinosis
• A few helminths, such as Strongyloides stercoralis and Hymenolepis nana multiply in the human host
• Zoonotic infections or zoonoses

  Parasitic infections which humans acquire from animals
• Domestic zoonoses


  Parasites that live normally in cycles involving domestic animals
• Feral or sylvatic zoonoses


  Parasites that live normally in cycles involving wild animals
• Reservoir host


  The vertebrate species in which the parasite passes its life cycle and which may act as the source of human infection
• Amplifier host


  Intermediate hosts in which metazoan parasites undergo multiplication
• Anthroponoses


  Infections with parasitic species that are maintained in humans alone