Parasites

• Parasite


  An organism that lives at the expense of another organism, called the host
• Pathogen


  Parasites that cause disease
• Parasitology


  The study of parasites
• Less than half the world's cultivable land is under cultivation, primarily because parasites endemic to (always present in) those lands prevent humans and domesticated animals from inhabiting some of them
• Parasitic infections in wild and domestic animals provide sources of human infection and cause debilitation and death among the animals, thus preventing the raising of cattle and other animals for food
• Division of parasites


  • Ectoparasites (live on the surface of other organisms)
  • Endoparasites (live within the bodies of other organisms)
• Division of parasites


  • Obligate parasites (must spend at least some of their life cycle in or on a host)
  • Facultative parasites (normally are free-living, but can obtain nutrients from a host)
• Parasites according to the duration of their association with their hosts

  • Permanent parasites (remain in or on a host once they have invaded it)
  • Temporary parasites (feed on and then leave their hosts)
  • Accidental parasites (invade an organism other than their normal host)
• Parasites' mechanisms for evading host defense mechanisms

  • Encystment (formation of an outer covering that protects against unfavorable environmental conditions)
  • Changing the parasite's surface antigens faster than the host can make new antibodies
  • Causing the host's immune system to make antibodies that cannot react with the parasite's antigens
  • Invading host cells, where the parasites are out of reach of host defense mechanisms
• Organisms considered as parasites

  • Protists
  • Fungi
  • Helminths
  • Arthropods
• Protists


  • Unicellular (though sometimes colonial), eukaryotic organisms with cells that have true nuclei and membrane-enclosed organelles
  • Most protists are microscopic, they vary in diameter from 5 µm to 5 mm
• Protists are a key part of food chains
• Autotrophic protists capture energy from sunlight
• Some heterotrophic protists ingest autotrophs and other heterotrophs
• Others decompose, or digest, dead organic matter, which then can be recycled to living organisms
• Some protists are parasitic and cause debilitation in large numbers of people and sometimes death, especially in poor countries that lack the resources to eradicate those protists
• Classification of protists


  • Plantlike protists (have chloroplasts; live in moist, sunny environments)
  • Funguslike protists (most are saprophytes; may be unicellular or multicellular)
  • Animal-like protists (heterotrophs; most are unicellular, most are free-living, but some are commensals or parasites)
• Intestinal & urogenital protozoa

  • Entamoeba histolytica (Sarcodina)
  • Giardia lamblia (Mastigophora)
  • Balantidium coli (Ciliophora)
  • Trichomonas vaginalis (Mastigophora)
• Entamoeba histolytica is found worldwide but is more common in tropical countries, especially in areas with poor sanitation
• Entamoeba histolytica is primarily transmitted by the fecal oral route through ingestion of the cyst from contaminated food and water
• Giardia lamblia is primarily transmitted through ingestion of the cyst from fecally-contaminated water and food
• The main source of Balantidium coli infection is water contaminated by pig feces and the mode of transmission is through the fecal oral route
• Trichomonas vaginalis causes urogenital infections and the main mode of transmission is through sexual intercourse
• Blood and tissue protozoa

  • Acanthamoeba castellani (Sarcodina)
  • Naegleria (Sarcodina)
  • Trypanosoma spp.
  • Plasmodium spp.
• Acanthamoeba castellani is a free living amoeba that causes inflammation of the brain substance and its meningeal coverings (meningoencephalitis)
• People acquire Acanthamoeba castellani infection usually while swimming in contaminated water, and eye infection occurs primarily in patients who wear contact lenses
• Naegleria is usually acquired transnasally when swimming in contaminated waters, penetrates the nasal mucosa and cribriform plate, enters the central nervous system, and produces a rapidly fatal meningitis and encephalitis (primary amoebic meningoencephalitis)
• Trypanosoma cruzi is found primarily in South and Central America and is transmitted by the bite of the reduviid or triatomid bug, causing Chagas Disease (American Trypanosomiasis)
• Trypanosoma brucei rhodesiense & Trypanosoma brucei gambiense involve the tsetse fly (Glossina) as the vector, with T. gambiense infection (West African or Gambian Sleeping Sickness) being chronic and T. rhodesiense infection (East African or Rhodesian Sleeping Sickness) being more rapidly fatal
• The vector and definitive host for Plasmodium spp. that cause malaria is the female Anopheles mosquito, with 5 Plasmodium species infecting humans
• Fungi


  • Many are saprophytes that digest dead organic matter and organic wastes
  • Some are parasites that obtain nutrients from the tissues of other organisms
  • Most fungi, such as molds and mushrooms, are multicellular, but yeasts are unicellular
  • Many fungi reach their hosts by producing spores that are carried by wind or water
  • Many fungi reproduce both sexually and asexually, but a few have only asexual reproduction
• In ecosystems, fungi are important decomposers
• In the health sciences, fungi are important as facultative parasites—they can obtain nutrients from nonliving organic matter or from living organisms
• Some fungi produce antibiotics that inhibit the growth of or kill bacteria
• Fungal parasites in plants cause diseases such as wilts, mildews, blights, rusts, and smuts and thereby produce extensive crop damage and economic losses
• Classification of mycoses (human fungal diseases)

  • Superficial diseases (affect only keratinized tissue in the skin, hair, and nails)
  • Subcutaneous diseases (affect skin layers beneath keratinized tissue and can spread to lymph vessels)
  • Systemic diseases (invade internal organs and cause significant destruction)

  
• Classification of fungi


  • Phylum Zygomycota (Bread Molds)
  • Phylum Ascomycota (Sac Fungi)
  • Phylum Basidiomycota (Club Fungi)
  • Phylum Deuteromycota (Fungi Imperfecti)
• Candida albicans causes vaginal yeast infections
• Trichophyton is associated with athlete's foot, and Aspergillus with opportunistic respiratory infections
• Amanita fungi produce toxins that can be lethal to humans