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
• Types of parasites


  • Ectoparasites (live on the surface of other organisms)
  • Endoparasites (live within the bodies of other organisms)
• Parasites by life cycle

  • 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 by duration of association with host

  • 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)
• Mechanisms for parasites to evade host defense

  • Encystment (formation of an outer covering that protects against unfavorable environmental conditions)
  • Changing surface antigens faster than host can make new antibodies
  • Causing host's immune system to make antibodies that cannot react with parasite's antigens
  • Invading host cells (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 are microscopic, varying in diameter from 5 μm to 5 mm
• Protists are a key part of food chains, with autotrophic protists capturing energy from sunlight and heterotrophic protists ingesting autotrophs and other heterotrophs or decomposing dead organic matter
• Some protists are parasitic, causing debilitation and sometimes death in large numbers of people, especially in poor countries that lack the resources to eradicate those protists
• Types 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 and free-living, some are commensals or parasites)
• Entamoeba histolytica


  Sarcodine protist found worldwide, more common in tropical countries with poor sanitation, transmitted by fecal-oral route through ingestion of cysts
• Giardia lamblia


  Mastigophoran protist primarily transmitted through ingestion of cysts from fecally-contaminated water and food
• Balantidium coli


  Ciliate protist with main source of infection being water contaminated by pig feces, transmitted by fecal-oral route
• Trichomonas vaginalis


  Mastigophoran protist that causes urogenital infections, mainly transmitted through sexual intercourse
• Acanthamoeba castellani


  Free-living amoeba that causes brain inflammation, acquired usually while swimming in contaminated water or through eye infection in contact lens wearers
• Naegleria


  Amoeba usually acquired transnasally when swimming in contaminated waters, penetrates nasal mucosa and causes fatal meningitis and encephalitis
• Trypanosoma cruzi


  Causes Chagas Disease, found primarily in South and Central America, transmitted by the bite of the reduviid or triatomid bug
• Trypanosoma brucei rhodesiense & Trypanosoma brucei gambiense

  Cause Sleeping Sickness, involve the tsetse fly (Glossina) as the vector, T. gambiense infection is chronic, T. rhodesiense infection is more rapidly fatal
• Plasmodium spp.


  Cause Malaria, the vector and definitive host is the female Anopheles mosquito, 5 species that infect humans
• Fungi


  • Many are saprophytes that digest dead organic matter, some are parasites that obtain nutrients from the tissues of other organisms
  • Most are multicellular, but yeasts are unicellular
  • Many reproduce both sexually and asexually
• Fungi are important decomposers in ecosystems, and some produce antibiotics that inhibit the growth of or kill bacteria
• Fungal parasites in plants cause diseases such as wilts, mildews, blights, rusts, and smuts, producing extensive crop damage and economic losses
• Classifications of human fungal diseases

  • Superficial diseases (affect only keratinized tissue in 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)
• Fungal phyla


  • Zygomycota (Bread Molds)
  • Ascomycota (Sac Fungi)
  • Basidiomycota (Club Fungi)
  • Deuteromycota (Fungi Imperfecti)
• Helminths


  • Bilaterally symmetrical, have a head and tail end, and have differentiated tissues in three distinct layers: ectoderm, mesoderm, and endoderm
  • Include flatworms and roundworms that parasitize humans
• Flatworms, also known as Platyhelminthes, are primitive worms usually no more than 1 mm thick, but some, such as tapeworms, can grow much longer
• Claviceps purpurea, a parasite of rye, produces the toxic substance ergot
• Ergot


  Can be used in small quantities to treat migraine headaches and induce uterine contractions, but in larger quantities it can kill yeast
• Cryptococcus causes opportunistic respiratory infections, which can be fatal if they spread to the central nervous system, causing meningitis and brain infection
• Phylum Deuteromycota


  • Common Name: Fungi Imperfecti
  • Examples: Soil organisms; various human pathogens
• Helminths


  • Bilaterally symmetrical, have a head and tail end, and their tissues are differentiated into three distinct tissue layers: ectoderm, mesoderm, and endoderm
  • Helminths that parasitize humans include flatworms and roundworms
• Flatworms


  Also known as Platyhelminthes, are primitive worms usually no more than 1 mm thick, but some, such as large tapeworms, can be as long as 10 m
• Flukes (trematodes)


  Blood dwelling
• Schistosoma spp. infection


  1. Eggs are found in fresh water contaminated with the feces or urine of infected humans

  2. Infection is acquired through skin penetration by the fork tailed cercaria (larval form)

  3. The parasite migrates into the bloodstream where they undergo maturation
• Clonorchis sinensis (Asian Liver Fluke, Chinese Liver Fluke) infection

  Humans acquire the infection by ingesting raw or undercooked freshwater fish containing the infective metacercariae
• Fasciola hepatica (Sheep Liver Fluke) infection


  Found worldwide, especially in sheep and cattle raising countries, and where humans consume raw watercress such as Asia, Europe, and the Middle East