CHAP 4

• Parasitology


  The study of the interaction between parasites and their hosts
• Parasitology


  • Tends to concentrate on eukaryotic parasites, such as lice, mites, protozoa and worms, with prokaryotic parasites and other infectious agents the focus of fields such as bacteriology, microbiology and virology
• Parasite


  A living organism which receives nourishments and shelter from another organism where it lives
• Parasite


  An organism that is entirely dependent on another organism (host)
• Pathogenicity

  • The pathogenic mechanism of parasitic infection varies according to species and quantity of parasites as well as parasite-host adaptation and host responses
  • The damage caused by parasites can be either confined within the parasitic site or extend into other parts in host
  • Damages are commonly caused by mechanical pressure, capturing nutrition, toxins, responses and anaphylaxis of host to the stimulation from parasites as well as passage for other pathogens invading the host
• Pathogen


  An organism that causes diseases to the host after infection
• Parasite


  An organism that lives on or in another organism of another species, usually deriving nutrients at the expense of its host
• Pathogens usually kill their host through diseases, while most parasites do not kill their host but do cause some damage
• Pathogens typically do not require a host to complete its life cycle, while parasites will have to depend on their host to complete their life cycle
• Pathogenicity


  The ability of an organism to infect another organism (host)
• Parasitism

  A kind of interaction between two species, where one species is benefitted, and the other is harmed
• Definitive or Primary Host


  The host which harbors the adult parasites or where the parasite replicates sexually
• Definitive hosts


  • Sheep for Fasciola gigantica
  • Dog for Echinococcus granulosus
  • Female anopheles mosquito for Plasmodium spp.
• Intermediate or Secondary Host


  The host which harbors the larval stages of a parasite or in which the parasite undergoes asexual multiplication
• Reservoir Host

  • A host which harbors the parasites, possibly grow, and multiply and serves as an important source of infection to other susceptible hosts
  • Reservoir hosts do not get the disease carried by the pathogen or it is asymptomatic and non-lethal
• Paratenic or Storage Host

  • A host that serves as a temporary refuge and vehicle for reaching an obligatory host, usually the definitive host
  • A paratenic host harbors the sexually immature parasite, but it cannot develop further in this host
• Incidental or Accidental Host


  A host organism that shelters the parasite, but since it can't progress the life cycle development, it is dead-end for it
• Symbiotic Relationships


  Host-parasite relationships in which the organisms (host and parasite) live in close proximity to each other and are dependent on each other in one or another way for their survival
• Types of Symbiotic Relationships

  • Parasitism
  • Mutualism
  • Commensalism
  • Phoresis
• Mutualism

  A symbiotic relationship where both organisms benefit from each other's existence
• Parasitism

  A non-mutual symbiotic relationship in which the parasite benefits at the expense of the host, while the host is harmed
• Commensalism

  A symbiotic relationship where one partner benefits while the second partner (the host) are neither helped nor harmed
• Most of the normal floras of the human body can be considered as commensals
• Parasitic Adaptation

  Any feature of an organism or its part which enables it to exist under conditions of its habitat
• Structural Adaptations of Endoparasites

  • Feeding organs are usually absent
  • Presence of well-developed piercing device to invade host tissue
  • Locomotory organs are generally absent or highly reduced
  • Presence of attachment organs like rostellum, hooks or suckers
  • Outer covering resistant to enzymatic digestion of host
  • Highly muscular pharynx for easy absorption of food materials from host
• Classification of Parasites


  • Ectoparasites (live on the surface of host body)
  • Endoparasites (live in the interior of host body)
  • Permanent Parasites (parasitic throughout life)
  • Temporary Parasites (visit host for short period)
  • Facultative Parasites (can live parasitic or non-parasitic)
  • Obligatory Parasites (cannot exist without parasitic life)
  • Occasional Parasites (attack unusual host)
• Main Classes of Parasites

  • Protozoa
  • Helminths
  • Ectoparasites
• Protozoa


  • Microscopic, one-celled organisms that can be free-living or parasitic in nature
  • Can multiply in humans, contributing to survival and serious infections from a single organism
  • Transmission through fecal-oral route or by arthropod vector
• Helminths


  • Large, multicellular organisms generally visible to the naked eye in adult stages
  • Cannot multiply in humans in adult form
• Main Groups of Helminths

  • Flatworms (Platyhelminths) - Trematodes (flukes) and Cestodes (tapeworms)
  • Thorny-headed Worms (Acanthocephalins)
  • Roundworms (Nematodes)
• Protozoa that live in a human's intestine are typically transmitted to another human through a fecal-oral route
• Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly)
• Helminths


  Large, multicellular organisms that are generally visible to the naked eye in their adult stages. Like protozoa, helminths can be either free-living or parasitic in nature. In their adult form, helminths cannot multiply in humans.
• Main groups of helminths

  • Flatworms (Platyhelminths) - include trematodes (flukes) and cestodes (tapeworms)
  • Thorny-headed worms (Acanthocephalins) - the adult forms reside in the gastrointestinal tract
  • Roundworms (Nematodes) - the adult forms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues
• Some consider the helminths to also include the segmented worms (annelids)—the only ones important medically are the leeches. Of note, these organisms are not typically considered parasites.
• Ectoparasites


  Organisms such as ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time
• Arthropods are important in causing diseases in their own right, but are even more important as vectors, or transmitters, of many different pathogens that in turn cause tremendous morbidity and mortality from the diseases they cause.
• Acanthamoebiasis


  A tiny ameba can cause an infection to the eye, the skin, and the brain. It can be found all over the world in soil and water. Individuals can get an infection if they have their contact lenses cleaned with tap water.
• Babesiosis


  A disease that is caused by parasites that are carried by ticks. The disease affects the red blood cells and poses a higher risk in summer around the Northeast and upper Midwest of the US.
• Balantidiasis


  A disease transmitted by Balatidium coli, which is a single-cell parasite that mostly infects pigs but can cause intestinal infections in humans rare cases. It can be spread by drinking contaminated water, or through direct contact with pigs.