Parasites (L4)

• Parasites


  Eukaryotic organisms that derive benefit from a host, often causing harm to the host
• Parasites


  • Different areas of infection - generally blood or intestines
  • Parasites that cause harm are pathogenic
  • Parasitic symbiosis - one member benefits, other harmed
  • Parasites that derive benefit from host without causing harm are called commensals

  
• Types of parasites


  • Protozoa
  • Helminths
  • Arthropods
• Protozoa


  • Most are usually <50μm
  • Unicellular eukaryotes
  • Nucleus has a membrane
  • Lack of cell wall
• Trophozoite


  The active feeding and multiplying stage of protozoa

  
• Oocyst


  The stage resulting from sexual reproduction of protozoa, characterised by a protective membrane or thickened wall

  
• Reproduction of protozoa


  1. Asexual reproduction through binary fission

  2. Sexual cycle involving production of gametes, fertilization to form zygote, encystation of zygote to form oocyst, formation of infective sporozoites within the oocyst

  
• Holozoic nutrition


  Protozoa require organic materials and can engulf particulate food
• Protozoa infections


  • Entamoeba histolytica - anaerobic parasitic protozoal commensal
  • Malaria caused by Plasmodium species

  
• Malaria treatment


  • Effective with quinine derivatives like quinine sulfate, chloroquine, mefloquine, primaquine
  • Drug resistance is a major problem
  • Control measures include eradication of infected mosquitos and preventative measures like mosquito nets
• Helminths


  • Worm-like parasites including flukes, tapeworms, and roundworms
  • Invertebrates that develop through egg, larval and adult stages
• Tapeworms (cestodes)


  • Flattened, elongated and consist of segments called proglottids
  • Vary in length from 2mm to 10m
  • Anatomically divided into scolex, neck, and strobila
  • Lack alimentary canal, substances enter across tegument
  • Hermaphroditic with male and female reproductive systems in each proglottid
• Tapeworm treatment


  • Praziquantel used to treat many Tenia infections
  • Expulsion of the scolex is a must
  • Good hygiene especially regarding animals and children
• Arthropods


  • Affect human disease as vectors, parasites, or by eliciting allergies
  • Examples include lice, fleas, mites, ticks
• Lice


  • Spend all of their life on one specific host, both male and female feed on blood
  • Only leave one host to transfer to another
  • Head lice cause itching due to sensitization to louse saliva
• Why can’t antimicrobial agents that target cell walls be used against protozoa?

  protozoa lack a cell wall
• Protozoa nutrition

  • Many have a permanent mouth
  • Pinocytosis often occurs to draw in fluid
  • Vacuoles in protozoa perform digestion
  • Metabolic pathway is similar to higher animals
• Entamoeba histolytica: invasion of the intestinal lining causes amoebic dysentery (amoebiasis), transmitted by faecal-oral route, infects GI tract (can pass into blood and infect liver)
• Malaria: caused by protozoal sporozoa, trophozoites are generally ring shaped and 1-2 micrometers, gametocytes are usually 7-14 micrometers, has two life cycles in humans and has an indirect life cycle
• Malaria life cycle


  1. Female mosquito injects sporozoites (present in the saliva of mosquito) into human

  2. Sporozoites circulate in blood for a short time and then settle in parenchymal cells (in liver)

  3. They infect parenchymal cells where they either remain dormant (hypnozoites) or multiply to produce merozoites

  4. After around 12 days, parenchymal cells rupture to release thousands of merozoites into the blood to infect red blood cells

  5. In red blood cells, the parasites mature into trophozoites, then transform to form daughter moerozoites

  6. Some merozoites transform into male and female gametocytes

  7. These circulate in the blood and are taken up by another female mosquito when they bite human for blood

  8. The female gametocyte transforms into ookinete, is fertilised and forms an oocyst, which produces sporozoites (sporozoites move into salivary glands and are ready to be injected into another human)
• What is pre-erythrocytic schizogony?

  Where sporozoites infect parenchymal cells in the liver (either remains dormant or multiply to produce merozoites)
• What do merozoites do in the red blood cells?

  • Replicate by binary fission (completely filling the red blood cell)
  • Releases chemical signal into the blood that synchronises all parasites
  • Parasite will rupture red blood cells all at the same time, releasing merozoites, gametocytes and toxins
• Why is malaria dangerous and hard to treat?

  In the body, there are no signs of malaria until toxins are detected (usually after around 12 days). Symptoms of malaria occur when toxins are present
• Which protozoal sporozoa is the most widespread species?

  Plasmodium vivax
• Which protozoal sporozoa is the most serious and is the cause of most fatal cases?

  Plasmodium falciparum
• Why do female mosquitos transmit malaria and not male?

  Female mosquitos need blood to nourish the egg
• Lice treatment:

  • Shampoo containing 1% benzene hexachloride
  • Mixture of pyrentins (0.2%) and pipronyl butoxide (2%)
  • Brushing out with a lice comb
• Features of protozoa

  • contract vacuole
  • pellicle
  • macronucleus
  • food vacuole
  • anal pore
  • trichocysts
  • gullet
  • micronucleus
  • cillia