Parasites

Parasites and Parasitism

• Parasites (endo-) or (ecto-) feed on living organisms while causing harm
• They gain benefits from them (e.g. unlimited supply of nutrients, H2O, constant temp)
• Can be bacteria, protoctists, viruses, fungi, arthropods, platyhelminthes
• Have structural and functional adoptions to their lifecycle
  • Specialised reproductive strategies
  • Modification of mouthparts, digestive enzymes and enzymes to allow attachment to the host and utilisation of host's food supply, blood or tissues
  • Resist attack by immune system
  • Reduction of unnecessary sensory organs and locomotory organs in the adult stage, as they live in protected, optimum conditions

Plasmodium (Malarial parasite)

• Single celled, intracellular parasite of the kingdom Proctista (class protozoa)
  • Development requires 2 hosts for its lifecycle
  • Female mosquito (unaffected) and human (affected)
  • Parasite is transferred by bites from the female mosquito
  • Mosquito releases anti-coagulants when piercing skin / prevents blot clotting
  • Parasites is injected into human in the form of sporozoites
  • If the human is infected with the malarial parasite, the mosquito takes up Plasmodium gametes in the blood on which the mosquito feeds
• Asexual reproduction phase
  • Occurs in human liver and red blood cells
  • Produces enormous quantities of parasites, merozoite stage
• Sexual phase
  • Occurs in the female mosquito
• [EXAM] Lives inside the liver and red blood cells
  • Survive for long periods because it is protected from the immune system
    • Inside the blood or liver cell; and
    • Surface antigen changes rapidly
  • Has no (need for) locomotory structures because
    • It is transported via blood stream
    • No need to move to find food
  • Has no (need for) mechanism for regulating its water content because
    • cytoplasm has same water potential as blood cell
• Symptoms and Treatment
  • Red blood cells are destroyed causing anaemia and fever
  • Temp peaks correspond to bursting of red blood cells
  • Anti-malarial drug chloroquine lowers fever; reduces number of parasites
  • Increasing resistance to the drug / combined drugs used
  • High mutation rate / memory cells or vaccines useless

Lifecycle

• Infection of Human
  • Infected female mosquitoes are feeding human blood
  • Secretes salvia which contains anticoagulants (anti blood-clotting agents)
  • Sporozoites (young malarial parasites) in the salvia enter blood
• Latent Period
  • Parasites migrate to liver cells / undergo asexual reproduction / develop into merozoites
  • Merozoites infect red blood cells / undergo asexual reproduction / produce more merozoites
• Outbreak
  • Merozoites burst out of red blood cells to infect more cells
  • This outburst is associated with bouts of fever
  • Some merozoites develop into gametocytes
• Infection of Female Mosquitos
  • Gametocytes are taken up by female mosquitos that feed on the infected person's blood
  • Inside the female mosquito, gametes produced by those gametocytes fuse (sexual reproduction phase)
  • Results in zygotes which develop into sporozoites
  • Sporozoites move into the salivary glands of the female mosquito where they can be injected into another person
• Cycle starts again

Schistosoma (endoparasite)

• Name of a genus of parasitic flatworms; causes the disease Bilharzia in humans
• Two hosts for lifecycle: fresh water snails (vector) and humans
• Penetrate the skin via enzymes damaging the host cell membrane
• Invades blood vessels, veins of the bladder region, abdomen and pelvis (Schistosoma haematobium) or the intestine (S. mansoni and S. japanicum)
• Adult schistosomes exist as separate males and females usually found attached to one another, to ensure mating and sexual reproduction
• Fertilised eggs are deposited in the blood vessels of the host
  • Huge number of eggs cause the vessels to rupture
  • Eggs are discharged into the intestine to reach the outside of the body
  • They work their way into adjacent organs (bladder, large intestine) for their way out
  • Within tissues they cause severe inflammation, blood in the urine (anaemia)
• Adult worm lacks locomotion and sensory organs
  • Unnecessary inside the host's body
  • Surrounded by nutrients in constant optimum conditions
• Adult worm manipulates host's immune system
  • Parasite coats itself with molecules from the host's red blood cells
  • The host recognises those cells as its own

Lifecycle

• Eggs pass out of an infected human via the urine or faeces
• Hatch into minute ciliated larvae (miracidia) / capable of swimming until they find and burrow into a water snail and grow into a structure called sporocyst
• Asexual reproduction occurs, producing free swimming larvae, cercariae, which borrow, aided by the secretion of digestive enzymes, into the human via the skin or feet
• Larvae migrate to the bladder or gut region where they may exist for many years producing a vast number of parasites