Protozoans

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Protozoans

• Protozoans include a wide diversity of taxa that
  do not form a monophyletic group but all are
  unicellular eukaryotes.
• Protozoa lack a cell wall, have at least one
  motile stage in their life cycle and most ingest
  their food.
• Protozoan cell is much larger and more complex
  than prokaryotic cell and contains a variety of
  organelles (e.g. Golgi apparatus, mitochondria,
  ribosomes, etc).

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Protozoans

• Eukaryotic cell was developed through
  endosymbiosis.
• In distant past aerobic bacteria appear to have
  been engulfed by anaerobic bacteria, but not
  digested. Ultimately, the two developed a
  symbiotic relationship with the engulfed aerobic
  bacteria becoming mitochondria and eukaryotic
  cells developed.
• In a similar fashion, ancestors of chloroplasts
  formed symbiotic union with other prokaryotes.

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Protozoans

• Protozoans include both autotrophs and
  heterotrophs. They include free-living and
  parasitic forms.
• Reproduction can be asexual by fission or
  budding or sexual by conjugation or syngamy
  (fusion of gametes).

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Protozoans

• The protozoa were once considered a single
  phylum, now at least 7 phyla are recognized.
• Were also once grouped with unicellular algae
  into the Protista, an even larger paraphyletic
  group.

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Movement in Protozoa

• Protozoa move mainly using cilia or flagella and
  by using pseudopodia
• Cilia also used for feeding in many small
  metazoans.

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Cilia and flagella

• No real morphological distinction between the two
  structures, but cilia are usually shorter and
  more abundant and flagella fewer and longer.
• Each flagellum or cilium is composed of 9 pairs
  of longitudinal microtubules arranged in a circle
  around a central pair.

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Cilia and flagella

• The collection of tubules is referred to as the
  axoneme and it is covered with a membrane
  continuous with the rest of the organisms cell
  membrane.
• Axoneme anchors where it inserts into the main
  body of the cell with a basal body.

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Figure 11.09a
Protein spoke
Dynein motor
Basal body
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Cilia and flagella

• The outer microtubules are connected to the
  central pair by protein spokes.
• Neighboring pairs of outer microtubules
  (doublets) are connected to each other by an
  elastic protein.

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Figure 11.09a
Protein spoke
Dynein motor
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Cilia and flagella

• Cilium is powered by dynein motors on the outer
  doublets. As these motors crawl up the adjacent
  doublet (movement is powered by ATP) they cause
  the entire axoneme to bend.
• The dynein motors do not cause the doublets to
  slide past each other because the doublets are
  attached to each other by the elastic proteins
  and the radial spokes and have little freedom of
  movement up and down. Instead the walking motion
  causes the doublets to bend.

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Flagella, intelligent design and irreducible
complexity

• Oddly, the humble flagellum has been dragged into
  the evolution culture wars!

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Flagella, intelligent design and irreducible
complexity

• The U.S. Supreme Court has prohibited the
  teaching of creationism in public schools as a
  violation of the establishment of religion
  clause of the constitution.
• Latest attempt to insert creationism into schools
  is the idea of Intelligent Design.

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Flagella, intelligent design and irreducible
complexity

• The concept of intelligent design is outlined
  most clearly in Michael Behes book Darwins
  Black Box.
• The central idea in intelligent design is that
  some structures in the body are so complex that
  they could not possibly have evolved by a gradual
  process of natural selection. These structures
  are said to irreducibly complex.

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Flagella, intelligent design and irreducible
complexity

• By irreducibly complex Behe means that a
  complex structure cannot be broken down into
  components that are themselves functional and
  that the structure must have come into existence
  in its complete form.

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Flagella, intelligent design and irreducible
complexity

• If structures are irreducibly complex Behe
  claims that they cannot have evolved.
• Thus, their existence implies they must have been
  created by a designer (i.e. God, although the
  designer is not explicitly referred to as such).

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Flagella, intelligent design and irreducible
complexity

• One of Behes main examples is flagella/cilia.
• Behe claims that because cilia are composed of at
  least half a dozen proteins, which combine to
  perform one task, and that all of the proteins
  must be present for a cilium to work and that
  cilia could not have evolved in a step-by step
  process of gradual improvement.

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Flagella, intelligent design and irreducible
complexity

• The flagellum is not, in fact, irreducibly
  complex.
• For example, the flagellum in eel sperm lacks
  several of the components found in other flagella
  (including the central pair of microtubules,
  radial spokes, and outer row of dynein motors),
  yet the flagellum functions well.

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Flagella, intelligent design and irreducible
complexity

• The whole irreducible complexity argument could
  in reality be recast as an argument of personal
  incredulity.
• I personally cannot imagine a sequence of steps
  by which this complex structure could have
  evolved. Therefore, it must have been created.

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Movement in Protozoa Pseudopodia

• Pseudopodia are chief means of locomotion of
  amoebas but are also formed by other protozoa and
  amoeboid cells of many invertebrates.
• In amoeboid movement the organism extends a
  pseudopodium in the direction it wishes to travel
  and then flows into it.

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Pseudopodia

• Amoeboid movement involves endoplasm and
  ectoplasm. Endoplasm is more fluid than
  ectoplasm which is gel-like.
• When a pseudopodium forms, an extension of
  ectoplasm (the hyaline cap) appears and endoplasm
  flows into it and fountains to the periphery
  where it becomes ectoplasm. Thus, a tube of
  ectoplasm forms that the endoplasm flows through.
  The pseudopodium anchors to the substrate and
  the organism moves forward.

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Figure 11.10
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Feeding in amebas

• Feeding in amoebas involves using pseudpodia to
  surround and engulf a particle in the process of
  phagocytosis.
• The particle is surrounded and a food vacuole
  forms into which digestive enzymes are poured and
  the digested remains are absorbed across the cell
  membrane.

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Phagocytosis
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Reproduction in protozoa

• The commonest form of reproduction is binary
  fission in which two essentially identical
  individuals result.
• In some ciliates budding occurs in which a
  smaller progeny cell is budded off which later
  grows to adult size.

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Binary fission in various taxa
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Sexual reproduction in protozoa

• All protozoa reproduce asexually, but sex is
  widespread in the protozoa too.
• In ciliates such as Paramecium, a type of sexual
  reproduction called conjugation takes place in
  which two Paramecia join together and exchange
  genetic material

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Diseases caused by protozoa

• Many diseases are caused by protozaon parasites
• These include
• Malaria (caused by a sporozaon)
• Giardia, Sleeping sickness (caused by
  flagellates)
• Amoebic dysentry (caused by amoebae)

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Malaria

• Malaria is one of the most important diseases in
  the world.
• About 500 million cases and an estimated 700,000
  to 2.7 million deaths occur worldwide each year
  (CDC).
• Malaria was well known to the Ancient Greeks and
  Romans. The Romans thought the disease was
  caused by bad air (in Latin mal-aria) from
  swamps, which they drained to prevent the
  disease.

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Malaria symptoms

• The severity of an infection may range from
  asymptomatic (no apparent sign of illness) to the
  classic symptoms of malaria (fever, chills,
  sweating, headaches, muscle pains), to severe
  complications (cerebral malaria, anemia, kidney
  failure) that can result in death.
• Factors such as the species of Plasmodium and the
  victims genetic background and acquired immunity
  affect the severity of symptoms.

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Malaria

• Despite humans long history with malaria its
  cause, a sporozoan parasite called Plasmodium,
  was not discovered until 1889 when Charles Louis
  Alphonse Laveran a French army physician
  identified it, a discovery for which he won the
  Nobel Prize in 1907.

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Malaria

• In 1897 an equally important discovery, the mode
  of transmission of malaria, was made by Ronald
  Ross.
• His identification of the Anopheles mosquito as
  the transmitting agent earned him the 1902 Nobel
  Prize and a knighthood in 1911.

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Plasmodium

• There are four species of Plasmodium P.
  falciparum, P. vivax, P.ovale and P. malariae.
• P. falciparum causes severe often fatal malaria
  and is responsible for most deaths, with most
  victims being children.

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Plasmodium

• Both Plasmodium vivax and P. ovale can go
  dormant, hiding out in the liver. The parasites
  can reactivate and cause malaria months or years
  after the initial infection.
• P. malariae causes a long-lasting infection. If
  the infection is untreated it can persist
  asymptomatically for the lifetime of the host.

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Life cycle of malaria

• Plasmodium has two hosts mosquitoes and humans.
• Sexual reproduction takes place in the mosquito
  and the parasite is transmitted to humans when
  the mosquito takes a blood meal.

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Life cycle of malaria humans

• The mosquito injects Plasmodium into a human in
  the form of sporozoites.
• The sporozoites first invade liver cells and
  asexually reproduce to produce huge numbers of
  merozoites which spread to red blood cells where
  more merozoites are produced through more asexual
  reproduction.
• Some parasites transform into sexually
  reproducing gametocytes and these if ingested by
  a mosquito continue the cycle.

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Plasmodium gametocyte
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Life cycle of malaria mosquitoes

• Gametocytes ingested by a mosquito combine in the
  mosquitos stomach to produce zygotes.
• These zygotes develop into motile elongated
  ookinites.
• The ookinites invade the mosquitos midgut wall
  where they ultimately produce sporozoites, which
  make their way to the salivary glands where they
  can be injected into a new human host.

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How Plasmodium enhances transmission rates

• The Plasmodium parasite engages in a number of
  manipulative behaviors to enhance its chances of
  being transmitted between hosts.
• Such manipulations are a common feature of
  parasite behavior, in general, as we will see
  throughout the semester.

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How Plasmodium enhances transmission rates

• Mosquitoes risk death when feeding and attempt to
  minimize risk and maximize reward when doing so.
• To obtain blood a mosquito must insert its
  proboscis through the skin and then locate a
  blood vessel. The longer this takes, the greater
  the risk.

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How Plasmodium enhances transmission rates

• As soon as the mosquito hits a blood vessel the
  hosts body responds by clotting the wound.
• Platelets clump around the proboscis and release
  chemicals which cause the platelets to clot
  together.

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How Plasmodium enhances transmission rates

• To slow clotting and speed feeding, mosquitoes
  inject anticoagulants including one called
  apyrase that unglues the platelets. They also
  inject other chemicals that expand the blood
  vessels.
• Plasmodium in the host helps the mosquito feed by
  releasing chemicals that also slow clotting. The
  parasites help increases the chances of the
  mosquito feeding successfully and sucking up the
  parasite.

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How Plasmodium enhances transmission rates

• Once in the mosquito Plasmodium needs about 10
  days to produce sporozoites that are ready to be
  injected into a human.
• During this time, to reduce the chances of the
  mosquito dying, Plasmodium apparently discourages
  its host from eating. Although how the parasite
  does this is not clear, mosquitoes containing
  ookinites abandon feeding attempts sooner than
  parasite-free mosquitoes.

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How Plasmodium enhances transmission rates

• Once sporozoites are in the salivary glands,
  however, Plasmodium wants the mosquito to bite
  and bite often.
• In the salivary gland the parasite cuts off the
  mosquitos anticoagulant apyrase supply. This
  makes it harder for the mosquito to feed so it is
  hungrier and bites more hosts.

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How Plasmodium enhances transmission rates

• As a result, an infected mosquito is twice as
  likely to bite two people in a single night as an
  uninfected mosquito is.
• As a result, the parasite is spread more widely.

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Behavior of Plasmodium in humans

• Plasmodium enters the blood stream through a
  mosquito bite.
• The parasite must avoid the hosts immune system.
  To do so while in the body it moves from one
  hiding place to another.
• The parasite moves first to the liver. Can get
  there in about 30 minutes, which is usually fast
  enough to avoid triggering the immune system.

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Behavior of Plasmodium in humans

• At the liver Plasmodium enters a liver cell.
• The cell responds by grabbing Plasmodium proteins
  and displaying the antigens on its cell surface
  in a special cup the major histocompatibility
  complex or MHC.

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Behavior of Plasmodium in humans

• The immune system recognizes the Plasmodium
  antigens and mounts an immune response.
• However, in a week before the immune system has
  mounted its full response the parasite has
  produced about 40,000 copies of itself and these
  burst out of the liver to seek red blood cells.

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Behavior of Plasmodium in humans

• The parasites leave the liver, reenter the
  bloodstream, and find a red blood cell to enter.
  
• Each parasite spends two days in a red blood cell
  consuming the hemoglobin and reproducing.

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Plasmodium in red blood cell


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Red blood cells

• Red blood cells (strictly red blood corpuscles)
  are a challenging environment to live in.
• They lack a nucleus and have little metabolic
  activity. As a result, they have few proteins
  for generating energy and also lack most of a
  normal cells channels for transporting fuel in
  and wastes out.

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Red blood cells

• Red blood cells are specialized to transport
  oxygen, which they carry by binding and wrapping
  in hemoglobin molecules.
• A red blood cell is pumped around the body by the
  heart and travels about 300 miles over its
  lifetime.

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Red blood cells

• Red blood cells are squeezed through slender
  capillaries and compressed to one fifth of their
  normal diameter before rebounding.
• To survive this squeezing, red blood cells have a
  network of proteins under their membrane that can
  fold like a concertina and allow the cell to
  stretch and squeeze as needed.

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Red blood cells

• Old red blood cells eventually lose their
  elasticity and become stiff.
• Those that show signs of such aging are filtered
  out as they pass through the spleen and destroyed.

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Behavior of Plasmodium in humans

• Plasmodium cannot swim but uses hooks to move
  along the blood vessels.
• At the parasites tip are sensors that respond
  only to young red blood cells and clasp on to
  proteins on the cells surface.

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Behavior of Plasmodium in humans

• The parasite uses a set of organelles
  concentrated at its apical end to gain entry. A
  suite of proteins are produced that cause the red
  blood cells membrane to open and let the
  parasite squeeze in.
• It takes only about 15 seconds for the parasite
  to get in.

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Plasmodium Sporozoite
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Behavior of Plasmodium in humans

• Inside in the red blood cell the Plasmodium
  consumes the hemoglobin. It takes in a small
  amount of hemoglobin, slices it apart with
  enzymes and harvests the energy released.
• The toxic core of the hemoglobin molecule is
  processed into an inert molecule called hemozoin.

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Behavior of Plasmodium in humans

• In order to reproduce, Plasmodium needs more than
  hemoglobin.
• It sets about modifying the red blood corpuscle
  so it can obtain amino acids and make proteins.
• The parasite builds a series of tubes that
  connect it to the surface of the cell and uses
  these to bring in materials from the blood steam
  and to pump out wastes.

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Behavior of Plasmodium in humans

• The parasite also produces proteins that help to
  maintain the red blood cells springiness for as
  long as possible so it is not eliminated by the
  spleen.
• After a few hours, however, the red blood cell
  has been too modified by the parasite to fool the
  spleen. The parasite now produces sticky latch
  proteins that glue the cell to blood vessel walls.

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Behavior of Plasmodium in humans

• Infected cells clump up in capillaries.
• After another day the contents of the cell have
  been used up. The cell ruptures and 16 new
  parasites burst out to infect other red blood
  cells.
• Some of these parasites transform into sexually
  reproducing gametocytes and, as mentioned
  previously, these if ingested by a mosquito will
  continue the cycle.

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Behavior of Plasmodium in humans

• While in the red blood cells Plasmodium is
  invisible to the immune system because the red
  blood cells have no MHC and cannot alert the
  immune system.
• The latch proteins however do stimulate the
  immune system.

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Behavior of Plasmodium in humans

• The latch protein is made by a single gene, but
  Plasmodium has over 100 such genes each of which
  produces a unique latch.
• In each generation some of the new parasites
  switch on a new latch gene and so the immune
  system is always playing catch up.

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Effects of malaria on human evolution

• The intense selection pressure imposed by malaria
  has resulted in a large number of mutations that
  provide protection against the parasite being
  selected for in humans.
• The best known is sickle cell anemia.

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Anti-malaria mutations Sickle cell anemia

• Sickle cell anemia is a condition common
• in West Africans (and African Americans of West
  African ancestry).
• In sickle cell anemia red blood cells are
• sickle shaped as a result of a mutation which
  causes hemoglobin chains to stick together.

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Anti-malaria mutations Sickle cell anemia

• People with the sickle cell allele are protected
  against Plasmodium because their hemoglobin under
  low oxygen conditions contracts into
  needle-shaped clumps.
• This contraction not only causes the sickling of
  the cell, but harms the parasite. Parasites are
  impaled on the clumps and the cell loses its
  ability to pump potassium, which the parasite
  needs.

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Anti-malaria mutations Sickle cell allele

• People with two copies of the sickle cell allele
  usually die young, but heterozygotes are
  protected against malaria.
• As a result the geographic distribution of the
  allele and malaria in Africa match quite closely.

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Anti-malaria mutations (G6PD) deficiency

• Glucose-6-phosphate dehydrogenase (G6PD)
  deficiency. There are hundreds of alleles known
  and with more than 400 million people affected
  G6PD deficiency is the commonest enzyme
  deficiency known.

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Anti-malaria mutations Thalassemia

• Geographic distribution suggests it protects
  against malaria and epidemiological evidence also
  supports this.
• People with G6PD-202A a reduced activity variant
  common in Africa have a significantly reduced
  risk of suffering severe malaria.

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Anti-malaria mutations Thalassemia

• Thalassemia People with thalassemia make the
  ingredients of hemoglobin in the wrong amounts.
• Too many or too few a or ß hemoglobin chains are
  produced and when they are assembled into
  hemoglobin molecules spare chains are left over.

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Other anti-malaria mutations Thalassemia

• Extra chains clump together and cause major
  problems in the cell. These clumps grab oxygen,
  but dont enclose it and the oxygen often escapes
  and because it is strongly charged, the oxygen
  damages other molecules in the cell.
• Severe thalassemia is fatal, but mild forms
  protect against malaria because the loose oxygen
  severely damages the parasite and renders it
  unable to invade new cells.

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Anti-malaria mutations Ovalocytosis

• Ovalocytosis Occurs in South east Asia and has
  same genetic rules and consequences as sickle
  cell anemia.
• People with ovalocytosis have blood cell walls
  that are so rigid they cant slip through
  capillaries. The rigid cell walls make it hard
  for the parasite to enter the cell and the cells
  rigidity appears to prevent the parasite pumping
  in phosphates and sulphates it needs to survive.

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Anti-malaria mutations

• One major advantage of these various
  anti-malarial mutations appears to be that they
  provide a natural vaccination program for
  children.
• By slowing the development of the parasite these
  mutations give a childs naïve immune system time
  to overcome Plasmodiums attempts to elude the
  immune system and mount an immune response. Mild
  cases of malaria thus immunize children to
  malaria and allow them to survive to adulthood.

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Mosquito nets save lives

• www.nothingbutnets.net or www.nothingbutnets.org
• 10 gets a net to a family. 100 of your
  donation goes to purchase and distribute nets.

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Human African Trypanosomiasis (Sleeping sickness)

• Sleeping sickness is a protozoan disease, which
  like malaria is spread by an insect vector, the
  tsetse fly.
• The disease is endemic to sub-Saharan Africa and
  an estimated 300,000 people are infected annually
  with about 40,000 deaths.
• The disease organism is Trypanosoma brucei.

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Trypanosoma forms in blood smear from patient
with African trypanosomiasis http//en.wikipedia.
org/wiki/FileTrypanosoma_sp._PHIL_613_lores.jpg
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Sleeping Sickness

• Symptoms
• Begins with fever, headaches, and joint pains.
• Lymph nodes may swell enormously and parasite
  numbers may be incredibly high. Greatly enlarged
  lymph nodes in the back of the neck are tell-tale
  signs of the disease.
• If untreated the parasite may cross the
  blood-brain barrier, which causes the
  characteristic symptoms the disease is named for.
  The patient becomes confused and the sleep cycle
  is disturbed with the patient alternating between
  manic periods and complete lethargy. Progressive
  mental deterioration is followed by coma and
  death.

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Sleeping Sickness

• Trypanosome levels in infected patients show a
  cycle of sharp peaks and valleys in parasite
  numbers of approximately a week in length.
• The cycle occurs because the immune system
  recognizes the glycoprotein in the trypanosomes
  coat and mounts an immune response to it, which
  eliminates parasites with that glycoprotein.

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Sleeping Sickness

• Trypanosomes, however, possess about 1,000
  different coat-building genes and periodically a
  new one is turned on by a trypanosome that
  produces a different coat, which the immune
  system doesnt recognize.
• Trypanosomes with this new coat reproduce
  undetected until the immune system can mount a
  response to the new coat.

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Sleeping Sickness

• If the first generation of trypanosomes to infect
  a host turned on their coat genes at random the
  immune system could learn to recognize the
  various possibilities quickly, remember them, and
  eliminate the parasite.
• Instead the coat-building genes are turned on in
  pre-set sequence. This means that the immune
  system every week or so is faced with a new coat
  that it has not seen before.

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Sleeping Sickness

• As a result of the sequential coat-switching, the
  immune system becomes chronically over-stimulated
  and begins to attack the hosts body.
• The overstimulation of the immune system and the
  movement of parasites into the central nervous,
  where they escape the immune system altogether,
  eventually kills the patient.