BYA3 SECTION 12.2

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 female mosquito, human liver and red blood cells
- Produces enormous quantities of parasites, merozoite stage
Sexual phase generates genetic diversity essential for evading host's immune system
[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
1. Infected female mosquitoes are feeding human blood
2. Secretes salvia which contains anticoagulants (anti blood-clotting agents)
3. Sporozoites (young malarial parasites) in the salvia enter blood
Latent Period
1. Parasites migrate to liver cells / Undergo asexual reproduction / Develop into merozoites
2. Merozoites infect red blood cells / Undergo asexual reproduction / Produce more merozoites
Outbreak
1. Merozoites burst out of red blood cells to infect more cells
2. This causes fever and other symptoms
3. Some merozoites develop into gametocytes
Infection of Female Mosquitos
1. Gamelocytes are taken up by female mosquitos that feed on the infected person's blood
2. Inside the female mosquito, gametes produced by those gametocytes fuse
3. Results in zygotes which develop into sporozoites
4. Sporozoites move into the salivary glands 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

Tuberculosis (Myobacterium tuberculosis)

Lung most common infected organ
Inhaling droplets exhaled from a carrier during coughing causes the infection
Latent period: bacteria may lie inactive for up to 30 years and become active as primary tuberculosis (TB)
Symptoms are fever, loss of weight and persistent coughing
Bacteria destroy lung tissue and cause accumulation of fluid in the pleural cavity
Coughing up blood is common because bacteria destroy lung tissue
Treatment
- Antibiotics for a six-month period to ensure eradication
- Combinations of drugs prevent development of resistant strains
- Vaccine is a live attenuated strain of TB

Salmonella

Symptoms: typhoid fever, intestinal infections
Food poisoning caused from uncooked poultry, beef, and eggs
"Salmonella enters the body in contaminated food/drink
The bacterium passes from the esophagus, through the stomach, into the intestine
It enters cells lining the small intestine to multiply \ population increases
Some bacteria die and release an endotoxin
This causes (symptoms) diarrhoea, vomiting, nausea, abdominal pain (food poisoning)"¹

References and Further Reading
1) AQA (2003) Mark Scheme June 2003 GCE Biology/Human Biology A Unit BYA3, [PDF]
AQA (2006) GCE Biology/Biology (Human) 2006 specification, [PDF]
BAKER, M, INDGE, B, & ROWLAND, M (2001) Further Studies in Human Biology. Hodder Arnold H&S

BYA3 SECTION:

12.2