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Module 4
Communicable diseases
• Communicable disease- disease that’s passed from one organism to another (within and possibly
between species).
• Caused by infective organisms- pathogens.
- E.g. bacteria, viruses, fungi and protoctista.
• Vectors carry pathogens from one organism to another e.g. water and insects.
• Cause of 23% of deaths world-wide.
• Major problem in domestic and wild animals and plants e.g. pets and farm animals- cows, goats
and sheep etc.
Bacteria
• Classified in 2 main ways:
1. Basic shape:
- Rod (bacilli).
- Spherical (cocci).
- Comma shaped (vibrios).
- Spiral (spirilla).
- Corkscrew (spirochaete).
2. Cell wall:
- Gram +ve = purple under light
microscopeà thick cell wall e.g.
staphylococcus aureus.
- Gram –ve= red- safranin counterstain.
Important to identify bacteria so we know what medication to prescribe etc.
Viruses:
• Non-living and are all pathogenic.
• 0.02- 0.3 micrometers.
• Structure- DNA/RNA surrounded by protein.
• Invade living cell, take over its biochemistry to make more copies of
virus rapidly- ultimate parasites (only active when inside host cell,
have little structure but take over whole host cell).
- Bacteriophage= viruses that attack bacteria.
Protoctista/ protista:
• Eukaryotic organism.
• Unicellular.
• Parasitic- use people or animals as their host e.g. malaria/ sleeping sickness or enter through
polluted water e.g. amoebic dysentery/ Giardia.
• Take over host e.g. plasmodium and Hb.
Fungi:
• Eukaryotic.
• Can be multicellular or unicellular.
• Can cause diseases in plants and animals.
• Reproduce- make millions of spores which spread huge distances.
Pathogens- mode of action:
- Damaging host tissues directly.
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, 1. Viruses- get into host cell’s DNA and takes over cells metabolism to make new viruses which burst
out of the cell and spread to infect other cells.
2. Protoctista- takes over cell and feed on cell contents as they reproduce e.g. plasmodium.
3. Fungi- digest living cells and destroy them.
Pathogens- mode of action:
- Producing toxins which damage host tissues- indirect.
1. Bacteria- produce toxins/poison as a by-product of their normal function, which damages host cells
e.g. by breaking down cell membranes/ inactivating enzymes/interfering with genetic material to
prevent cell division.
2. Some fungi- produce toxins to cause disease.
Plant diseases:
Plant disease Caused by Host Description
Ring rot - Gram positive - Potatoes. - Damages leaves, tubers, fruits.
bacterium. - Tomatoes. - No cure.
- Clavibacter - Aubergines. - If ring rot infects a field, potatoes can’t grow
michiganenesis. on that field for 2 years.
Tobacco - Virus. - Tobacco plants. - Damages leaves, flowers, fruit and stops
mosaic virus - Tomatoes. growth and reduces yield.
(TMV) - Peppers. - No cure.
- Cucumbers. - Resistant crop strains available.
- 150 other plant
species.
Potato blight - Fungus like - Potatoes. - Destroys leaves, tubers and fruit.
protoctist. - No cure.
- Phytophthora - Resistant strains.
infestans. - Careful management and chemical
treatments can reduce infection risk.
Black sigatoka - Fungus. - Bananas. - Hyphae penetrates and digests leaves,
- Mycosphaerella - Plantains. turning them black.
fijiensis. - Resistant strains being developed.
- No cure.
- Good husbandry and fungicide can control
speed.
PLANT DISEASES CANNOT BE TREATED/ CURED.
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,Animal diseases:
Animal disease Caused by Host Description
TB - Bacteria. - Humans. - Destroys lung tissue.
- Mycobacterium - Cows. - Suppresses immune system.
tuberculosis. - Pigs. - Curable by antibiotics.
- M.bovis. - Badgers. - Preventable by improving living conditions +
- Deers. vaccinations.
Meningitis - Bacteria. - Humans- - Causes septicaemia- purple/red rash when
- Streptococcus young glass is pressed.
pneumoniae. children and - 10% of infected ppl die.
- Neisseria teenagers. - 25% recover but still have permanent
meningitidis. damage.
- Antibiotics cure it if treated early.
- Vaccines can protect against it in some
forms.
HIV - Virus. - Humans. - Destroys T cells so you are susceptible to
- Non- human other infections.
primates. - No vaccine/ cure.
- Anti-retroviral drug to slow down
progression.
Flu - Virus. - Mammals- - Destroys ciliated epithelial cells, leaving
- Orthomyxoviridae humans, airways open to secondary infections- can
spp. pigs, birds be fatal to young/old.
and - 3 main strains- A, B, C (A- most virulent).
chickens. - Classified by surface proteins e.g. A (H1N1).
- Mutate regularly.
- No cure.
Malaria - Protoctista. - Mosquitoes. - Reproduce in female mosquito- after 2
- Plasmodium. - Humans. blood meals, she lays eggs and is passed on.
- Spread by bites of - Invades erythrocytes, liver and brain-
anopheles increases vulnerability to other infections.
(female) - No vaccines/ limited cures.
mosquito. - Preventative measures (insecticides,
mosquito nets/ door screens).
Ring worm - Different fungi - Cattle. - Causes grey- white, crusty infectious,
affect different - Dogs. circular areas of skin- itchy.
species. - Cats. - Anti- fungal cream is effective cure.
- Trichophyton - Humans.
verrucosum.
Athletes foot - Fungus. - Humans. - Grows on and digests the warm, moist skin
- Tinia pedia. between toes- causes cracking and scaling-
itchy and sore.
- Anti-fungal cream is an effective cure.
Direct transmission:
- Pathogen directly transferred from one individual to another.
1. Direct contact (contagious diseases):
- Exchange bodily fluids e.g. STD.
2. Inoculation:
- Skin-to-skin contact. E.g. ringworm/athletes foot.
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, 3. Ingestion:
- Microorganisms from faeces transmitted on hands e.g. diarrhoeal diseases.
Indirect transmission:
- Pathogen indirectly transferred from one individual to another through another medium e.g. air/
H2O.
1. Tomites:
- Sharing inanimate objects such as bedding, socks or cosmetics e.g. athletes foot, gas gangrene.
2. Droplet infection (inhalation):
- Inhaling droplets of saliva/ mucus containing pathogens from someone sneezing e.g. flu/ TB.
3. Vectors:
- Often, but not always animal carrying pathogen e.g. dogs transmit rabies, mosquitoes transmit malaria.
Transmission between animals and humans:
• Bird flu strain H1N1 and bucellosis passed from sheep to humans.
• We can minimise contact and wash hands to reduce infection rates.
• People can act as vectors of some animals’ diseases e.g. foot and mouth disease.
Factors affecting transmission of communicable diseases in animals:
1. Overcrowded conditions.
2. Poor nutrition.
3. Compromised immune system system e.g. people with HIV/ babies/ old people.
4. Poor waste disposal provides breeding site for vectors.
5. Climate change introducing new vectors e.g. higher temps= optimum for mosquitoes. If it gets hot
here, we could get malaria etc.
6. Culture and infrastructure.
7. Socioeconomic factors.
Preventing spread of communicable diseases:
- Keep clean/ shower everyday/ wash hands.
- Have probiotics- yogurt etc.
- Multivitamins- nutrition.
- Hand sanitiser.
- Dispose of waste properly.
Transmission of pathogens between plants:
• Plant pollen and seeds move widely.
• Less well developed immune system than humans.
- Direct transmission:
• Direct contact of a healthy plant with any part of a diseased plant.
- Indirect transmission:
1. Soil contamination:
- Pathogens and reproductive spores from protoctista and fungi spread to crops.
- Some survive composting process and infection cycle is completed when contaminated compost is
used.
2. Vectors:
- Wind-spores carried to other plants.
- Water-spores swim on the surface water of leaves and spread by raindrop splashes.
- Animals- insects/ birds carry spores. Aphids inoculate pathogens directly into plant tissues.
- Humans- transmitted by hands, clothing, formites, farm machinery.
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