4.1.1 Common Pathogens & Communicable Diseases

• A disease is an illness or disorder of the body or mind that leads to poor health
• Each disease is associated with a set of signs and symptoms
• Communicable/infectious diseases are caused by pathogens and are transmissible (can be spread between individuals within a population)
• Both plants and animals can be affected by pathogens

Infectious & non-infectious diseases table

• Many microorganisms are pathogens including:
  • Bacteria
  • Viruses
  • Fungi
  • Protoctists
• To control disease, it is very important to know what pathogen is causing it

Bacteria

• Bacteria are a diverse range of prokaryotic organisms
• Some bacteria are non-pathogenic (they do not cause any disease or damage) while others are pathogenic
• Pathogenic bacteria do not always infect the hosts of cells, they can remain within body cavities or spaces
• M. tuberculosis causes tuberculosis (TB) in humans
  • The bacteria infect the lungs, causing a chronic cough and bloody mucus
  • It is a disease often associated with poor hygiene and sanitation
  • M. bovine in cows can also transmit to humans to cause TB
• N. meningitidis causes bacterial meningitis in humans
  • Very few bacteria can cross the barrier created by the meninges (the tissue that surrounds the brain and spinal cord) however N. meningitidis crosses this barrier to cause acute inflammation
  • Inflammation of the meninges causes symptoms such as fever, headache, neck stiffness and a characteristic rash
• Ring rot diseases in potato plants are caused by bacterial pathogens
  • The bacteria infect the vascular tissue and prevent the transport of water, causing the plant to wilt and die
  • The infection spreads into the potato tubers where the vascular tissue is arranged in a ring, producing the characteristic black ring of rot

Viruses

• Viruses do not have a cellular structure
  • This means they can't respire, produce ATP, replicate genetic material or synthesise protein
• They infect host cells and hijack their machinery to replicate their own genetic material and proteins
• The first virus ever discovered was the Tobacco Mosaic Virus (TMV)
• TMV infects several plant species
  • It causes a distinct yellowing of the leaves which produces a mosaic pattern
• Three different influenza viruses infect humans to cause the flu
  • Influenza A, influenza B and influenza C infect the cells that line the airways
  • They cause a high temperature, body aches and fatigue
  • Influenza A is the virus that causes the most cases of flu globally
    • It has a capsid that surrounds 8 single-stranded molecules of RNA
• The human immunodeficiency virus (HIV) infects specific cells of the immune system
  • It is an enveloped retrovirus
  • The viral enzyme reverse transcriptase produces single-stranded DNA from its viral RNA
  • DNA polymerase synthesises double-stranded DNA from this single-stranded DNA
  • The double-stranded DNA is inserted into the host DNA and can remain inactive for many years
  • Once activated the DNA provirus is used to synthesise new viruses

Protoctista

• Protists are unicellular eukaryotes
• Plasmodium falciparum is a protist that causes severe forms of malaria in humans
  • The parasite is spread by mosquitoes
  • Infected individuals experience fever, chills and fatigue
• P. infestans causes the infamous potato blight
  • The pathogen is unusual as it has some fungal characteristics
  • It is transmitted via spores
  • The first signs of potato blight are small, dark brown marks on the leaves which quickly increase in size and number
  • The protist destroys potato and tomato crops leaving them completely inedible

Fungi

• Fungi have a similar structure to plants
  • Their eukaryotic cells have cell walls and large central vacuoles
  • However, instead of being made of separate cells, their bodies consist of filaments known as hyphae
  • These hyphae form a network and spread throughout a host/soil
• Fungal diseases are much more common in plants than animals
• Cattle ringworm and athletes foot are fungal diseases that exist on the surface of the skin
• Fungal diseases in plants tend to be much more serious and can threaten entire crops
• Black Sigatoka is a fungal disease in bananas
  • It spreads through the leaves of the plant, reducing its ability to photosynthesise
  • The lack of photosynthesis causes parts of the leaf to die; producing black streaks
  • Eventually, the whole leaf dies

Common pathogens and related communicable diseases in humans table

Common pathogens and related communicable diseases in plants table