Topic 5.3: Pathogens and Disease

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5.3.1 Pathogen
 
  • A pathogen is an organism or virus that causes a disease.
  
   
5.3.2 Pathogens and diseases
 
Type
  
Pathogen
  
Disease
      
 

 

 

  • Virus

 

 

  • Adeno virus

 

 

  • Common cold
adeno virus    
     
     
salmonella

 

  • Bacteria

 

  • Salmonella

 

  • Food poisoning
   
           
ringworm

 

  • Fungi

 

  • Epidermophyta floccosum

 

  • Tinea faciei (ringworm)

 

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plasmodiium

 

  • Protozoa

 

  • Plasmodium falciparum

 

  • Malaria

 

CDC Atlanta

    
               
sheep fluke

 

  • Flatworm (fluke)

 

  • Fasciola hepatica (sheep fluke)

 

  • Fascioliasis

 

CDC Atlanta

        
             
nematodes

 

  • Roundworms (Nematodes)

 

  • Toxocara canis

 

  • Trichuriasis

 

      
 
5.3.3 Methods of disease transmission
                   
  • Water
  • Food
  • Air borne (aerosol droplet)
  • Touch
  • Sexual transmission
  • Vectors ( animals)
 
 
5.3.4 Human Bacterial Disease
   
   

Salmonellosis

  • Salmonella enteritidis is the disease causing agent
  • Transmission:Through poultry and their eggs, especially when uncooked (e.g. mayonnaise and ice cream), or lightly-cooked (e.g. meringue and soft-boiled eggs); or through un-pasteurised milk. Other sources are contamination of foods and water by human or household pet faeces. Low infectivity (106-107 cells required for infection).
  • Effects:The bacteria colonise small intestine and remain there producing exotoxins Sudden signs 15-48 h after eating infected food. This is much faster than typhoid because the bacteria remain in the gut. Symptoms include moderate diarrhoea, vomiting, abdominal pain, mild fever and headache. Recovery usually in 2-3 days.
    
   
     
     
5.3.5 Antibiotics Effectiveness

 

    

Antibiotics block specific metabolic pathways found in bacteria, but not in eukaryotic cells.

  • Some antibiotics are cidal (bacteriocidal, fungicidal, etc.), which means they kill the microbes, while others are static (bacteriostatic, fungistatic, etc.), which means they stop further growth, but don't kill existing cells. Both are useful medically, because if the growth of an infective pathogen is stopped, the body's immune system will be able to kill it. These graphs show how the two kinds of drug affect bacterial growth curves.
    
               
  • Bacteria, as prokaryotes, have many unique features not present in eukaryotes, so antibiotics can be selectively toxic by targeting such features as the bacterial cell wall, 70S ribosomes, and enzymes that are specific to bacteria. In this way the human eukaryotic cells are unaffected.
    

Viruses reproduce using the host cell metabolic pathways that are not affected by antibiotics.

  • Viruses do not have metabolic pathways like bacteria and therefore antibiotics do not work on viruses.
  • Viruses can only be treated by their specific anti-microbial agent and antibiotics should never be prescribed for viral infections (such as flu).

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5.3.6 The cause, transmission and social implications of AIDS

 

    
          The cause:
  • The agent of AIDS is the retro virus HIV
  • This infects and replicates inside the T-helper cell (white lymphocyte)
  • The virus disables the T-lymphocyte so that it cannot respond to the diseases for which it is specific.
  • The infected person is therefore vulnerable to opportunistic infection by the disease against which protection has been lost.
   
     
       
     
      The Transmission:
  • Through infected semen or vaginal secretions during sexual activity.
  • Infected blood in transfusions or contaminated needles.
  • Infected mothers can also pass the virus on to their children through the placenta or milk.
  • Before 1985 many hospital patients, especially haemophiliacs, became infected through blood trans-fusions, but since 1985 all blood donations in the UK are tested for HIV.
  • Many drug addicts have been infected through sharing needles. By far the most important method of transmission of HIV world-wide is unprotected sexual intercourse.
  • HIV cannot survive in air and therefore cannot be transmitted by skin contact or kissing.
    
     
    Social implications :
  • HIV has a long latency of 8-10 years, during which time there are no symptoms, but the individual is infectious
  • After this period the person starts to shows mild symptoms of the AID-related complex (ARC), such as tiredness, fever, weight loss and diarrhoea.
  • This is followed by the more serious symptoms of full-blown AIDS. Since the immune system no longer functions the patient has no defence against a variety of opportunistic infections.
  • There is as yet no cure or vaccination for AIDS, though drugs like AZT can delay its onset for many years.
  • Vaccinations are difficult because the HIV genome is highly variable (probably because reverse transcriptase make many base copying errors).
  • Prevention of AIDS has concentrated on "safe sex" education (using condoms and reducing promiscuity), not sharing needles, and screening blood transfusions.
    
                     

 

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