Infectious Diseases By Andoh Wilson

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Infectious DiseasesByAndoh Wilson
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Infectious Disease Terms

1. Epidemiology
2. Epidemic
3. Endemic
4. Pandemic
5. Pathogen
6. Opportunist
7. Nosocomial
8. virulence

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Normal Micro flora its importance

1. Prevent the growth of pathogens
2. Stimulate the immune system to produce antibodies
   that cross-react with invading pathogens
3. Aid in digestion of cellulose in ruminants.
4. Produce essential nutrients

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Kochs Postulates

1. The same pathogen must be present in every case
   of the disease
2. The pathogen must be isolated from the diseased
   host and grown in pure culture
3. The pathogen from the pure culture must cause the
   disease when it is introduced into a healthy but
   susceptible organism.
4. The pathogen must be isolated from the inoculated
   animal and be shown to be the original organism.

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Modifications to Kochs Postulates

1. Some infectious agents cannot be cultured e.g.
   prions
2. Some pathogens have non-virulent strains whose
   presence does not link them to a disease. E.g.
   non encapsulated Diplococcus pneumoniae

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Types of Pathogens

1. Bacteria
2. Gram positive
3. Gram negative
4. Acid-Fast e.g. Mycobacteria
5. Spherical described as cocci
6. Rod shaped described as bacilli

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Gram Positives

• Unique Features
• Thick peptidoglycan wall
• No periplasmic space
• No outer membrane (capsule)
• E.g. Streptococcus pyogenes, Staphylococcus
  aureus, Bacillus anthracis, Clostridium tetani

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Gram Positive wall
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Gram Negatives

• Unique features
• Thin peptidoglycan wall
• Has periplasmic space containing different
  degradative enzymes such as deoxyribonucleases,
  ?-lactamases and proteases
• Outer membrane containing lipid A, an endotoxin
• E.g. Neisseria, Salmonella typhi, E. coli,
  Yersinia pestis, Vibrio cholerae

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Gram Negative wall
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Types of pathogens

• Parasites (Eukaryotic Pathogen)
• Fungi e.g. Candida, Aspergillus
• Protozoa e.g. Plasmodium, Schistosoma
• Worms e.g. Ascaris, Taenia

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Types of pathogens

• Viruses
• Are pieces of DNA or RNA surrounded by protein
  coat. The may be
• Encapsulated e.g. HIV, HBV, measles, mumps,
  influenza, rabies
• Non-encapsulated e.g.adenoviruses, HPV, Polio

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Viruses
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Modes of transmission

1. Direct contact e.g. touching, handshaking, or
   sexual intercourse
2. Indirect contact e.g. food, water or droplets in
   air
3. Animal vectors e.g. insect bites in malaria,
   plague and oncho, dog bite in rabies

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Pathogenesis

• Sequence of activities
• Transmission of causative agent to susceptible
  host
• Adherence of the agent to a target tissue
• Colonization and invasion
• Damage to host by toxins or other mechanisms
• Exit from host
• Survival outside host long enough for step 1 to
  occur.

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Virulent Factors

• For all pathogens there is an infective dose and
  a lethal dose.
• Virulent factors that confer pathogenicity
  include
• Pili that facilitate attachment
• Capsules that interfere with phagocytosis
• Exotoxins
• Endotoxins
• Proteases that break down antibodies
• Ability to vary antigens to evade antibodies

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Bacterial Pathogenesis

1. Toxin production. Toxins fall into two
   categories exotoxins and endotoxins.
2. Invasiveness, where bacteria grow to large
   numbers locally and produce enzymes that damage
   host tissues.

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exotoxins

1. Heat labile (60-100 degrees for 30 mins) proteins
   produced and released by both gram positive and
   gram negative bacteria.
2. Produced by bacteria such as Clostridium
   (neurotoxins) and Bacillus (enterotoxin) () and
   E. coli and Vibrio (enterotoxin) (-)

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endotoxins

1. Are heat stable (100 degrees for 1 hr)
   lipopolysaccharide produced only by gram ve
   bacteria. They remain attached to cell wall.
2. Cause fever and shock and is of lower toxicity
   compared to exotoxins.
3. Produced by bacteria such as Salmonella

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cholera

1. Causative Agent Vibrio cholerae
2. Symptoms severe diarrhoea up to 20 liters a day
   of rice water stool, vomiting, muscle cramps
   caused by loss fluid and electrolytes.
3. Pathogenesis Vibrio adheres to the small
   intestinal lining, multiply and produce the
   enterotoxin choleragen which causes the
   accumulation of cAMP. An increased secretion of
   water and electrolyte from the cells results

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Cholera

• 4. Epidemiology Feacally contaminated water,
  crabs and vegetables fertilized with human
  faeces. Has been eradicated most developed
  countries but a new strain discovered in 1992 is
  threatening another pandemic.

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Cholera

• 5. Incubation period 12-48 hours
• 6. Lab diagnosis Microscopy, culture of sample
  from faeces or vomit.
• 7. Prevention Purification of water, washing of
  hands.
• 8. Treatment administration of solution of
  glucose and electrolyte orally or intravenously
  tetracycline antibiotic orally

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malaria

1. Causative Agent Plasmodium (4 species)
2. Symptoms (Clinical features) fever, chills,
   anaemia, headache, nausea, shivering, convulsions
   (esp. in under 5 yr olds) enlarged spleen.
3. Pathogenesis site of action of pathogen include
   liver, RBC, brain. The vector, female Anopheles
   mosquito, transfer pathogen during feeding.

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malaria

• 4. Epidemiology Endemic in 91 tropical and
  subtropical countries. Invade the liver 1st and
  move to reproduce in RBCs resulting in their
  rupture and the associated chills.
• 5. Incubation Period 1 2 weeks.
• 6. Lab diagnosis Microscopy.

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An Infected RBC
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Malaria - Prevention

1. Reduce the number of mosquitoes destruction of
   larvae and adult mosquitoes by biological and
   chemical control methods
2. Avoid being bitten protective clothing and
   creams, treated bed nets
3. Use of drugs to prevent infection
   chemoprophylaxis

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Malaria -Treatment

• Combination therapy Artesunate Amodiaquine

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Tuberculosis

1. Pathogen M. tuberculosis (pulmonary TB) M.
   bovis(GI TB)
2. Transmission airborne droplets (NB MTB is
   dessication resistant and survives in dried
   sputum) unpasteurized milk.
3. Clinical features prolonged coughing sometimes
   with bloody sputum, shortness of breath, fever,
   sweating , weight loss

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Tuberculosis

• 4. No toxin production. Pathogenicty is by
  invasiveness that produce characteristic lesions
  in the lungs.
• 5. Epidemiology pathogen triggers acute
  inflammatory response forms tubercle giant
  cells containing MTB and surrounded by epithelial
  cells. Tubercles heals by fibrosis and
  calcification. Can desseminate via bloodstream to
  other internal organs