Infectious DiseasesInfectious Diseases

ByByAndoh WilsonAndoh Wilson

Infectious Disease TermsInfectious Disease Terms

1. Epidemiology2. Epidemic3. Endemic4. Pandemic5. Pathogen6. Opportunist7. Nosocomial8. virulence

Normal Micro flora & its importanceNormal Micro flora & its importance

1. Prevent the growth of pathogens2. 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

Koch’s PostulatesKoch’s Postulates1. 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.

Modifications to Koch’s PostulatesModifications to Koch’s 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

Types of PathogensTypes of Pathogens

1. Bacteria1. Gram positive2. Gram negative3. Acid-Fast e.g. Mycobacteria

1. Spherical described as cocci2. Rod shaped described as bacilli

Gram PositivesGram Positives

• Unique Features– Thick peptidoglycan wall– No periplasmic space– No outer membrane (capsule)– E.g. Streptococcus pyogenes,

Staphylococcus aureus, Bacillus anthracis, Clostridium tetani

Gram Positive wallGram Positive wall

Gram NegativesGram 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

Gram Negative wallGram Negative wall

Types of pathogensTypes of pathogens

2. Parasites (Eukaryotic Pathogen)1. Fungi e.g. Candida, Aspergillus2. Protozoa e.g. Plasmodium, Schistosoma3. Worms e.g. Ascaris, Taenia

Types of pathogensTypes of pathogens

3. Viruses1. Are pieces of DNA or RNA surrounded by

protein coat. The may be2. Encapsulated e.g. HIV, HBV, measles,

mumps, influenza, rabies3. Non-encapsulated e.g.adenoviruses, HPV,

Polio

VirusesViruses

Modes of transmissionModes of transmission1. 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

PathogenesisPathogenesis• Sequence of activities1. Transmission of causative agent to

susceptible host;2. Adherence of the agent to a target tissue;3. Colonization and invasion;4. Damage to host by toxins or other

mechanisms;5. Exit from host;6. Survival outside host long enough for step

1 to occur.

Virulent FactorsVirulent Factors• For all pathogens there is an infective

dose and a lethal dose.• Virulent factors that confer pathogenicity

include;1. Pili that facilitate attachment;2. Capsules that interfere with phagocytosis3. Exotoxins4. Endotoxins5. Proteases that break down antibodies6. Ability to vary antigens to evade antibodies

Bacterial PathogenesisBacterial 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.

exotoxinsexotoxins

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) (-)

endotoxinsendotoxins

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

choleracholera1. Causative Agent: Vibrio cholerae2. 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

CholeraCholera

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.

CholeraCholera5. Incubation period: 12-48 hours6. 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

malariamalaria

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.

malariamalaria

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.

An Infected RBCAn Infected RBC

Malaria - PreventionMalaria - 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

Malaria -TreatmentMalaria -Treatment

• Combination therapy: Artesunate Amodiaquine

TuberculosisTuberculosis1. 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

TuberculosisTuberculosis

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