Infectious diseases

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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

Direct contact e.g. touching, handshaking, or sexual intercourse

Indirect contact e.g. food, water or droplets in air;

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

Toxin production. Toxins fall into two categories; exotoxins and endotoxins.

Invasiveness, where bacteria grow to large numbers locally and produce enzymes that damage host tissues.

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Exotoxins

Heat labile (60-100 degrees for 30 mins) proteins produced and released by both gram positive and gram negative bacteria.

Produced by bacteria such as Clostridium (neurotoxins) and Bacillus (enterotoxin) (+) and E. coli and Vibrio (enterotoxin) (-)

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endotoxins

Are heat stable (100 degrees for 1 hr) lipopolysaccharide produced only by gram –ve bacteria. They remain attached to cell wall.

Cause fever and shock and is of lower toxicity compared to exotoxins.

Produced by bacteria such as Salmonella

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Cholera

Causative Agent: Vibrio cholerae