Parasites and Pathogens

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Parasites and Pathogens

• Chapter 21

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Microbes

• Any microscopic organism or entity
• Microbes are beneficial (nutrient cycling
  economic, aesthetic, and health contributions)
• Microbes also can exhibit pathogenicity the
  ability to cause disease

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Pathogens

• Emerging diseases - microbes that recently became
  pathogenic by the acquisition of
• Antibiotic resistance
• Virulence factors
• Toxin production, e.g. E. coli and C. tetani
• Adhesion properties
• Invasion mechanisms. Systemic diseases more
  serious than localized, e.g. certain Salmonella

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Epidemiology

• Study of factors that influence transmission of
  disease
• Infectious diseases caused by presence of a
  pathogen /or its product(s)
• Contagious diseases are transmissible by direct
  contact
• Acute infections occur quickly with severe
  symptoms chronic infections occur longer with
  less severe symptoms

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Modes of Transmission

• Direct Contact w/ body fluids or lesions, e.g.
  HIV, hepatitis, herpes
• Airborne as droplet nuclei or in dust, e.g.
  measles
• Vehicle (formite), or inanimate objects, e.g.
  cold virus(es), Salmonella, tetanus
• Vector-borne, animals that transmit other
  pathogens, e.g. malaria, Lyme disease, rabies

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The extent of infectious disease

• Sporadic limited
• Epidemic larger area
• Pandemic worldwide
• Endemic always present

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Infectious Agents

• Viruses
• Bacteria
• Fungi
• Protozoans unicellular eukaryotes
• Helminths parasitic worms

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Viruses

• Viruses are acellular and obligate
• Very small 10X smaller than bacteria
• Composed of a capsid and nucleic acid, and
  sometimes an envelope
• Viruses use host cells machinery to reproduce
  themselves
• Some viruses can exhibit latency

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Viral disease Influenza

• Flu tends to be epidemic
• Flu viruses display two antigens on their protein
  spikes
• Body produces antibodies in response to these
  antigens
• Body is protected against these particular flu
  viruses in the future.

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• Why are people affected by flu every year?

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Influenza

• Antigenic shift mutations bring about small
  changes in antigens.
• Antigenic drift two viruses infect same cell and
  recombine their genomes. Antigens so different
  that no one is immune.

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Viral diseases Herpes

• Example of chronic disease and latency
• Herpes virus stored in spine after initial
  infection. Inserts itself into host DNA.
• Stress can cause new outbreak, as virus migrates
  to site of original infection
• HSV-1, HSV-2, Varicella-zoster

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Infectious diseases Common cold

• About half the cases caused by Rhinovirus.
• Other viruses cause cold-like symptoms
• Probably (at least) 200 different cold viruses
  (doesnt include strains)
• Cold symptoms increase during cold winter reason
  not known
• Hand-to-hand contact most common form of
  transmission antibacterial soap ineffective

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Infectious diseases Measles

• Maybe most contagious disease
• Chance of unvaccinated child becoming infected
  (after exposure)?
• 100
• 50 million infected, 4 million deaths
• 1/1,000 develop encephalitis
• Developed countries 1/3000 fatal, undeveloped
  10-15

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Infectious diseases Smallpox

• Very deadly 20-40 fatality rate
• Eradicated in 1970s through use of Jenner
  vaccine
• New threat?

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Prions

• Even smaller than viruses infectious particles
• Nervous degenerative and wasting diseases
• Causes bovine spongiform encephalitis
• Transmission via ingestion of infected tissue
• Animal-to-human transmission disputed

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Infectious disease Bacteria

• Bacteria are prokaryotes lack a nucleus and
  possess a cell wall
• Rod-shape bacteria are termed bacilli,
  sphere-shaped are cocci.
• Gram staining also used to classify bacteria.

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Bacteria Streptococcus

• Gram-positive, chains, airborne, causes more
  diseases than any other bacterium
• Strep throat, scarlet fever, rheumatic fever,
  pneumonias, impetigo, tooth decay, meningitis,
  kidney damage, and flesh-eating disease.

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Bacteria food poisoning

• Vomiting and/or diarrhea
• Salmonella
• Staphylococcus
• Clostridium botulinum

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Other agents Fungi

• Mycoses are diseases caused by fungi.
• Difficult to treat.
• Tineas cutaneous diseases such as ringworm and
  athletes foot
• Candida albacans normal microflora that can
  cause vaginal yeast infections and oral thrush.

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Other agents Protozoans Helminths

• Malaria
• Flatworms and blood flukes
• See Figures 21.18 and 21.20

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Antibiotics

• Penicillins (ampicillin, amoxicillin, etc.)
  inhibit peptidoglycan production, Erythromycin
  target the ribosomes
• Allergic reactions
• Danger to microflora
• Prevention of natural immunity
• Growing resistance