Emerging Diseases: Causes and Characteristics

Emerging Diseases

• An emerging disease is a newly identified infection, or a previously recognized infection that has expanded into a new ecological niche, often accompanied by a significant change in pathogenicity.
• Many emerging diseases are zoonotic, meaning an animal reservoir incubates the organism, with only occasional transmission into human populations.
• Causes of emerging diseases include:
• Newly identified species (e.g. HIV and AIDS)
• Newly identified strains that have evolved from a known infection (e.g. influenza)
• Ecological changes that alter the composition and size of reservoirs (e.g. Lyme disease)
• Spread to a population in a new area of the globe (e.g. West Nile fever)
• Re-emerging infections like drug-resistant tuberculosis
• Nosocomial (hospital-acquired) infections, such as Methicillin-resistant S. aureus

Disease Transmission

• Main modes of transmission:
• Contact transmission
• Indirect transmission – vehicle or vector
• Horizontal (vs vertical)

Contact Transmission

• Direct Contact Transmission: person-to-person transmission (touching, kissing, sexual intercourse) without an intermediate object.
• Examples: Hepatitis A, Smallpox, Staphylococcal infections, mononucleosis, sexually transmitted diseases like syphilis or HIV/AIDS.
• Indirect Contact Transmission: the microbe is transferred via a non-living object or fomite, such as towels, eating utensils, thermometers, stethoscopes, bedding, clothes, money, and needles.
• Examples: Herpes simplex virus, Cytomegalovirus, Giardia, Impetigo.
• Droplet Transmission: microbes are spread in mucus droplets that travel short distances (less than 1 meter).
• Examples: respiratory viruses (e.g. influenza, adenovirus, respiratory syncytial virus, human metapneumovirus), Bordetella pertussis, Pneumococci, Diphtheria, and Rubella.

Vehicle Transmission

• Transmission of disease via a medium such as water, food, air, blood, body fluids, and intravenous fluids.
• Waterborne Transmission: usually caused by water contaminated with sewage.
• Airborne Transmission: not to be confused with droplet transmission, is due to inhalation of small pathogens and particles that are suspended in air and can travel long distances.

Antimicrobial Agents

• Antibacterial agents: type of antimicrobial drug used in the treatment and prevention of bacterial infections.
• May either kill (bactericidal) or inhibit the growth of bacteria (bacteriostatic).
• Classified as either broad-spectrum or narrow-spectrum depending on how many types of microorganisms are naturally susceptible to their action.
• Classified depending on their chemical structure and site of action:
• Molecules that inhibit cell wall synthesis.
• Molecules that inhibit the function of the bacterial plasma membrane.
• Molecules that inhibit the synthesis of nucleic acids.
• Molecules that inhibit the synthesis of proteins.

Antiviral Agents

• Major steps targeted by common antiviral agents:
• Attachment and entry: inhibit fusion of viral envelope or attachment to receptor.
• Nucleic acid synthesis: molecules that target viral DNA and RNA polymerases.
• Assembly and budding: inhibit viral proteins needed for virion maturation and/or release.

Antimicrobial Drug Resistance

• Antimicrobial resistance is the ability of a microorganism to survive and multiply in the presence of an antimicrobial agent that would normally inhibit or kill this particular kind of organism.
• Bacterial resistance strategies:
• By preventing the drug from reaching its target by reducing its ability to penetrate the cell.
• By inactivation of the drug via modification or degradation.
• By expulsion of the drug from the cell via general or specific efflux pumps.
• By modification of the drug's target site within the bacteria.
• Viral resistance strategies:
• Results from spontaneous mutations in the viral genome during viral replication.
• Mutations are within the target of the antiviral drug.
• The error-prone polymerase enzyme in RNA viruses causes these viruses to develop resistance more frequently than DNA viruses.
• Special concern during extended therapy for chronic infections (e.g. HIV, HBV, and HCV).

Prevention of Disease Transmission

Immunological Memory

• First exposure to antigen: clonal expansion, memory cells are long-lived, and continue to reproduce.
• Second exposure to antigen: stronger and more rapid response.

Types of Adaptive Immunity

• Adaptive immunity can be:
• Active: natural (exposure to infectious agent) or artificial (vaccination).
• Passive: natural (maternal antibodies) or artificial (injected antibodies).

Artificial Passive Immunisation

• Immunity is transferred by administration of antibodies to a non-immune individual.