Zoonoses, Reservoirs, and Pathogenesis

Zoonoses

  • Zoonoses are diseases transmitted from animals to humans.
  • Emerging diseases are often zoonotic.
  • Routes of acquisition:
    • Direct contact with animals or their waste products.
    • Consuming improperly processed, refrigerated, or cooked animals.
    • Arthropod vectors (especially blood-sucking insects like mosquitoes).
  • Humans are usually dead-end hosts for zoonotic pathogens.
  • Zoonotic diseases are difficult to eradicate because of the challenge of controlling animal reservoirs (e.g., mosquitoes, rodents, bats).

Common Zoonoses

  • Helminths (worms): Reservoirs include dogs, sheep, cattle, and pigs.
  • Protozoans (e.g., malaria, toxoplasmosis): Various animal reservoirs.
  • Fungal infections (e.g., ringworm): Contact with domestic animals.
  • Bacterial microbes:
    • Bacillus anthracis (anthrax): Domestic livestock.
    • Yersinia pestis (bubonic plague): Rodents.
    • Lyme disease: Ticks found in deer; also birds, rodents, reptiles.
  • Viral reservoirs:
    • Rabies: Bats, skunks, foxes, or dogs.

Human Carriers

  • Active Carriers: Infected individuals transmitting the pathogen.
    • May or may not show signs/symptoms (asymptomatic).
    • Asymptomatic individuals can still be infectious.
    • Example: SARS-CoV-2 (COVID-19), where asymptomatic individuals spread the virus.
    • Some may have a dormant period (e.g., AIDS).
  • Passive Carriers: Not infected, but contaminated; mechanically transmit the pathogen.
    • Example: Healthcare worker transferring pathogens on their skin/clothes to a new host.
    • Healthy carriers often have natural resistance or defense systems.

Non-Living Reservoirs

  • Soil, water, and food are common non-living reservoirs.
  • Contamination often due to feces or urine (human or animal).
  • Example: Bacillus anthracis spores in soil.
  • Water and food: Common sources of foodborne illnesses.

Pathogenesis

  • Pathology: The study of disease.
  • Pathogenesis: Stages of disease development.

Key Definitions

  • Contamination: Presence of microbes in or on the body (does not necessarily mean infection).
  • Infection: Microbe has evaded host defenses, multiplied, and established itself in the body.
    • May or may not result in disease.
  • Disease: A step further; occurs after infection.

Stages of Pathogenesis

  1. Exposure or Contact
  2. Adhesion or Colonization: Microbe adheres to cells/tissues to establish a colony.
  3. Invasion: Pathogen moves beyond the original point of exposure, deeper into the body.
  4. Infection: Microbe establishes itself.
Pathogen Requirements for Disease
  • Gain entry into the host.
  • Travel to a location where it can establish an infection.
  • Evade/overcome the host immune system.
  • Cause damage (results in disease).
  • Complete cycle: Pathogen exits the host and transmits to a new host.

Portals of Entry

  • Sites where pathogens enter the body.
Major Pathways
  • Skin
    • Effective physical barrier, but compromised by breaks, insect bites, punctures.
  • Mucous Membranes
    • Line body cavities exposed to the environment (eye, ear, nose, mouth).
    • Connected to respiratory and gastrointestinal tracts.
    • Also found in the reproductive system (vagina), urinary system (urethra), and anus.
  • Placenta
    • Relevant to the fetus; connects mother and fetus.
  • Parenteral Route
    • Circumvents usual portals; pathogens enter through openings or cuts in the skin.
    • Example: Pathogens entering through cuts from a knife or nail.

Skin

  • Outer layer is dead cells, an effective barrier.
  • Parenteral route: Pathogens enter through cuts/openings.
  • Some pathogens burrow or secrete chemicals to digest the skin.

Mucous Membranes

  • Moist, warm environment hospitable to pathogens.
  • Respiratory Tract: Most common site of entry (nose, mouth, eyes).
    • Viruses can enter through the conjunctiva of the eyes.
    • Avoid touching the face to prevent pathogen entry.
  • Gastrointestinal Tract: Pathogens must survive the acidic environment.

Placenta

  • Tissue connecting mother and fetus.
  • Most pathogens cannot cross, but some can (2% of pregnancies).
  • Infections can cause spontaneous abortion, birth defects, or premature birth.
  • Examples of pathogens that can cross the placenta:
    • Protozoan: Toxoplasmosis
    • Bacterium: Syphilis
    • Viruses: Chickenpox, Hepatitis B, HIV, Rubella, Cytomegalovirus, Herpes

Parenteral Route

  • Not a true portal of entry but a means to circumvent normal portals.
  • Injury deposits pathogens into tissues.
  • Example: Tattoo needles puncturing the skin.

Adhesion or Colonization

  • Pathogen attaches to cells or tissues using adhesion factors.
  • Adhesion Factors: Specialized structures or attachment molecules.
    • Ligands on viruses and bacteria bind to host cell receptors (host specificity).
    • Example: Virion spike proteins.
  • Preventing adhesion can prevent disease.
    • Block adhesion factors or receptors.
    • Example: Cranberry juice prevents bacteria from adhering to urinary tract walls, preventing UTIs.
  • Microbes lacking attachment proteins become avirulent.

Ligands and Receptors

  • Ligands on microbes recognize glycoprotein receptors on host cells, enabling attachment.
  • Without attachment, microbes are flushed out by bodily fluids.
  • Example: E. Coli cells attaching to the mucous membrane of the intestines.

Biofilms

  • Formed through adhesion factors.
  • Bacteria attach to tissues/cells or other bacteria.

Invasion

  • Pathogen moves/spreads to establish an infection.
  • Spreads through local tissues or disseminates throughout the body.
  • Virulence factors (exoenzymes, toxins) facilitate colonization and damage.
  • Virulence Factors: Proteins/chemicals that allow the cell to invade and establish an infection.
    • Some pathogens produce exoenzymes or toxins.
    • Some produce factors that protect against the immune system (cloaking devices).
  • Virulence factors determine the degree of damage.

H. Pylori Example

  • Helicobacter pylori (H. pylori) invades the stomach lining.
  • Produces virulence factors to pass through the mucus barrier.
    • Releases urease to neutralize stomach acid.
    • Liquefies mucin, allowing bacteria to swim through the protective barrier.
  • H. pylori is a common cause of stomach ulcers.

Infection

  • Pathogen establishes itself and multiplies after exposure, adhesion, and invasion.

Types of Infections

  • Local: Confined to a small area near the portal of entry.
  • Focal: Localized, but pathogens/toxins can spread to a secondary location.
  • Systemic: Throughout the whole body.
  • Type of infection depends on the virulence factors of the pathogen.

Recap Questions

  • All of the following are examples of zoonotic diseases except what?
    • Ringworm, rabies, chickenpox, or malaria.
    • Answer: Chickenpox (transmitted human to human).
  • Which of the following is an example of a parenteral route of infection?
    • Inhaling contaminated air? Pathogens crossing the placenta? An infant being exposed to normal flora? Or stepping on a nail.
    • Answer: Stepping on a nail (bypasses normal portals).
  • What's the most frequently used portal of entry?
    • Respiratory tract, skin, parenteral route, or placenta?
    • Answer: Respiratory tract (eyes, nose, mouth).