25.Pathogenesis of Infectious Process -Development, Infectious Dose,

Pathogenesis of Infectious Process -Development, Infectious Dose, Portals of Entry, Routes of Dissemination and Localization of Microbes in the Host.

Pathogenesis is the mechanisms by which a pathogen causes disease in a host. It describes the origin and development of a disease.

It starts when a pathogen comes into contact with a suitable host. The host must be susceptible.
The pathogen needs to enter the body, often by overcoming physical barriers like skin or mucous membranes.
It then adheres to host tissues using adhesion factors (like pili or capsules).
Next, it may invade host cells or tissues.
The pathogen must evade or overcome host immune defenses.
Colonization is the establishment of a stable population on skin or mucous membranes.
Pathogens cause tissue injury. Damage can be direct (e.g., by toxins like exotoxins, endotoxins, cytotoxins, neurotoxins, enterotoxins) or by triggering damaging inflammation.
Disease development depends on the pathogen's virulence factors, the host's immune status, and the ability to repair damage.

This is the minimum number of pathogens needed to cause an infection.
The dose varies greatly between pathogens.
Some require a low infectious dose (e.g., Shigella spp. with as few as 10-100 cells), while others need a high infectious dose (e.g., cholera requiring larger numbers, or enteric salmonellae >10^6 bacteria).
It's influenced by pathogen virulence, host immune status, and route of transmission.

These are the main routes pathogens use to enter the body.
Common portals include:
Respiratory tract: via airborne microbes.
Digestive tract: via contaminated food or water.
Skin: particularly if it's compromised by cuts, abrasions, or bites.
Genital & Urinary Tract: relevant for UTIs and STIs.
Parenteral Routes: direct entry beneath the skin from wounds, injections, or arthropod bites.

This is how a pathogen spreads from the initial entry site.
Local spread: Pathogen remains and spreads only in the immediate area.
Systemic spread: Pathogens spread throughout the body. Specific systemic routes include:
Lymphatic spread: Through the lymphatic system.
Hematogenous spread: Through the bloodstream (bacteremia, viremia).
Neural spread: Along nerves (seen with viruses like rabies or herpesviruses).
Cell-to-cell spread: Directly from one host cell to an adjacent one.

After dissemination, microbes often settle and cause infection in specific tissues or organs.
This is called tissue-specific tropism. Pathogens infect specific tissues based on factors like receptors, environment, or host response.
Examples include HIV targeting CD4+ T-cells, Hepatitis B targeting liver cells, and Neisseria meningitidis targeting the brain/meninges.