Chain of Infection

Chain of Infection

• Serves as a model to illustrate how infections are transmitted between individuals.
• Consists of several interconnected links that represent steps in infection transmission.
• If any link in the chain is broken, the infection cannot be transmitted.
• Infectious agents are the pathogens like bacteria, viruses, or fungi which cause infection.
• Transmission initiates when a pathogen is present and capable of infecting a host
• Reservoirs are environments or hosts like humans, animals, surfaces, or soil, where pathogens live and reproduce.
• Pathogens cannot survive long enough to infect without a reservoir.
• Portal of exit refers to how pathogens exit the reservoir, such as through bodily fluids, respiratory droplets, or broken skin.
• Pathogens need to exit the infected host to infect the next one
• Mode of transmission is how the infection spreads from one person or reservoir to another.
• Direct transmission is person-to-person contact.
• Indirect transmission occurs through contaminated surfaces, air, or vectors like insects.
• Portal of entry is how the pathogen enters a new host, possibly through breaks in the skin, mucous membranes like the nose or mouth, or the respiratory tract
• Susceptible hosts are individuals who can potentially become infected.
• Host vulnerability depends on factors like immune status, age, pre-existing health conditions, and vaccination status

Example of Infection Transmission

• A person infected with a flu virus (infectious agent) sneezes (portal of exit).
• Droplets travel through the air (mode of transmission).
• Droplets land in another person's nose or mouth (portal of entry).
• If the second person is not immune (susceptible host), they can become infected.

Preventing Viral Spread

• Breaking links can prevent the spread of infection.
• Disinfecting surfaces breaks the mode of transmission.
• Wearing masks blocks the portals of exit and entry.
• Practicing hand hygiene prevents the agent’s transfer.
• Vaccinating individuals reduces susceptibility.

Modes of Transmission

• Contact transmission is the transfer of microbes by physical touch.
• Direct contact involves physical skin-to-skin contact between an infected or colonized individual and a susceptible host like C-diff
• Indirect contact involves contact between a susceptible host and a contaminated object, like soiled linen, equipment, or unwashed hands like C-diff, MRSA, VRE.
• Droplet transmission produces droplets via coughing, sneezing, or talking from influenza virus.
• Airborne transmission has airborne particles containing microbes where the susceptible host inhales them, like Coronavirus
• Vehicle transmission is when a single contaminated source (water, medications, food) transmits infections to multiple hosts for example, Salmonella.
• Vectorborne transmission: insects transmit microbes to humans, for example Lyme Disease

Body's Defenses Against Infection

• Skin acts as a barrier to microorganisms and provides antibacterial activity.
• Mouth serves as a mechanical barrier, washing away particles with microbial inhibitors (lysozyme).
• Eyes blink to prevent pathogen entry; tearing helps wash particles away.
• Respiratory tract traps inhaled microbes and sweeps them outward in mucus for expectoration or swallowing.
• Urinary tract washes away microorganisms on lining of bladder and urethra.
• Gastrointestinal tract destroys microorganisms via acid.
• Vagina inhibits microorganism growth.

Inflammatory Response

• Blood vessels supplying the injured area constrict and then dilate, increasing blood flow, leading to redness, heat, and swelling.
• Local vasodilation delivers RBCs and WBCs to the injured site.
• Injury causes tissue necrosis resulting in the body producing histamines, bradykinin, prostaglandin, and serotonin.
• A result is increased permeability, fluid, protein, and cells that enter the interstitial space, causing edema.
• Pain results from edema pushing on nerve endings, causing temporary loss of function.
• Accumulation of fluids, dead tissue cells, and WBCs form an exudate at the site of inflammation.
• Serous exudate is clear, watery plasma.
• Sanguineous exudate is bloody drainage.
• Serosanguineous exudate is thin, watery drainage tinged with blood.
• Purulent exudate is thick drainage containing pus.
• Tissue repair involves regeneration and scar tissue formation.

Localized vs Systemic Infections

• Localized infections are restricted to one area of the body, affecting a specific part or organ.
• Localized infections do not spread throughout the entire body.
• Symptoms include redness (erythema), swelling (edema), pain, heat, pus or abscess, and loss of function.
• Skin infections, urinary tract infections, ear infections, and sinus infections are examples of localized infections.
• Systemic infections affect the entire body or multiple organ systems, spreading beyond the initial site.
• Spread occurs through the bloodstream or lymphatic system
• Symptoms of a systemic infection can include fever, chills, fatigue, body aches, headache, nausea and vomiting, increased heart rate, low blood pressure, shortness of breath, and confusion or disorientation.
• Sepsis, bloodstream infections, pneumonia, meningitis, influenza, and severe viral infections are examples of systemic infections.

Risk Factors for Infection

• Age, especially in the elderly.
• Poor diet and reduced intake of protein and other nutrients.
• Emotional or physical.
• Disease processes like leukemia including AIDS, lymphoma, and aplastic anemia.
• Medical therapies like prescriptions.

Nosocomial Infections

• Infections are acquired while receiving treatment for other conditions in a healthcare setting.
• Invasive procedures and medical devices can lead to infection include catheters, wound drains, and feeding tubes.
• Weakened immune systems in immunocompromised individuals, the elderly, and newborns increase the risk of infection.
• Antibiotic use and resistance contributes to nosocomial infections.
• Cross-contamination from healthcare workers due to lack of hygiene or inadequate sterilization practices.
• Hospital environments expose patients to contaminated surfaces, equipment, and airborne transmission.
• Patient mobility and hygiene practices affect the risk.
• Transmission from visitors.

Medical vs Surgical Asepsis

• Medical asepsis (clean technique) involves practices and procedures to reduce the number and spread of microorganisms, focusing on cleanliness and infection prevention without total sterility, like hand hygiene, disinfectants, using PPE, and proper disposal of contaminated materials.
• Surgical asepsis (sterile technique) eliminates all microorganisms from the environment to prevent infection, ensuring no pathogens are present, like sterilizing instruments and equipment, maintaining a sterile field, practicing strict hand hygiene, and wearing sterile gloves and gowns.

Elements of the Infection Chain and Prevention

• For infectious agents like bacteria and viruses: prompt treatment, decontamination, and rapid identification of the organism.
• For reservoirs like people and equipment: proper hygiene, sanitation, disinfection, and sterilization.
• For the portal of exit like excretions and secretions: hand hygiene, proper PPE, and waste disposal.
• For mode of transmission like contact, droplets, and airborne routes: hand hygiene, disinfection/sterilization, proper food handling, and isolation precautions.
• For the portal of entry like the respiratory tract and GI tract: aseptic technique, wound care, and hand hygiene.
• For susceptible hosts like the elderly, immunocompromised, and infants: treatment of primary disease and recognition of high-risk patients.

Additional Information

• Resident skin microorganisms are usually nonpathogenic but can cause infection if they enter deep tissues during surgery or when a patient is immunocompromised.
• Resident microorganisms survive and multiply without causing harm and are not usually removed by hand hygiene.
• Transient microorganisms attach to the skin by contact with another person or object and can be removed with proper hand hygiene.
• Virulence is the ability to produce disease.

The chain of infection is a model that illustrates how infections are transmitted. It includes infectious agents, reservoirs, portals of exit, modes of transmission and susceptible hosts. Breaking any link prevents infection transmission.